Presentation type:
OS – Ocean Sciences

EGU23-9125 | ECS | Orals | MAL13 | OS Division Outstanding Early Career Scientist Award Lecture

Atlantic Multidecadal Variability and the Implications for North European climate 

Florian Börgel, H. E. Markus Meier, Matthias Gröger, Cyril Dutheil, Monika Rhein, Leonard Borchert, and Hagen Radtke

The North Atlantic exhibits temperature variations on multidecadal time scales, summarized as the Atlantic Multidecadal Variability (AMV). The AMV plays an essential role in regional climate and is a crucial driver of the low-frequency variability in Northern Europe.

In this talk, I will first discuss the characteristic ocean-atmosphere interaction preceding an AMV maximum event. In the following, I will disentangle the seasonal impact of the AMV and show that a significant fraction of the variability in Baltic Sea winter temperatures is related to the AMV. The strong winter response can be linked to the interaction between the North Atlantic Oscillation, the Atlantic Meridional Overturning Circulation (AMOC), and the AMV. In contrast, the AMVs' impact on other seasons remains small.

How to cite: Börgel, F., Meier, H. E. M., Gröger, M., Dutheil, C., Rhein, M., Borchert, L., and Radtke, H.: Atlantic Multidecadal Variability and the Implications for North European climate, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9125, https://doi.org/10.5194/egusphere-egu23-9125, 2023.

EGU23-16418 | Orals | MAL13 | Fridtjof Nansen Medal Lecture

Rewriting the tale of deep-ocean upwelling 

Alberto C. Naveira-Garabato

Since the seminal work of Walter Munk in the 1960s ('Abyssal Recipes'), oceanographers have believed that the upwelling of cold, abyssal waters that regulates the deep ocean's ability to sequester heat and carbon for decades to millennia is mainly driven by centimetre-scale turbulent mixing associated with breaking internal waves in the ocean interior. Measurements of deep-ocean turbulence over the last >20 years, however, have contested this scenario, and instead suggest that mixing by breaking internal waves drives *downwelling* of abyssal waters. Inspired by this conundrum, recent theoretical investigations have developed an alternative view of the role of mixing in sustaining deep-ocean upwelling. In this new view, upwelling is driven by highly localised turbulence within thin (typically tens of metres thick) layers near the seafloor, known collectively as the bottom boundary layer. In the BLT Recipes experiment, we recently set out to test this new view, and figure out how it works, by obtaining the first set of concurrent, systematic measurements of (1) large-scale mixing and upwelling, (2) their interior and bottom boundary layer contributions, and (3) the processes underpinning these contributions, in a representative deep-ocean basin (the Rockall Trough, in the Northeast Atlantic). This talk will review the insights emerging from the BLT Recipes experiment, and offer an outlook on how they might re-shape our understanding of the way in which turbulence sustains deep-ocean upwelling.

How to cite: Naveira-Garabato, A. C.: Rewriting the tale of deep-ocean upwelling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16418, https://doi.org/10.5194/egusphere-egu23-16418, 2023.

OS1 – Ocean Circulation and Climate

EGU23-442 | ECS | Posters on site | OS1.1

Time-Lapse Volumetric Seismic Imaging of Water Masses at a Major Oceanic Confluence in the South Atlantic Ocean 

Xiaoqing Chen, Nicky White, and Andy Woods

Water-mass interaction processes within the Southern Ocean strongly influence the global oceanic circulation system.  For example, the western side of the South Atlantic Ocean is dominated by the confluence between the Brazil Current (BC) and Falkland/Malvinas Current (MC). At this confluence, tropical/subtropical (i.e. warm and salty) waters are transported southward by the BC where they interact with subantarctic (i.e. cold and fresh) waters transported northward by the MC. This interaction creates a highly dynamic frontal system that is characterized by complex water mass interactions and intense diapycnal mixing. Here, we exploit time-lapse volumetric seismic imaging of the Brazil-Malvinas Confluence (BMC) in order to elucidate the detailed thermohaline structure of this critical region. Careful signal processing of a ~25 terabyte survey, acquired during February 2013, reveals a spectacular northeastward dipping oceanic front that extends as deep as ~1800 m. Significantly, a deep transient mesoscale eddy is embedded in this front. This eddy appears to grow and decay over ~11 day period and it has a maximum diameter of ~40 km. Time-lapsed imagery also reveals mesoscale to sub-mesoscale complexity at all depths. Long wavelength temperature fields extracted from our acoustic velocity measurements reveal a pattern of cool anomalies on the MC side together with a steep and fanning temperature gradient close to the front but above the eddy, indicative of heat transfer. Evolution of this prominent eddy embedded in the front can be independently investigated using velocity fields calculated from the GLORYS12v1 product for the period of interest. Tracked particles, which are released daily through the confluence area down to 1800 m, flow along the MC from 40° S  to 36° S and are deflected clockwise by the BMC. This flow suggests that the observed eddy is cyclonic and related to MC recirculation, as a result of the combination of the steep continental slope and geometry of the BMC. In this way, cooler water masses are juxtaposed against the front. A simple one-dimensional steady-state model is used to examine heat transfer across the front. Our results highlight the importance of combining high quality three-dimensional seismic imagery with hydrographic observations in order to elucidate the fluid dynamics of complex oceanic fronts.

How to cite: Chen, X., White, N., and Woods, A.: Time-Lapse Volumetric Seismic Imaging of Water Masses at a Major Oceanic Confluence in the South Atlantic Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-442, https://doi.org/10.5194/egusphere-egu23-442, 2023.

EGU23-533 | ECS | Posters on site | OS1.1

Weakened AMOC upper limb compensated by strengthened South Atlantic subtropical gyre  circulation in CESM1-LE simulations 

Fernanda Marcello, Marcos Tonelli, Bruno Ferrero, and Ilana Wainer

The upper limb of the Atlantic Meridional Overturning Circulation (AMOC) brings shallow interocean contributions to replenish the North Atlantic export of deepwaters. It is primarily formed in the southern South Atlantic where the converging entrainment of Pacific and Indian Ocean waters meet and incorporate into the South Atlantic subtropical gyre (SASG). Here, the human-induced response of AMOC and SASG near-surface pathways is illustrated according to CESM1 Large Ensemble simulations from 1920 to 2100, where future projections derive from the most aggressive (yet most realistic) scenario in assumed fossil fuel use and greenhouse gas emissions. In terms of flow redistribution, it is shown that the AMOC upper limb weakens not because less waters are being imported from the adjacent ocean basins — but because they are being mostly directed to recirculate in the southwestern portion of a distorted SASG, turning back southward after reaching the South Atlantic western boundary.

How to cite: Marcello, F., Tonelli, M., Ferrero, B., and Wainer, I.: Weakened AMOC upper limb compensated by strengthened South Atlantic subtropical gyre  circulation in CESM1-LE simulations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-533, https://doi.org/10.5194/egusphere-egu23-533, 2023.

EGU23-558 | ECS | Orals | OS1.1

Characterizing the spatial coherence of mesoscale eddies using in-situ data 

Yan Barabinot and Sabrina Speich

Ocean mesoscale eddies are ubiquituos in the global ocean. They are responsible of about 80% of the total eddy kinetic energy and are suggested to exert a significant impact on air-sea interactions, ocean large-scale circulation, weather and marine ecosystems. They have been qualified as "coherent" structures as they can leave for months if not years propagating in the ocean interior. As ocean observations are very sparse, they have been essentially characterized from satellite altimetry fields, which provides access to a limited number of surface characteristics of only those eddies having an imprint on sea surface height.  Observations of mesoscale eddies 3D structure, or even 2D vertical sections are rare.  On the other hand, accurate description of ocean eddies from high-resolution ocean numerical simuation are also limited. In general, they have been accoubted for via statistics, instead of individual descriptions as the latter is difficult as they move away from fixed positions. In this work we present a detailed study of ocean eddies (surface and subsurface intensified) sampled during 10 oceanographic cruises which have a sufficient horizontal spatial resolution of the vertical eddy sampling - 9 in the Atlantic Ocean (during experiments EUREC4A-OA, M124, MSM60, MSM74, M160, HM2016611, KB2017606, KB 2017618), and one in the Indian (during the Physindien 2011 experiment). Our study characterizes the eddy core and boundary in a generic way using diagnostics based on active (PV, oxygen) and passive (temperature, salinity) tracers. Despite the different resolutions of the eddy sampling in the 9 studied regions, we show that the 3D boundary of an eddy behaves like a frontal zone characterized by the Ertel PV where the water mass trapped in the eddy joins with the surrounding waters. Whatever the  origin and size of the eddy are, the core is homogeneous in properties with the anomaly maximum located at depth, which makes its altimetric characterization difficult. Moreover, these analyses provide a new metrix for defining the coherence of an ocean eddy, a concept that has been always ill-defined because of the elusive character and undersampling of these structures.

 

How to cite: Barabinot, Y. and Speich, S.: Characterizing the spatial coherence of mesoscale eddies using in-situ data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-558, https://doi.org/10.5194/egusphere-egu23-558, 2023.

Mesoscale ocean turbulence is the best-known expression of Chaotic Intrinsic Variability (CIV), which spontaneously emerges from the unstable ocean circulation regardless of the atmospheric variability. Substantial amounts of CIV are also found up to the scale of basins and decades, potentially produced by large-scale baroclinic instability or resulting from spatiotemporal inverse cascade processes.

A 56-year atmospherically-forced 50-member 1/4° large ensemble simulation of the global eddying ocean/sea-ice system has been performed to explore these phenomena using the NEMO model. We first show that the low-frequency large-scale (LFLS) CIV has climate-relevant imprints over most of the globe, is largest in western boundary currents and south of about 30°S, and competes with (and in certain zones exceeds) the atmospherically-forced ocean variability (AFV) in terms of amplitude.

However, the separability of AFV and CIV is questionable in certain cases. Concepts from dynamical system and information theories are leveraged to avoid this separation, and to probabilistically describe the ocean variability as an atmospherically-modulated oceanic "chaos". The partly random character of multi-scale ocean fluctuations in the eddying regime questions the attribution of observed signals to sole atmospheric drivers, the turbulent ocean predictability and its potential influence in high-resolution coupled simulations.

How to cite: Penduff, T.: Describing the ocean variability as an atmospherically-modulated oceanic "chaos", EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2013, https://doi.org/10.5194/egusphere-egu23-2013, 2023.

EGU23-2308 | ECS | Posters on site | OS1.1

On AMOC and climate feedback: Evidence from Coupled and Slab-ocean models 

Kai-Uwe Eiselt and Rune Graversen

It has recently been established that in numerical model experiments climate sensitivity and feedback change over time and that this time dependence may result from a so-called “pattern effect”, i.e., changing patterns of surface warming. The Atlantic Meridional Overturning Circulation (AMOC) influences surface warming patterns as it redistributes energy across the globe. Thus, it may be an important factor for climate feedback change over time.

In this study, members of the Coupled Model Intercomparison Project (CMIP) phases 5 and 6 are investigated and two groups of models distinguished, one with weak and one with strong feedback change over time. It is found that the model groups differ significantly in the AMOC response to quadrupling of CO2. To investigate if the difference in AMOC development between the two groups may be responsible for the differences in feedback, experiments with a slab-ocean model (SOM) are performed where the AMOC change is mimicked by changing the ocean heat uptake pattern. Especially in the Northern-Hemisphere Extra-Tropics the differences between the CMIP model groups are found to be qualitatively reproduced but other factors are needed to explain differences in the Southern Hemisphere and the Tropics.

How to cite: Eiselt, K.-U. and Graversen, R.: On AMOC and climate feedback: Evidence from Coupled and Slab-ocean models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2308, https://doi.org/10.5194/egusphere-egu23-2308, 2023.

MaCOM model takes the international advanced numerical model NEMO as the power core, coupled with the sea ice model, with the horizontal grid resolution better than 10 km and a total of 75 layers in vertical direction. On this basis, a comprehensive integrated numerical forecasting system with data collection system as the root, ensemble assimilation system as the backbone, forecasting system as the branch and product production system as the terminal has been developed, forming a distributed and loosely coupled tree operation and maintenance architecture with four subsystems: data collection, data assimilation, numerical forecasting and product distribution.

In order to test the MaCOM model forecasting effect, the MaCOM model is used to make day-by-day forecasts of temperature, salt, current, sea surface height and other variables of the global ocean for the whole year of 2020. This experiment focuses on evaluating the model performance, and to avoid differences in assimilation systems, the global 1/12 resolution day-by-day analysis field of the PSY4 model v3r1 version of the Mercator Center in France is selected as the initial field of the model; the GFS meteorological forecast field data is used as the model upper surface forcing field to drive the model; the model is run from the forecast moment with a forecast time limit of 7 days, and after each forecast process the The forecast results are interpolated to the standard latitude and longitude grid and depth after each forecast process; other settings of the model remain unchanged.The model forecasts are compared using the GOV IV-TT (The GODAE Oceanview Intercomparison and Validation Task Team) Class 4 standard method, which is commonly used to evaluate the performance of forecast systems and forecast skill. The statistics used in the evaluation are based on the comparison of model forecasts with observations, including root mean square error (RMES), bias (Bias), and anomaly correlation, as well as comparing forecasts with climatology and persistence.The following conclusions were obtained from the 2020  evaluation:

  • The MaCOM model sea surface temperature forecasts are less biased and closer to the live observations, with RMSE around 0.6℃ and better forecast stability, and PSS and CSS show that the model has obvious positive skill.
  • The vertical structure test of the MaCOM model shows that the RMSE is around 0.6℃, and the forecastability of temperature profiles in the Southern Ocean, Indian Ocean, South Pacific, North Pacific and other Southern Hemisphere regions is better than that of the PSY4 model.
  • The RMSE of sea surface height anomaly of MaCOM model is around 0.05m, which is smaller than that of PSY4. The PSS test indicates that the forecasting skill of MaCOM model for sea surface height anomaly needs further improvement.
  • MaCOM has better forecasts than PSY4 for sea surface temperature, vertical structure of temperature and salt, and sea surface height anomalies; among them, it has effective forecasting techniques for vertical structure of temperature and salt and sea surface temperature, and can better simulate the weather-scale variability, which has good operational application value.

How to cite: Qi, D. and Chen, D.: The Forecast Performance Evaluation of numerical prediction model of ocean temperature and salt flow (MaCOM), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2562, https://doi.org/10.5194/egusphere-egu23-2562, 2023.

We analyzed physical oceanic parameters gathered by a mooring array at mesoscale spatial sampling deployed in Argentine Basin within the Ocean Observatory Initiative, a National Science Foundation Major Research Facility. The array was maintained at 42°S 42°W, a historically sparsely sampled region with small ocean variability, during 34 months from March 2015 to January 2018. The data documented four anticyclonic extreme structures events in 2016 and the presence of near-inertial waves (NIWs) trapped at depth within the anticyclones. Although the four anticyclonic structures had different characteristics (size, vertical extension, origin, lifetime, Rossby Number) they all featured low Richardson values well below the mixed layer associated to NIWs. Low Richardson values suggest favorable conditions for mixing. The anticyclonic features act as mixing structures at the pycnocline bringing heat and salt from the South Atlantic Central Water to the Antarctic Intermediate Waters. The extreme events were unique in the 29-year-long satellite altimetry record at the mooring site. However, the Argentine Basin is populated with many anticyclones and mixing associated to trapped NIWs probably plays an important role in setting up the upper water masses characteristics in the Basin.

How to cite: Artana, C. and Provost, C.: Intense Anticyclones and Near Inertial Trapped Waves at the Global Argentine Basin Array of the Ocean Observatory Initiative, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3318, https://doi.org/10.5194/egusphere-egu23-3318, 2023.

EGU23-3615 | Orals | OS1.1

The role of internal variability and feedbacks controlling AMOC stability 

Anastasia Romanou, David Rind, Jeff Jonas, Ron Miller, Maxwell Kelley, Gary Russell, Clara Orbe, Larissa Nazarenko, Rebecca Latto, and Gavin A. Schmidt

A bi-stable mode of the Atlantic Meridional Overturning Circulation (AMOC) is found in a 10-member ensemble simulation of the SSP2-4.5 scenario using the NASA GISS-E2-1-G climate model. Local feedbacks in the subpolar North Atlantic region in conjunction with internal variability in sea-ice transport and melt play a critical role in causing the divergent behavior of the AMOC in the ensemble members. While other fully coupled models have demonstrated the important role of surface freshening in leading to AMOC shutdown, either through hosing experiments or increased precipitation and greenhouse gas warming at high latitudes, in the GISS simulations, there are no external freshwater perturbations. This is the first time that a CMIP-class model has shown such a bifurcation across an initial condition ensemble.

 

How to cite: Romanou, A., Rind, D., Jonas, J., Miller, R., Kelley, M., Russell, G., Orbe, C., Nazarenko, L., Latto, R., and Schmidt, G. A.: The role of internal variability and feedbacks controlling AMOC stability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3615, https://doi.org/10.5194/egusphere-egu23-3615, 2023.

EGU23-4006 | Orals | OS1.1

On the Entrainment of Glacial Meltwater by the Gulf Stream 

Olivier Marchal and Alan Condron

The fate of glacial meltwater introduced into the ocean is an important problem both in paleoceanography and in modern oceanography. A long-standing question in paleoceanography concerns the evolution and consequences of the glacial meltwater delivered from the great ice sheets which covered a large fraction of North America and Europe during glacial periods. Although the associated rise in mean sea level of about 130 m has long been estimated, the pathways and impacts of the glacial meltwater for ocean circulation and climate remain poorly understood. Notably, the ocean components of climate models do not generally have a spatial resolution that is fine enough to properly simulate coastal phenomena which are known to contribute to the offshore export of shelf water in the modern ocean.

Here we apply a regional eddy-revolving numerical model of ocean circulation in order to explore the pathways of glacial meltwater emanating from the St. Lawrence Channel – a major ice stream of the Laurentide Ice Sheet of North America during the last glacial period. Emphasis is placed on the offshore entrainment of glacial water by the Gulf Stream (GS), which according to paleoceanographic observations detached from the continental slope near Cape Hatteras, as it does today. First, a simulation of the eddying circulation in the glacial western North Atlantic is obtained by integrating the regional model to statistical steady state under glacial atmospheric forcing. Second, a series of glacial water discharge experiments are conducted for various assumptions about the discharge, including its volume transport, its density, and its seasonal timing. Mechanisms of glacial water export away from the slope are identified, such as the eastward entrainment by (anticyclonic) warm core rings and the subsequent incorporation of the glacial water into the GS offshore. The implications of our results for the interpretation of sediment records from the Laurentian Fan and for the simulation of glacial water discharges in paleoclimate models are then clarified.

How to cite: Marchal, O. and Condron, A.: On the Entrainment of Glacial Meltwater by the Gulf Stream, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4006, https://doi.org/10.5194/egusphere-egu23-4006, 2023.

EGU23-4562 | Posters on site | OS1.1

A regime shifts in North Pacific subtropical mode water formation 

Sang-Yeob Kim, Young-Oh Kwon, Wonsun Park, and Ho Jin Lee

A regime shift in the formation mechanisms of the North Pacific subtropical mode water (NPSTMW) was investigated using 50-year (1960-2009) ocean general circulation model (OGCM) and 2,000-year fully coupled atmosphere–ocean–sea ice model (Kiel Climate Model; KCM). We found that primary driving mechanism for NPSTMW formation is alternated between air–sea interaction (ASI) and ocean dynamics (OD) from two model simulations. In the OGCM simulation, we revealed that the local air-sea interaction process is a main driver of the NPSTMW formation prior to late-1980s, while ocean dynamics including the vertical entrainment become dominant since then. In the KCM simulation, the relative importance of two (ASI and OD) has periodically alternated in multidecadal timescales of approximately 50–70 years. The regime shift of the NPSTMW formation was closely related to the meridional (50 years) and zonal (70 years) movements of the Aleutian Low (AL). When AL shifted to the south or east, it induced the sea surface height anomalies propagating westward from the central North Pacific and preconditions for the NPSTMW formation, thus the ocean dynamics became relatively more important.

How to cite: Kim, S.-Y., Kwon, Y.-O., Park, W., and Lee, H. J.: A regime shifts in North Pacific subtropical mode water formation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4562, https://doi.org/10.5194/egusphere-egu23-4562, 2023.

Based on data collected from 14 air–sea buoys in the Gulf Stream, this study presents an examination of how the hourly air–sea turbulent heat fluxes vary on subdaily timescales under persistent marine atmospheric boundary layer (MABL) stability conditions. The annual mean magnitudes of the subdaily variations in the latent heat (LH) and sensible heat (SH) at all stations are determined to be 40 and 15 Wm-2, respectively. Under near-neutral conditions, the hourly anomalies of the air–sea humidity and temperature differences are the major drivers of the subdaily variations in LH and SH, respectively, followed by nonlinear effects and wind anomalies. Wind anomalies play dominant roles in determining the subdaily variations in LH and SH when the MABL is stable. In contrast, the contributions of the hourly anomalies of the air–sea differences in humidity and temperature are secondary but also significant. For a convectively unstable MABL, the wind anomalies control the subdaily variations in LH, whereas the subdaily variations in SH are dominated by the air–sea temperature anomalies. Accordingly, the above mechanism also controls the subdaily magnitudes. Quantitative estimates of the above relations are given in this study. However, compared to the observations when using daily mean SST, the subdaily variations in the reanalysis are found to be underestimated on average by 17% and 5% for LH and SH, respectively. Resolving the subdaily variations contributes significantly to the mean LH/SH estimates. For near-neutral and unstable MABLs, the subdaily contributions are O(100) and O(20) Wm-2 for LH and SH, respectively, while they are O(10) Wm-2 for LH/SH under stable conditions.

How to cite: Song, X., Xie, X., Yan, Y., and Xie, S.-P.: Observed subdaily variations in air–sea turbulent heat fluxes under different marine atmospheric boundary layer stability conditions in the Gulf Steam, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4621, https://doi.org/10.5194/egusphere-egu23-4621, 2023.

EGU23-4760 | Orals | OS1.1

Future changes in Antarctic coastal polynyas and bottom water formation simulated by a high-resolution coupled model 

Hyein Jeong, Sun-Seon Lee, Hyo-Seok Park, and Andrew Stewart

Antarctic coastal polynyas produce Dense Shelf Water, a precursor to Antarctic Bottom Waters (AABW) that supply the global abyssal circulation. Recent studies suggest that increasing atmospheric CO2 concentrations will weaken AABW export by suppressing heat loss to the atmosphere. However, future projections of DSW formation are hindered by the small spatial scales of atmosphere-sea ice-ocean interactions in polynyas. Here, using a high-resolution ocean-ice-atmosphere coupled model, this study shows that wintertime sea ice production rates are still active under elevated CO2 concentrations, although delayed freeze-up decreases autumn sea ice production. In winter, Antarctic coasts exhibit a nonlinear response CO2 concentration: doubling CO2 decreases sea ice production only by around 6–8%, versus 10–30% under CO2 quadrupling. Despite continued sea ice production in winter, doubling or quadrupling CO2 substantially freshens Dense Shelf Water, primarily due to increased precipitation, implying a shutdown of AABW formation.

How to cite: Jeong, H., Lee, S.-S., Park, H.-S., and Stewart, A.: Future changes in Antarctic coastal polynyas and bottom water formation simulated by a high-resolution coupled model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4760, https://doi.org/10.5194/egusphere-egu23-4760, 2023.

EGU23-7387 | ECS | Orals | OS1.1

Diagnosing the AMOC slowdown in a coupled model: a cautionary tale 

Justin Gérard and Michel Crucifix

It is now established that the increase in atmospheric CO2 is likely to cause a weakening, or perhaps a collapse of the Atlantic Meridional Overturning Circulation (AMOC). To investigate the mechanisms of this response in CMIP5 models, Levang and Schmitt (2020) have estimated offline the geostrophic streamfunction in these models, and decomposed the simulated changes into a contribution caused by the variations in temperature and salinity. They concluded that under a warming scenario, and for most models, the weakening of the AMOC is fundamentally driven by temperature anomalies while freshwater flux changes actually act to stabilize it.

However, given that both ocean temperature and salinity are expected to respond to a forcing at the ocean surface, it is unclear to what extent the diagnostic is informative about the nature of the forcing. To clarify this question, we used the Earth system Model of Intermediate Complexity (EMIC) cGENIE, which is equipped with the C-GOLDSTEIN friction-geostrophic model (Marsh et al. (2011)). First, we reproduced the experiments simulating the RCP8.5 warming scenario and observed that cGENIE behaves similarly to the majority of the CMIP5 models considered by Levang and Schmitt (2020), with the response dominated by the changes in the thermal structure of the ocean.

Next, we considered hysteresis experiments associated with (1) water hosing and (2) CO2 increase and decrease. In all experiments, changes in the ocean streamfunction appear to be primarily caused by the changes in the temperature distribution, with variations in the 3-D distribution of salinity compensating only partly for the temperature contribution. These experiments reveal also limited sensitivity to changes in the ocean's salinity inventory. That the diagnostics behave similarly in CO2 and freshwater forcing scenarios suggests that the output of the diagnostic proposed in Levang and Schmitt (2020) is mainly determined by the internal structure of the ocean circulation, rather than the forcing applied to it.

Our results illustrate the difficulty of inferring any information about the applied forcing from the thermal wind diagnostic and raise questions about the feasibility of designing a diagnostic or experiment that could identify which aspect of the forcing (thermal or haline) is driving the weakening of the AMOC.

Acknowledgements

This is a contribution to the WarmAnoxia project funded by the Belgian National Fund of Scientific Research.

References:

Levang, S. J. and Schmitt, R. W. (2020). What causes the amoc to weaken in cmip5? Journal of Climate, 33(4):1535–1545.

Marsh, R., Müller, S., Yool, A., and Edwards, N. (2011). Incorporation of the c-goldstein efficient climate model into the genie framework:" eb_go_gs" configurations of genie. Geoscientific Model Development, 4(4):957–992.

How to cite: Gérard, J. and Crucifix, M.: Diagnosing the AMOC slowdown in a coupled model: a cautionary tale, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7387, https://doi.org/10.5194/egusphere-egu23-7387, 2023.

EGU23-7637 | Posters on site | OS1.1

Accelerated North Atlantic climate variability triggered by increased seasonal melt 

Marilena Oltmanns, Sheldon Bacon, Penny Holliday, and Ben Moat

Freshwater plays a key role in the Arctic - North Atlantic climate system. On the one hand, it has been suggested as a precursor of large-scale weather extremes and as a potential trigger of rapid climate changes in the past. On the other hand, it is, itself, a sensitive climate change indicator that increases in response to the melting cryosphere. Yet, future risks arising from enhanced glacial and sea ice melt remain difficult to assess due to the complexity of the involved ice-ocean-atmosphere feedbacks and the interference of signals on different timescales. Combining observations, models, theory, and a sophisticated statistical approach, we demonstrate the central role of freshwater anomalies in North Atlantic climate variability over the last 70 years, assess the extent to which they have been contributing to weather extremes, and discuss the risk of a more fundamental climate change under increased freshwater fluxes in future.

How to cite: Oltmanns, M., Bacon, S., Holliday, P., and Moat, B.: Accelerated North Atlantic climate variability triggered by increased seasonal melt, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7637, https://doi.org/10.5194/egusphere-egu23-7637, 2023.

EGU23-8246 | Posters on site | OS1.1

Changing Atlantic Freshwater Transports in Response to Future Climate Projections 

Jennifer Mecking, Sybren Drijfhout, and Bablu Sinha

Changes in Atlantic meridional freshwater transports have been hypothesized to play an important role in Atlantic Meridional Overturning Circulation (AMOC) stability, redistributing precipitation and evaporation and dynamic sea level changes.  Freshwater transports can be altered by both changes in ocean circulation and changes in salinity.  In this study CMIP5 and CMIP6 models are analyzed to investigate how salinity and velocity changes impact the freshwater transports in the ScenarioMIP future projections.  In the multi-model means of CMIP5 and CMIP6 data there is only a slight increase (< 0.1Sv) in southward freshwater transport across the Atlantic basin.  This slight increase comes from a balance of changes in overturning (zonal mean) and azonal freshwater transports.  The changes in overturning and azonal freshwater transports are largest in the subtropical Atlantic where the salinity driven azonal changes drive an increase in southward freshwater transport which are almost completely counteracted by a velocity driven overturning freshwater transport.  Changes in these freshwater transports are larger in CMIP6 relative to CMIP5, which is especially noticeable in the velocity driven overturning changes (due to the larger AMOC weakening in CMIP6) and salinity driven azonal changes (due to a comma shaped freshening pattern in the North Atlantic, typically associated with a weakening of the AMOC).

How to cite: Mecking, J., Drijfhout, S., and Sinha, B.: Changing Atlantic Freshwater Transports in Response to Future Climate Projections, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8246, https://doi.org/10.5194/egusphere-egu23-8246, 2023.

EGU23-8749 | ECS | Orals | OS1.1

An abrupt transition in the Antarctic sea ice–ocean system 

F. Alexander Haumann, François Massonnet, Paul R. Holland, Mitchell Bushuk, Ted Maksym, Will Hobbs, Michael P. Meredith, Ivana Cerovečki, Thomas Lavergne, Walter N. Meier, Marilyn Raphael, and Sharon Stammerjohn

Over the past decade, Antarctic sea ice extent exhibited a sequence of record maxima, followed by a rapid decline in 2015/16, and record minima since. In this presentation, we show that this sudden and remarkable ice loss marks an abrupt transition from a high to a low ice state that cannot be explained by year-to-year variability. Instead, it is most likely associated with a longer term variability arising from ice–ocean feedbacks. The abrupt transition was preceded by a multi-decadal increase in persistence and variance of the sea ice anomalies, an increasing upper Southern Ocean density stratification, and an accumulation of heat at the subsurface; suggesting a decoupling of the surface from the subsurface ocean. During this period, the sea ice anomalies shifted from being structured predominantly regionally and seasonally to a largely circumpolar and interannual regime. In 2015/16, the upper ocean density stratification in the ice-covered region suddenly weakened, leading to a release of heat from the subsurface, contributing to the sea ice decline during winter. Our analysis suggests that the sudden sea ice loss in 2015/16, and the persisting low ice conditions since, arose from a systematic change in the physical state of the coupled circumpolar ice–ocean system. This change will have wide implications for global climate, ecosystems, and the Antarctic Ice Sheet.

How to cite: Haumann, F. A., Massonnet, F., Holland, P. R., Bushuk, M., Maksym, T., Hobbs, W., Meredith, M. P., Cerovečki, I., Lavergne, T., Meier, W. N., Raphael, M., and Stammerjohn, S.: An abrupt transition in the Antarctic sea ice–ocean system, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8749, https://doi.org/10.5194/egusphere-egu23-8749, 2023.

EGU23-9602 | Orals | OS1.1

An Atlantic-Pacific Seesaw in Circulation and Biogeochemistry 

James Rae, Charlotte O'brien, Louisa Bradtmiller, Andrea Burke, Holger Gebhardt, William Gray, Eloise Littley, Robb Wills, Xu Zhang, Michael Sarnthein, and David Thornalley

By regulating the supply of carbon, nutrients, and heat, ocean circulation at high latitudes plays a critical role in global climate.  During the last ice age, the Atlantic’s overturning circulation was repeatedly perturbed, associated with major changes in climate, but little is known of the response of biogeochemistry and circulation in the Pacific.  Here we present new high-resolution data that illuminate the coupled changes in circulation, CO2 and nutrient supply, and biological productivity associated with rapid climate change events at northern high latitudes.  We show that abrupt stadial cold events are consistently associated with pulses of enhanced nutrient supply and diatom productivity at mid latitudes in the North Atlantic.  Abrupt changes are also seen in the North Pacific, but are anti-phased, with peaks of productivity and nutrient supply occurring during abrupt interstadial warming.  Using model simulations, we show that these productivity changes can be explained by abrupt switches in the mode of overturning circulation, with weakened overturning associated with accumulation of nutrients in the subsurface waters that supply the surface via winter mixing and upwelling, alongside a southward shift of nutrient-rich subpolar waters.  Our results demonstrate the persistent operation of an Atlantic-Pacific seesaw in overturning circulation and biogeochemistry on centennial to millennial timescales and provide a valuable test for simulation of interlinked changes in circulation, biogeochemistry, and climate.

How to cite: Rae, J., O'brien, C., Bradtmiller, L., Burke, A., Gebhardt, H., Gray, W., Littley, E., Wills, R., Zhang, X., Sarnthein, M., and Thornalley, D.: An Atlantic-Pacific Seesaw in Circulation and Biogeochemistry, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9602, https://doi.org/10.5194/egusphere-egu23-9602, 2023.

EGU23-10855 | ECS | Orals | OS1.1

Increasing role of Kuroshio-Oyashio Extension variations as a conveyor of decadal ocean oscillation to seasonal air-sea heat exchange since the late 1980s 

Youngji Joh, Thomas Delworth, Andrew Wittenberg, Xiaosong Yang, Anthony Rosati, Nathaniel Johnson, and Liwei Jia

The Kuroshio-Oyashio Extension (KOE) is the North Pacific oceanic frontal zone where air-sea heat and moisture exchanges allow strong communication between the ocean and atmosphere. Using satellite observations and reanalysis datasets, we show that the KOE surface heat flux variability constitutes an essential component of the seasonal and decadal Pacific ocean/atmosphere variability. We first show a strong covariability between the winter air-sea heat exchange and decadal fluctuations of the Kuroshio Extension (KE) sea surface height (SSH; the SSH reflects upper-ocean heat content anomalies). Interannual to decadal variations of ocean subsurface heat content become strongly connected to the surface during early winter (i.e., November-December-January, NDJ), where they influence the strong ocean-to-atmosphere heat transfer over the KOE. During the early winter (NDJ), the enhanced Aleutian-Low-like atmospheric circulation associated with KE SSH helps to induce a substantial sea-air temperature difference through northwesterly winds over the warm ocean surface. The analysis over an extended time period (i.e., 1959-2022) exhibits that the KOE upward latent and sensible heat flux anomalies have been significantly enhanced since the late 1980s mainly due to increasing variance of the oceanic variability (e.g., KOE sea surface temperature) rather than atmospheric forcing changes (e.g., Aleutian Low). Our findings suggest that winter KOE heat flux variations can be useful climate proxies (e.g., KE SSH) as a physical indicator that links the subsurface ocean and atmosphere.

How to cite: Joh, Y., Delworth, T., Wittenberg, A., Yang, X., Rosati, A., Johnson, N., and Jia, L.: Increasing role of Kuroshio-Oyashio Extension variations as a conveyor of decadal ocean oscillation to seasonal air-sea heat exchange since the late 1980s, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10855, https://doi.org/10.5194/egusphere-egu23-10855, 2023.

EGU23-11728 | ECS | Orals | OS1.1

Using super-residual heat transport to elucidate ocean heat storage in a resolution hierarchy of models 

Jiheun Lee, Till Kuhlbrodt, Remi Tailleux, and Dave Storkey

The large spread in projections of ocean heat uptake found in CMIP simulations is known to be problematic, leading to large uncertainties in the projected future ocean heat storage. This study introduces a new diagnostic, super-residual transport (SRT), to trace ocean heat uptake processes consistently in different models. SRT is the contribution to ocean heat uptake associated with residual mean advection and isopycnal diffusion, and therefore explains large and mesoscale heat transports in terms of spatial scale regardless of model resolution. We compare two different resolutions (eddy-parameterising and eddy-present models) of the global coupled HadGEM3-GC3.1 models to investigate performance of ocean heat uptake simulation and suggest where focus should be applied in model development.

We find that high-latitude regions show substantial inter-resolution differences in SRT of the mean state. Due to strong along-isopycnal heat uptake poleward of 50°S, a large amount of heat is stored in the Southern Ocean with little sea surface warming. The ocean heat uptake in the mixed layer is stronger and deeper near Drake passage in the eddy-present model which has steeper isopycnal surfaces of the Southern Ocean. The deep ocean warming varies with model resolution due to different properties of deep water formation in Weddell Sea and North Atlantic, which provides different paths from the surface to the bottom of the ocean. We demonstrate that mesoscale eddy advection due to baroclinic instability, implemented by Gent-McWilliams parameterisation, is key to understanding the differences in warming Antarctic Bottom Water and North Atlantic Deep Water across resolutions.

In the context of CO2-forced change, SRT shows much higher similarity across model resolutions than in the mean state. For both model resolutions, the mixed layer warming driven by SRT is much reduced in the high-latitude Southern Ocean. This results mainly from slumping of isopycnals, which brings excessive heat further northward of 50°S and then downward by enhanced Deacon cell. Consistent with our findings in the mean state, deep ocean warming penetrated to the bottom of the Southern Ocean is only observed in the eddy-present model. An important implication of this result is that better agreement across model resolutions in AMOC strength and North Atlantic warming is achieved in CO2-induced SRT. This suggests that whether ocean mesoscale is explicitly resolved or parameterised becomes less influential with respect to the patterns of ocean warming as the climate warms, which results from abrupt changes in mean circulation and reduced effect of Gent-McWilliams parameterisation.

How to cite: Lee, J., Kuhlbrodt, T., Tailleux, R., and Storkey, D.: Using super-residual heat transport to elucidate ocean heat storage in a resolution hierarchy of models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11728, https://doi.org/10.5194/egusphere-egu23-11728, 2023.

EGU23-11881 | ECS | Posters on site | OS1.1

Modelling of Arabian Sea processes and investigation of Turbulent Kinetic Energy using Modular Ocean Model 

Rajesh Chauhan, Manasa Behera, and Sridhar Balasubramanian

Arabian sea (AS), a north Indian Ocean basin plays a significant role in the transfer of energy and moisture flux during the Indian summer monsoon. It is crucial to understand mixing in AS that affects ocean properties and subsequently the interaction between ocean and atmosphere. A huge amount of energy from the atmosphere in the form of winds is enforced to the ocean surface of Arabian Sea in summer monsoon which develops various large scale features such as Somali current, the great whirl, Socotra eddy and helps in churning the ocean layers. Thus in this study, standalone Ocean circulation model, Modular Ocean Model (MOM5) is used to study the dynamics and energetics of Arabian Sea in regional ocean domain. Regional AS with domain extent between 38 to 79⁰ E in longitude and 10⁰ S to 31⁰ N is chosen and open boundary condition is implemented at the southern and eastern part of the lateral boundaries for smooth exchange with the open ocean. Grid resolution is 0.25x0.25 ⁰ in horizontal and varies in vertical depth from 5m near surface to 500m near ocean bottom. Model is initialised from state of rest with an annual average Temperature and Salinity profile as background state and forced with 10 years climatology of daily average momentum flux from NASA JPL ECCO2 and heat fluxes from WHOI and precipitation from TRMM. At the lateral boundaries sea surface height anomaly is prescribed at 7 days interval to maintain the mass conservation. At lateral boundaries, vertical profiles of temperature and salinity are also prescribed at 5 days interval obtained from SODA. Model run is integrated for 10 years as spin up and then restarted for 5 years with instantaneous data from same source. The instantaneous 5-year output data is analysed to investigate the circulation and energetics in AS. It is observed that model very well represents the Somali current and south-eastward net water transport during summer monsoon and current reversal in winter monsoon with reversing winds and weak currents during boreal spring and fall. Salinity which plays dominant role in AS is also represented well in the model. Model produces a positive warm bias in the equatorial and south-western part of the domain which could be due to improper latent heat flux exchange. Investigation of Turbulent kinetic Energy (TKE) reveals that TKE is strong along Somali coast in summer monsoon and relatively weak in winter monsoon due to strong winds. Dissipation also shows strong signatures along Somali coast and quite strong features in equatorial region in winter monsoon. This indicates that AS is largely influenced by momentum flux exchange that in turn influences the energy budget.

How to cite: Chauhan, R., Behera, M., and Balasubramanian, S.: Modelling of Arabian Sea processes and investigation of Turbulent Kinetic Energy using Modular Ocean Model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11881, https://doi.org/10.5194/egusphere-egu23-11881, 2023.

EGU23-13140 | ECS | Orals | OS1.1

Overturning and heat transport variations in the South Atlantic in an ocean reanalysis ensemble and other estimates 

Jonathan Baker, Richard Renshaw, Laura Jackson, Clotilde Dubois, Doroteaciro Iovino, Hao Zuo, Renellys Perez, Shenfu Dong, Marion Kersalé, Michael Mayer, Johannes Mayer, Sabrina Speich, and Tarron Lamont

The variability of the South Atlantic meridional overturning circulation and meridional heat transport measured across 34.5°S during 2013–2017 differs significantly between observational and ocean reanalysis estimates. Variability in an ocean reanalysis ensemble and an eddy-resolving reanalysis is similar to an altimeter-based estimate, but smaller than energy-budget and mooring-based estimates. Over 1993–2020, there is no long-term trend in the ensemble-mean overturning and heat transport, although there are inter-model differences, whereas the altimeter-based and energy-budget estimate transports increase over this period. Time-mean overturning volume transport (and the depth of maximum overturning) across 34.5°S in the ensemble and observations are similar, whereas the corresponding mean heat transports differ by up to 0.3 PW. The seasonal cycle of these transports varies between estimates, due to differences in the methods for estimating the geostrophic flow and the sampling characteristics of the observational approaches. The baroclinic, barotropic and Ekman MOC components tend to augment each other in mooring-based estimates, whereas in other estimates they tend to oppose each other so the monthly-mean, inter-annual and seasonal MOC anomalies have a greater magnitude in the mooring-based estimates. Thus, the mean and variation of real world South Atlantic transports, and the amplitude of their fluctuations, are still uncertain. Ocean reanalyses may be useful tools to understand these differences and the mechanisms that control volume and heat transport variability in the South Atlantic, a region critical for determining the global overturning pathways and inter-basin transports.   

How to cite: Baker, J., Renshaw, R., Jackson, L., Dubois, C., Iovino, D., Zuo, H., Perez, R., Dong, S., Kersalé, M., Mayer, M., Mayer, J., Speich, S., and Lamont, T.: Overturning and heat transport variations in the South Atlantic in an ocean reanalysis ensemble and other estimates, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13140, https://doi.org/10.5194/egusphere-egu23-13140, 2023.

EGU23-13731 | Orals | OS1.1

Easterlies/Westerlies convergence in the tropical Pacific triggering El Niño initiation? 

Sandro Carniel and Gianluca Eusebi Borzelli

Climate change and climate variability play a relevant role on the occurrence of conflicts in several parts of the world, including the tropics, where events of flooding and droughts are dictated by El Niño Southern Oscillation (ENSO). In order to better analyze possible relations between conflicts hot-spots and ENSO impacts on society and security, a better understanding of the dynamics of the latter is needed. ENSO is the result of an ocean-atmosphere feedback, which produces an irregular oscillation between a warm (El Niño) and a cold (La Niña) phase, peaking in boreal winter and recurring every 2-5 years. During La Niña phase, intensified trade winds accumulate warm water in to the west of the Pacific, leading to droughts in the northern US and catastrophic floods in regions such as northern Australia. During El Niño warm phase, westerly winds advect warm waters eastward towards the coasts of America, generating dry conditions in northern US and Canada, and wetter periods in the US Gulf Coast areas. Most of the studies on ENSO focused on the coupling between changes in the depth of the main thermocline, heat content in the surface layer of the water column and oceanic feedback on the zonal wind pattern. According to these works, the subsurface memory of the ocean (i.e. the heat stored in the surface layer), depends on the depth of the thermocline and the zonal shape of the isothermal surfaces is sustained by the dynamical balance between the zonal pressure gradient and the trade winds. This process systematically transfers heat westward and “charges” the western Pacific, which is then “discharged” through the action of eastward propagating internal Kelvin Waves (KW). While westerly wind events are known to play an important role in the generation of KW associated with El Niño, much less is known on the role of easterly winds. Here we show that the encountering between Westerlies and Easterlies determines the convergence, providing the initial forcing exciting internal, downwelling Rossby and Kelvin waves. Only KW formed east of 175oE  reach the eastern Pacific boundary and determine an El Niño events, that become the more intense the more the waves are formed eastward, indicating a “zonal position” triggering of El Niño. It is shown here that the zonal shifts of the Easterlies/Westerlies convergence region displaces zonally in phase with region of the deep atmospheric convection and with the Southern Oscillation Index, indicating that changes in the large scale pressure system, the zonal position of westerly wind events, the easterly wind variability, the position of the deep atmospheric convection and El Niño are all intimately related features of the whole tropical Pacific climate system.

Funding from the STO Office of Chief Scientist 907EUR30 is gratefully acknowledged.

How to cite: Carniel, S. and Eusebi Borzelli, G.: Easterlies/Westerlies convergence in the tropical Pacific triggering El Niño initiation?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13731, https://doi.org/10.5194/egusphere-egu23-13731, 2023.

EGU23-13931 | Orals | OS1.1

Breaking down the overturning circulation to local mixing 

Knut Klingbeil, Erika Henell, Ulf Gräwe, and Hans Burchard

I will present an analytical relation that directly shows how mixing locally drives the overturning circulation. The theory is based on the application of the well-known Water Mass Transformation framework to each local water column. The budgets for volume and tracer content mapped to tracer space are supplemented by a budget for the squared tracer. Mixing is defined by the destruction term of squared tracer, which is equivalent to the decay of tracer variance. I will present maps of the simulated diahaline mixing and the associated diahaline exchange velocity in the Baltic Sea. In addition, our numerical model offers to separately diagnose the mixing due to turbulence parameterizations and the spurious mixing due to discrete transport schemes. This enables us to also quantify the amount of spuriously induced overturning circulation. The planned application to diapycnal exchange and the global overturing circulation will be outlined.

How to cite: Klingbeil, K., Henell, E., Gräwe, U., and Burchard, H.: Breaking down the overturning circulation to local mixing, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13931, https://doi.org/10.5194/egusphere-egu23-13931, 2023.

The rates and mechanisms of ocean mixing are important controls on how the oceans function; yet, our understanding of mixing in the ocean is significantly limited by complex variability in mixing rates and processes and by a scarcity of direct observations. In the Arctic Ocean, the challenges involved in understanding mixing space-time geography and its implications are significant: mixing measurements are especially sparse, and latitude, ice, and stratification make the mixing environment unique. In this talk, I’ll discuss various ways we are mapping Arctic Ocean mixing rates and deriving insights into what sets their variability in space and in time using pan-Arctic measurements from a variety of autonomous instrument platforms and the archived data record. I’ll also show results from our experiments with realistic ocean models to argue that this map matters both to our understanding of Arctic Ocean functioning and our ability to make robust predictions of climate change.

How to cite: Waterman, S.: Filling in the Map: Arctic Ocean mixing space-time geography & its implications, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13961, https://doi.org/10.5194/egusphere-egu23-13961, 2023.

Vertical velocities are several orders of magnitude smaller than the horizontal ones when looking at patterns larger than the sub-mesoscales in the open ocean. Hence, direct measurement attempts of open ocean w are scarce. Methods for estimating w in the real ocean combine theory and observation-based fields. In the present work, climatological circulation patterns in Linear Vorticity Balance (LVB: βv=f∂w/∂z) are first identified in an eddy-permitting OGCM. Then, for such regime of circulation, we show that it is possible to reconstruct a robust w field for the climatological mean.

In the first part, we present a thorough baroclinic analysis of the climatological LVB. Below the Mixed Layer, the LVB holds to first order in the tropical and subtropical gyres interior and part of subpolar and austral circulation throughout the water column. Within western boundary currents, the equatorial band, areas of the subpolar gyres and the Circumpolar Circulation, significant departures occur due to the dominance of other terms in the vorticity budget, such as nonlinearities or friction. Although the ocean transport adjustment occurs on time scales constrained by basin-crossing times of Rossby waves, we show that the LVB often holds at much shorter time scales of a few years. When the climatology is reduced, the LVB's strength to describe the ocean circulation is relatively maintained. However, the time-dependent of the vorticity balance becomes significant and impacts the vorticity balance in western boundary currents and western tropical regions.

These results allow us to reconstruct the interannual variability of w for flows in LVB using geostrophic meridional velocities and satellite wind fields within large fractions of the global ocean. In the last part, we explore the differences at regional scale between our observation-based reconstruction and two other available estimates of w: one produced by an ocean reanalysis and the other reconstructed with observations and the Omega equation theory.

How to cite: Cortés Morales, D. and Lazar, A.: Estimating the interannual variability of vertical velocity within the global ocean thermocline from observation-based geostrophic meridional velocities., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14367, https://doi.org/10.5194/egusphere-egu23-14367, 2023.

EGU23-14931 | Orals | OS1.1

New improvements for monitoring the Ocean Heat Content and the Earth Energy imbalance (MOHeaCAN). 

Florence Marti, Alejandro Blazquez, Benoit Meyssignac, Michaël Ablain, Anne Barnoud, Robin Fraudeau, Victor Rousseau, Jonathan Chenal, Gilles Larnicol, Julia Pfeffer, Marco Restano, Jérôme Benveniste, Gérald Dibarboure, and Francois Bignalet-Cazalet

The Earth energy imbalance (EEI) at the top of the atmosphere is responsible for the accumulation of energy in the climate system. While necessary to better understand the Earth’s warming climate, measuring the EEI is challenging as it is a globally integrated variable whose variations are small (0.5-1 W.m−2) compared to the amount of energy entering and leaving the climate system (~ 340 W.m-2). Accuracies better than 0.1 W.m−2 are needed to evaluate the temporal variations of the EEI at decadal and longer time-scales. The CERES experiment provides EEI time variations with a typical uncertainty of ± 0.1 W.m−2 and shows a trend in EEI of 0.50 +/- 0.47 W.m−2 per decade over the period 2005-2019.

The combination of space altimetry and space gravimetry measurements provides an estimate of the ocean heat content (OHC) change which is an accurate proxy of EEI (because >90% of the excess of energy stored by the planet in response to the EEI is accumulated in the ocean in the form of heat). 

In Marti et al. (2021), the global OHC was estimated at global scales based on the combination of space altimetry and space gravimetry measurements over 2002-2016. Changes in the EEI were then derived with realistic estimates of its uncertainty.

Here we present the improvements brought to the global OGC and EEI over an extended period (2002-2021), such as the calculation of the expansion efficiency of heat over the total water column, the improvement of ocean mass solution, the empirical correction of the wet tropospheric correction of Jason-3 altimeter measurements (Barnoud et al., 2022).

The space geodetic GOHC-EEI product based on space altimetry and space gravimetry is available on the AVSIO website at https://doi.org/10.24400/527896/a01-2020.003.

 

References:

Barnoud A., Picard B., Meyssignac B., Marti F., Ablain M., Roca R. Reducing the uncertainty in the satellite altimetry estimates of global mean sea level trends using highly stable water vapour climate data records. Submitted to JGR: Oceans.

Marti, F., Blazquez, A., Meyssignac, B., Ablain, M., Barnoud, A., Fraudeau, R., Jugier, R., Chenal, J., Larnicol, G., Pfeffer, J., Restano, M., and Benveniste, J.: Monitoring the ocean heat content change and the Earth energy imbalance from space altimetry and space gravimetry, Earth Syst. Sci. Data, 14, 229–249, https://doi.org/10.5194/essd-14-229-2022, 2022.



How to cite: Marti, F., Blazquez, A., Meyssignac, B., Ablain, M., Barnoud, A., Fraudeau, R., Rousseau, V., Chenal, J., Larnicol, G., Pfeffer, J., Restano, M., Benveniste, J., Dibarboure, G., and Bignalet-Cazalet, F.: New improvements for monitoring the Ocean Heat Content and the Earth Energy imbalance (MOHeaCAN)., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14931, https://doi.org/10.5194/egusphere-egu23-14931, 2023.

The Atlantic Meridional Overturning Circulation (AMOC) transports heat and salt between the tropical Atlantic and Arctic Oceans. The interior of the North Atlantic Subpolar Gyre (SPG) is responsible for the much of the water mass transformation in the AMOC, and the export of this water to intensified boundary currents is crucial for projecting air-sea interaction onto the strength of the AMOC. However, the magnitude and location of exchange between the SPG and the boundary remains unclear. We present a novel climatology of the SPG boundary using quality controlled CTD and Argo hydrography, defining the SPG interior as the oceanic region bounded by 47° N and the 1000m isobath.  From this hydrography we find geostrophic flow out of the SPG around much of the boundary with minimal seasonality.  The horizontal density gradient is reversed around West Greenland, where the geostrophic flow is into the SPG.  Surface Ekman forcing drives net flow out of the SPG in all seasons with pronounced seasonality, varying between 2.45 ± 0.73 Sv in the summer and 7.70 ± 2.90 Sv in the winter.  We estimate heat advected into the SPG to be between 0.14 ± 0.05 PW in the winter and 0.23 ± 0.05 PW in the spring, and freshwater advected out of the SPG to be between 0.07 ± 0.02 Sv in the summer and 0.15 ± 0.02 Sv in the autumn. These estimates approximately balance the surface heat and freshwater fluxes over the SPG domain. Overturning in the SPG varies seasonally, with a minimum of 6.20 ± 1.40 Sv in the autumn and a maximum of 10.17 ± 1.91 Sv in the spring, with surface Ekman the most likely primary driver of this variability.  The density of maximum overturning is at 27.30 kgm-3, with a second, smaller maximum at 27.54 kgm-3.  Upper waters (σ0 < 27.30 kgm-3) are transformed in the interior then exported as either intermediate water (27.30-27.54 kgm-3) in the North Atlantic Current (NAC) or as dense water (σ0 > 27.54 kgm-3) exiting to the south.  Our results support the present consensus that the formation and pre-conditioning of subpolar Mode Water in the north-eastern Atlantic is a key determinant of AMOC strength.

How to cite: Jones, S., Fraser, N., Cunningham, S., Fox, A., and Inall, M.: Observation-based estimates of volume, heat and freshwater exchanges between the subpolar North Atlantic interior, its boundary currents and the atmosphere, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15026, https://doi.org/10.5194/egusphere-egu23-15026, 2023.

EGU23-15466 | ECS | Orals | OS1.1

Mixing Processes in the Dotson Ice Shelf Outflow 

Tiago Dotto, Rob Hall, Peter Sheehan, Gillian Damerell, Yixi Zheng, Lars Boehme, Sharon Stammerjohn, and Karen Heywood

The Dotson Ice Shelf (DIS) shows high rates of basal melting in recent decades. Relatively warm ocean currents access the sub-ice shelf cavity and interact with the base of the ice shelf providing heat for its melting. The water mass transformation associated with the mixture of warm water and meltwater creates buoyant plumes that shallow as they flow out from the cavity. Here, we show that high turbulent kinetic energy (TKE) dissipation rates (up to order 10−7 W kg−1) and diapycnal eddy diffusivities (up to order 10−2 m2 s−1) are associated with the outflow current from DIS. Four high-resolution Vertical Microstructure Profile (VMP) and ship-based Acoustic Doppler Current Profiler (SADCP) sections were conducted in January and February 2022 at the western side of DIS spanning the outflow as it hugs the steep topographic slope. Near-bed TKE dissipations rates are elevated by up to 3 orders of magnitude and elevated mixing rates are also observed mid-water column around the edges of the outflow. These elevated TKE are associated with friction near the bed and current shear at the outflow boundary. In this presentation, we explore the consequences for dissipation of physical and biogeochemical properties.

How to cite: Dotto, T., Hall, R., Sheehan, P., Damerell, G., Zheng, Y., Boehme, L., Stammerjohn, S., and Heywood, K.: Mixing Processes in the Dotson Ice Shelf Outflow, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15466, https://doi.org/10.5194/egusphere-egu23-15466, 2023.

EGU23-16257 | ECS | Posters on site | OS1.1

Sensitivity of the observed and modeled discrepancy in tropical Pacific Sea Surface temperatures to the time interval 

Shreya Dhame, Dirk Olonscheck, and Maria Rugenstein

The time-evolving pattern of ocean surface warming in the Pacific Ocean affects the radiation budget and estimates of global climate sensitivity. Over the historical period, models consistently show a different equatorial Pacific SST warming pattern than observed.

Some studies attribute the large discrepancies between the observed and modeled SST trends in the Pacific Ocean, in recent decades, to systemic model biases and their response to greenhouse gas forcing or model biases in the spatial and temporal pattern of multi-decadal variability (e.g., Seager et al, 2019, 2022; Wills et al, 2022). Other studies find that the observed warming pattern can be explained by internal variability (e.g., Olonscheck et al, 2020; Watanabe et al, 2021). Here, we examine whether these analyses of regional temperature changes in the tropical Pacific Ocean are sensitive to the time interval selected to calculate the multi-decadal trends and whether the sensitivity to the time interval can explain the conflicting results of previous studies.

Olonscheck, D., Rugenstein, M., & Marotzke, J. (2020). Broad consistency between observed and simulated trends in sea surface temperature patterns. Geophysical Research Letters47(10), e2019GL086773.

Seager, R., Cane, M., Henderson, N., Lee, D. E., Abernathey, R., & Zhang, H. (2019). Strengthening tropical Pacific zonal sea surface temperature gradient consistent with rising greenhouse gases. Nature Climate Change9(7), 517-522.

Seager, R., Henderson, N., & Cane, M. (2022). Persistent discrepancies between observed and modeled trends in the tropical Pacific Ocean. Journal of Climate, 1-41.Watanabe, M., Dufresne, J. L., Kosaka, Y., Mauritsen, T., & Tatebe, H. (2021). Enhanced warming constrained by past trends in equatorial Pacific sea surface temperature gradient. Nature Climate Change11(1), 33-37.

Wills, R. C., Dong, Y., Proistosecu, C., Armour, K. C., & Battisti, D. S. (2022). Systematic Climate Model Biases in the Large‐Scale Patterns of Recent Sea‐Surface Temperature and Sea‐Level Pressure Change. Geophysical Research Letters49(17), e2022GL100011.

How to cite: Dhame, S., Olonscheck, D., and Rugenstein, M.: Sensitivity of the observed and modeled discrepancy in tropical Pacific Sea Surface temperatures to the time interval, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16257, https://doi.org/10.5194/egusphere-egu23-16257, 2023.

EGU23-314 | ECS | Posters on site | OS1.2

Current feedback in an anticyclonic eddy during the passage of cold frontal systems over the Gulf of Mexico 

Ivonne García Martínez and Julio Sheinbaum Pardo

In autumn and winter, the dynamics and thermodynamics of the anticyclonic eddies detached from the Loop Current in the Gulf of Mexico are strongly influenced by the passage of cold fronts and Northerly winds. In turn, the interaction between the wind and these anticyclonic eddies modulates vertical nutrient fluxes, biomass, and phytoplankton community distribution at mesoscale and sub-mesoscale. In this work, the physical mechanisms of eddy-cold front interactions are analysed based on high-resolution (3 km) numerical simulations of the NEMO (Nucleous for European Modeling of the Ocean) model, contrasting simulations that partially include or not the effect of ocean currents on the wind stress (the so-called current feedback). This analysis is part of a work in progress focused on developing and implementing a high-resolution ocean-atmosphere coupled model for the Gulf of Mexico.

How to cite: García Martínez, I. and Sheinbaum Pardo, J.: Current feedback in an anticyclonic eddy during the passage of cold frontal systems over the Gulf of Mexico, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-314, https://doi.org/10.5194/egusphere-egu23-314, 2023.

EGU23-514 | ECS | Orals | OS1.2

Impact of Bay of Bengal mesoscale eddies on Indian Summer Monsoon Rainfall 

Kiran Vg, Suryachandra A Rao, and Prasanth A Pillai

The northern Indian Ocean serves as an ideal space to study the interaction between Ocean and atmosphere as it accommodates unique and versatile oceanic conditions and the largest monsoonal circulation in the world. The South Asian Monsoon System, the largest of its kind, directly impacts the lives and livelihoods of billions of people living in the Indian Subcontinent. Various factors that influence its strength and characteristics have been studied extensively throughout the years. But, owing to its complex and dynamic nature, a comprehensive understanding and accurate monsoon prediction remain a work in progress. Several Oceanic components that play a part in monsoon processes have been identified. Our study focuses on the Bay of Bengal, distinguished from other oceans due to its highly stratified upper layers. Through this study, we aim to understand the Impact of mesoscale eddies in the Bay of Bengal on the Indian Summer monsoon.

 

The influence of oceanic mesoscale eddies on the circulation and precipitation directly over them has been addressed through different studies after the advent of high-resolution satellite data. The current research focuses on the large-scale influence of the eddies in the Bay of Bengal on the seasonal rainfall during the Indian summer monsoon(ISM). Indices were created using the Okubo Weiss parameter to understand the inter-annual variation of eddies (classified according to polarities and regions of occurrence). These indices correlated with the ISM system suggested that Anticyclonic eddies in the  Western Bay of Bengal strongly influenced wind and rainfall patterns over the monsoon region. The Anticyclonic Eddy activity that peaked during the El Nino years countered the suppression of rainfall by El Nino through enhanced synoptic activity in BoB. The low-pressure system formation and propagation in BoB were found to be stronger in the years having more anticyclonic eddies. The warming created by the warm-core Anticyclonic eddies initiates an Anticyclonic(Clockwise) circulation around the region, which feeds back into the existing oceanic conditions. This coupled Ocean-Atmospheric system mediated through the mesoscale eddies needs to be further analyzed through stand-alone and coupled modeling experiments. Improving the representation of the mesoscale processes in the Northern Indian Ocean can serve as a crucial step in improving the monsoon prediction systems.

How to cite: Vg, K., A Rao, S., and A Pillai, P.: Impact of Bay of Bengal mesoscale eddies on Indian Summer Monsoon Rainfall, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-514, https://doi.org/10.5194/egusphere-egu23-514, 2023.

The Bay of Bengal is a unique tropical ocean basin because of seasonally reversing monsoon
winds; copious freshwater discharge from nearby continental rivers helps barrier layer formation. 
Again, due to its capacity to keep warm SST beyond 28oC, the bay is prone to tropical cyclones
during the transition between two monsoons. Geographically, the basin is land bounded on three 
sides and connected to global oceans through its southern boundary. Vicinity to the equatorial 
The Indian Ocean facilitates the propagation of Kelvin waves through its rim as coastally trapped 
waves. Hence, local and remote forcings make the bay an active basin for brewing mesoscale 
features like eddies. Eddies play a vital role in the bay's upper ocean mesoscale dynamics (O[100s 
Km]). Surface intensified eddies are well studied, but very little is known about subsurface 
circulations. Limited literature reports active subsurface eddy fields in the basin. Observations by 
a RAMA buoy at 90oE, 15oN from 2007 to 2020 shows a thermocline bulge. For about a month, 
this peculiar subsurface feature is characterized by the doming (denting) of the seasonal 
thermocline's upper (lower) part. The bulge is a regular seasonal feature during the winter 
monsoon as denoted by time series analysis of D26 (depth of 26oC isotherm) – D12 (depth of 
26oC isotherm) from RAMA temperature. This research, using a suite of in-situ moored buoy 
observations, satellite altimetry, OSCAR surface current, near-surface ASCAT wind and HYCOM 
re-analysis data, suggests a possible mechanism for the formation of thermocline bulge. Usually, 
it isn't easy to detect a subsurface feature from the sea surface variables like SST or SLA. But 
eddy-wind interactions can lead to the local generation of lens-shaped features in the 
thermocline of a pre-existing surface-intensified anti-cyclonic eddy. Observations show the 
simultaneous development of a surface anti-cyclone off the Irrawaddy delta (hereafter referred 
to as ICAE) and upwelling-favourable winter monsoon winds in the background. The interaction 
of background wind stress with the IACE facilitates the formation of a bulge by doming the 
seasonal thermocline at the eddy core. The thermocline bulge starts its westward journey along 
with the parent eddy due to Rossby wave forcing in December and crosses the RAMA buoy in 
mid-January. Three factors are responsible for a bulged IACE off the Myanmar coast: 1. the arrival 
of coastal Kelvin waves due to intense remote equatorial forcing by Wrytki jets, 2. eddy 
separation from the coast, and 3. Ekman suction (or upwelling) at the centre of IACE due to local 
"eddy-wind" interaction during late fall to winter. The IACE that wraps a bulged thermocline in 
its core is an example of seasonal mode-water ACE or intra-thermocline eddy during the winter, 
typical in higher latitudes but only recently observed in tropical basins like the Bay of Bengal.

How to cite: Kalita, B. K. and Vinayachandran, P.: Role of coastally trapped waves of remote origin and local eddy-wind interaction in the formation of seasonal thermocline bulge in the Bay of Bengal, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-713, https://doi.org/10.5194/egusphere-egu23-713, 2023.

In this study, two years (2021-2022) of High Frequency Radar (HFR) sea surface current data (30 min time resolution) and modelled near-bottom wind data (1 h time resolution) in the Gulf of Trieste (Northern Adriatic Sea) are analysed through a superstatistical (a superposition of different statistics) approach.
 
Three distinct main wind forcing regimes are present in the Gulf of Trieste: Bora, Sirocco and low wind. Bora and Sirocco are strong winds whose characteristics are different: the Bora is a cold wind that blows in gusts from the East-North-East with a short fetch, the Sirocco is a warm wind that blows from the South with a fetch along the entire Adriatic.
 
The currents in the Gulf of Trieste are forced and highly dependent on such variable wind conditions. It results in a succession of different sea current dynamics on different time scales, asking for a superstatistical analysis of the sea surface current data. From the oceanic signal it is possible to extract two different time scales: a relaxation time τ, the time the system spends to reach the local equilibrium and a larger timescale T, the time for which the signal is locally gaussian. This permits extracting a slowly varying β(t) strictly connected to the original time series’ local variance σ2-1. Neither β nor σ2 show well known PDFs and have algebraic tails. Contrary to what one might expect, they show a universal behaviour with respect to the different wind regimes blowing over the Gulf of Trieste.

How to cite: Flora, S., Ursella, L., and Wirth, A.: Superstatistical analysis of sea surface currents in the Gulf of Trieste, measured by HF Radar, and its relation to wind regimes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1063, https://doi.org/10.5194/egusphere-egu23-1063, 2023.

EGU23-1996 | ECS | Posters on site | OS1.2

Resolution sensitivity of tropical turbulent fluxes and precipitation in NorESM models 

Fangxing Tian, Noel Keenlyside, Ingo Bethke, Shunya Koseki, and Fei Li

We evaluated the ¼° model NorESM1.3 in which the well-known “double-ITCZ problem” in the Pacific is mitigated. However, excessive precipitation is produced in the northern branch of the ITCZ. The excessive precipitation is consistent with overevaluated latent heat flux in the tropical ocean. Further analysis shows that in NorESM1.3, the latent heat flux is too sensitive to the surface wind. The increased sensitivity in the ¼° model is partly contributed by small-scale air-sea interaction. The sensitivity of latent heat flux to surface wind, with the scale finer than 2.5°, is up to 40 (Wm-2 / ms-1), which is almost twice of that with scale coarser than 2.5°. This study helps to understand extra air-sea interaction resolved by higher-resolution models, and helps to tune and correct the related model bias.  

How to cite: Tian, F., Keenlyside, N., Bethke, I., Koseki, S., and Li, F.: Resolution sensitivity of tropical turbulent fluxes and precipitation in NorESM models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1996, https://doi.org/10.5194/egusphere-egu23-1996, 2023.

EGU23-2108 | ECS | Posters on site | OS1.2

Impact of Diurnal Warm Layers on Atmospheric Convection 

Radomyra Shevchenko, Cathy Hohenegger, and Mira Schmitt

The phenomenon of sea surface temperature (SST) anomalies created by oceanic diurnal warm layers has been extensively studied for the last decades, but the assessment of its importance for atmospheric convection has come within reach only very recently, thanks to the development of kilometre-scale simulations. We use the output of a global coupled simulation with a 5km horizontal grid spacing and near-surface ocean layers of order O(0.5m) to explicitly resolve both atmospheric convection and diurnal warm layers. As expected, the simulations produce daily SST fluctuations of up to several degrees. The increase of SST during the day causes an abrupt afternoon increase of atmospheric moisture due to enhanced latent heat flux. This increase is followed by an increase in cloud cover and cloud liquid water content. However, although the daily SST amplitude is exaggerated in comparison to reanalysis, the impact on cloud cover and cloud liquid water content only lasts for 5-6 hours. Moreover, the global daily average of these quantities is not influenced by their increase. All in all, we conclude that the global short-timescale impact of diurnal warm layers is negligible.

How to cite: Shevchenko, R., Hohenegger, C., and Schmitt, M.: Impact of Diurnal Warm Layers on Atmospheric Convection, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2108, https://doi.org/10.5194/egusphere-egu23-2108, 2023.

EGU23-2957 | Orals | OS1.2 | Highlight

Extreme air-sea turbulent heat fluxes over the global oceans: determination, implications and mechanisms 

Sergey Gulev, Konstantin Belyaev, and Natalia Tilinina

Extreme surface turbulent heat fluxes affecting convective processes in subpolar ocean regions may amount to 1000-3000 W/m2. Their quantitative estimation is critically important for many oceanographic and meteorological applications. Extreme turbulent fluxes are largely responsible for the vertical mixing in the ocean, especially in the subpolar latitudes where deep convection forms intermediate waters. Over western boundary currents and their extension regions very strong turbulent heat fluxes may result in local responses in the lower atmosphere on time scales from several hours to days and spatial scales from several kilometers to several tens of kilometers. Accurate estimation of extreme turbulent fluxes also strongly relates to the sampling problem especially for the poorly and irregularly sampled regions. Estimation of extreme turbulent fluxes is thus requires knowledge of probability distribution of fluxes. We suggest a concept for determination of extreme surface turbulent heat fluxes based upon theoretical probability distributions, which allow for accurate estimation of extreme fluxes. In this concept the absolute extremeness of surface turbulent fluxes is quantified from the Modified Fisher-Tippett (MFT) distribution. Further we extend MFT distribution to a fractional distribution, quantifying the so-called relative extremeness, representing the fraction of surface flux accumulated during continuous time (e.g. months, season) due to most intense surface fluxes (e.g. the strongest 1% of flux events). We provide explicit form of the fractional distribution and effective algorithms for parameter estimation.

Further we demonstrate applications of the concept for the global ocean using reanalyses and Voluntary Observing Ship (VOS) data for the period 1979 onwards. Global climatologies reveal that the regions with the strongest relative extremeness are not collocated with the strongest mean fluxes. Moreover, interannual variability of the absolute and relative flux extremes is not necessarily correlated with variability of mean fluxes. Growing mean flux may result in both increase and decrease of absolute and relative extremes that has implications for estimates of linear trends which may have different signs for mean fluxes, absolute and relative flux extremes – the situations found for the western boundary currents and major convections sites. Suggested concept has also profound implications for comparative assessments of surface turbulent fluxes from reanalyses (ERA5, ERA-Interim, CFSR, MERRA2, NCEP-DOE, JRA55) and satellite (IFREMER, J-OFURO, HOAPS, SEAFLUX) products. High mean fluxes in some products are not necessarily associated with the strongest absolute and relative extremeness in the same products and vice versa. Also a new concept allows for accurate estimation and minimization of sampling biases in VOS and satellite flux products. Finally, we analyzed the mechanisms responsible for forming extreme surface turbulent fluxes associated with cold air outbreaks in the rear parts of midlatitudinal cyclones under locally high winds and strong air-sea temperature gradients.

How to cite: Gulev, S., Belyaev, K., and Tilinina, N.: Extreme air-sea turbulent heat fluxes over the global oceans: determination, implications and mechanisms, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2957, https://doi.org/10.5194/egusphere-egu23-2957, 2023.

EGU23-3127 | ECS | Orals | OS1.2

Develop an ocean-atmosphere-wave regional coupled model Windwave version 1.0 for predicting wind and wave conditions of the Northwest Pacific Ocean 

Shu Fu, Wenyu Huang, Runqing Lv, Zifan Yang, Tianpei Qiu, Danyi Sun, Jingjia Luo, Xiaomeng Huang, Haohuan Fu, Yong Luo, and Bin Wang

A new ocean-atmosphere-wave regional coupled model named Windwave version 1.0 for simulating and predicting winds and waves has been developed for the Northwest Pacific Ocean. In particular, the global-to-regional nesting technique is adopted for the ocean component to alleviate the bias due to the inconsistency in the lateral boundary. This paper is devoted to describing the coupling details of Windwave and the initialization scheme and assessing its basic performance, especially in predicting surface winds and significant wave heights (SWH) on the weather timescale. The control experiment set contains 31 experiments for August 2020, with seven typhoons passing through the Northwest Pacific. Each experiment starts at 0:00 am UTC of each day and runs for three days. Experiment results show that the new coupled model performs well in predicting surface winds, SWH, surface air temperature, and sea surface temperature on the weather timescale. In particular, the Root Mean Square Errors (RMSEs) of surface winds at 10 m height over the Northwest Pacific of the control experiment are 1.82 m s-1, 2.22 m s-1, and 2.59 m s-1, respectively, at lead times of 24 h, 48 h, and 72 h. Meanwhile, the RMSEs of SWH at lead times of 24 h, 48 h, and 72 h are 0.39 m, 0.43 m, and 0.51 m. In addition, we have explored the impacts of the different sea surface aerodynamic roughness parameterization schemes on predicting surface winds and SWH. In total, five different sea surface aerodynamic roughness parameterization schemes are adopted, corresponding to one control set and four sensitivity sets of experiments. Under normal conditions, the sea surface aerodynamic roughness parameterization scheme considering the effects of wind-wave direction tends to perform better for winds and waves, while that depending on wave age and SWH tends to perform worse. Under extreme wind and wave conditions, the schemes considering the effects of wind-wave direction and that considering wave age and peak wave length have better performance. These findings can provide new insights for developing a more advanced sea surface aerodynamic roughness parameterization scheme.

How to cite: Fu, S., Huang, W., Lv, R., Yang, Z., Qiu, T., Sun, D., Luo, J., Huang, X., Fu, H., Luo, Y., and Wang, B.: Develop an ocean-atmosphere-wave regional coupled model Windwave version 1.0 for predicting wind and wave conditions of the Northwest Pacific Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3127, https://doi.org/10.5194/egusphere-egu23-3127, 2023.

EGU23-3608 | Orals | OS1.2 | Highlight

Lessons learned from the EUREC4A-OA experiment on the impact of ocean small scales on air-sea interactions in the Northwest Tropical Atlantic 

Sabrina Speich, Johannes Karstensen, Gilles Reverdin, Léa Olivier, Pablo Fernandez, Pierre L'Hégaret, Solange Coadou, Rémi Laxenaire, Dongxiao Zhang, Chelle Gentemann, Peter Landschutzer, Jacqueline Boutin, Hugo Bellenger, Claudia Pasquero, Agostino Meroni, Matteo Borgnino, Claudia Acquistapace, and Laurent Bopp

EUREC4A-OA is a large international project, connecting experts of ocean and atmosphere observations and modelling to enhance the understanding of key ocean and air-sea processes at the and to improve the skill of forecasts and future projections.

The core of EUREC4A-OA has been a one-month (Jan/Feb 2020) field study in the western tropical North Atlantic Ocean where high-resolution, synchronized observational data have been collected using cutting-edge technology on ships, airplanes and autonomous vehicles. EUREC4A-OA investigates heat, momentum, water and CO2 transport within the ocean and exchanges across the air/sea interface using innovative high-resolution ocean observations and a hierarchy of numerical simulations. EUREC4A-OA focuses on ocean dynamics at the small-scale (0.1–100 km) and related atmospheric boundary layer processes. EUREC4A-OA is centered on the tropics where the primary external time scale affecting air-sea exchange is the diurnal cycle. However, the internal ocean and atmosphere dynamics convolute the diurnal, synoptic, seasonal and longer time scales to climate variability.

The talk will present some of the results we obtained so far from the observations collected during the field experiment and from numerical simulations. The analyses carried out revealed with unprecedented detail the particular characteristics of the ocean small-scale dynamics, enlightening that such scales are also very active in the tropical regions and not only over the mid and higher latitudes ocean.  Observations and models also unveil that the ocean small scales is important in contributing to the exchanges of heat, freshwater and CO2 between the ocean and the atmosphere. Moreover, the evaluation of the intensity of the coupling between the ocean and the atmosphere assessed from data and high-resolution simulations show that they are very important and intimately linked with the 3D structure of the small-scale ocean dynamics. The project has also provided preliminary results in terms of parametrization of different processes influencing the ocean and atmosphere exchanges that have been uncovered by the EUREC4A-OA field experiment. Notably a better representation of the small-scale freshwater patches due to precipitation has been introduced in the French Earth-System model that improves the overall simulations of air-sea interactions and clouds. A similar parametrization is now been introduced to take into account these physical processes in air-sea fluxes of CO2.

How to cite: Speich, S., Karstensen, J., Reverdin, G., Olivier, L., Fernandez, P., L'Hégaret, P., Coadou, S., Laxenaire, R., Zhang, D., Gentemann, C., Landschutzer, P., Boutin, J., Bellenger, H., Pasquero, C., Meroni, A., Borgnino, M., Acquistapace, C., and Bopp, L.: Lessons learned from the EUREC4A-OA experiment on the impact of ocean small scales on air-sea interactions in the Northwest Tropical Atlantic, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3608, https://doi.org/10.5194/egusphere-egu23-3608, 2023.

EGU23-3693 | Orals | OS1.2

Seasonal buildup and downward transfer of Warm Pool heat content by wind-driven ocean mixing 

Noel Gutierrez Brizuela, Yi Xia, Shang-Ping Xie, Matthew Alford, and James Moum

Ocean heat stored in the Western Pacific Warm Pool (WPWP) helps drive some of the foremost modes of weather and climate variability including ENSO, Intraseasonal Oscillations, and tropical cyclones. To understand the associated coupled mechanisms that regulate ocean temperature, we use reanalysis and a novel moored microstructure dataset yielding estimates of the turbulent ocean heat flux (Jq(z)) to describe how WPWP mixing is regulated by seasonal, intraseasonal, and synoptic-scale atmospheric disturbances. It is argued that observed variations in Jq(z) impact seasonal-to-synoptic trends in SST and mixed-layer depth. Jq(z) is weakest during Spring (dry season), when heat fluxes into the ocean (Qnet > 0) create a shallow mixed layer (ML) of warm water that lays undisturbed atop the seasonal thermocline. In the Summer, westerly winds associated with the Asian Monsoon create favorable conditions for tropical depression-like (TD-like) storms, which in turn cause episodic spikes in Jq(z) that gradually deepen the ML and momentarily cool sea surface temperature (SST) while the background SST continues to rise. Towards the end of the summer, SST at our mooring was greater than 30.7 °C but rapidly dropped and stabilized below 29.5 °C after a strong pulse of the Madden-Julian Oscillation (MJO) cooled the upper ocean and deepened the ML for almost 15 days in a row. Enhanced upper ocean mixing continued to be episodic throughout the Fall as TD-like storms and intraseasonal disturbances moved over the mooring site. Mixing remained high throughout the Winter, when cold outbreaks forced the upper ocean at high frequencies and mean convective cooling (Qnet < 0) further contributed to mixing. A similar seasonality is observed at the thermocline level, where Jq(z) is enhanced by storm-driven near-inertial internal waves (NIWs) whose power peaks between Fall and Winter. While intraseasonal wind bursts had the greatest impact on near-surface mixing, synoptic disturbances generated greater-amplitude NIWs and thus had a greater potential to mix temperature gradients in the permanent thermocline. Lastly, we use reanalysis data to argue that storm-driven mixing shapes interannual relations between SST, storm activity, and the Asian Monsoon. 

How to cite: Gutierrez Brizuela, N., Xia, Y., Xie, S.-P., Alford, M., and Moum, J.: Seasonal buildup and downward transfer of Warm Pool heat content by wind-driven ocean mixing, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3693, https://doi.org/10.5194/egusphere-egu23-3693, 2023.

Reanalysis and current generation global coupled climate models have shown a dominant role of atmospheric forcing for the ocean such as stronger winds that increase turbulent heat flux and consequently cools sea surface temperatures (SSTs) . While satellite observations and eddy-resolving global coupled climate models have additionally shown clear imprints of mesoscale ocean forcing on the atmosphere, such as warm SST anomalies that can destabilise the overlying atmosphere and enhance surface winds by transferring momentum from aloft to the surface (vertical mixing mechanism; VMM). With winds blowing along downwind or crosswind SST gradients, this can subsequently induce wind stress divergence or curl respectively, particularly over mesoscale ocean features. The dominance of forcing from either atmosphere or ocean indicate a spatial scale dependency for the coupling variability of air-sea interactions.
 
Using a global coupled 5-km ICON-ESM simulation, we derive a global distribution map of the air-sea coupling associated with the VMM, and investigate the spatial and temporal scale dependency of the vertical mixing mechanism. Choosing various regions over the ocean, we evaluate the frequency-wavenumber cross-spectra between downwind SST gradients and windstress divergence in order to identify the dominant temporal and spatial scales between them. For example, we found that over the tropical Pacific, such interactions are prevalent on spatial scales of about 300-3000km and longer than 10-days timescale, while over the Gulf stream region, they are dominant at scales of roughly 100-1000km and longer than 5-days timescale. This is the first time that the dominant spatial and temporal scales for the vertical mixing mechanism in various regions of the world's ocean is quantified. 

How to cite: Putrasahan, D. and von Storch, J.-S.: Geographical distribution and spatio-temporal scale dependence of air-sea coupling via the vertical mixing mechanism, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4685, https://doi.org/10.5194/egusphere-egu23-4685, 2023.

EGU23-4800 | ECS | Posters on site | OS1.2

Intense atmospheric frontogenesis by air-sea interaction captured at Ieodo Ocean Research Station during Typhoon Lingling (2019) 

Sinil Yang, Il-Ju Moon, Hyo-Jun Bae, Baek-Min Kim, Do-Seong Byun, and Hwa-Young Lee

The Ieodo Ocean Research Station (Ieodo ORS) is a fixed marine observation platform at the boundary of the Yellow and East China Seas. In 2019, a Category 4 Typhoon Lingling passed by the Ieodo ORS very closely. At that time, the Ieodo ORS observed Sea Surface Temperature (SST) cooling of 4.5°C by vertical mixing and negative turbulent heat flux (i.e., the sum of sensible and latent heat fluxes) due to the SST cooling. In this study, uncoupled and coupled simulations were conducted to examine the role of air-sea interactions in changes in atmospheric frontogenesis around the typhoon passage. In the coupled simulation, SST cooling up to 6°C occurred over the dangerous semicircle due to vertical mixing induced by wind stress. Strong wind stress also enhanced the SST gradient and, therefore, contributed to the formation of a steeper atmospheric frontal zone. Moreover, the comparison with reliable datasets supports the physical linkage between SST cooling and atmospheric frontogenesis by utilizing the meridional theta-e gradient and moisture convergence zone. Therefore, we hope to improve our understanding of atmospheric frontogenesis by air-sea coupling processes in developing a coupled atmosphere-ocean modeling system from the simulation results.

How to cite: Yang, S., Moon, I.-J., Bae, H.-J., Kim, B.-M., Byun, D.-S., and Lee, H.-Y.: Intense atmospheric frontogenesis by air-sea interaction captured at Ieodo Ocean Research Station during Typhoon Lingling (2019), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4800, https://doi.org/10.5194/egusphere-egu23-4800, 2023.

EGU23-4940 | Orals | OS1.2

The rate of information transfer as a measure of ocean-atmosphere interactions 

David Docquier, Stéphane Vannitsem, Alessio Bellucci, and Claude Frankignoul

Exchanges of momentum, energy and mass between the ocean and atmosphere are of large importance in regulating the climate system. Here we apply the Liang-Kleeman rate of information transfer to quantify interactions between the upper ocean and lower atmosphere over the period 1988-2017 at monthly time scale in two different case studies. In the first case study, we investigate dynamical dependencies between sea-surface temperature (SST), SST tendency and turbulent heat flux in satellite observations. We find a strong two-way influence between SST or SST tendency and turbulent heat flux in many regions of the world, with largest values in eastern tropical Pacific and Atlantic oceans, as well as in western boundary currents. The total number of regions with a significant influence of turbulent heat flux on SST and SST tendency is reduced when considering the three variables, suggesting an overall stronger ocean influence compared to the atmosphere. In the second case study, we focus on the influence of ocean heat transport convergence (dynamical influence) and net surface heat flux (thermodynamical influence) on upper ocean heat content tendency in three global climate models with at least two different ocean resolutions. We find that low-resolution model configurations (1° in the ocean) show a much larger number of regions with a significant dynamical influence compared to high-resolution model configurations. The reason for the large difference in dynamical influence between low and high resolutions partly comes from the spatial distribution of ocean velocity field, which displays a larger spatial variability at high resolution.

How to cite: Docquier, D., Vannitsem, S., Bellucci, A., and Frankignoul, C.: The rate of information transfer as a measure of ocean-atmosphere interactions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4940, https://doi.org/10.5194/egusphere-egu23-4940, 2023.

EGU23-5161 | ECS | Orals | OS1.2

Partial control of the Gulf of Mexico dynamics by the current feedback to the atmosphere 

Marco Larrañaga, Lionel Renault, and Julien Jouanno

The feedback of ocean surface currents to the atmosphere (CFB) has been shown to correct long-lasting biases in the representation of ocean dynamics by providing an unambiguous energy sink mechanism. However, CFB effects on the Gulf of Mexico oceanic circulation, mainly dominated by the Loop Current and large anticyclonic eddies that shed from it), remain unknown. Here, twin ocean-atmosphere eddy-rich coupled simulations, with and without CFB, are performed over 24 years (1993-2016) to assess to which extent CFB modulates the dynamics of the Gulf of Mexico. We show that CFB damps the mesoscale activity by roughly 20% over the Gulf of Mexico through the eddy killing mechanism and the associated transfer of momentum from mesoscale currents to the atmosphere, but also by modifying the production of eddy kinetic energy via barotropic and baroclinic instabilities. This energy adjustment results in increasing the mean Loop Current penetration into the Gulf of Mexico and plays a key role in determining the shedding of Loop Current Eddy and their subsequent preferential trajectories and properties.

How to cite: Larrañaga, M., Renault, L., and Jouanno, J.: Partial control of the Gulf of Mexico dynamics by the current feedback to the atmosphere, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5161, https://doi.org/10.5194/egusphere-egu23-5161, 2023.

EGU23-7362 | ECS | Orals | OS1.2

A Preliminary Forecast Study of the North Indian Ocean Tropical Cyclones Using a Coupled Atmosphere-Ocean Model 

Aref Farhangmehr, Sarmad Ghader, Abbas Ali Ali Akbar Bidokhti, and Zahra Ranji

Tropical cyclones (TCs) are severe weather marvels, occur in warm tropical waters. These phenomena are among the most influential atmospheric-oceanic events in subtropics regions as the northern part of the Indian Ocean, affecting the Arabian Sea and the Oman Sea, which often cause severe damage to the coastal areas. The interaction between the atmosphere and the upper ocean plays an important role in the structure of TCs, in which successful connections between ocean circulations and the intensity and track of TCs have been identified. TCs derive their energy primarily from the release of latent heat through evaporation and sea spraying in the atmosphere boundary layer. This implies that the presence of a moisture source, with sufficiently warm sea surface temperature (frequently above 26°C) is required to sustain the flux of moisture from the ocean to the atmosphere. The most visible effect of TC passage is the cooling of the sea surface temperature (SST) as the response of the ocean mixed layer (OML) temperature. This decrease in SST has a negative feedback on the intensity of TCs, as it suppresses the heat exchange flux between the atmosphere and the ocean, consequently it can affect the TC track.

To investigate the effect of the temperature field on TCs structure, TC Shaheen (2021) with unusual track and entry into the Gulf of Oman is studied. In this regard, Weather Research and Forecasting (WRF) model was used with two different configurations. First, WRF was ran standalone and SST field was only adopted from global models as initial conditions and were not updated during the simulation. Then, as the second configuration, WRF model was coupled with an ocean finite volume model and the SST field was updated online during the simulation. The initial conditions of the ocean temperature, salinity and velocity field were taken from GOFS 3.1 global reanalysis product from the HYCOM Consortium. Primary result for the selected event implies that SST correction during TC simulation with WRF improves air-sea heat flux and has a pronounced effect on the TCs’ intensity and track predictions. Cold wake underside of the TC led to a remarkable heat flux loss from ocean surface into the storm. Hence, the TC size is reduced and the maximum wind speed of the storm is intensified.

How to cite: Farhangmehr, A., Ghader, S., Ali Akbar Bidokhti, A. A., and Ranji, Z.: A Preliminary Forecast Study of the North Indian Ocean Tropical Cyclones Using a Coupled Atmosphere-Ocean Model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7362, https://doi.org/10.5194/egusphere-egu23-7362, 2023.

EGU23-8166 | Orals | OS1.2

Observing the Air-Sea Transition Zone using Combined Uncrewed Systems and Other Conventional Platforms 

Chidong Zhang and the Saildrone Hurricane Observations Mission Team

Full understanding of air-sea interaction requires observations not only at the air-sea interface but also through the entire air-sea transition zone (the upper ocean, air-sea interface, and marine atmospheric boundary layer as a single identity). Our capabilities of observing collocated and simultaneous profiles through the entire column of the air-sea transition zone are currently limited. This study explores the feasibility of using combined uncrewed robotic systems in conjunction with conventional platforms to provide such collocated and simultaneous profiles of the air-sea transition zone. An introduction is given to experimental deployments of uncrewed surface vehicles (saildrones), underwater vehicles (gliders), aerial vehicles in coordination with profiling floats, surface drifters, airborne dropsondes, and moored buoys to observe the air-sea transition zone. Based on the preliminary results and lessons learned, a vision of potential capabilities of observing the air-sea transition zone in the future using combined uncrewed systems is offered.

How to cite: Zhang, C. and the Saildrone Hurricane Observations Mission Team: Observing the Air-Sea Transition Zone using Combined Uncrewed Systems and Other Conventional Platforms, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8166, https://doi.org/10.5194/egusphere-egu23-8166, 2023.

EGU23-8293 | Orals | OS1.2

Beyond Mesoscale off Mauretania 

Heiner Dietze, Ulrike Löptien, Robinson Hordoir, and Matthias Renz

Hitherto unresolved oceanic sub-mesoscale variability may retard air-sea exchange of heat and carbon in contemporary IPCC-type model projections. Here, we set out to put this hypothesis to the test in the Atlantic in a region off Mauretania that is renown for its complex coastal dynamics. Results from a suite of coupled ocean-circulation biogeochemical models suggest that the oceanic bottleneck between the atmosphere and the vast abyssal stocks of heat and carbon is relatively insensitive in the range from mesoscale and to sub-mesoscale resolution. More specifically we find that the sensitivity of effective vertical mixing to changes in spatial resolution is comparable to infamous uncertainties associated with contemporary numerical algorithms.

How to cite: Dietze, H., Löptien, U., Hordoir, R., and Renz, M.: Beyond Mesoscale off Mauretania, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8293, https://doi.org/10.5194/egusphere-egu23-8293, 2023.

EGU23-10356 | ECS | Posters on site | OS1.2 | Highlight

The Competing Forces of Hurricane-Induced Ocean Cooling 

Lev Looney and Gregory Foltz

It is known that oceanic conditions can play a crucial role in the intensification of tropical cyclones (TCs) when atmospheric conditions are conducive. However, the relative roles of ocean temperature and salinity stratification on ocean mixing and TC-induced sea surface temperature (SST) cooling are unclear. Temperature stratification has competing effects on cooling from ocean mixing: stronger stratification leads to cooler water near the surface which can enhance SST cooling (thermodynamic effect), yet it also leads to resistance to vertical mixing due to a stronger density gradient and increased static stability (mixing effect). In contrast, salinity stratification almost always acts to reduce mixing and cooling. To investigate the mechanisms that control the amount of ocean cooling under a storm, we use a one-dimensional mixed layer model, initialized with different oceanic profiles and forced with cyclones of various intensities, translation speeds, and sizes. We then compare output from the mixed layer model with observations. Results consistently show that the thermodynamic effect (changes in vertical temperature gradient with density gradient held constant) is 2-3 times that of the mixing effect (changes in density stratification with temperature stratification held constant). Furthermore, we find that translation speed and storm size are the two most important factors for SST cooling, followed by temperature stratification, maximum wind speed, and mixed layer depth, respectively. These results emphasize the importance of temperature stratification over most of the tropical cyclone basins and the often overlooked role of storm size.

How to cite: Looney, L. and Foltz, G.: The Competing Forces of Hurricane-Induced Ocean Cooling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10356, https://doi.org/10.5194/egusphere-egu23-10356, 2023.

EGU23-11795 | ECS | Orals | OS1.2

Atmospheric and Oceanic Responses to Surface Current Coupling near the Kuroshio Current 

Ajin Cho, Hajoon Song, and Hyodae Seo

The so-called frontal-scale air-sea interaction describing the atmospheric responses to oceanic fronts has been mainly discussed in the context of interactions between sea surface temperature and surface winds. The ocean current also influences the surface winds, which can significantly affect the atmosphere, especially in regions of energetic ocean currents and mesoscale activities as in the western boundary current systems. This study uses an atmosphere-ocean coupled model to analyze how the Kuroshio Current affects the momentum and turbulent heat fluxes and the atmospheric boundary layer and how these responses feed back to the ocean and atmosphere in this region. The ocean current coupling influences the path of Kuroshio extension and the eddy activities by mechanical and thermal current feedbacks. Mechanical current feedback reduces momentum flux and damps eddy kinetic energy (EKE) by reducing wind work as expected. On the other hand, the thermal current feedback associated with turbulent heat fluxes injects EKE by baroclinic energy conversion. Overall, the shift of Kuroshio Current and the change of eddy activities impact the region of strong turbulent heat release to the atmosphere, which can eventually trigger changes in weather systems.

How to cite: Cho, A., Song, H., and Seo, H.: Atmospheric and Oceanic Responses to Surface Current Coupling near the Kuroshio Current, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11795, https://doi.org/10.5194/egusphere-egu23-11795, 2023.

EGU23-13039 | ECS | Orals | OS1.2

Mesoscale and Submesoscale air-sea interactions in the lower North Atlantic trades 

Carlos Conejero, Lionel Renault, and Hervé Giordani

In the past decades, numerous studies have shown that oceanic mesoscale activity, over scales of O(50–250) km, has a strong influence on the atmosphere through both the Thermal FeedBack (TFB) and the Current FeedBack (CFB). However, at the submesoscale, over scales of O(1–10) km, both TFB and CFB are not well understood, mainly due to technical barriers (observation and simulation). Here, a realistic high-resolution coupled oceanic model (dx = 1 km), including tidal forcing and river discharge, and atmospheric (dx = 2 km) model in the lower North Atlantic trades region over a period of 1-year (from July 2019 to June 2020) is used to assess the atmospheric response to submesoscale processes. We used classic coupling coefcients between the ocean and the atmosphere to quantify spatial and temporal variabilities of the TFB and CFB coupling.

Our results show that, similar to oceanic mesoscale activity, at the submesoscale, both TFB and CFB have an imprint on the low-level wind, surface stress and turbulent heat fluxes. On the one hand, the linear relationship between surface wind (stress) curl and surface current vorticity existing at the mesoscale regime is also supported at the submesoscale. At submesoscale, CFB, as at the mesoscale, is acting as a sink of energy from the ocean to the atmosphere, acting as an submesoscale eddy killer. Furthermore, the magnitude of surface stress curl introduced by submesoscale processes is greater by ~17 % than that presented by mesoscale processes, which is explained by a reduction of the wind response by ~55 %. On the other hand, the linear relationship between wind stress magnitude, or latent heat flux, and sea surface temperature (SST) anomalies, widely present at the mesoscale, is also found at the submesoscale. Similar results are found when considering wind stress curl/divergence and crosswind/downwind SST gradients coupling coefficients. However, the magnitude of the corresponding TFB coupling at submesoscale is reduced by ~30 % with respect to those at mesoscale.

Overall, our results emphasize the significant impact of both oceanic currents and strong SST fronts on the local wind/stress and latent heat flux response at the submesoscale regime.

How to cite: Conejero, C., Renault, L., and Giordani, H.: Mesoscale and Submesoscale air-sea interactions in the lower North Atlantic trades, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13039, https://doi.org/10.5194/egusphere-egu23-13039, 2023.

EGU23-14929 | ECS | Posters on site | OS1.2

Overview of aerosol observations from the Marion Dufresne Atmospheric Program – Indian Ocean (MAP-IO) program 

Meredith Dournaux, Pierre Tulet, Joris Pianezze, Karine Sellegri, and Jérôme Brioude

The study of marine aerosols size distribution and cloud condensation nuclei (CCN) properties is of major interest as they influence clouds life and clouds radiative properties, particularly in the remote ocean which remains poorly documented. Several short campaigns focusing on specific regions as phytoplankton bloom regions, pristine regions or remote areas influenced by continental air masses took place to address this issue. However, long sampling periods targeting different in-situ conditions had not been realized.

In this context, the MAP-IO program was launched with the aim of providing a large new set of marine aerosol observations (size distribution from 10 nm to 10 µm and CCN properties) on different sea state and meteorological conditions. Thus, the Marion Dufresne vessel has been equipped with a set of various instruments described in Tulet et al. (in preparation) or on the website www.mapio.re. Two years after the launch of the program, we now have aerosol observations (about 200 days) over an area covering 50 ° of latitudes and extending from the Tropics to the upper Southern Ocean. 

These measurements were first used to investigate the size distribution and CCN variability of marine aerosols according to local conditions (sea states and wind speed).The results highlight that at the lowest latitudes (south of 50 °S) the minimum concentration of CCN tends to increase when the wind speed exceeds approximately 12 m s-1, which is consistent with the literature as sea salt  emissions are mechanically driven by local conditions and tend to be predominant from 10 m s-1. .When analyzing the size distributions of aerosols according to the wind speed during a 5-day storm that occurred in the Southern Ocean, we found that: (1) the number of particles with a diameter less than 500 nm is predominant and stable over the full range of wind speeds (4 to 33 m s-1), (2) the number of aerosols with diameter greater than 500 nm remains low under 10 m s-1 and increases from 10 m s-1 to 33 m s-1 to finally reach the concentration of the particles with diameter less than 500 nm at 33 m s-1. 

Taking this first analysis into account, further work will focus on average size distributions per region, season, origin of air masses (from simulated FLEXPART back trajectories) and wind speed conditions. Analysis of these distributions is unique due to the size of the database, the variability of regions encountered and knowing that the measurements were carried out with the same experimental device. 

 Finally, to deepen the study, the activation diameter of marine aerosols will be determined and the hygroscopicity parameter Kappa-Köhler will be calculated (Petters and Kreidenweis, 2007) in this case to distinguish sea salts (Kappa~1.2) from organic matter (0.01<Kappa<0.5).

How to cite: Dournaux, M., Tulet, P., Pianezze, J., Sellegri, K., and Brioude, J.: Overview of aerosol observations from the Marion Dufresne Atmospheric Program – Indian Ocean (MAP-IO) program, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14929, https://doi.org/10.5194/egusphere-egu23-14929, 2023.

EGU23-15444 | ECS | Posters virtual | OS1.2

Surface attached vortices as a proxy for gas transfer in free-surface turbulent flow 

Omer Babiker, Ivar Bjerkebæk, Anqing Xuan, Lian Shen, and Simen A Ellingsen

Gas transfer between ocean and atmosphere is largely governed by the turbulence in the topmost centimetres beneath the free surface. It has been frequently observed that areas of strong positive divergence of the surface-tangential velocity field correspond to efficient surface renewal and consequently increased transfer of gas across the interface. Patches of strong positive surface divergence occur in the form of intermittent upwelling events visible as ``boils'' on the surface.

It has been qualitatively observed that surface-attached ``bathtub'' vortices tend to appear at the edges of upwelling boils, hence a correlation between the density of these long-lived vortices and average surface divergence might be expected. Surface-attached vortices leave imprints on the surface which are particularly simple to detect: they live for a long time compared to turbulence turn-over, and their imprints are in the form of nearly circular dimples.

From direct numerical simulations, we show that a very high correlation exists between the time-dependent number of surface-attached vortices and the mean square of the surface divergence. A correlation coefficient of over 0.9 is found, where peaks in the number of vortices occur a little time after the peak in surface divergence, approximately half of the integral timescale of the bulk turbulence.

We use a newly developed method whereby the surface-attached vortices are identified with high precision and accuracy from their surface imprint only. Thus, observation of surface dimples can act as a proxy for surface divergence, with the prospect of remote sensing of gas transfer rate.

How to cite: Babiker, O., Bjerkebæk, I., Xuan, A., Shen, L., and Ellingsen, S. A.: Surface attached vortices as a proxy for gas transfer in free-surface turbulent flow, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15444, https://doi.org/10.5194/egusphere-egu23-15444, 2023.

EGU23-16033 | Posters on site | OS1.2

Investigating the Impact of Sea Waves on Offshore Wind Simulation: A Study Using the WRF Model 

Mohammadreza Mohammadpour Penchah, Mostafa Bakhoday Paskyabi, and Hai Bui

Previous studies showed that interaction between the atmosphere and sea is very important for simulating offshore wind, due to the effects of sea waves on momentum, mass and energy exchanges. In numerical weather prediction models, this effect is typically represented by a parameter known as roughness length. However, many atmospheric models do not take into account the impact of waves on roughness length over the sea.

In this study, we aimed to investigate the effects of waves on offshore wind simulation by applying some new roughness length formula in Weather Research and Forecasting (WRF) to consider wave characteristics in roughness length calculation. The simulations were compared with observations during some cases of misalignment and alignment of wind and wave directions at FINO1 station. We compared wind speed and wind power density of the simulated and observed data using met mast and lidar data.

How to cite: Mohammadpour Penchah, M., Bakhoday Paskyabi, M., and Bui, H.: Investigating the Impact of Sea Waves on Offshore Wind Simulation: A Study Using the WRF Model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16033, https://doi.org/10.5194/egusphere-egu23-16033, 2023.

EGU23-16379 | Posters on site | OS1.2

The diurnal warm layer and its consequences for the upper ocean: from EUREC4A-OA observations and the global coupled ICON-ESM 

Florian Schuette, Diego Lange, Dian Putrasahan, Ruben Carrasco, Pierre L'Herguet, Dongxiao Zhang, Sabrina Speich, Jin-Song von Storch, and Johannes Karstensen

The uppermost 0-20m depth of the ocean within the mixed layer (ML) were investigated on diurnal scales using data collected during the EUREC4A campaign in the western tropical Atlantic. The results are compared against data from the global coupled Earth System model ICON. In both datasets is the diurnal impulse generator the penetrating shortwave solar radiation, heating the first meters of the ocean. During day on top of the ML a stably stratified near-surface layer, called the diurnal warm layer (DWL), can be formed. Depending on the wind conditions or the amount of incoming solar radiation the depth of such a DWL can reach from several centimeters to tens of meters. Associated to the stable stratification (and the wind) shear is produced which propagates down with time. At that point, the model and the observations start to differ. Using high-resolution current measurements of ADCP’s mounted on saildrones the detailed structure of the descending shear layer is observed. The cycle of shear instability leads the diurnal mixing cycle, typically by 2–3 h, consistent with the time needed for instabilities to grow and break (observed by microstructure measurements). In the morning, the turbulence decays and the upper ocean restratifies. At this point, wind accelerates the near-surface flow to form a new unstable shear layer, and the cycle begins again. Since the study area is located around 15°N, the excited layers are affected by the Coriolis force, which causes the descending shear layer to rotate around the inertial frequency of 1.8 days. Compared to the global coupled earth system model, these processes cannot be represented in such detail here. This leads to lower shear (and also mixing) at the different time and depth. Different model configurations show that even small differences in the upper 20m of the ocean, such as those observed, can lead to quite large changes in the model, e.g., a different strength of the ocean current system down to 1000m depth.

How to cite: Schuette, F., Lange, D., Putrasahan, D., Carrasco, R., L'Herguet, P., Zhang, D., Speich, S., von Storch, J.-S., and Karstensen, J.: The diurnal warm layer and its consequences for the upper ocean: from EUREC4A-OA observations and the global coupled ICON-ESM, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16379, https://doi.org/10.5194/egusphere-egu23-16379, 2023.

EGU23-16810 | ECS | Posters on site | OS1.2

Comparison of ECMWF-ERA5 turbulent Air-Sea Fluxes and related environmental variables to data from to the OCARINA wave following platforms 

Saïd Benjeddou, Denis Denis Bourras, Boris Dewitte, Christopher Luneau, and Philippe Fraunié

The ECMWF-ERA5 reanalysis is amongst the best products to access the hourly state and trend of the global Atmosphere, Wave field, and Ocean over several decades, for many scientists and in many studies. In the proposed presentation, we will compare the turbulent momentum and heat exchange flux values output from the reanalysis to corresponding estimates that were not assimilated in the model. Those estimates were computed from data collected with a wave following platform that was deployed in several basins since 2012, including the Chile-Peru upwelling region in 2014. Not only the fluxes and the bulk variables will be statistically compared, but the focus will also be laid on the sensitivity of the results to the surface current, to the proximity of coast and, where it applies, to the direction of the wind (onshore, offshore and parallel to the coast).

How to cite: Benjeddou, S., Denis Bourras, D., Dewitte, B., Luneau, C., and Fraunié, P.: Comparison of ECMWF-ERA5 turbulent Air-Sea Fluxes and related environmental variables to data from to the OCARINA wave following platforms, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16810, https://doi.org/10.5194/egusphere-egu23-16810, 2023.

EGU23-16883 * | ECS | Orals | OS1.2 | Highlight

Identifying an Evaporative Thermal Refugium for the Preservation of Coral Reefs in a Warming World—The Gulf of Eilat (Aqaba) 

Shai Abir, Hamish A. McGowan, Yonathan Shaked, and Nadav Lensky

Coral bleaching events are more frequent and severe as global temperatures rise and marine heat waves are more frequent. However, quantifying the surface energy fluxes in coral reefs at various geoclimatic regions and the mechanisms by which the air-water interactions regulate water temperature is rare. We measure surface energy fluxes over coral reefs using Eddy Covariance towers in two contrasting geo-climatic regions: The typical setup of humid/tropical coral reefs (Heron Reef, Great Barrier Reef, Australia) and the rarer desert coral reef (Golf of Eilat, Israel). We analyze how the surface heat fluxes regulate the temperature of shallow coral reef environments. We show that in the desert reefs, the dry air overlying the shallow coral reef results in extremely high evaporation rates which in turn results in extensive cooling of the water. In humid/tropical reefs, evaporation is suppressed by humidity and is limited in the ability to offset the heating of the water. The extreme difference in evaporative cooling in desert versus tropical reefs is key in the response to marine heat waves. Marine heat waves which impose thermal stress on corals are a result of synoptic-scale circulation variations which suppress evaporation and increase heating. The most severe marine heatwave ever measured at the Gulf of Eilat (August 2021) was found to be caused by low evaporation rates, which resulted from synoptic circulation with weak winds and high humidity. After the onset of water temperature rise and the return of the dry winds, evaporation instantly cooled the water overlying the corals- relieving potential stress. Whereas, at the humid/tropical Heron Reef (Great Barrier Reef, Australia) evaporation solely is unable to reduce the high water temperature and therefore coral heat induce stress events are inevitably longer and more frequent. We conclude that evaporative cooling is a key mechanism protecting coral reefs located in deserts from extreme high-water temperatures, thereby representing possible thermal refugium for corals against background global warming.

How to cite: Abir, S., McGowan, H. A., Shaked, Y., and Lensky, N.: Identifying an Evaporative Thermal Refugium for the Preservation of Coral Reefs in a Warming World—The Gulf of Eilat (Aqaba), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16883, https://doi.org/10.5194/egusphere-egu23-16883, 2023.

EGU23-187 | ECS | Orals | OS1.3

A parameter space for evaluating oceanic convection regimes 

Alexandre Legay, Bruno Deremble, Thierry Penduff, and Pierre Brasseur

Oceanic convection, parameterized through vertical mixing schemes, is still not well captured by ocean general circulation models. A preliminary step necessary to improve these schemes is to evaluate and compare how the models behave for different forcing regimes. Literature often proposes single-case comparison (either on a specific location or a specific time or with a specific metrics). The goal of our work is to propose a more systematic framework allowing evaluations and comparisons over a larger range of forcing regimes. For doing so, we define a parameter space which has been derived thanks to a theoretical 1D model of the mixed-layer depth (MLD) evolution. This parameter space is formed by two dimensionless numbers : λs which describes the relative contribution of the buoyancy flux and the wind in the surface layer, and the Richardson number Rh which characterizes the stability of the water column at the mixed layer base. In this presentation, I will highlight the key features of this parameter space and I will illustrate its physical robustness with an ensemble of 1D simulations. These simulations were conducted by applying a 10 years JRA55-do 1.4.0 atmospheric forcing within a 1D standalone code making use of the NEMO Turbulent Kinetic Energy + Enhanced Vertical Diffusivity (TKE + EVD) scheme. Then, I will present a test case to study the impact of the horizontal resolution on the convection regimes for a TKE + EVD scheme in 1D, 1°, 1/12° and 1/60° realistic NEMO simulations. I will define convective regimes by sorting the values according to the normalized evolution of the mixed layer depth dt MLD / MLD and I will show that these regimes are almost kept in the parameter space between 1D and 1° but become generally less convective / more restratifiying when increasing the resolution, highlighting the restratification processes by lateral fluxes. Moreover, I will show that the dynamics in the Mediterranean is much more affected by the increase of resolution than the Labrador sea, suggesting that it involves more intense lateral restratification processes.

How to cite: Legay, A., Deremble, B., Penduff, T., and Brasseur, P.: A parameter space for evaluating oceanic convection regimes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-187, https://doi.org/10.5194/egusphere-egu23-187, 2023.

EGU23-726 | ECS | Orals | OS1.3

Ocean small-scale fronts in the Northwestern Tropical Atlantic: Assessment from the EUREC4A-OA/ATOMIC field experiment 

Solange Coadou, Sabrina Speich, Sebastiaan Swart, Chelle Gentemann, Dongxiao Zhang, and Johannes Karstensen

Upper-ocean fronts are dynamically active features of the global ocean that have significant implications for air-sea interactions, vertical mass and heat transfers, stratification and phytoplankton production and export. They have a large range of spatial scales from submesoscale (1 – 10 km) to mesoscale (10 – 100s km) characterized by temporal variability from days to months. The small dimensions and short duration of these structures have limited our capacity of observing, modelling and understanding fully these processes and their impact.

The EUREC4A-OA/ATOMIC field experiment, that took place during January-February 2020 in the Northwest Tropical Atlantic, has tried to address this challenge. In particular, five Saildrones, which are uncrewed platforms instrumented to measure the air-sea interface, have been deployed. This region showed to be a well-suited laboratory to investigate horizontal density surface gradients over a wide range of scales. Strongly affected by the outflow of the Amazon River, the generation of fine-scale horizontal thermohaline gradients is favored by the stirring of this freshwater input by large anticyclonic eddies (a.k.a. North Brazil Current Rings). The distribution of these frontal structures highlights the presence of very intense gradients, including at the smaller spatial scales. The coherence of temperature and salinity fronts was estimated by a wavelet transform analysis. It reveals that large-scale density fronts are primarily controlled by horizontal variations in salinity but with increasing temperature-salinity coherence at the small scales range of the spectrum (O (0.1 km)) for strong gradients whereas they are poorly correlated for weaker fronts.

Our study shows that processes such as the mixed layer depth, the diurnal cycle, and air-sea exchanges are strongly affected by these small-scale frontal regimes. The parallel and quasi synchronous tracks of a 4-Saildrone formation provide a detailed picture of the upper ocean vorticity, divergence, and strain from their ADCP current measurements. Overall the methodology that has been developed could be extended on other datasets in order to assess the phenomenology of fine-scale structures in other dynamical regions.

How to cite: Coadou, S., Speich, S., Swart, S., Gentemann, C., Zhang, D., and Karstensen, J.: Ocean small-scale fronts in the Northwestern Tropical Atlantic: Assessment from the EUREC4A-OA/ATOMIC field experiment, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-726, https://doi.org/10.5194/egusphere-egu23-726, 2023.

EGU23-2545 | ECS | Orals | OS1.3

Surface cooling as an internal wave generator in high latitudes. 

Matheus Azevedo and Yujiro Kitade

Heat-forced convection is a phenomenon observed frequently in high-latitude oceans. An inherent part of the Global Ocean Conveyor, it affects the global climate state over a wide range of spatial and temporal scales while being fundamentally tied to the diurnal cycle. Despite the importance of convective phenomena, most ocean general circulation models do not fully resolve it, instead parametrizing convection with adjustment schemes that remove static instability in the water by mixing vertically adjacent grid cells. However, the mixed layer response to daily-averaged fluxes is not necessarily the same as the average response to the diurnal cycle. Neglecting the diurnal cycle replaces periodic nightly convective pulses with chronic mixing that does not reach as deep. (Soloviev and Klinger, 2008).

Furthermore, the current understanding of upper-layer processes does not elucidate the consequences of the oscillatory behavior of the diurnal convection at the boundaries of the mixed layer. To address this issue, we devised a numerical experiment to investigate whether an upward heat flux is enough to generate internal waves capable of propagation despite their original forcing having a non-propagating period (~24 hours).   

To reproduce the surface cooling-induced convection and the consequent internal wave generation, we formulated a 2-D model incorporating non-hydrostatic dynamics. Although pressure is the most computationally intensive term to calculate in such models, we could exclude it from our calculation by employing the Navier-Stokes equation with a rigid-lid, incompressible, and Boussinesq approximation, and cross-differentiating the equation system to reach a single equation defined in terms of vorticity and stream function. The model was set with a 60s time step, implemented using a leap-frog scheme, constant step Δx=200m for the horizontal and Δz=5m for the vertical axis, over a 40000 x 2000 m domain. The bottom and lateral boundaries were respectively set to a reflective non-slip and a cyclic boundary. The inertial period for the domain was set at 13.81h, simulating the 60°S latitude. The experiment started from a stratified condition and was forced using a sinusoidal heat-flux function at the middle of the domain with a diurnal period and varying amplitudes.

Our experiment indicates that internal waves are generated at the boundary of the mixed layer by nonlinear wave-wave interactions of the diurnal and inertial periods. The enhancement of the near-inertial period was observed as well as the generation of higher frequency waves of 8, 6 and 4 hours. These waves travel far beyond their generation site and propagate down to 2000 m deep, as deep as the vertical domain allows.

The internal waves observed in the numerical experiment might play an important role in enhancing mixing in the ocean interior at high latitudes, especially during the winter. This mechanism could also help to explain deep and bottom ocean variability and establish a pathway for the upper layer and deep ocean interaction.

How to cite: Azevedo, M. and Kitade, Y.: Surface cooling as an internal wave generator in high latitudes., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2545, https://doi.org/10.5194/egusphere-egu23-2545, 2023.

EGU23-2874 | ECS | Orals | OS1.3

Local variability of internal wave driven upwelling at four remote Pacific Reefs 

Megan O'Hara, Peter Robins, Gareth Williams, and Mattias Green

Internal wave driven upwelling has been shown to deliver cool, nutrient-rich water (‘cold pulses’) to the shallow shores of remote reefs, providing potential thermal refugia and nutrient enrichment. However, the spatial variability within nearby islands is yet to be explored. Two methods were used to quantify the distribution of cold pulse events between two sets of remote tropical Pacific Islands: Kingman Reef and Palmyra Atoll (66 km apart), and Howland and Baker Islands (70 km apart); the two groups of islands are ~1700 km apart.

Using data from subsurface temperature loggers (STR’s) from 2008 to 2018, moored on the forereefs of our four Islands, we show that there were clear differences in upwelling behaviour. Around Palmyra Atoll, the northwest and west logger sites are <1 km apart and have a difference in degree cooling hours (DCH – hours in a day during which cold pulses were present) of up to 0.6 per day. The temperature drop at these sites differs by up to 1°C per cold pulse. Kingman Reef showed up to 0.5 DCH difference between sites (~4 km apart) per day, with temperature drop differences of up to 2°C per pulse. In contrast, Howland and Baker Islands showed up to 3 DCH difference per day between islands, whereas the temperature drop around Baker Island differed by up to 2°C per pulse. During the very strong 2015/2016 El Niño, Palmyra showed an increase of up to 1.3 DCH in a day, whereas Kingman reached <0.2 DCH per day. Howland and Baker Islands showed a similar response during this El Niño event but differed during normal ENSO phases. For example, during 2013/2014, Baker Island showed a maximum of up to 2.75 DCH per day, whereas Howland Island did not reach past 0.5 DCH per day.

We conclude that cold pulse behaviour varies between geographically close reefs and so one reef’s data cannot be used as a proxy for other islands and reefs. Subsequently, we hypothesise that the slope angle of the reef may be correlated to the presence of cold pulse activity, and that increased cold pulses may be able to mitigate the effects of a global warming on reefs with specific characteristics.

How to cite: O'Hara, M., Robins, P., Williams, G., and Green, M.: Local variability of internal wave driven upwelling at four remote Pacific Reefs, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2874, https://doi.org/10.5194/egusphere-egu23-2874, 2023.

EGU23-3283 | ECS | Orals | OS1.3 | Highlight

Sub-frontal niches of marine plankton driven by transport and trophic interactions at ocean fronts 

Inès Mangolte, Marina Lévy, and Mark Ohman

Observations and theory have suggested that ocean fronts are ecological hotspots, associated with higher diversity and biomass across many trophic levels. The hypothesis that these hotspots are driven by frontal nutrient injections is seemingly supported by the frequent observation of opportunistic diatoms at fronts, but the behavior of the rest of the plankton community is largely unknown.
Here we investigate the organization of planktonic communities across fronts by analyzing 8 high resolution transects in the California Current Ecosystem containing extensive data for 24 groups of bacteria, phytoplankton and zooplankton.
We find that a distinct frontal plankton community characterized by enhanced biomass of not only diatoms and copepods but many other groups of plankton such as chaetognaths, rhizarians and appendicularians emerges over most fronts. Importantly, we find spatial variability at a finer scale (typically 1-5 km) than the width of the front itself (typically 10-30 km) with peaks of different plankton taxa at different locations across the width of a front. Our results suggest that multiple processes, including both horizontal stirring and biotic interactions, are responsible for creating this fine-scale patchiness.

How to cite: Mangolte, I., Lévy, M., and Ohman, M.: Sub-frontal niches of marine plankton driven by transport and trophic interactions at ocean fronts, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3283, https://doi.org/10.5194/egusphere-egu23-3283, 2023.

In the last decade, three persistent warm blob events (2013–2014, 2015, and 2019–2020) in the Northeast Pacific (NEP) have been hotly debated given their substantial effects on climate, ecosystem, and socioeconomy. This study investigates the changes of such long-lived NEP warm blobs in terms of their intensity, duration, structure, and occurrence frequency under Shared Socioeconomic Pathway (SSP) 119 and 126 low-warming scenarios of the Coupled Model Intercomparison Project Phase 6. Results show that the peak timing of the warm blobs shifts from cold season to boreal summer. For the summer-peak warm blobs, their maximum intensity increases by 6.7% (10.0%) under SSP119 (SSP126) scenario, but their duration reduces by 31.0% (20.4%) under SSP119 (SSP126) scenario. In terms of their vertical structure, the most pronounced temperature signal is located at the surface, and their vertical penetration is mostly confined to the mixed layer, which becomes shallower in warming climates. Based on a mixed-layer heat budget analysis, we reveal that shoaling mixed layer depth plays a dominant role in driving stronger intensity of the warm blobs under low-warming scenarios, while stronger magnitude of ocean heat loss after their peaks explains the faster decay and thus shorter duration. Regarding occurrence frequency, the total number of the warm blobs does not change robustly in the low-warming climates. Following the summer peak of the warm blobs, extreme El Niño events may occur more frequently under the low-warming scenarios, possibly through stronger air-sea coupling induced by tropical Pacific southwesterly anomalies.

How to cite: Shi, J., Tang, C., and Zhang, Y.: Changes and mechanisms of long-lived warm blobs in the Northeast Pacific in low-warming climates, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3723, https://doi.org/10.5194/egusphere-egu23-3723, 2023.

Two extremely low surface chlorophyll concentration events in the southeast Arabian Sea (SEAS, 6oN-15oN, 72oE-77oE) during summers of 2015 and 2019 have been found since 1998. Although warm sea surface temperature (SST) and low nutrients are the direct cause for the anomalously low surface chlorophyll concentration, the physical processes leading to the warm SST anomalies during 2015 and 2019 summer are different. Satellite observations, model outputs and reanalysis data are used to explore the related mechanisms. In 2019, the combined effects of northward local wind anomaly due to extreme positive IOD and westward-propagating downwelling Kelvin wave driven by the easterly anomaly in eastern Sri Lanka weaken the upwelling in the SEAS, leading to warm SST anomaly and suppressing the upward transport of the subsurface nutrients to the surface. A weaker positive IOD occurred in 2015, leading to stronger upwelling in the SEAS than during 2019. Yet, seawater in the SEAS experienced extreme warming (lowest SST exceeded 28.5oC) due to the development of super El Niño in 2015. The significant seawater warming can shoal mixed layer and prevent the nutrients in the subsurface from reaching surface, which is unfavorable for the chlorophyll growth. The thermal balance analysis suggests that the extreme warming in the SEAS was mainly related to more downward shortwave radiation.

How to cite: Huang, H.: Negative surface chlorophyll concentration anomalies in the southeastern Arabian Sea during 2015 and 2019 summers, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4833, https://doi.org/10.5194/egusphere-egu23-4833, 2023.

EGU23-7758 | ECS | Posters on site | OS1.3 | Highlight

Seasonal variation in the barrier layer of the Banda Sea 

Mochamad Furqon Azis Ismail and Johannes Karstensen

The Banda Sea is crucial to the circulation of the world's oceans and atmosphere due to its location within the equatorial regions of the Indonesian Maritime Continent. It links the Pacific and Indian Oceans' circulation via the Indonesian Throughflow and contributes to driving atmospheric conditions via heat and moisture fluxes. Strong salinity-stratified barrier layers have the potential to play a significant role in air-sea interaction by separating the base of the mixed layer from the top of the thermocline and reducing the exchange of surface heat and momentum with the ocean's subsurface. In this study, we present the seasonal variability of barrier layer thickness (BLT) and its formation mechanism in the Banda Sea using the eddy-resolving ocean reanalysis Bluelink version 2020 (BRAN2020) for 1993 to 2021 and air-sea flux data. The findings show that the BLT is a persistent feature in the Banda Sea with a strong seasonal cycle. The BLT maxima appear in the southeast monsoon season period from May to July and the minima in the pre-northwest monsoon season from October-November. The spatial distribution of BLT is zonally oriented along the sea surface salinity (SSS) front from the west to the east of the Banda Sea. We suggest that the horizontal advection of low salinity water from the Java Sea and precipitation contributes to the formation of BLT formation and variability in the Banda Sea.

How to cite: Ismail, M. F. A. and Karstensen, J.: Seasonal variation in the barrier layer of the Banda Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7758, https://doi.org/10.5194/egusphere-egu23-7758, 2023.

EGU23-8162 | Posters on site | OS1.3

The mixed layer depth in Ocean Model Intercomparison Project (OMIP) high resolution models 

Anne Marie Tréguier, Clement de Boyer Montégut, Eric Chassignet, Baylor Fox-Kemper, Andy Hogg, Doroteaciro Iovino, Andrew Kiss, Julien le Sommer, Camille Lique, Pengfei Lin, Hailong Liu, Guillaume Serazin, Dmitry Sidorenko, Steve Yeager, and Qiang Wang

The ocean mixed layer is the interface between the ocean interior and the atmosphere or sea ice, and plays a key role in climate variability. Numerical models used in climate studies should therefore have a good representation of the mixed layer, especially its depth (MLD). Here we use simulations from the Ocean Model Intercomparison Project (OMIP), which have been forced by a common atmospheric state, to assess the realism of the simulated MLDs. For model validation, an updated MLD dataset has been computed from observations using the fixed density threshold recommended by the OMIP protocol. We evaluate the influence of horizontal resolution by using six pairs of simulations, non-eddying (typically 1° resolution) and eddy-rich (1/10° to 1/16° resolution). In winter, low resolution models exhibit large biases in the deep water formation regions. These biases are reduced in eddy-rich models but not uniformly across models and regions. The improvement is most noticeable in the mode water formation regions of the northern hemisphere, where the eddy-rich models produce a more robust MLD and deep biases are reduced. The Southern Ocean offers a more contrasted view, with biases of either sign remaining at high resolution. In eddy-rich models, mesoscale eddies control the spatial variability of MLD in winter. Contrary to an hypothesis that the deepening of the MLD in anticyclones would make the MLD deeper globally, eddy-rich models tend to have a shallower MLD in the zonal mean. In summer, a deep MLD bias is found in all the non-eddying models north of the equator; this bias is greatly reduced at high resolution. In addition, our study highlights the sensitivity of the MLD computation to choice of a reference level and the spatio-temporal sampling, which motivates new recommendations for MLD computation in future model intercomparison projects.

How to cite: Tréguier, A. M., de Boyer Montégut, C., Chassignet, E., Fox-Kemper, B., Hogg, A., Iovino, D., Kiss, A., le Sommer, J., Lique, C., Lin, P., Liu, H., Serazin, G., Sidorenko, D., Yeager, S., and Wang, Q.: The mixed layer depth in Ocean Model Intercomparison Project (OMIP) high resolution models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8162, https://doi.org/10.5194/egusphere-egu23-8162, 2023.

EGU23-8346 | ECS | Posters on site | OS1.3

Variability of Warming at Mixed Layer Base in the Subtropical South Indian Ocean Salinity Maxima Region 

Madhu Kaundal, Jithendra Raju Nadimpalli, and Mihir Kumar Dash

Subtropical South Indian Ocean salinity maxima region plays an important role in transporting temperature anomalies towards north along the isopycnals following geostrophic pathways. In this study, interannual and decadal changes in temperature and salinity at the base of mixed layer during austral winters are investigated for the Argo era. Winter time deep mixed layer allows these Temperature/Salinity (T/S) changes to penetrate to the permanent pycnocline. Interannual changes in the mixed layer depth (MLD) are mostly driven by convective buoyancy and wind forcing. Contribution of different atmospheric and oceanic forcing to the changes in mixed layer temperature and salinity are shown using mixed layer budget calculation. It is observed that net heat flux term dominates the temperature changes whereas meridional advection plays a important role in driving salinity changes in the mixed layer. Mixed layer T/S changes are subducted to the permanent pycnocline mainly by lateral induction process because of large meridional MLD gradient. Density compensated anomalies also contribute to the T/S changes at the bottom of the mixed layer. Interannual temperature anomalies due to spiciness and heaving are further explored.

How to cite: Kaundal, M., Nadimpalli, J. R., and Dash, M. K.: Variability of Warming at Mixed Layer Base in the Subtropical South Indian Ocean Salinity Maxima Region, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8346, https://doi.org/10.5194/egusphere-egu23-8346, 2023.

EGU23-8431 | Posters on site | OS1.3

Imaging small-scale ocean dynamics at interfaces of the Earth System with the SeaSTAR Earth Explorer 11 mission candidate 

Christine Gommenginger, Adrien C. H. Martin, David L. McCann, Alejandro Egido, Kevin Hall, Petronilo Martin-Iglesias, and Tânia Casal

SeaSTAR is a satellite mission candidate for ESA Earth Explorer 11 that proposes to measure small-scale ocean dynamics below 10 km at ocean/atmosphere/land/ice interfaces of the Earth System. SeaSTAR products consist of high-resolution images of total surface current vectors and wind vectors of unprecedented resolution (1 km) and accuracy over a wide swath. A key objective of SeaSTAR is to characterize, for the first time, the magnitude, spatial structure, regional distribution and temporal variability of upper ocean dynamics on daily, seasonal and multi-annual time scales, with particular focus on coastal seas, shelf seas and Marginal Ice Zone boundaries. The mission addresses an urgent need for new measurements of small-scale ocean processes to help understand and model their impacts on air-sea interactions, horizontal water pathways, vertical mixing and marine productivity. High-resolution imaging of total currents with collocated wind and waves data would bring new means of validating and developing models to improve operational forecasts and climate projections. The presentation will outline the key elements of the mission and the latest status of the mission concept evolution, with the technical solutions and trade-offs that are being considered. We will also present the latest results of the SEASTARex airborne campaign in Iroise Sea using the OSCAR (Ocean Surface Current Airborne Radar) demonstrator.

How to cite: Gommenginger, C., Martin, A. C. H., McCann, D. L., Egido, A., Hall, K., Martin-Iglesias, P., and Casal, T.: Imaging small-scale ocean dynamics at interfaces of the Earth System with the SeaSTAR Earth Explorer 11 mission candidate, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8431, https://doi.org/10.5194/egusphere-egu23-8431, 2023.

EGU23-8577 | ECS | Posters on site | OS1.3

On the ability of OMIP models to simulate the seasonal cycle of the ocean mixed layer depth in pan-Arctic Seas 

Sofía Allende, Thierry Fichefet, Hugues Goosse, and Anne-Marie Tréguier

In this study, we assess the ability of the ocean-sea ice general circulation models that participated in the Ocean Model Intercomparison Project (OMIP) to simulate the seasonal cycle of the ocean mixed layer depth in pan-Arctic seas. We focus on the central Arctic Ocean, Beaufort, Chukchi, East Siberian, Laptev, Kara, and Barents Seas. All models underestimate the mixed layer depth by about 15m on average during summertime compared to the MIMOC (Monthly Isopycnal/Mixed layer Ocean Climatology) observational data. In fall and winter, differences of several tens of meters are noticed between the models themselves, and between the models and the observational data. Some models generate too deep mixed layers, while others produce too shallow mixed layers. The magnitude of these inter-model variations differs depending on the sea under consideration.

In almost all the seas, OMIP models with similar ocean stratification compared to MIMOC observational data display the best mixed layer depth at the end of the winter. Furthermore, all models simulate more or less the same sea ice mass balance and thus salt flux into the ocean during sea ice freezing. We argue that the discrepancies between models are not so much linked to the surface salt balance but rather to the accuracy with which those models reproduce the ocean stratification. To substantiate this behavior, we apply a simple conceptual model, which simulates the fall/winter month-to-month evolution of the mixed layer depth in ice-covered regions. In almost fully sea ice-covered regions such as the central Arctic Ocean, Beaufort, and Chukchi Seas, this simplified dynamics captures very well the behavior of the general circulation models, and this highlights that the main difference between the models is the ocean stratification. At the same time, in the East Siberian, Laptev, and Kara Seas, inter-model variations are not explained by the differences in ocean stratification, even though they contain a significant concentration of sea ice. In not fully sea ice-covered regions, such as the Barents Sea, the mixed layer depth dynamics is different: the retreat of the ice cover during summer is more significant than in fully covered regions, hence favoring exchanges with the atmosphere.

How to cite: Allende, S., Fichefet, T., Goosse, H., and Tréguier, A.-M.: On the ability of OMIP models to simulate the seasonal cycle of the ocean mixed layer depth in pan-Arctic Seas, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8577, https://doi.org/10.5194/egusphere-egu23-8577, 2023.

EGU23-9721 | ECS | Posters on site | OS1.3

Effects of Ocean Mesoscale Processes on Biogeochemistry 

Lucas Casaroli, Tatiana Ilyina, and Fatemeh Chegini

Mesoscale processes contemplate movements in the ocean ranging from tens to hundreds of kilometers. At this scale it is possible to observe phenomena such as eddies, vortices, fronts among others. These processes are of great importance to biogeochemical cycles as they, for example, can affect the transport of nutrients to the euphotic zone by vertical movements, alter the mixed layer depth through vertical displacement of isopycnals, as well as trap biological, chemical and physical properties inside eddies and meanders.
Ocean General Circulation Models (OGCM’s) either resolve the mesoscale eddies by increasing the model resolution or parameterize them. The chosen approach regarding the eddies comes with some caveats as it can lead to simulations limited to small time periods or substantial simplifications of the physical processes, which in turn can alter results and obtain a different configuration for ocean and atmosphere dynamics. Regarding biogeochemical tracers, how eddies are represented in the model bring different outcomes, showing solutions that are model dependent such as ocean regions acting as a net source or sink of nutrients, oxygen and carbon. Hence we can’t accurately constrain the ocean’s role as a carbon sink. Not only the results are model dependent, but also resolution dependent. Ocean models with higher resolutions indicate that the vertical profiles of salinity and temperature are substantially altered by mesoscale activity, thus it is expected that biogeochemical tracers are altered by eddy induced disturbances.
We present some preliminary results of the output of the HAMburg Ocean Carbon Cycle model (HAMOCC; Ilyina et al 2013, Jungclaus et al 2020) in a 40 km and 10 km resolution on a global setup. As a result we show how changing the resolution affect on the upper ocean and mixed layer the major biogeochemical tracers and overall the carbon cycle.

How to cite: Casaroli, L., Ilyina, T., and Chegini, F.: Effects of Ocean Mesoscale Processes on Biogeochemistry, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9721, https://doi.org/10.5194/egusphere-egu23-9721, 2023.

EGU23-9840 | ECS | Orals | OS1.3

Observing and Modeling the variability of DWLs during the summer Monsoon in the Northern Indian Ocean 

Siddhant Kerhalkar, Amit Tandon, Tamara Schlosser, J.Thomas Farrar, Andrew Lucas, Leah Johnson, Verena Hormann, and Luca Centurioni

Diurnal Warm Layers (DWLs) play an important role in coupling the atmosphere and the ocean, but their observations in the freshwater dominated Northern Indian Ocean in summer Monsoons are rare. This study focuses on the following aspects of DWLs observed during a 5-day suppressed atmospheric convection phase of the southwest monsoon season in 2019: (i) DWL observations using innovative drifting flux profilers to simultaneously measure high resolution shear and stratification as well as the surface meteorological forcing variables to compute air-sea fluxes (ii) Observed spatial gradients of SST over 1-100 km scales and (iii) Modeling using the popular one-dimensional models increasing in complexity. These observations show regions of marginal shear instability at the DWL base in agreement with previous studies in the tropical Pacific. The commonly used constant stratification assumption within the DWL (e.g. Fairall et al. 1996) breaks down in scenarios with weaker winds and salinity-driven stratification. The vertical structure of DWLs is therefore explored using k-e turbulence closure scheme in General Ocean Turbulence Model (GOTM) framework. Insights from model-observation comparisons show that for days with similar wind speeds, the DWL response can vary based on whether warm water or freshwater advection plays a role. Notably, warm water advection leads to deeper DWLs, whereas the freshwater advection traps the DWL to shallower depths. Further, spatial differences of O(1 C) in diurnal cycles of SST are observed over O (1-100 km), showing remarkable lateral inhomogeneity in the evolution of DWLs. 

How to cite: Kerhalkar, S., Tandon, A., Schlosser, T., Farrar, J. T., Lucas, A., Johnson, L., Hormann, V., and Centurioni, L.: Observing and Modeling the variability of DWLs during the summer Monsoon in the Northern Indian Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9840, https://doi.org/10.5194/egusphere-egu23-9840, 2023.

EGU23-10370 | ECS | Posters on site | OS1.3

Atmospheric weather patterns and their contributions to the fall stratification breakdown on the Southern New England shelf 

Lukas Lobert, Glen Gawarkiewicz, and Al Plueddemann

High-wind events predominantly cause the rapid breakdown of seasonal stratification on mid-latitude continental shelfs. It is well established that downwelling-favorable wind forcing, i.e., wind vectors with the coastline to their right (on the northern hemisphere), leads to enhanced coastal destratification. A categorization scheme for high-wind events has identified the two atmospheric weather patterns that locally cause such favorable wind conditions on the Southern New England shelf and have the largest contribution to the annual breakdown of stratification in the region. These patterns are i) cyclonic storms that propagate south of the continental shelf and cause strong anticyclonically rotating winds, and ii) persistent large-scale high-pressure systems over eastern Canada causing steady north-easterly winds. Despite both patterns generally producing downwelling-favorable winds on the shelf, the two patterns differ in their wind direction steadiness and tend to produce opposite temperature and salinity contributions to destratification, implying differences in the dominant processes driving ocean mixing. We hypothesize that local mechanical mixing and surface cooling dominate for cyclonic storms due to their strong wind energy input and shear production. In contrast, the weaker but steady downwelling-favorable winds from high-pressure systems can lead to an enhanced cross-shelf Ekman cell that advects salty and less buoyant Slope Water onto the continental shelf. To assess which process dominates for the different impactful high-wind event patterns, we apply a simplified two-dimensional mixed-layer model framework that incorporates horizontal buoyancy gradients across the shelfbreak front. The model allows to determine the stratification change caused by one-dimensional surface forcing (wind stress and surface buoyancy flux) and Ekman-driven advection individually. Observations from moorings and glider transects across the shelfbreak, provided by the Ocean Observatories Initiative Coastal Pioneer Array (2015-2022) at the Southern New England shelfbreak, allow a comparison to investigate the importance of along-shelf processes for predicting shelf stratification changes on synoptic to intra-seasonal timescales.

How to cite: Lobert, L., Gawarkiewicz, G., and Plueddemann, A.: Atmospheric weather patterns and their contributions to the fall stratification breakdown on the Southern New England shelf, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10370, https://doi.org/10.5194/egusphere-egu23-10370, 2023.

EGU23-10563 | ECS | Posters on site | OS1.3

Buoy Observations of Turbulent Mixing in the northwestern subtropical Pacific Ocean 

Hsin-I Lin and Yiing-Jang Yang

Strong winds from typhoons decrease sea surface temperatures, and this cooling area is called the cold wake. It is well known that the primary mechanisms causing this phenomenon include turbulent mixing and Ekman pumping that result in the upwelling of cold water in the lower layer. The amplitude of surface cooling is influenced by the typhoon’s moving speed, strength, the radius of the storm, and the pre-typhoon conditions of the upper ocean. The cooling phenomenon affects air-sea interactions, but observing the upper ocean in such an extreme environment is challenging. To understand the physical process of mixing, and to improve the predictions of numerical models, more observations of turbulent mixing are needed. In addition, the passage of eddy also affects the occurrence of background conditions and turbulent mixing. The northwestern subtropical Pacific Ocean is an area where typhoons are prevalent, and eddies often pass through it. Therefore, this is a suitable area to study turbulent mixing when typhoons and eddies pass by.

These data were obtained from the surface buoy and the ADCP subsurface moorings located in the northwestern subtropical Pacific Ocean. In September 2022, category 5 typhoon Hinnamnor passed the buoy site during observation period. The upper ocean profiles of temperature and current were obtained, in order to estimate the Richardson numberwhen the typhoon and the cold eddy passed by. The observation results show that the peak value of the probability distribution of the Richardson number was about 3 to 4, and the probability of being less than 0.25 was about 12% at a depth of 20 m before the passage of Typhoon Hinnamnor. When the buoy system was within the 34-knot wind radius (R34) of a typhoon, the peak value of the probability distribution of the Richardson number decreased to slightly smaller than 0.25, and the probability of being less than 0.25 is about 62% at a depth of 20 m. At a depth of 75 m, the probability distribution of the Richardson number did not significantly change within the 34-knot wind radius (R34) of a typhoon,and it was not even close to 0.25. It shows that typhoon-induced turbulent mixing has no effect at this depth. In addition, during the normal period without a cold eddy, the mixed layer was deeper than the depth of 20 m. Marginal instability was evident within the mixed layer, in which the probability distribution of the Richardson number oscillated around 0.25. During the passage of the cold eddy, the upwelling of cold water made the surface mixed layer thinner, and stratification was more stable. Therefore, the cold eddy would prohibit turbulent mixing. The probability distribution of the Richardson number shifted to a larger value. However, the probability distribution of the Richardson number at a depth of 75 m did not change significantly. As a result, the observed cold eddy had no effect on the turbulent mixing at this depth. These results will be presented herein in detail.

How to cite: Lin, H.-I. and Yang, Y.-J.: Buoy Observations of Turbulent Mixing in the northwestern subtropical Pacific Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10563, https://doi.org/10.5194/egusphere-egu23-10563, 2023.

EGU23-11167 | ECS | Orals | OS1.3

Unveiling the Recent Changes in Barrier Layer Dynamics over the Arabian Sea Mini Warm Pool 

Sankar Prasad Lahiri and Vimlesh Pant

The reversal of monsoon wind and restriction of further northward oceanic heat transport makes the Indian Ocean unique compared to the other two tropical oceans. The circulation over the north Indian Ocean (NIO) also reverses following this change in the wind pattern. Two basins of NIO, i.e., the Bay of Bengal and the Arabian Sea, witness distinct physical and dynamical properties in response to this wind pattern and freshwater influx, although they lie within the same latitudinal band. The focus of this study is over the south-eastern Arabian Sea (SEAS) (7.5 - 12.5°N, 72.5 - 76.5°E), where sea surface temperature (SST) of more than 29.5°C is observed during late April and early May. This warm temperature over the SEAS is associated with the formation of a monsoon onset vortex that influences the onset of the Indian Summer Monsoon. Previous studies have suggested that high SST over the SEAS is independent of the tropical Indian Ocean warm pool. This high SST region is referred to as the Arabian Sea Mini Warm Pool (ASMWP). The development of ASMWP starts in November when the coastal Kelvin wave packets initiate the formation of an equatorward flowing boundary current along the east coast of India, East Indian Coastal Current (EICC). EICC transports the low saline Bay of Bengal water to the SEAS, resulting in a strong haline stratification which leads to the formation of a barrier layer. Once this layer forms, it restricts the vertical mixing of water in the mixed layer with the thermocline water. The objective of this study is to observe the recent change in the dynamics of this barrier layer thickness (BLT) over SEAS. Using reanalysis data from Copernicus Marine Services, the seasonal and yearly evolution of BLT is analyzed from 1993 to 2018. This study calculates the isothermal layer depth (ILD) based on the 1°C temperature criteria. The density change is computed following this temperature change which is used to calculate mixed layer depth (MLD). The monthly climatology suggests the presence of thick BLT (i.e., ILD - MLD) over SEAS from December to February, although some remnant is present in March. A seasonal average (December - February) of BLT suggests a significant increasing trend from 1993 to 2018. Although the MLD is not showing any significant changes, the ILD is witnessing a substantial increase over these years. The effect of the ILD increase is also reflected in the stratification and heat content. Using geostrophic eddy kinetic energy, the energetics of the EICC in October-November are noticed in three different regions along the southeast coast of India and south of Sri Lanka. The influence of local forcings on the dynamics of the BLT is investigated to understand the mechanism behind this evolution of BLT and its role in ASMWP variability.

How to cite: Lahiri, S. P. and Pant, V.: Unveiling the Recent Changes in Barrier Layer Dynamics over the Arabian Sea Mini Warm Pool, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11167, https://doi.org/10.5194/egusphere-egu23-11167, 2023.

EGU23-14268 | ECS | Posters on site | OS1.3

Observations of Sea Ice Melt and Ice-Ocean Boundary Layer Heat Fluxes in the Marginal Ice Zone North of Fram Strait 

Simon F. Reifenberg, Wilken-Jon von Appen, Ilker Fer, Christian Haas, Mario Hoppmann, and Torsten Kanzow

Given the prospect of a merely seasonally ice-covered Arctic Ocean in the future and a consequential new quality of atmosphere-ocean coupling, understanding and quantifying oceanic processes that contribute to sea ice melt is of particular relevance.

A region of intense melting is the marginal ice zone (MIZ) north of Fram Strait, where inflowing warm Atlantic Water meets sea ice advected southward by the Transpolar Drift. We present observations of the ice-ocean boundary layer (IOBL) from a cruise of German research vessel Polarstern to that region in summer 2022, where we gathered continuous-in-time hydrographic observations from autonomous drifting stations on three separate ice floes, supplemented by intense observation periods of vertical microstructure profiles and ice cores from crewed stations during three revisits per floe throughout the drifting period.

The three occupied floes were oriented on a line approximately perpendicular to the ice edge, initially about 25 km apart from each other, with the southernmost floe located 75 km away from the edge. The drifting instrument platforms cover a common time period of approximately two weeks, under relatively quiescent atmospheric conditions. First results show that, while the floes exhibited similar drift trajectories dominated by superimposed diurnal and semidiurnal oscillations, the evolution of key IOBL variables, such as stratification, melt rates, friction velocity, and turbulent fluxes, varied considerably – both in time and among the occupied floes.

We plan to assess how this observed variability relates to other measured properties of sea ice (e.g., ice roughness, ice thickness distribution, floe size distribution) and of the upper ocean (e.g., ice-ocean velocity shear, turbulence, surface waves, internal waves and tides) and their interaction, in order to put our preliminary findings into the broader context: ocean controls on sea ice melt in the marginal ice zone north of Fram Strait.

How to cite: Reifenberg, S. F., von Appen, W.-J., Fer, I., Haas, C., Hoppmann, M., and Kanzow, T.: Observations of Sea Ice Melt and Ice-Ocean Boundary Layer Heat Fluxes in the Marginal Ice Zone North of Fram Strait, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14268, https://doi.org/10.5194/egusphere-egu23-14268, 2023.

EGU23-14300 | ECS | Posters on site | OS1.3

Assessing the physical processes controlling oxygen subduction in CMIP6 models 

Benedict Blackledge, Oliver Andrews, Esther Portela, Rory Bingham, and Damien Couespel

Approximately half of the total loss of upper ocean oxygen over the recent past has been driven by its reduced solubility in a warming ocean. The remainder can be explained by less well constrained changes to ocean circulation, mixing and biogeochemical processes. Model based studies have shown that the choice of mixing parameterisations as well as biogeochemical factors, can introduce substantial differences in the distribution of oxygen within an individual Earth System Model (ESM).  Model Intercomparison Projects such as the latest Coupled Model Intercomparison Project Phase 6 provide an opportunity to explore processes controlling oceanic oxygen in a multi-model framework. Here we apply an oxygen transport decomposition to seven CMIP6 ESMs, using a well-established framework for the transport of tracers from the surface mixed layer into the ocean interior. We show that despite a close agreement in the oxygen concentration at the mixed-layer base, the transports to the ocean interior vary greatly between models. ESMs with similar physical ocean model components are clearly identifiable based on the spatial distribution of oxygen transport, both in the globally integrated transport terms and their inter-annual variability. Applying this decomposition to CMIP6 pre-industrial control experiments, we find the total oxygen subduction ranges between +0.6 to +1.1PMol yr-1, in agreement with an observationally based estimate. Despite broad agreement in the total magnitude of oxygen subduction, the inter-model range for individual transport terms is often large (+0.69 PMol yr-1 to -0.23 PMol yr-1 for vertical advection), implying a high degree of model uncertainty as to the physical processes controlling interior oxygen. We also characterise variability in oxygen transport terms and find that interannual variability in advective transport depends on the term and the model family. Lateral advection displays the greatest model-model difference in interannual variability, by a factor of ~6 between the most and least variable model. Mixed-layer entrainment of oxygen shows closer agreement between models, with interannual variability in this term differing by a factor of ~1.4. We recommend that future model intercomparisons including ocean biogeochemistry archive the relevant transport, production and consumption terms for key biogeochemical variables such as oxygen.

How to cite: Blackledge, B., Andrews, O., Portela, E., Bingham, R., and Couespel, D.: Assessing the physical processes controlling oxygen subduction in CMIP6 models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14300, https://doi.org/10.5194/egusphere-egu23-14300, 2023.

EGU23-14823 | ECS | Orals | OS1.3

How subsurface and double-core anticyclones intensify the winter mixed layer deepening in the Mediterranean sea 

Alexandre Barboni, Solange Coadou-Chaventon, Alexandre Stegner, Briac Le Vu, and Franck Dumas

The mixed layer is the uppermost layer of the ocean, connecting the atmosphere to the subsurface ocean through atmospheric fluxes. It is subject to pronounced seasonal variations: it deepens in winter due to buoyancy loss and shallows in spring while heat flux increase and restratify the water column. A mixed layer depth (MLD) modulation over this seasonal cycle has been observed within mesoscale eddies. 

Taking advantage of the numerous Argo floats deployed and trapped within large Mediterranean anticyclones over the last decades, we reveal for the first time this modulation at a 10-day temporal scale and free of the smoothing effect of composite approaches. The analysis of 16 continuous MLD time series inside 13 long-lived anticyclones at a fine temporal scale brings to light the importance of the eddy preexisting vertical structure in setting the MLD modulation by mesoscale eddies. Extreme MLD anomalies of up to 330m are observed when the winter mixed layer connects with a preexisting subsurface anticyclonic core, greatly accelerating mixed layer deepening. The winter MLD sometimes does not achieve such connection but homogenizes another subsurface layer, then forming a multi-core anticyclone with spring restratification. A MLD restratification delay is always observed, reaching more than 2 months in 3 out the 16 MLD timeseries. The water column starts to restratify outside anticyclones while mixed layer keeps deepening and cooling at the eddy core for a longer time. 

These new elements provide direct observation of double-core anticyclone formation, which dominant formation mechanism was previously considred to be vertical alignement, and provides new keys for understanding anticyclone vertical structure evolution.

How to cite: Barboni, A., Coadou-Chaventon, S., Stegner, A., Le Vu, B., and Dumas, F.: How subsurface and double-core anticyclones intensify the winter mixed layer deepening in the Mediterranean sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14823, https://doi.org/10.5194/egusphere-egu23-14823, 2023.

EGU23-15118 | Orals | OS1.3

Dissipation due to ageostrophic turbulence in the upper-ocean mixed layer 

Nils Brüggemann, Leonidas Linardakis, and Peter Korn

In our study, we diagnose dissipation by ageostrophic turbulence in the upper ocean. To this end, we use the Max Planck Institute's ocean model ICON-O with a telescoping grid configuration, where the resolution is enhanced up to values smaller than 600m over large areas of the North Atlantic. This allows to represent parts of the ageostrophic turbulence spectrum associated with submesoscale instabilities and eddies. We diagnose the dissipation associated with the ageostrophic eddies and investigate to what end ageostrophic turbulence is providing an efficient energy transfer towards smaller scales. We find that such an energy transfer and the associated dissipation is strongly enhanced within the upper-ocean and within and south of the Gulf Stream front. Attempts are made to develop parameterizations for the ageostrophic downscale energy flux to couple this energy dissipation with other ocean energy reservoirs. Therewith, we aim to obtain a more realistic view on the ocean energy cycle.

How to cite: Brüggemann, N., Linardakis, L., and Korn, P.: Dissipation due to ageostrophic turbulence in the upper-ocean mixed layer, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15118, https://doi.org/10.5194/egusphere-egu23-15118, 2023.

EGU23-17246 | ECS | Posters on site | OS1.3

Dynamics and impact of diurnal warm layers in the ocean 

Mira Schmitt, Sutanu Sarkar, Hieu T. Pham, and Lars Umlauf

Thin Diurnal Warm Layers (DWLs) form near the surface of the ocean on days of large solar radiation,
weak to moderate winds, and small surface waves. DWLs are characterized by complex dynamics,
and are relevant to the ocean especially by modifying surface-layer mixing and atmosphere-ocean
fluxes. Here, we use idealized Large Eddy Simulations (LES) and second-moment turbulence
modelling, both including the effects of Langmuir turbulence, to identify the key non-dimensional
parameters of the problem, and explore DWL properties and dynamics across a wide parameter
space. Comparison of LES and the second-moment turbulence models shows that the latter provide
an accurate representation of the DWL structure and dynamics. We find that, for equilibrium wave
conditions, Langmuir effects are significant only in the Stokes layer very close to the surface. While
we see pulses in the turbulent stresses and shear in the LES, there are no relevant effects of
Langmuir turbulence on DWL bulk properties and total entrainment. Results of the parameter space
analysis agree with the midday scaling by Pollard et al. (1986), however, with modified model
coefficients and deviations of up to 30% especially at high-latitudes. We develop non-dimensional
expressions for the strength and timing of the DWL temperature peak in the afternoon, and discuss
the mixing efficiency and energetics of DWLs in the presence of Langmuir turbulence.

How to cite: Schmitt, M., Sarkar, S., Pham, H. T., and Umlauf, L.: Dynamics and impact of diurnal warm layers in the ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17246, https://doi.org/10.5194/egusphere-egu23-17246, 2023.

EGU23-17247 | Posters on site | OS1.3 | Highlight

Subducting filaments in frontal zones in the Western Mediterranean Sea: Physical, turbulent and biological evidences 

Francesco Marcello Falcieri, Lorenzo Pasculli, and Giovanni Testa

Oceanic frontal areas are well known as sites prone to the generation of submesoscale instabilities that can lead to surface waters subduction along isopycnal surfaces well below the mixed layer. Those processes can play an important role in the vertical exchange of physical properties and biogeochemical tracers.
In the framework of the CALYPSO DRI research initiative (Coherent Lagrangian Pathways from the Surface Ocean to Interior), turbulent dissipation rates characterizing subducting filaments originated form frontal areas were studied with a free falling microstructure profiler. Microstructure profiles, along ancillary data, were collected on several transects along and across frontal areas and mesoscale eddies in the Western Mediterranean Sea during two cruises: one in the Alboran Sea (March/April 2019) and one in the Balearic Sea (February/March 2022).
The presence of subducting filaments moving along isopycnal surfaces was identified at depths between 100 and 250 m by the combined analysis of physical (i.e. temperature and salinity), chemical (i.e. dissolved oxygen) and biological properties (i.e. high chlorophyll concentration well below the mixed layer and the deep chlorophyll maximum). The majority of the subducting filaments were characterized by turbulent kinetic energy dissipation rates (TKE, values of 10-7 W·m-2) much higher than rates generally observed at such depths. The TKE values were found in conjunction with an increase in Brunt Vaisala frequency and low Thorpe scale values. The same conclusion can be drawn from Turner angle values.

How to cite: Falcieri, F. M., Pasculli, L., and Testa, G.: Subducting filaments in frontal zones in the Western Mediterranean Sea: Physical, turbulent and biological evidences, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17247, https://doi.org/10.5194/egusphere-egu23-17247, 2023.

EGU23-234 | ECS | Posters on site | OS1.4

Future Arctic Ocean atmosphere-ice-ocean momentum transfer and impacts on ocean circulation 

Morven Muilwijk, Tore Hattermann, Sigrid Lind, and Mats Granskog

Over the last few decades, the Arctic has experienced surface warming at more than twice the global rate and extensive sea ice loss. The reduced sea ice cover affects the mechanical and thermodynamical coupling between the atmosphere and the ocean. A commonly repeated hypothesis is that a thinner and more mobile sea ice cover will increase momentum transfer, resulting in a spin-up of upper Arctic Ocean circulation and enhanced vertical mixing. In general, sea ice protects the ocean from interaction with the atmosphere, and a thinning and shrinking sea ice cover implies a more direct transfer of momentum and heat. For example, several observational studies show a more energetic ocean after strong wind events over open water than wind events over ice-covered water. However, previous modeling studies show that seasonality is very important and that the total momentum transfer can decrease with more open water because the ice surface provides greater drag than the open water surface. We here present numerical simulations of future scenarios with the Norwegian Earth System Model (NorESM) and show how the momentum transfer is projected to change with changing sea ice and wind conditions in various regions of the Arctic Ocean. We then compare our results with output from other CMIP6 models and present how different wind conditions and the diminishing ice cover impacts the upper ocean circulation. 

How to cite: Muilwijk, M., Hattermann, T., Lind, S., and Granskog, M.: Future Arctic Ocean atmosphere-ice-ocean momentum transfer and impacts on ocean circulation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-234, https://doi.org/10.5194/egusphere-egu23-234, 2023.

EGU23-2446 | ECS | Orals | OS1.4

Drivers of Laptev Sea interannual variability in salinity and temperature from satellite data 

Phoebe Hudson, Adrien Martin, Simon Josey, Alice Marzocchi, and Athanasios Angeloudis

Arctic surface air temperatures are warming twice as fast as global average temperatures. This has caused ocean warming, an intensification of the hydrological cycle, snow and ice melt, and increases in river runoff. Rivers play a central role in linking the components of the water cycle and Russian rivers alone contribute ~1/4 of the total freshwater to the Arctic Ocean, maintaining the halocline that covers the Arctic and dominates circulation. Increases in river runoff could further freshen this layer and increase Arctic Ocean stratification. However, the increase in atmosphere-ocean momentum transfer with sea ice loss could counteract or alter this pattern of circulation, mixing this cold fresh water with the warm salty water that currently sits below it. Understanding the interplay between these changes is crucial for predicting the future state of the Arctic system. Historically, studies trying to understand the interplay between these changes have been challenged by the difficulty of collecting in situ data in this region.

 

Over most of the globe, L-band satellite acquisitions of sea surface salinity (SSS), such as from Aquarius (2011–2015), SMOS (2010- present), and SMAP (2015-present), provide an idea tool to study freshwater storage and transport. However, the low sensitivity of L-band signal in cold water and the presence of sea ice makes retrievals at high latitudes a challenge. Nevertheless, retreating Arctic sea ice cover and continuous progress in satellite product development make the satellite based SSS measurements of great value in the Arctic. This is particularly evident in the Laptev Sea, where gradients in SSS are strong and in situ measurements are sparse. Previous work has demonstrated a good consistency of satellite based SSS data against in situ measurements, enabling greater confidence in acquisitions and making satellite SSS data a truly viable potential in the Arctic. Therefore, this project aims to combine satellite data, particularly SMAP and SMOS sea surface salinity (SSS) data, with model output to improve our understanding of interactions between the components of the Arctic hydrological cycle and how this is changing with our changing climate.

 

The Laptev Sea was chosen as an initial region of focus for analysis as the Lena river outflows as a large, shallow plume, which is clearly observable from satellite SSS data. The spatial pattern of the Lena river plume varies considerably interannually, responding to variability in atmospheric and oceanic forcing, sea ice extent, and in the magnitude of river runoff.  Numerical model output and satellite products confirm what has previously been suggested from in-situ data: wind forcing is the main driver of river plume variability.

How to cite: Hudson, P., Martin, A., Josey, S., Marzocchi, A., and Angeloudis, A.: Drivers of Laptev Sea interannual variability in salinity and temperature from satellite data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2446, https://doi.org/10.5194/egusphere-egu23-2446, 2023.

EGU23-3244 | Orals | OS1.4

Stable oxygen isotopes from the MOSAIC expedition show vertical and horizontal variability of sea-ice and river water signals in the upper Arctic Ocean during winter 

Dorothea Bauch, Nils Andersen, Ellen Damm, Alessandra D'Angelo, Ying-chih Fang, Ivan Kuznetsov, Georgi Laukert, Moein Mellat, Hanno Meyer, Benjamin Rabe, Janin Schaffer, Kirstin Schulz, Sandra Tippenhauer, and Myriel Vredenborg

Our aim is to better understand how local winter modification and advected signals from the Siberian Shelf affect the structure of the upper Arctic Ocean along the Transpolar Drift (TPD). Hereto we use stable oxygen isotopes of the water (δ18O) in combination with salinity to quantify river water contributions (fr) and changes due to sea-ice formation or melting (fi) in the upper ~150m of the water column during the MOSAIC drift. Furthermore, ratios of fi/fr at identical salinities can be used to distinguish waters remnant from the previous summer and those modified locally.

Within the ongoing winter we observed salinification and deepening of the mixed layer (ML) due to sea-ice related brine release together with interleaving waters at the base of the ML and within the main halocline. These interleaving waters with variable sea-ice and river water signals are observed for the first time and have not been observed during summer expeditions before.

The MOSAIC floe drifted in and out of the freshwater-rich part of the TPD and into the Atlantic regime throughout the winter. Despite these strong regime changes the sea-ice related brine content accumulated during the ongoing winter remained visible within the water column. Budgets derived by integration of signals over the upper 100m result in ~1 to 5 m of pure sea-water (34.92 salinity and 0.3‰ δ18O) removed from the water column for ice formation and are much higher than ice thicknesses of ~0.5 to 2 m observed for the MOSAIC floe. For further evaluation scaling factors have to be considered accounting e.g. for the different densities of ice and water as well as for the lower salinity in the halocline relative to pure sea-water. Therefore, our analysis indicates a lower limit of the advected signal relative to local winter modification within the Arctic Ocean halocline.

How to cite: Bauch, D., Andersen, N., Damm, E., D'Angelo, A., Fang, Y., Kuznetsov, I., Laukert, G., Mellat, M., Meyer, H., Rabe, B., Schaffer, J., Schulz, K., Tippenhauer, S., and Vredenborg, M.: Stable oxygen isotopes from the MOSAIC expedition show vertical and horizontal variability of sea-ice and river water signals in the upper Arctic Ocean during winter, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3244, https://doi.org/10.5194/egusphere-egu23-3244, 2023.

EGU23-3465 | Orals | OS1.4

Ocean heat increase and sea ice reduction in the Fram Strait conveys Arctic Ocean change 

Laura de Steur, Hiroshi Sumata, Dmitry Divine, Mats Granskog, and Olga Pavlova

The sea ice extent in the Arctic Ocean has reduced dramatically with the last 16 years (2007-2022) showing the 16 lowest September extents observed in the satellite era. Besides a declining sea ice cover and increase in ocean heat content in summer, the winter sea ice concentration and thickness have also become more vulnerable to changes. We present results from the Fram Strait Arctic Outflow Observatory showing that the upper ocean temperature in the East Greenland Current in the Fram Strait has increased significantly between 2003 and 2019. While the cold Polar Water now contains more heat in summer due to lower sea ice concentration and longer periods of open water upstream, the warmer returning Atlantic Water has shown a greater presence in winter the central Fram Strait, impacting the winter sea ice thickness and sea ice extent. These processes combined result in a reduced sea ice cover downstream along the whole east coast of Greenland both in summer and winter, which has consequences for winter-time ocean convection in the Greenland Sea.

How to cite: de Steur, L., Sumata, H., Divine, D., Granskog, M., and Pavlova, O.: Ocean heat increase and sea ice reduction in the Fram Strait conveys Arctic Ocean change, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3465, https://doi.org/10.5194/egusphere-egu23-3465, 2023.

EGU23-3547 | Orals | OS1.4

An increasingly turbulent Arctic Ocean? 

Tom P. Rippeth

Historically, the Arctic Ocean has been considered an ocean of weak turbulent mixing. However, the decline in seasonal sea ice cover over the past couple of decades has led to increased coupling between the atmosphere and the ocean, with potential enhancement of turbulent mixing. Here, we review studies identifying energy sources and pathways that lead to turbulent mixing in an increasingly ice-free Arctic Ocean. We find the evolution of wind-generated, near-inertial oscillations is highly sensitive to the seasonal sea ice cycle, but that the response varies greatly between the continental shelves and the abyssal ocean. There is growing evidence of the key role of tides and continental shelf waves in driving turbulent mixing over sloping topography. Both dissipate through the development of unsteady lee waves. The importance of the dissipation of unsteady lee waves in driving mixing highlights the need for parameterization of this process in regional ocean models and climate simulations.

How to cite: Rippeth, T. P.: An increasingly turbulent Arctic Ocean?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3547, https://doi.org/10.5194/egusphere-egu23-3547, 2023.

EGU23-4159 | ECS | Orals | OS1.4

Modes of decadal variability in observed Arctic sea-ice concentration 

Jakob Dörr, Marius Årthun, David B. Bonan, and Robert C. J. Wills

The Arctic sea ice cover is strongly influenced by internal variability on decadal time scales, affecting both short-term trends and the timing of the first ice-free summer. Several mechanisms of variability have been proposed, but the contributions of distinct modes of decadal variability to regional and pan-Arctic sea-ice trends has not been quantified in a consistent manner. The relative contribution of forced and unforced variability in observed Arctic sea ice changes also remains poorly quantified. Here, we identify the dominant patterns of winter and summer decadal Arctic sea-ice variability in the satellite record and their underlying mechanisms using a novel technique called low-frequency component analysis. The identified patterns account for most of the observed regional sea ice variability and trends, and thus help to disentangle the role of forced and unforced sea ice changes since 1979. In particular, we separate a mode of decadal ocean-atmosphere-sea ice variability, with an anomalous atmospheric circulation over the central Arctic, that accounts for approximately 30-50% of the accelerated decline in pan-Arctic summer sea-ice area between 2000 and 2012. For winter, we find that internal variability has so far dominated decadal trends in the Bering Sea, while it plays a smaller role in the Barents and Kara Seas. These results, which detail the first purely observation-based estimate of the contribution of internal variability to decadal trends in sea ice, suggest a lower estimate of the internal variability contribution than most model-based assessments.

How to cite: Dörr, J., Årthun, M., Bonan, D. B., and Wills, R. C. J.: Modes of decadal variability in observed Arctic sea-ice concentration, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4159, https://doi.org/10.5194/egusphere-egu23-4159, 2023.

EGU23-4822 | ECS | Orals | OS1.4

Ocean response to reduced Arctic sea ice in PAMIP simulations. 

Sourav Chatterjee, Julia Selivanova, Tido Semmler, and James A. Screen

Arctic Amplification (AA) – the greater warming of the Arctic than the global average - is a prominent feature of past and projected future climate change. AA exists due to multiple positive feedbacks involving complex interactions among different components of Arctic atmosphere, ocean, and cryosphere. The loss of sea ice is a key driver of AA. Sea ice loss and resultant AA can influence the global climate system, way beyond the Arctic. The atmospheric response to sea ice loss has been studied extensively. In comparison, the oceanic response has received less attention and our understanding of it is imprecise. Here, we utilize the fully coupled model simulations from the Polar Amplification Model Comparison Project (PAMIP) to explore the oceanic response to projected Arctic sea ice loss at 2o C global warming.

The sea surface warming signal is maximum in the Barents-Kara Sea region in all three models analysed. Results suggest that the observed northward propagation of the Arctic ‘cooling machine’ (region of intensive heat loss to the atmosphere) is largely driven by the reduced sea ice over the northern Barents Sea. Simultaneously, the atmospheric response with stronger south-westerlies over the Norwegian Seas and southern Barents Sea reduces the heat loss therein. This may partly explain the bipolar spatial structure of heat loss in the Norwegian seas and the Northern Barents-Kara Sea. This seesaw heat loss pattern can result in a warmer inflow of Atlantic Waters from the Norwegian Sea to the northern Barents Sea as projected by CMIP6 models. The mixed layer depth response in these regions is consistent with the heat loss patterns, with a deepening of the mixed layer in regions of enhanced heat loss and vice versa. The surface ocean dynamic response is most prominent in the Beaufort Sea. With reduced sea ice, the Beaufort gyre circulation is strengthened due to larger wind forcing and accumulates freshwater within. As a result, surface salinity response shows maximum freshening in this region. In summary, preliminary results from the coupled simulations under the PAMIP protocol indicate that the observed and projected changes in the Arctic Ocean during the 21st century are strongly driven by the reduction in sea ice.

How to cite: Chatterjee, S., Selivanova, J., Semmler, T., and Screen, J. A.: Ocean response to reduced Arctic sea ice in PAMIP simulations., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4822, https://doi.org/10.5194/egusphere-egu23-4822, 2023.

EGU23-4972 | Posters on site | OS1.4

An 8-year time series of mesozooplankton fluxes in Kongsfjorden, Svalbard 

Patrizia Giordano, Alessandra D'Angelo, Kyle Mayers, Jasmin Renz, Ilaria Conese, Stefano Miserocchi, Federico Giglio, and Leonardo Langone

In Arctic regions, the food availability for epi-pelagic fauna is strictly influenced by environmental stressors, such as solar radiation, ice cover, glacial and watershed runoffs. This study presents an 8-year time-series (2010-2018) of mesozooplankton collected from an automatic sediment trap in the inner part of Kongsfjorden, Svalbard, at ~87m depth. The aim of this study is to observe the temporal variability in the abundance of epipelagic mesozooplankton species, collected as active flux (swimmers). Reference meteorological and hydrological data are also presented as environmental stressors, to evaluate possible relationships with zooplankton populations. A principal component analysis (PCA) applied to the dataset revealed that the physical and chemical characteristics of seawater affected the mesozooplankton abundance and composition. Collectively, this result highlighted the role of the thermohaline characteristics of the water column on the Copepods behavior, and the correlation between siliceous phytoplankton and Amphipods. Overall, the zooplankton within inner Kongsfjorden did not show a clear seasonal trend, suggesting their high adaptivity to extreme environmental conditions. Although the swimmer fluxes have decreased from 2013 onwards, an increase in community diversity has nevertheless been observed, probably due to copepods decline and subsequent higher food availability. Despite the decreasing magnitude of the zooplanktonic community over time, we recorded the intrusion of subarctic boreal species, such as Limacina retroversa, since 2016. The uniqueness of this dataset is an 8-year uninterrupted time series, which provides correlations between environmental and biological parameters in a poorly studied region. Under a warming Kongsfjorden scenario, with increasing submarine and watershed runoff, and the rapid Atlantification of the fjord, major changes in mesozooplankton communities are expected in the medium to long-term due to their adaptation to environmental changes and the introduction of alien species.

How to cite: Giordano, P., D'Angelo, A., Mayers, K., Renz, J., Conese, I., Miserocchi, S., Giglio, F., and Langone, L.: An 8-year time series of mesozooplankton fluxes in Kongsfjorden, Svalbard, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4972, https://doi.org/10.5194/egusphere-egu23-4972, 2023.

EGU23-4988 | ECS | Posters on site | OS1.4

Spatial and temporal distribution of organic matter in central Arctic: Insights from biomarker proxy 

Akanksha Singh, Sze Ling Ho, and Ludvig Löwemark

Studies have shown that Arctic sea-ice conditions influence the earth’s energy budget by affecting its albedo and global ocean circulation. It also exerts a strong control on the local primary productivity. In addition, by drifting sea ice, it facilitates the transport of sediment and organic matter (OM) from marginal seas across the Arctic Ocean. Over the past decades, there have been several studies on sediment cores from Central Arctic where the major source of OM was shown to be terrigenous. The presence of this elevated terrigenous OM is driven by the transport of sediments and OM from marginal seas to the Central Arctic via drifting ice. However, our understanding of the processes involved in the transport of OM to the central Arctic is still limited. In this study, in order to better understand the pathways of OM transport, we examine spatial and temporal variations in OM flux to the central Arctic. We use organic carbon and biomarker proxies, namely n-alkanes and Glycerol dialkyl glycerol tetraether (GDGT) to explore the spatial and temporal (Marine Isotope Stage 1, 2 and 3) variation of terrigenous input versus marine primary productivity in the central Arctic. To understand the transport of OM in the Central Arctic, biomarkers in 100 samples collected from 9 central Arctic cores were investigated. The presence of terrestrial organic matter in the central Arctic region was confirmed by the high values of the BIT index, which virtually all reached above 0.5 with a maximum of 0.9. The spatial pattern of both terrestrial and marine OM showed higher concentrations at the central Lomonosov ridge and reduced values towards the Lomonosov Ridge off Greenland, with lowest concentrations from the cores located at Morris Jesup Rise (MJR). The pattern of declining terrestrial biomarker concentrations from the central Arctic to MJR, which is closer to the Fram Strait and marks the exit of the Arctic Ocean, are likely caused by sea-ice drift patterns. The sea ice would have been transported by the Transpolar Drift, which allows terrigenous material entrained in the dirty sea ice to get transported towards central Arctic. This spatial pattern remains same for all three studied Marine Isotope Stages. Looking at the temporal variation of the OM into the central Arctic, compared to MIS 3 and MIS 2, TOC as well as both marine and terrestrial biomarkers show enhanced concentration during MIS 1 all over the central Arctic. These increased biomarker concentrations reflect that MIS 1 was warmer with less extensive sea-ice cover than MIS 2 and MIS 3.

How to cite: Singh, A., Ho, S. L., and Löwemark, L.: Spatial and temporal distribution of organic matter in central Arctic: Insights from biomarker proxy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4988, https://doi.org/10.5194/egusphere-egu23-4988, 2023.

EGU23-5197 | ECS | Orals | OS1.4

A high-resolution view on mesoscale eddy activity in the Eurasian Basin 

Vasco Müller, Qiang Wang, Sergey Danilov, Nikolay Koldunov, Xinyue Li, and Thomas Jung

Mesoscale eddies might play a substantial role for the dynamics of the Arctic Ocean, making them crucial for understanding future Arctic changes and the ongoing ‘atlantification’ of the Arctic Ocean. However, simulating high latitude mesoscale eddies in ocean circulation models presents a great challenge due to their small size and adequately resolving mesoscale processes in the Arctic requires very high resolution, making simulations computationally expensive.

Here, we use a seven-year simulation from the unstructured‐mesh Finite volumE Sea ice-Ocean Model (FESOM2) with 1-km horizontal resolution in the Arctic Ocean. This very high-resolution model setup can be considered eddy resolving and has previously been used to investigate the distribution of eddy kinetic energy (EKE) in the Arctic. Now, with a simulation spanning several years, we evaluate the changes of EKE in the Eurasian Basin and the connection to other properties like sea-ice cover, baroclinic conversion rate and stratification. EKE seasonality is influenced predominantly by sea-ice changes, while monthly anomalies have different drivers for different depths levels. The mixed layer is strongly linked to the surface and thus to sea-ice variability. Deeper levels on the other hand are shielded from the surface by stratification and influenced more strongly by baroclinic conversion.

How to cite: Müller, V., Wang, Q., Danilov, S., Koldunov, N., Li, X., and Jung, T.: A high-resolution view on mesoscale eddy activity in the Eurasian Basin, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5197, https://doi.org/10.5194/egusphere-egu23-5197, 2023.

EGU23-5605 | Posters on site | OS1.4

CMIP6/OMIP simulations of the Arctic Ocean and the impact of resolutions 

Chuncheng Guo, Qi Shu, Qiang Wang, Aleksi Nummelin, Mats Bentsen, Alok Gupta, Yang Gao, and Shaoqing Zhang

Underlying the polar climate system are a number of closely coupled processes that are interconnected through complex feedbacks on a range of temporal and spatial scales. Observations are limited in these inaccessible and remote areas, and understanding of these processes often relies on regional and global climate modelling. However, large uncertainties remain due to unresolved key processes in both the regional and global contexts.

In this presentation, we first show that large model spread and biases exist in simulating the Arctic Ocean hydrography from the latest CMIP6/OMIP experiments. Our results indicate that there are almost no improvements compared with the previous CORE-II experiments (with similar OMIP-like protocol) which were thoroughly assessed by the ocean modelling community. The model spread and biases are especially conspicuous in the simulation of subsurface halocline and Atlantic Water, the latter often being too warm, too thick, and too deep for many models. The models largely agree on the interannual/decadal variabilities of key metrics, such as volume/heat/salt transport across main Arctic gateways, as dictated by the common atmospheric forcing reanalysis.

We then examine a hierarchy of global models with horizontal resolutions of the ocean on the order of 1-deg, 0.25-deg, and 0.1-deg. For the 0.1-deg resolution, we take advantage of a recent unprecedented ensemble of high-resolution CESM simulations, as well as NorESM simulations of similar ocean resolution but of shorter integration. High(er) resolutions show signs of improvements and advantages in simulating the Arctic Ocean, but certain biases remain, which will be discussed together with the challenges of high-resolution simulations in the region.

How to cite: Guo, C., Shu, Q., Wang, Q., Nummelin, A., Bentsen, M., Gupta, A., Gao, Y., and Zhang, S.: CMIP6/OMIP simulations of the Arctic Ocean and the impact of resolutions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5605, https://doi.org/10.5194/egusphere-egu23-5605, 2023.

EGU23-5780 * | ECS | Orals | OS1.4 | Highlight

Marine Heatwaves in the Arctic Ocean: drivers, feedback mechanisms and interactions with sea ice 

Benjamin Richaud, Eric C.J. Oliver, Xianmin Hu, Sofia Darmaraki, and Katja Fennel

Arctic regions are warming at a rate faster than the global average. Superimposed on this trend, marine heatwaves and other extreme events are becoming more frequent and intense. Simultaneously the sea ice phenology with which these events interact is also changing. While sea ice can absorb atmospheric heat by melting and therefore acts as a heat buffer for the ocean, meltwater-induced stratification and albedo changes can provoke positive feedbacks on the heat content of the upper ocean. Disentangling those effects is key to better understanding and predicting the present and future state of the Arctic Ocean, including how it responds to forcing by extreme events. Using a three-dimensional regional ice-ocean coupled numerical model, we calculate a two-layer heat budget for the surface mixed layer of the Arctic Ocean, using a novel approach for the treatment of residuals. We present a statistical overview of the dominant drivers of marine heatwaves at the regional scale as well as more in-depth analyses of specific events in key regions of interest. The characteristics of marine heatwaves under different sea ice conditions is also considered, to identify anomalous ice-ocean interactions. Finally, potential feedback mechanisms are investigated to verify their existence and quantify their importance.

How to cite: Richaud, B., Oliver, E. C. J., Hu, X., Darmaraki, S., and Fennel, K.: Marine Heatwaves in the Arctic Ocean: drivers, feedback mechanisms and interactions with sea ice, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5780, https://doi.org/10.5194/egusphere-egu23-5780, 2023.

EGU23-6012 | ECS | Posters on site | OS1.4

Winter Atlantic Water intrusions in Kongsfjorden: atmospheric triggering and oceanic preconditioning 

Francesco De Rovere, Jacopo Chiggiato, Leonardo Langone, Angelo Rubino, and Davide Zanchettin

Kongsfjorden is an Arctic fjord in Svalbard facing the West Spitsbergen Current (WSC) transporting warm and salty Atlantic Water (AW) through the Fram Strait to the Arctic. In this work, winter AW intrusions in Kongsfjorden occurring in the 2010-2020 decade are assessed by means of oceanographic and atmospheric observations, provided by in-situ instrumentations and reanalysis products. Winter AW intrusions are relatively common events, bringing heat and salt from the open ocean to the fjord interior; they are characterized by water temperatures rising by 1-2 °C in just a few days. Several mechanisms have been proposed to explain winter AW intrusions in West Spitsbergen fjords, tracing back to the occurrence of energetic wind events along the shelf slope. Here we demonstrate that the ocean plays a fundamental role as well in regulating the inflow of AW toward Kongsfjorden in winter.

Winter AW intrusions in 2011, 2012, 2016, 2018 and 2020 occurred by means of upwelling from the WSC, triggered by large southerly winds blowing on the West Spitsbergen Shelf (WSS) followed by a circulation reversal with northerly winds. Southerly winds are generated by the setup of a high pressure anomaly over the Barents Sea. In these winters, fjord waters are fresher and less dense than the AW current, resulting in the breakdown of the geostrophic control mechanism at the fjord mouth, allowing AW to enter Kongsfjorden. The low salinity signal is found also on the WSS and hence is related to the particular properties of the Spitsbergen Polar Current (SPC). The freshwater signal is hypothesized to be linked to the sea-ice production and melting in the Storfjorden and Barents Sea regions, as well as the accumulation of glaciers’ runoff. The freshwater transport toward West Spitsbergen is thus the key preconditioning factor allowing winter AW intrusions in Kongsfjorden by upwelling, whilst energetic atmospheric phenomena trigger the intrusions. 

Winter 2014 AW intrusion shows a different dynamic, i.e., an extensive downwelling of warm waters in the fjord lasting several weeks. Here, long-lasting southerly winds stack surface waters toward the coast. The fjord density is larger than the WSC density, forcing the AW intrusion to occur near the surface, then spreading vertically over the water column due to heat loss to the atmosphere. We hypothesize the combination of sustained Ekman transport and the shallower height of the WSC on the water column to be the key factor explaining the AW intrusion in this winter. 

After mixing with the initial AW inflow, fjord waters undergo heat loss to the atmosphere and densification. The water column becomes denser than the WSC, restoring the geostrophic control mechanism and blocking further intrusions of AW.

How to cite: De Rovere, F., Chiggiato, J., Langone, L., Rubino, A., and Zanchettin, D.: Winter Atlantic Water intrusions in Kongsfjorden: atmospheric triggering and oceanic preconditioning, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6012, https://doi.org/10.5194/egusphere-egu23-6012, 2023.

EGU23-6564 | ECS | Orals | OS1.4

Impact of an isolated summer storm on sea ice and ocean conditions in the Canadian Basin 

Emma Bent, Camille Lique, and Peter Sutherland

The Arctic Ocean has undergone a rapid decrease of sea ice extent for decades and studies have shown that the storm activity has increased in the Arctic. Regions that are seasonally ice-opened experience a greater forcing at the surface, which affects the upper-ocean through mixing, turbulence and air-sea interactions. Previous studies have shown the local and short term impacts of wind and waves on sea ice through negative or positive feedback mechanisms. For instance, increased air-sea flux during the freezing season can lead to a cooling of the upper-ocean and favor ice formation, while an increase in wind forcing can modify the vertical profile of the mixed layer, leading to melting or formation of ice. Given the potential of the mixed layer properties to be modified locally by an increased wind/wave forcing, we question whether this type of forcing could have a seasonal effect on the mixed layer and therefore on the sea ice.

We thus use a 1D coupled ocean-sea ice model (NEMO1D-SI³) to study, in the seasonal ice zone of the Beaufort Sea, the immediate change and the seasonal evolution of the mixed layer when forced by an idealized summer storm. The response of sea ice is also examined. We conduct the experiment for a range of storms varying in intensity, duration and date of forcing. Compared to a situation with no increased forcing, we first find that summer storms thicken the mixed layer through mixing which increases the upper-ocean heat content. In the fall, ice formation is consequently delayed for a maximum of 2 weeks compared to a situation with no increased forcing. Secondly, we show that storm-induced thick mixed layers isolate the sea ice from sub-surface warm waters, allowing for efficient ice growth. Ice is consequently thicker at the end of winter compared to a situation with no increased forcing (maximum difference of 10 cm). Thirdly, we find that these results are amplified for storms that happen earlier in summer and have a strong momentum input to the ocean. Our results suggest that localized storms could be a significant driver of the seasonal evolution of the mixed layer and the sea ice.

How to cite: Bent, E., Lique, C., and Sutherland, P.: Impact of an isolated summer storm on sea ice and ocean conditions in the Canadian Basin, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6564, https://doi.org/10.5194/egusphere-egu23-6564, 2023.

EGU23-6699 | ECS | Orals | OS1.4

Investigating ventilation and saturation dynamics in the Arctic Ocean using noble gas tracer techniques 

Yannis Arck, Lennart Gerke, Edith Engelhardt, Florian Freundt, Julian Robertz, Stanley Scott, David Wachs, Markus Oberthaler, Toste Tanhua, and Werner Aeschbach

Timescales of ventilation of the Arctic Ocean are still only poorly known. The commonly used tracers for ocean ventilation studies like CFCs and SF6 are limited to young water masses that are either close to the surface or in highly ventilated deep waters. The radioisotope 39Ar with its half-life of 269 years covers time scales of 50 to 1000 years, perfectly suitable to investigate ventilation timescales of deep and intermediate water masses within the Arctic Ocean. The new measurement technique called Argon Trap Trace Analysis (ArTTA) only requires samples sizes of a few liters of ocean water, instead of the previous low-level counting method, which required about 1000 liters of water. The benefit for ocean studies is evident, much more samples can be taken during one cruise if ArTTA is applied. This enables a better resolution of the water column in great depths at the desired sampling location in the Arctic Ocean. Combined with the additional data of the CFC-12 and SF6 measurements, ventilation timescales of the complete water column from surface to bottom are obtained by constraining transit time distributions via this multi-tracer approach.

Another focus of this study is the saturation of all gaseous transient tracers. It is determined by surface conditions as well as interior mixing processes. Measurements of stable noble gas isotopes (He, Ne, Ar, Kr, Xe) are used to determine possible saturation anomalies that arise during air bubble dissolution, rapid cooling and subduction, or ice formation and subsequent interior mixing of water masses. These saturation distortions for different boundary conditions are of key importance to correct the input function for gas tracers in the Arctic Ocean and hence to constrain the ventilation timescales. The uncertainty of the age distributions will be reduced, and ocean circulation models can be improved.

This contribution presents first stable and radioactive noble gas results of the project Ventilation and Anthropogenic Carbon in the Arctic Ocean (VACAO), which is part of the Synoptic Arctic Survey carried out in summer 2021 on the Swedish icebreaker Oden.

How to cite: Arck, Y., Gerke, L., Engelhardt, E., Freundt, F., Robertz, J., Scott, S., Wachs, D., Oberthaler, M., Tanhua, T., and Aeschbach, W.: Investigating ventilation and saturation dynamics in the Arctic Ocean using noble gas tracer techniques, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6699, https://doi.org/10.5194/egusphere-egu23-6699, 2023.

EGU23-6724 | Orals | OS1.4

On the realism of Arctic Ocean transports in CMIP6 

Susanna Winkelbauer, Michael Mayer, and Leopold Haimberger

This contribution evaluates key components of the Arctic energy budget as represented by the Coupled Model Intercomparison Project Phase 6 (CMIP6) against reanalyses and observations.

The Arctic regions are characterized by a net energy loss to space, which is balanced by northward heat transports in atmosphere and ocean. Mean and variability in the oceanic northward heat transports have major impacts on the state and change of the Arctic Ocean and sea ice. Therefore, an accurate representation of oceanic transports in climate models is a key feature to realistically simulate the Arctic climate. However, the nature of curvilinear ocean model grids and the variety of different grid types used in the CMIP ensemble, make the calculation of oceanic transports on their native grids difficult and time consuming. We developed new tools that enable the precise calculation of volume, heat, salinity and ice transports through any desired oceanic sections or straits for a large number of CMIP6 models as well as ocean reanalyses. Our tools operate on native grids and hence avoid biases that often arise from interpolation to regular grids. Those tools will be made available as open-source Python package enabling easy and effortless calculations of oceanic transports.

In the work presented here, we use the newly developed tools to compare oceanic heat transports (OHT) through the main Arctic gateways from CMIP6 models and reanalyses to those gained from observations and analyze them concerning their annual means, seasonal cycles and trends. We find strong connections between the Arctic’s mean state and lateral OHT, with variations in OHT having major effects on the sea ice cover and ocean warming rate.

Results help us to understand typical model biases. For instance, many models feature systematic biases in oceanic transports in the Arctic main gateways, e.g., some models feature to high sea ice extents due to the underestimation of heat transports entering the Arctic through the Barents Sea Opening. Using those results it is possible to generate physically based metrics to detect outliers from the model ensemble, which may be useful in reducing the spread of future projections of Arctic change.

How to cite: Winkelbauer, S., Mayer, M., and Haimberger, L.: On the realism of Arctic Ocean transports in CMIP6, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6724, https://doi.org/10.5194/egusphere-egu23-6724, 2023.

EGU23-7774 | ECS | Posters on site | OS1.4

Upper Arctic Ocean properties and water mass pathways during the year-round MOSAiC expedition in the context of historical observations 

Myriel Vredenborg, Wiebke Körtke, Benjamin Rabe, Maren Walter, Sandra Tippenhauer, and Oliver Huhn

The Arctic Ocean is characterized by complex processes coupling the atmosphere, cryosphere, ocean and land, and undergoes remarkable environmental changes due to global warming. To better understand this system of physical, biogeochemical and ecosystem processes, as well as recent changes was the aim of the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) ice drift conducted year-round from autumn 2019 to autumn 2020. Here, we focus on the properties and circulation pathways of upper Arctic Ocean water masses that have been found to change in recent decades, likely in response to changes in sea ice, surface fluxes, and advection of air masses under Arctic amplification.

We use hundreds of hydrographic profiles obtained with two Conductivity Temperature Depth (CTD) systems mounted to rosette water samplers from the drifting ship and at a remote location on the ice to investigate the properties of the polar mixed layer, halocline waters and warm water of Atlantic origin (“Atlantic Water”) in the Eurasian Arctic during the MOSAiC campaign. Additionally, we analyse chemical tracers (noble gases and anthropogenic tracers CFC-12 and SF6) measured from water samples taken with both CTD/Rosette systems to identify pathways of the water masses. We compare these observations with a comprehensive dataset of historical hydrographic data from the region to put our findings into a long-term context.

We find a shoaling and thickening of the Atlantic-Water layer compared to historical observations, as well as signatures of interleaving at the core of the warm Atlantic Water that slowly get eroded during the drift. Along the MOSAiC track the hydrographic data show convective lower halocline waters that are typically formed north of Fram Strait and further downstream, as well as advective-convective lower halocline waters typically formed in the Barents Sea. We see a change in lower halocline properties in the eastern Amundsen Basin compared to historical observations, that could either be caused by local formation or a change in circulation. Further, we use the chemical tracers to investigate possible pathways and formation regions of the observed water masses.

How to cite: Vredenborg, M., Körtke, W., Rabe, B., Walter, M., Tippenhauer, S., and Huhn, O.: Upper Arctic Ocean properties and water mass pathways during the year-round MOSAiC expedition in the context of historical observations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7774, https://doi.org/10.5194/egusphere-egu23-7774, 2023.

EGU23-8320 | ECS | Posters on site | OS1.4

Tracing Atlantic water exiting the Fram Strait and its transit in the Arctic Ocean by isolating reprocessing-derived 236U and colored dissolved organic matter 

Gang Lin, jixin Qiao, Rafael Gonçalves‐Araujo, Peter Steier, Paul Dodd, and Colin Stedmon

The Fram Strait, located between Svalbard and Greenland is an important gateway for exchange of salt and heat between the Arctic Ocean and the North Atlantic Ocean and is also a geographically crucial region for investigating Atlantic water transport pathways and transit times, which are necessary to understand the progress of environmental changes in the Arctic. 236U from the two European nuclear reprocessing plants (RPs) at La Hague (LH) and Sellafield (SF) provides a unique signal in Atlantic water for studying its circulation pattern in the Arctic Ocean. In this study we first isolate RP-derived 236U (236URP) using the characteristic 233U/236U signature and then use colored dissolved organic matter (CDOM) to indicate transit pathways and therefore constrain the selection of appropriate 236URP input functions. High CDOM absorbance in the Fram Strait reflects the passage of Atlantic water transported to the Arctic by the Norwegian Coastal Current (NCC) and subsequently along the Siberian shelf where the Ob, Yenisei and Lena rivers supply terrestrial organic matter with high CDOM levels. Conversely low CDOM water represents Atlantic water that has remained off the shelf. Based on CDOM absorbance, potential temperature (θ) and water depth the path of a given body of Atlantic water could be determined and an appropriate RP input function selected so that transit times could be estimated. Waters with high CDOM levels sourced from the NCC and Barents Sea branch water (BSBW) had an average Atlantic water transit time of 12 years. Waters with low CDOM,  θ < 2 °C, and depth < 1500 m were sourced from the Norwegian Atlantic Current (NwAC), had little interaction with riverine freshwater with an advective Atlantic water transit time of 26 years.

How to cite: Lin, G., Qiao, J., Gonçalves‐Araujo, R., Steier, P., Dodd, P., and Stedmon, C.: Tracing Atlantic water exiting the Fram Strait and its transit in the Arctic Ocean by isolating reprocessing-derived 236U and colored dissolved organic matter, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8320, https://doi.org/10.5194/egusphere-egu23-8320, 2023.

EGU23-9367 | ECS | Posters on site | OS1.4

Wind forcing and tides mediate transport of ocean heat from Storfjordrenna to the Arctic domain of the Barents Sea 

Kjersti Kalhagen, Ragnheid Skogseth, Ilker Fer, Till M. Baumann, and Eva Falck

The Barents Sea is undergoing changes with impacts on the physical environment, e.g., the seasonal sea ice formation and extent and with large consequences for the ecosystems. There are knowledge-gaps concerning the complex pathways of Atlantic Water (AW) through the Barents Sea and the associated distribution of heat and nutrients. Records from a mooring deployed between September 2018 and November 2019 on the 70 m deep saddle between Edgeøya and Hopen islands in the Svalbard archipelago show sporadic exchange between the AW-influenced trough Storfjordrenna and the Arctic domain of the north-western Barents Sea. Forced by sea surface anomalies, the observed currents show a tendency for eastward transport across the saddle year-round. However, the eastward overflow into the Barents Sea is strongly mediated by wind forcing: The predominant north-northeasterly winds with corresponding geostrophic adjustment to Ekman transport tend to hamper and sometimes even reverse this cross-saddle current. Weaker and/or southerly winds on the other hand tend to enhance the eastward flow into the Barents Sea. The strength and shape of the overflow current vary substantially on seasonal and sub-seasonal timescales: during autumn and winter, the current is strong and barotropic, while during summer, the current is weaker and more baroclinic. On shorter time scales, the strongest oscillations occur during the ice-free autumn with a periodicity of a few days. When the area has a partial sea ice cover in winter, the strength decreases and the periodicity increases to a week or more. Further analysis of variability in temperature and current velocity shows that cross-saddle transport of positive temperature anomalies (indicating heat from waters of Atlantic origin) is evident in frequency bands associated with various drivers of mesoscale variability, such as eddies, synoptic events, and tides. There are indications that the studied area will become an increasingly important location for heat transport into the interior of the Barents Sea: A comparison between historical and recent hydrographic records show that AW is warming and shoaling in the water column in Storfjordrenna, which suggests that AW will be more easily transported across the saddle by the mentioned drivers. Furthermore, the ongoing changes in the large-scale weather patterns resulting in more southerly and southwesterly winds is hypothesized to affect the strength and persistence of the overflow on the saddle between Edgeøya and Hopen islands.

How to cite: Kalhagen, K., Skogseth, R., Fer, I., Baumann, T. M., and Falck, E.: Wind forcing and tides mediate transport of ocean heat from Storfjordrenna to the Arctic domain of the Barents Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9367, https://doi.org/10.5194/egusphere-egu23-9367, 2023.

EGU23-9887 | ECS | Posters virtual | OS1.4

An updated observational record of Davis Strait ocean transports, 2004-2017 

Jed Lenetsky, Craig Lee, Clark Richards, and Alexandra Jahn

The Davis Strait, located in Southern Baffin Bay between Greenland and the Canadian Arctic Archipelago, is a key gateway of oceanic exchange between the Arctic and North Atlantic Oceans. Large fluxes of fresh Arctic Waters through the Davis Strait potentially influence deep-water formation in the Labrador Sea, with implications for the strength of the Atlantic Meridional Overturning Circulation. From 2004-2017, and 2020-present, ocean temperatures, salinities, and velocities have been measured along a moored array spanning the entire strait, allowing for ocean transports to be assessed over both the continental shelves and central channel. Here we will present new data from 2011-2017, extending the previously published data for 2004-2010. Furthermore, the whole record has been updated, filling spatial and short temporal data gaps using average temperature, salinity, and velocity sections from high resolution Seaglider surveys from 2004 to 2010. These updated volume, freshwater, and watermass transports will increase understanding of changing oceanic conditions in Baffin Bay, as well as local and remote physical mechanisms that govern the Davis Strait throughflow on synoptic to interannual timescales.

How to cite: Lenetsky, J., Lee, C., Richards, C., and Jahn, A.: An updated observational record of Davis Strait ocean transports, 2004-2017, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9887, https://doi.org/10.5194/egusphere-egu23-9887, 2023.

Everything that happens in the Arctic Ocean, be it of physical, biological, or chemical nature, is constrained by the vertical distribution of heat and salt. In this talk, I will share recent results and on-going work aimed at examining questions directly related to vertical mixing below sea ice: (1) How accurately are the physical properties of the Canada Basin simulated in climate models? (2) How do observed changes to the size and speed of a sea ice floe and ocean stratification impact ocean mixing in 2D numerical simulations? (3) Can we, for the first time, examine seasonal ice-ocean boundary layer dynamics in a 20 m × 10 m × 3 m outdoor saltwater pool?

Our results indicate that the majority of climate models do not accurately simulate the surface freshening trend observed in the Canada Basin between 1975 and 2006-2012, nor do they simulate heat from Pacific Water in the same region. We suggest that both of these biases can be partly attributed to unrealistically deep vertical mixing in the models. We next explore one possible source of this model bias related to decadal changes to the underside of ice floes, called ice keels. Results from idealized numerical simulations highlight the importance of ice keel depth, which controls the range over which ocean mixing occurs, as well as ice keel speed and ocean stratification. Further, we estimate that observational uncertainties related to ice keel depth may translate into uncertainties in the sign of current and future changes to below-ice momentum transfer into the ocean. Lastly, we present the instrument setup for our 2022-2023 pilot experiment and on-going outreach work at the Sea-ice Environmental Research Facility (SERF) in Canada. This is a unique facility centres around an outdoor saltwater pool where sea ice evolves under natural atmospheric conditions in a semi-idealized and well-instrumented setting.

How to cite: Rosenblum, E. and the Team: Exploring ice-ocean boundary layer dynamics in climate models, idealized simulations, and outdoor lab experiments, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10302, https://doi.org/10.5194/egusphere-egu23-10302, 2023.

EGU23-10365 | Orals | OS1.4

Causal Mechanisms of Rising Sea Level and Increasing Freshwater Content of the Beaufort Sea 

Ichiro Fukumori, Ou Wang, and Ian Fenty

Over the last two decades, sea-level across the arctic’s Beaufort Sea has been rising an order of magnitude faster than its global mean. This rapid sea-level rise is mainly a halosteric change, reflecting an increase in Beaufort Sea’s freshwater content. The rising volume of freshwater is greater than that associated with the Great Salinity Anomaly of the 1970s, raising the prospect of future disruptions in large-scale ocean circulation and climate. Here we provide a new perspective of this Beaufort Sea variation using a global data-constrained ocean and sea-ice model of the Estimating the Circulation and Climate of the Ocean (ECCO) consortium. Causal relationships are quantified using the model’s adjoint. Controlling processes are elucidated analyzing property budgets.

The study reveals the multi-decadal variation to be driven jointly by change in wind stress and sea-ice melt. Strengthening anticyclonic winds surrounding the Beaufort Sea intensify the ocean’s lateral Ekman convergence of relatively fresh near-surface waters. The strengthening winds also enhance convergence of sea-ice and ocean heat that increase the amount of Beaufort Sea’s sea-ice melt. Whereas the region’s direct wind-driven kinematic anomalies equilibrate over weeks, sea-ice-melt-driven diabatic changes persist for years owing to Beaufort Sea’s semi-enclosed gyre circulation. The growing disparity between where sea-ice forms and where it melts results in this rare example of melting floating ice causing large-scale sea-level rise. The spin-up difference suggests that, on their own, the sea-ice-melt-driven diabatic change will last much longer than the direct wind-driven kinematic anomaly.

The study highlights the importance of observations and the utility of ECCO’s modeling system. While ocean and sea-ice observations are essential in diagnosing the change, the study also points to a need for expanded observations of the atmosphere, especially the winds that act on the ocean/sea-ice system. ECCO is implementing a novel “point-and-click” interface for analyzing its modeling system, such as conducted here, without requirements for expertise in numerical modeling, and invites exploitation of its new utility (https://ecco-group.org).

How to cite: Fukumori, I., Wang, O., and Fenty, I.: Causal Mechanisms of Rising Sea Level and Increasing Freshwater Content of the Beaufort Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10365, https://doi.org/10.5194/egusphere-egu23-10365, 2023.

Black carbon (BC) is one of the most important absorbing particles in the atmosphere. BC can reduce the albedo of snow/ice and enhance the absorption of solar radiation at ultraviolet (UV) and visible wavelengths when it deposited on snow/ice surface. The deposition of BC can lead to an acceleration of the melting of snow/ice. To quantify the changing process of BC in snow/ice and its contribution to the melting of snow/ice, a series of sensitivity numerical experiments including the impacts of BC species (hydrophobic and hydrophilic), deposition rate, and scavenging efficiency of BC was completed using the Icepack one-dimensional column model of CICE. Further, we evaluate the effects of BC deposition on Arctic albedo and ice thickness, forced by ERA5 reanalysis data and BC deposition rate from CMIP6, including two simulation results of the historical experiments with GISS-E2 model and EC-Earth3 model. The results indicate that the hydrophobic BC can cause a reduction of snow/ice albedo by 0.43% in the melting season, which is 35% larger than hydrophilic BC with the same deposition rate. When only the hydrophilic BC was considered, the impact on scavenging efficiency halved to BC content in snow/ice is similar to double the deposition rate in the melting season. Additionally, the 2D model results indicate that the existence of BC in snow could enhance the absorption of solar radiation in the snow layer and reduce the transmittance of radiation to the ice layer, leading to a thicker ice thickness before the melting season. The thermodynamic impact of BC is more significant in the marginal ice zone than that in the central Arctic, especially from Barents Sea to Laptev Sea. In this paper, we quantify the effects of BC on the melting of Arctic snow and sea ice and discuss the problems of the parameterizations of BC’s effect. This may contribute to the improvement of the sea ice model.

Key words: Black carbon; CICE model; Sensitivity experiment; Scavenging efficiency; Albedo

How to cite: Wang, Y. and Su, J.: Sensitivity study of the effects of black carbon on Arctic sea ice using CICE sea-ice model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10572, https://doi.org/10.5194/egusphere-egu23-10572, 2023.

EGU23-10826 | Posters on site | OS1.4

13-Year Observation of the CH4 across the sea surface in the Western Arctic Ocean 

Tae Siek Rhee, Young Shin Kwon, Mi-Seon Kim, Scott Dalimore, Charles Paull, Jong Kuk Hong, and Young Keun Jin

Methane (CH4) is one of the most important greenhouse gases on Earth. Recent finding of the strong CH4 emissions in the Arctic Seas with shrinking the sea ice may amplify the Arctic warming leading to the positive feedback in the Arctic climate. Korea Polar Research Institute (KOPRI) has ongoing interest in Arctic environmental conditions including the potential release of the CH4 from the seabed to the water column and finally, further to the atmosphere. During the last 13 years throughout a series of campaigns on the Korean ice-breaker, R/V Araon, we measured CH4 concentrations at the surface ocean and overlying air in summer season to estimate the emissions from the western arctic seas including the Chukchi Sea, the Beaufort Sea, and the East Siberian Sea. We compare each of these seas and the Central Arctic Ocean covering the deep Arctic Ocean basin. The surface ocean showed super-saturation almost everywhere with respect to the CH4 in the overlying air. Nonetheless, we have insufficient regional coverage to assess any possible saturation anomaly trend in each sea. Flux densities of outgassing CH4 are modestly larger than the global mean value of the continental shelf except for the Central Arctic Ocean where the CH4 emission is slightly lower. Our estimate of CH4 emission in the East Siberian Sea is far larger than other Arctic Seas abiding by the previous observations, but its magnitude is far lower due likely to the distance from the hot spot area. Future methane flux studies should be extended to shallow, nearshore environments where rate of permafrost degradation should be greatest in response to ongoing marine transgression.

How to cite: Rhee, T. S., Kwon, Y. S., Kim, M.-S., Dalimore, S., Paull, C., Hong, J. K., and Jin, Y. K.: 13-Year Observation of the CH4 across the sea surface in the Western Arctic Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10826, https://doi.org/10.5194/egusphere-egu23-10826, 2023.

EGU23-10840 | Posters on site | OS1.4

Upper Arctic Ocean Properties and Relationships with Sea Ice in CMIP6 Historical Simulations 

Wei Cheng, Cecilia Bitz, Lettie Roach, Edward Blanchard-Wriggleworth, Mitch Bushuk, and Qiang Wang

While current-generation CMIP and OMIP models have clear biases in their upper Arctic Ocean hydrography, it is less clear how these biases impact the models' ability to simulate the observed Arctic sea ice mean state and trends. In this study we seek to quantify cross-relationship between sea ice and ocean states in CMIP6 historical simulations and identify common model behaviors. Multi-model mean (MMM) simulations exhibit accelerated changes in the ice and ocean system since the late 20th century. Underlying the MMM is strong inter-model variation in the simulated ice and ocean mean states and their temporal variability including trends. Despite such inter-model differences, all models show a similar ratio between sea ice reduction and upper ocean warming such that models with higher ocean warming also have higher SIE reduction and vice versa. Our results also highlight the urgent needs of reliable Arctic Ocean observations or data products in order to better contextualize modeling results.

How to cite: Cheng, W., Bitz, C., Roach, L., Blanchard-Wriggleworth, E., Bushuk, M., and Wang, Q.: Upper Arctic Ocean Properties and Relationships with Sea Ice in CMIP6 Historical Simulations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10840, https://doi.org/10.5194/egusphere-egu23-10840, 2023.

EGU23-10871 | Orals | OS1.4

A First Look at Surface Ocean Measurements during the SASSIE Field Campaign in 2022 

Julian Schanze and the Salinity and Stratification at the Sea Ice Edge (SASSIE)

The NASA Salinity and Stratification at the Sea Ice Edge (SASSIE) field campaign took during place between August and October of 2022. Using three major components, the aim is to understand the relationship between both haline and thermal stratification and sea-ice advance, and to test the hypothesis that a significant fresh layer at the surface can accelerate the formation of sea ice by limiting convective processes. The three components of the field campaign include: 1) A one-month shipboard hydrographic and atmospheric survey in the Beaufort Sea, 2) A concurrent airborne campaign to observe ocean salinity, temperature, and other parameters from a low-flying aircraft, and 3) The deployment of autonomous assets, buoys, and floats that are able to observe both the melt season and the sea ice advance.

Here, we focus on the novel results from the month-long research cruise aboard the R/V Woldstad that took place during September and October of 2022, particularly measurements of salinity and temperature at radiometric depths (1-2 cm) from the salinity snake instrument. These measurements will be contextualized with all other components of the cruise, including uCTD, air-sea flux, airborne, and satellite data to examine the effects of stratification on ocean dynamics in the Beaufort Sea near at the sea ice edge.

How to cite: Schanze, J. and the Salinity and Stratification at the Sea Ice Edge (SASSIE): A First Look at Surface Ocean Measurements during the SASSIE Field Campaign in 2022, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10871, https://doi.org/10.5194/egusphere-egu23-10871, 2023.

EGU23-11483 | ECS | Posters on site | OS1.4

The Sea Ice Drift Forecast Experiment (SIDFEx): Introduction and applications 

Valentin Ludwig and Helge Gößling and the SIDFEx Team

We introduce the Sea Ice Drift Forecast Experiment (SIDFEx) database. SIDFEx is a collection of close to 180,000 lagrangian drift forecasts for the trajectories of specified assets (mostly buoys) on the Arctic and Antarctic sea ice, at lead times from daily to seasonal scale and mostly daily resolution. The forecasts are based on systems with varying degrees of complexity, ranging from free-drift forecasts to forecasts by fully coupled dynamical general circulation models. Combining several independent forecasts allows us to construct a best-guess consensus forecast, with a seamless transition from systems with lead times of up to 10 days to systems with seasonal lead times. The forecasts are generated by 13 research groups using 23 distinct forecasting systems and sent operationally to the Alfred-Wegener-Institute, where they are archived and evaluated. Many systems send forecasts in near-real time.

One key purpose when starting SIDFEx in 2017 was to find the optimal starting position for the Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC). Over the years, more applications evolved: During MOSAiC, the SIDFEx forecasts were used for ordering high-resolution TerraSAR-X images in advance, with a hit rate of 80%. During the Endurance22 expedition, we supported the onboard team with near-real time forecasts, contributing to the success of the mission. Currently, we evaluate drift forecasts for several buoys of the MOSAiC Distributed Network (DN). We know that there is skill in predicting the location of single buoys. Now, we extend this to studying the deformation of the polygon spanned by the DN buoys. Deformation is derived from the spatial velocity derivatives of the buoy array. We find low correlation coefficients between the deformation in the models and the observed deformation for a small-scale DN configuration, but larger and significant correlations around 0.7 for larger configurations and an Arctic-wide buoy array.

How to cite: Ludwig, V. and Gößling, H. and the SIDFEx Team: The Sea Ice Drift Forecast Experiment (SIDFEx): Introduction and applications, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11483, https://doi.org/10.5194/egusphere-egu23-11483, 2023.

EGU23-12014 | Posters on site | OS1.4

Summer Net Community Production in the northern Chukchi Sea: Comparison between 2017 and 2020 

Doshik Hahm, Soyeon Kwon, Inhee Lee, Keyhong Park, Kyoung-Ho Cho, Jinyoung Jung, Taewook Park, Youngju Lee, Chanhyung Jeon, and Seongbong Seo

The Arctic Ocean experiences warming-induced processes, such as the decrease in sea-ice extent and freshening of the surface layer. While these processes have the potential to alter primary production and carbon export to the deep layer, the changes that will likely occur in them  are still poorly understood. To assess the potential changes in net community production (NCP), a measure of biological carbon export to the deep layer, in response to climate change, we observed the O2/Ar at the surface of the northern Chukchi Sea in the summers of 2017 and 2020. The NCP estimates derived from O2/Ar measurements were largely in the range of 1 -- 11 mmol O2 m-2 d-1 in the northern Chukchi and Beaufort Seas, close to the lower bounds of the values in the global oceans. The average NCP of 1.5 ± 1.7 mmol O2 m-2 d-1 in 2020 was substantially lower than 7.1 ± 7.4  mmol O2 m-2 d-1  in 2017, with the most pronounced decrease occurring in the ice-free region of the northern Chukchi Sea; the NCP of the ice-free region in 2020 was only 12% of that in 2017. The decrease in 2020 was accompanied by a lower salinity of >2, which resulted in shallower mixed layer depths and stronger stratification. We speculated that the anomalously low pressure near the east Russian coast and the lack of strong winds contributed to the strong stratification in 2020. With a continuing decrease in the extent of sea ice, the northern Chukchi Sea will likely experience earlier phytoplankton blooms and nitrate exhaustion. Unless winds blow strong enough to break the stratification, the biological carbon export in late summer is likely to remain weak.  

How to cite: Hahm, D., Kwon, S., Lee, I., Park, K., Cho, K.-H., Jung, J., Park, T., Lee, Y., Jeon, C., and Seo, S.: Summer Net Community Production in the northern Chukchi Sea: Comparison between 2017 and 2020, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12014, https://doi.org/10.5194/egusphere-egu23-12014, 2023.

EGU23-12032 | ECS | Orals | OS1.4

Anthropogenic Carbon in the Arctic Ocean: Perspectives from different TTD Approaches and Tracer Pairs 

Lorenza Raimondi, Anne-Marie Wefing, and Núria Casacuberta Arola

At present, it is well-known that the fast increase in atmospheric carbon dioxide (CO2) concentrations resulting from human activities (Cant), drives the dramatic changes observed in our environment such as global warming and ocean acidification. The Arctic Ocean has been identified as one of the fastest-changing regions of the world ocean and can therefore be considered as a sentinel for future global scenarios.

Here, Cant-rich waters coming from the Atlantic Ocean become isolated from the atmospheric input of CO2 as they flow at an intermediate depth below the mixed layer, making the Arctic Ocean a key region for intermediate-to-long-term storage of Cant. Despite having such an important role, the magnitude of the Cant inventory and its change over time in the region is yet not fully understood, particularly if we are to consider future changes in ice coverage and therefore ocean circulation.

A way of estimating oceanic Cant inventories is by applying the so-called Transit Time Distribution (TTD) method, which implies the use of transient tracers such as the anthropogenically produced CFC-12 and SF6.

In this work we present a new estimate of Cant inventory for the Arctic Ocean in 2015 assessed with the TTD method using both well-established tracers (CFC-12 and SF6, both having a global source) as well as novel ones (anthropogenic radionuclides 129I and 236U, both having primarily a point-like source represented by European nuclear reprocessing plants, as well as a global one represented by the global fallout from nuclear bomb testing).

The TTD was here applied following a relatively novel approach to infer the statistical parameters that describe the age distribution within a water sample, the mean (G) and the width (D). Unlike the “classical TTD” approach, the one used in this study allows the statistical parameters of the TTD to be constrained for each individual sample rather than finding values that are most representative of the region and time studied. We first show a comparison of the two TTD approaches by comparing mean and mode ages as well D/G ratios of this study (new TTD method) to those presented in Rajasakaren et al. 2019 (classical TTD method), using CFC-12 and SF6 as our tracers’ pair. We then compare TTD results obtained from the two tracers’ pairs, CFC-12/SF6 and 129I-/236U, using the new TTD method.

Finally, we estimate and compare Cant concentrations and inventories obtained with the two pairs of transient tracers to one-another as well as to previous estimates of Cant in the region by Rajasakaren et al (2019) obtained with the “classical TTD”. This study demonstrates for the first time the feasibility of using anthropogenically produced radionuclides with input functions and chemical properties different than CO2 as proxies for Cant estimates.  

How to cite: Raimondi, L., Wefing, A.-M., and Casacuberta Arola, N.: Anthropogenic Carbon in the Arctic Ocean: Perspectives from different TTD Approaches and Tracer Pairs, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12032, https://doi.org/10.5194/egusphere-egu23-12032, 2023.

EGU23-12592 | ECS | Posters on site | OS1.4

Seasonality and regionality of the vertical structure of the water column in the Arctic Ocean. 

Lucia Gutierrez-Loza and Siv K. Lauvset

The Arctic Ocean is rapidly changing in response to high temperatures and increased atmospheric greenhouse gas concentrations.  As part of these changing conditions, sea-ice loss and increased freshwater inputs are expected to impact the mixing processes and the characteristics of water column in the Arctic region, directly modulating the nutrient availability and primary productivity in the surface water.

Here, we investigate the spatial and temporal variations of the vertical structure of the water column using high-resolution model outputs for the period 2000-2099. We focus on the Atlantic sector of the Arctic, an increasingly temperature-stratified region, where we evaluate the changes on nutrient availability and carbonate chemistry in the upper ocean. Changes in the regionality and seasonality under a medium- to high-end emission scenario (SSP3-7.0), transitioning towards a sea-ice free Arctic, will be used to further understand the upper ocean mixing processes and their impacts on the local and regional biogeochemistry.

How to cite: Gutierrez-Loza, L. and Lauvset, S. K.: Seasonality and regionality of the vertical structure of the water column in the Arctic Ocean., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12592, https://doi.org/10.5194/egusphere-egu23-12592, 2023.

EGU23-12658 | Orals | OS1.4

Arctic Ocean mixing maps inferred from pan-Arctic observations 

Stephanie Waterman, Hayley Dosser, Melanie Chanona, Nicole Shibley, and Mary-Louise Timmermans

Quantifying ocean mixing rates in the Arctic Ocean is critical to our ability to predict upwards oceanic heat flux, freshwater distribution, and circulation. However, direct ocean mixing measurements in the Arctic are sparse and cannot characterize the high spatiotemporal variability typical of ocean mixing. Further, latitude, ice, and stratification make the Arctic Ocean mixing environment unique, with all of double-diffusive (DD), internal wave (IW)-driven and non-turbulent mixing processes playing a role.

In this work, we use year-round temperature and salinity data from Ice-Tethered Profilers (ITPs), as well as an archived record of ship-based measurements, to construct highly-resolved, pan-Arctic maps characterizing the relative prevalence of DD, IW-driven and non-turbulent mixing mechanisms based on thermohaline staircase identification and estimations of turbulence intensity. We next quantify pan-Arctic maps of estimates of average effective vertical diffusivity inferred from these observations that account for all of DD, IW-driven, and non-turbulent mixing processes. Finally, focusing on the water column segment directly above the Atlantic Water (AW) temperature maximum, we use this mixing regime characterization and regime-specific estimates of effective diffusivity to compute estimates of the pan-Arctic distributions of average vertical heat and buoyancy flux from the AW layer.

We find that estimates of effective vertical diffusivities are highly variable in both space and time. Although variability in diffusivity reflects both variations in the prevalence of the various mixing processes and variability in the strength of IW-driven mixing, the prevalence of the mixing mechanisms (predominantly DD and non-turbulent in the basins vs. IW-driven on the shelf) sets the dominant large-scale spatial patterns and the notable shelf-basin contrast. Estimated heat fluxes out of the AW layer also exhibit distinct regional patterns set by mixing mechanism prevalence and regional patterns in the vertical temperature gradient. Buoyancy fluxes from DD mixing compete with the destabilizing effects of IW-driven mixing in the basins, a competition that may be an important control on stratification in the Arctic Ocean interior.

These results are significant as they show that mixing mechanism prevalence is an important consideration in computing robust estimates of average effective diffusivity. They further suggest that the sensitivity of mixing rates to changing environmental conditions may have important regional dependencies owing to differing prevalence of the various mixing processes.

How to cite: Waterman, S., Dosser, H., Chanona, M., Shibley, N., and Timmermans, M.-L.: Arctic Ocean mixing maps inferred from pan-Arctic observations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12658, https://doi.org/10.5194/egusphere-egu23-12658, 2023.

EGU23-13807 | Posters on site | OS1.4

The MOSAiC webODV: Interactive online data exploration, visualization and analysis 

Sebastian Mieruch, Ingrid Linck Rosenhaim, and Reiner Schlitzer

In the frame of the M-VRE (The MOSAiC virtual research environment, https://mosaic-vre.org) project we have set up a webODV application, to serve data from the arctic MOSAiC (https://mosaic-expedition.org) expedition.

webODV is deployed at AWI's computing center under https://mvre.webodv.cloud.awi.de. MOSAiC data have been retrieved from the long-term archive Pangaea (https://pangaea.de). To get the most out of the data with webODV, we have harmonized, aggregated and compiled the datasets into different separated and interdisciplinary data collections.

webODV is operated interactively in the browser via the mouse and keyboard (no programming), it's fast, efficient and easy to use for exploring, visualizing, analyzing, downloading data, creating map projections, scatter plots, section plots, surface plots and station plots and many more.

webODV supports the FAIR data principles and analyses and visualizations are fully reproducible using our so-called "xview" files that can be shared among colleagues or attached to publications. We provide real-time sharing, full author traceability and downloadable lists of all the DOI's used in the analysis or the respective .bib or .ris files including all citations. Extensive documentation is available at https://mosaic-vre.org/docs as well as video tutorials at https://mosaic-vre.org/videos/webodv.

How to cite: Mieruch, S., Linck Rosenhaim, I., and Schlitzer, R.: The MOSAiC webODV: Interactive online data exploration, visualization and analysis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13807, https://doi.org/10.5194/egusphere-egu23-13807, 2023.

EGU23-14133 | ECS | Posters on site | OS1.4

Seasonal and regional sensitivity of Arctic sea ice 

Markus Ritschel and Dirk Notz

We examine the seasonal and regional evolution of sea-ice coverage in the Arctic in response to changes in the forcing. Using satellite and reanalysis data in combination with CMIP6 model simulations, we build on previous studies that have found a strong linear relationship between the September sea-ice area of the northern hemisphere and global atmospheric air temperature (TAS) as well as anthropogenic CO2 emissions. Instead of focusing on the whole Arctic and September sea ice only, we perform sensitivity analyses on higher-resolved regional and seasonal scales, aiming to identify the atmospheric and oceanic drivers that govern the evolution of sea-ice coverage on these scales and to derive simple empirical relationships that describe the impact of these processes. We find clear linkages also on these higher-resolved scales, with different regions and different seasons showing diverse sensitivities of sea-ice area evolution with respect to TAS and anthropogenic CO2. Furthermore, we use a multivariate metric to quantify the "quality" of a single simulation matching the observations, thereby considering the different sensitivities of all seasons of the year. Building the combined covariance matrix of observations and simulations as a measure of the joint uncertainties, we can determine how "close" to the observations every single member of the simulations is. This allows us to separate models whose sensitivities are in overall good agreement with the observations from those that are apparently not capable of properly simulating the response of the sea ice to the forcing throughout all months. Based on our findings we can infer the dominant drivers that force Arctic sea-ice evolution on a regional and seasonal scale and also derive projections for the future evolution of Arctic sea ice for different climate scenarios based on simple empirical relationships that can directly be estimated from observational records.

How to cite: Ritschel, M. and Notz, D.: Seasonal and regional sensitivity of Arctic sea ice, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14133, https://doi.org/10.5194/egusphere-egu23-14133, 2023.

EGU23-16107 | Posters on site | OS1.4

Oceanic gyres in the Arctic 

Yevgeny Aksenov, Stefanie Rynders, Alex Megann, A.J. George Nurser, Chris Wilson, and Andrew C. Coward

The Arctic can be seen as a two-layer ocean: thin (<100m) mixed layer at the surface, and the rest of the weakly-stratified ~5-km water column, separated from the surface waters by the Arctic halocline. The weak subsurface ocean stratification results in most of the ocean flow being depth-uniform and guided by bathymetry. One way to look at the Arctic long-term, large-scale ocean circulation is examining the Arctic gyres and cross-ocean currents, such as the Trans-Polar Drift. Wilson et all 2021[1] show how gyres, saddle points and flow separation structures “separatrices” in the surface ocean circulation changes between years and how these affect cross-basin Arctic oceanic connectivity. We extend the method to the subsurface oceanic flow and examine barotropic circulation in the present-day Arctic Ocean using global NEMO model (Nucleus for European Modelling of the Ocean) at 3-km horizontal resolution. The closed-gyre detection method allows us to map positions of the principal Arctic gyres and quantify their strength. The Montgomery potential analyses complements the study by giving us an insight in the geostrophic flows of the Atlantic and Pacific waters. The results suggest a large year-to-year variability of the Arctic gyres and the changes in the Arctic – the Nordic Sea connectivity, which impacts exports of the freshwater, heat, and biogeochemical tracers from the Arctic.

This work has been funded from LTS-S CLASS (Climate–Linked Atlantic Sector Science, grant NE/R015953/1), from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 820989 (project COMFORT), from the project EPOC, EU grant 101059547 and UKRI grant 10038003 and from the UK NERC project CANARI (NE/W004984/1).

Reference

[1] Wilson, C., Aksenov, Y., Rynders, S. et al. Significant variability of structure and predictability of Arctic Ocean surface pathways affects basinwide connectivity. Commun. Earth. Environ. 2, 164 (2021). https://doi.org/10.1038/s43247-021-00237-0.

How to cite: Aksenov, Y., Rynders, S., Megann, A., Nurser, A. J. G., Wilson, C., and Coward, A. C.: Oceanic gyres in the Arctic, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16107, https://doi.org/10.5194/egusphere-egu23-16107, 2023.

EGU23-987 | ECS | Orals | OS1.5

Tracing Arctic outflow through the Fram Strait and its interaction with North Atlantic waters 

Dipanjan Dey, Robert Marsh, and Sybren Drijfhout

The Arctic region is warming four times quicker than the global average, a phenomenon known as the Arctic amplification. Some studies suggested that this warming may lead to seasonally ice-free Arctic Ocean by 2050 which will have potentially devastating consequences for Arctic oceanography, marine ecosystems and the Atlantic Meridional Overturning Circulation (AMOC). The relation between the slowdown of the AMOC and the Arctic Ocean is believed to be linked with enhanced freshwater outflow primarily through the Fram Strait which increases the stratification over sites of deep convection in the Irminger Sea. Earlier studies have also confirmed a link between deep water formation and freshwater release from the Arctic. In the current study, our objectives are to understand how and where the Arctic outflow is changing temperature, salinity and density, moving into the North Atlantic, during the historical period and in a warmer future climate. We use the Lagrangian parcel tracing algorithm, TRACMASS, to trace both the southward flows from Fram Strait and North Atlantic flows into the Nordic Sea. The results quantify how and where Arctic outflow increases temperature and salinity, and decreases density, in transit. This is primarily associated with mixing between the cold, fresh outflow and the relatively warmer, saltier Atlantic waters at Denmark Strait, despite some surface cooling in transit from Fram to Denmark Straits that is due to net surface heat loss and sea ice melting.

How to cite: Dey, D., Marsh, R., and Drijfhout, S.: Tracing Arctic outflow through the Fram Strait and its interaction with North Atlantic waters, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-987, https://doi.org/10.5194/egusphere-egu23-987, 2023.

EGU23-1169 | ECS | Orals | OS1.5

Improved simulation of extratropical North Atlantic atmosphere-ocean variability in HighResMIP models 

Casey Patrizio, Panos Athanasiadis, Claude Frankignoul, Dorotea Iovino, Simona Masina, Luca Famooss Paolini, and Silvio Gualdi

The simulated North Atlantic atmosphere­–ocean variability is assessed in a subset of models from HighResMIP that have either low-resolution (LR) or high-resolution (HR) in their atmosphere and ocean model components. In general, the LR models overestimate the low-frequency variability of subpolar sea-surface temperature (SST) anomalies and underestimate their correlation with the NAO compared to ERA5 reanalysis. These biases are substantially reduced in the HR models, and it is shown that the improvements are related to a reduction of intrinsic (non-NAO-driven) variability of the subpolar ocean circulation.

To understand the mechanisms behind the overestimated intrinsic subpolar ocean variability in the LR models, a link is demonstrated between the biases in subpolar ocean variability and known biases in the mean state of the Labrador-Irminger seas. Supporting previous studies, the Labrador-Irminger seas are found to be too cold and too fresh in the LR models compared to observations from EN4 and the HR models. This causes upper-ocean density and hence convection anomalies in this region to be more salinity-controlled in the LR models versus more temperature-controlled in the HR models. It is hypothesized that this may cause the excessive subpolar ocean variability in the LR models by 1) promoting a positive feedback between subpolar upper-ocean salinity, convection and Atlantic Meridional Overturning Circulation (AMOC) anomalies, and 2) weakening the negative feedback between subpolar upper-ocean temperature, convection and AMOC anomalies that is apparent in the HR models. The results overall suggest that mean ocean biases play an important role in the simulation of the variability of the extratropical ocean.

How to cite: Patrizio, C., Athanasiadis, P., Frankignoul, C., Iovino, D., Masina, S., Famooss Paolini, L., and Gualdi, S.: Improved simulation of extratropical North Atlantic atmosphere-ocean variability in HighResMIP models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1169, https://doi.org/10.5194/egusphere-egu23-1169, 2023.

Climate models are a valuable tool to study the interaction between ocean and atmosphere. Nevertheless, they are known to suffer from various biases and uncertainties. In the subpolar North Atlantic typical biases among models from the Coupled Model Intercomparison Project phase 6 (CMIP6) are found in the mean surface temperature and salinity, and in the mean sea ice concentration. These biases will affect the air-sea interaction.

In this study, we are investigating the diversity of CMIP6 models with respect to their response of the Atlantic Meridional Overturning Circulation (AMOC) to the North Atlantic Oscillation (NAO) in pre-industrial control experiments. This response is sensitive to the mean spiciness of the North Atlantic. Thus, we focus on two categories of models: Models that are spicy (warm-salty) and models that are minty (cold-fresh) within the subpolar gyre of the North Atlantic. Spicy models tend to have a lower sea ice cover in the Labrador Sea (LS) and larger LS heat loss during a positive NAO, compared to minty models. Also, spicy models have a weaker stratification in the LS. Sub-surface density changes 1 to 3 years after the NAO are larger in the spicy models and establish a zonal density gradient that can cause a stronger delayed AMOC response that is also more coherent across latitudes.

Although some metrics seem to be more realistic in the spicy models, other characteristics seem less realistic compared to the minty models, like the mixed layer depth relative importance between the eastern and the western subpolar North Atlantic. This could be a sign for how some mean states or processes might be right for the wrong reasons and stresses the need for model improvement.

How to cite: Reintges, A., Robson, J., Sutton, R., and Yeager, S.: Spiciness of the subpolar North Atlantic affects the response of the Atlantic Meridional Overturning Circulation to the North Atlantic Oscillation in CMIP6 models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1249, https://doi.org/10.5194/egusphere-egu23-1249, 2023.

EGU23-1359 | ECS | Orals | OS1.5

A Lagrangian study on the structure and pathways of the Irminger Current 

Nora Fried, Caroline A. Katsman, and M. Femke de Jong

The Irminger Current (IC), located over the western flank of the Reykjanes Ridge, is a contributor to the northward volume transport related to the Atlantic Meridional Overturning Circulation.

Previous studies showed that the IC is associated with a region of enhanced eddy kinetic energy. Using high-resolution mooring data from 2014 – 2020 combined with satellite altimetry, a strong intensification in volume transport of the IC in August 2019 could be attributed to the presence of mesoscale eddies in the vicinity of the moorings. At this time, altimetry showed an anticyclone lingering next to a cyclone in the mooring array, which intensified northward velocities within the IC. This example shows that mesoscale variability can directly impact the transport variability of the IC.

Further research presented here uses the high-resolution model POP (Parallel Ocean Program, 1/10°) to investigate the pathways of the IC up- and downstream of the mooring array. Here, the focus lies on determining the origin of waters feeding the IC and the role of mesoscale eddies in shaping the current and its pathways using Lagrangian particle tracking with the Ocean Parcels software. First results from a backtracking experiment reveal different origins for the water masses feeding the respective cores of the IC. Waters of the eastern core mostly originate from the eastern side of the Reykjanes Ridge. The western core appears to contain a substantial amount of waters from the interior Irminger Sea that partly recirculate from the Labrador Sea.

Additionally, we explore the mesoscale variability within the whole eastern Irminger Sea to investigate the potential impact of mesoscale eddies on restratification in the central Irminger Sea.  We focus on identifying the characteristics of variability along the ridge using the output of the Lagrangian particle tracking in POP. First results from a forward experiment show stronger mesoscale activity in the western core than the eastern core, which is in line with available mooring observations.

How to cite: Fried, N., Katsman, C. A., and de Jong, M. F.: A Lagrangian study on the structure and pathways of the Irminger Current, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1359, https://doi.org/10.5194/egusphere-egu23-1359, 2023.

EGU23-1639 | ECS | Posters on site | OS1.5

Impacts of AMOC slowdown on European circulation patterns 

Andrea Vito Vacca, Katinka Bellomo, and Jost von Hardenberg

The Atlantic meridional overturning circulation (AMOC) is a vital component of the global climate system regulating heat, carbon, and freshwater distribution. Most models predict a weakening of the AMOC throughout the 21st century, although there is significant uncertainty about its magnitude and the related regional climate impacts. In particular, the response of large-scale atmospheric circulation to the AMOC slowdown is still largely unknown, with implications for weather extremes and associated societal risks. The purpose of this study is to enhance our understanding of the impacts of an AMOC slowdown on atmospheric patterns with a focus on the Euro-Atlantic region, where the influence of AMOC is particularly relevant.

We analyse changes in an ensemble of idealised abrupt-4xCO2 climate model simulations from the CMIP archives with respect to the preindustrial climate. We split the models into groups according to their AMOC response to the 4xCO2. Through rigorous statistical testing, we attribute the differences in the simulated climate impacts to the difference in the AMOC response. Specifically, we find that models that simulate a larger AMOC decline feature minimum warming in the subpolar North Atlantic (North Atlantic Warming Hole or NAWH), a southward shift of the ITCZ, and a poleward strengthening of the mid-latitude jet stream. Instead, models that simulate a smaller AMOC decline feature enhanced North Atlantic warming, an intensification of the hydrological cycle but no southward shift in the ITCZ, and smaller displacements of the mid-latitude jet.  

To better characterize the large-scale atmospheric response at daily timescales, we use k-means clustering and self-organising maps to assess the changes in weather regimes over the Euro-Atlanic sector, including the NAO.  We further compare weather regimes’ frequency of occurrence and persistence between the two groups, attributing the differences to the AMOC decline. 

Our results indicate that the AMOC is a key source of large inter-model uncertainty in the simulation of future climate change impacts. Further observational campaigns may thus help us alleviate model biases and provide constraints on a number of societally relevant climate change impacts.

How to cite: Vacca, A. V., Bellomo, K., and von Hardenberg, J.: Impacts of AMOC slowdown on European circulation patterns, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1639, https://doi.org/10.5194/egusphere-egu23-1639, 2023.

EGU23-1796 | Orals | OS1.5

Intermittent Behavior in the AMOC-AMV Relationship  

Alessio Bellucci, Denis Mattei, Paolo Ruggieri, and Luca Famooss Paolini

The connection between the Atlantic meridional overturning circulation (AMOC) and the Atlantic multidecadal variability (AMV) is inspected in a suite of pre-industrial integrations from the 6th phase of the Coupled Model Inter-comparison Project (CMIP6), using a change-point detection method to identify different AMOC-AMV co-variability regimes. A key finding of this study is that models robustly simulate multi-decadal windows where the AMV and the AMOC are essentially uncorrelated. These regimes coexist with longer periods with relatively high correlation. Drops and recoveries of correlation are found to be often abrupt and confined in a temporal window of the order of 10 years. Phenomenological evidence suggests that the no-correlation regimes may be explained by drops in the variance of the AMOC: a less variable meridional heat transport leads to a suppressed co-variability of the AMV, leaving a larger role for non-AMOC drivers, consistent with a non-stationary AMOC-stationary noise interpretative framework.

How to cite: Bellucci, A., Mattei, D., Ruggieri, P., and Famooss Paolini, L.: Intermittent Behavior in the AMOC-AMV Relationship , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1796, https://doi.org/10.5194/egusphere-egu23-1796, 2023.

EGU23-1821 | Orals | OS1.5

Deep Ocean Circulation in the Subpolar North Atlantic Observed by Acoustically-tracked Floats 

Sijia Zou, Amy Bower, M. Susan Lozier, and Heather Furey

As part of the Overturning in the Subpolar North Atlantic Program, 122 acoustically-tracked subsurface floats were deployed at 1800-2800 dbar to understand the deep ocean circulation in the subpolar North Atlantic. Gridded mean velocity and eddy kinetic energy (EKE) maps have been constructed using velocity vectors derived from the floats. The mean velocity field reveals a relatively strong deep boundary current around Greenland and in the Labrador Sea, with a weaker deep boundary current over the eastern flank of the Reykjanes Ridge, and near-zero mean flow over the western flank, implying a discontinuous deep boundary current across the subpolar basin. Over most of the subpolar basin, deep EKE resembles that at surface, albeit with smaller magnitudes. A surprising finding about deep EKE is an elevated EKE band east of Greenland. This high EKE band is possibly attributed to the combined influence from propagating Denmark Strait Overflow Cyclones, variability of the wind-driven recirculation offshore of southeast Greenland, and/or topographic waves. The float-based flow fields constructed in this study provide an unprecedented view on the kinematic properties of the large-scale deep circulation in the subpolar North Atlantic.

How to cite: Zou, S., Bower, A., Lozier, M. S., and Furey, H.: Deep Ocean Circulation in the Subpolar North Atlantic Observed by Acoustically-tracked Floats, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1821, https://doi.org/10.5194/egusphere-egu23-1821, 2023.

EGU23-2035 | ECS | Orals | OS1.5

A Lagrangian view of seasonal overturning variability in the eastern North Atlantic subpolar gyre. 

Oliver J. Tooth, Helen L. Johnson, Chris Wilson, and Dafydd G. Evans

The Atlantic Meridional Overturning Circulation (AMOC) plays a critical role in the global climate system through the uptake and redistribution of heat, freshwater and carbon. At subpolar latitudes, recent observations show that the strength of the AMOC is dominated by water mass transformation in the eastern North Atlantic Subpolar Gyre (SPG). Both observations and ocean reanalyses show a pronounced seasonality of the AMOC within this region. However, the distribution of the strength and seasonality of overturning across the individual circulation pathways of the eastern SPG remains poorly understood. To investigate the nature of this seasonal overturning variability, we use Lagrangian water parcel trajectories evaluated within an eddy-permitting ocean sea-ice hindcast simulation.

By introducing a novel Lagrangian measure of the density-space overturning, we show that water mass transformation along the circulation pathways of the eastern SPG accounts for 8.9 ± 2.2 Sv (55%) of the mean strength of AMOC in the eastern subpolar North Atlantic. Our analysis highlights the crucial role of water parcel recirculation times in determining the magnitude of the strength and seasonality of overturning. We find that upper limb water parcels flowing northwards into the eastern SPG participate in a recirculation race against time to avoid wintertime diapycnal transformation into the lower limb of the AMOC. Upper limb water parcels sourced from the central and southern branches of the North Atlantic Current typically recirculate on interannual timescales (1-5 years) and thus determine the mean strength of overturning within this region. The seasonality of Lagrangian overturning is explained by a small collection of water parcels, recirculating rapidly (≤ 8.5 months) in the upper Central Iceland and Irminger Basins, whose along-stream transformation is dependent on their month of arrival into the eastern SPG.

How to cite: Tooth, O. J., Johnson, H. L., Wilson, C., and Evans, D. G.: A Lagrangian view of seasonal overturning variability in the eastern North Atlantic subpolar gyre., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2035, https://doi.org/10.5194/egusphere-egu23-2035, 2023.

EGU23-2777 | ECS | Orals | OS1.5

Impacts of Atlantic Multi-decadal Variability on the mid-latitude atmosphere 

Matthew Patterson and Tim Woollings

The large amplitude of low-frequency sea surface temperature (SST) variability in the North Atlantic, often known as Atlantic Multi-decadal Variability (AMV), raises the question of what impact this phenomenon has on atmospheric circulation. However, the coupled nature of AMV, makes disentangling the influence of the ocean on the atmosphere and that of the atmosphere on the ocean, challenging. This problem is further confounded by the relatively short observational record, when considering decadal to multi-decadal timescales.

To address this, we utilize information from both SSTs and ocean-atmosphere turbulent heat fluxes,  in a single index, to separate the influences that the ocean and atmosphere have on one another. This technique is then applied to both free-running coupled simulations and observations. This methodology will help further our understanding of North Atlantic variability on long timescales.

How to cite: Patterson, M. and Woollings, T.: Impacts of Atlantic Multi-decadal Variability on the mid-latitude atmosphere, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2777, https://doi.org/10.5194/egusphere-egu23-2777, 2023.

EGU23-2883 | ECS | Orals | OS1.5

The Atlantic meridional overturning circulation at 35N from deep moorings, floats, and satellite altimeter 

Isabela Le Bras, Joshua Willis, and Ian Fenty

From 2004 to 2014, the Line W moorings measured a 0.7 Sv/yr slowing of the Deep Western Boundary Current (DWBC) offshore of Cape Cod, Massachusetts. Here, we combine these deep mooring observations with float and satellite altimeter data and find that this DWBC change corresponded to a slowing of the cross-basin Atlantic Meridional Overturning Circulation (AMOC) of about 0.3 Sv/yr. Our AMOC transport time series corresponds well with the ECCO state estimate, particularly when the Line W mooring data influences our reconstruction of upper ocean volume fluxes. We compare our 35N time series with a similar time series at 41N as well as with the 26N RAPID AMOC, and find AMOC declines across datasets during this time period. The relative magnitudes of these declines are consistent with interdecadal variability originating in the Labrador Sea. We find that though our integrated overturning estimate agrees well with ECCO, the structure of the deep flow differs substantially. While we cannot rule out a decreasing AMOC trend during the 20th century, we find that natural variability is too large to detect a net AMOC decrease in direct observations or the ECCO ocean model since 2004.

How to cite: Le Bras, I., Willis, J., and Fenty, I.: The Atlantic meridional overturning circulation at 35N from deep moorings, floats, and satellite altimeter, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2883, https://doi.org/10.5194/egusphere-egu23-2883, 2023.

EGU23-3118 | ECS | Posters on site | OS1.5

Climate response to Atlantic meridional energy transport variations 

Weimin Jiang, Guillaume Gastineau, and Francis Codron

The climate impacts of fluctuations in the Atlantic meridional overturning circulation (AMOC) are studied using an atmosphere-ocean general circulation model (AOGCM). In two experiments, the baroclinic component of the North Atlantic Ocean currents is modified online to reproduce typical strong and weak AMOC conditions found in a preindustrial control simulation using the same model. These experiments are compared with slab ocean model (SOM) experiments that use heat flux corrections from the coupled model in the Atlantic Ocean. The main impacts of a strong AMOC include widespread warming in the Northern Hemisphere and a northward shift of the intertropical convergence zone (ITCZ). SOM experiments show similar atmospheric responses to AMOC-related heat flux anomalies, but with much larger impacts in the tropics. 
The atmospheric changes are driven by an anomalous cross-equatorial Hadley circulation transporting energy southward and moisture and heat northward. In the AOGCM, changes in the Indo-Pacific Ocean circulation and heat transport, driven by the wind stress associated with the abnormal Hadley cell, damp the atmospheric response. In the SOM simulations, the lack of Indo-Pacific transport and of ocean heat storage leads to larger atmospheric changes, that are further amplified by a positive tropical low cloud feedback. 

How to cite: Jiang, W., Gastineau, G., and Codron, F.: Climate response to Atlantic meridional energy transport variations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3118, https://doi.org/10.5194/egusphere-egu23-3118, 2023.

We investigated wintertime convection evolution in the past two decades over the Greenland Sea. This area is a major location regarding dense water production and supply of the lower limb of the Atlantic Meridional Overturning Circulation, a key component of the global climate.
Previous studies mentioned an increase in Greenland Sea wintertime convection intensity during the 2000s in comparison with the previous decade till the mid 2010s. Here, we further document the ongoing oceanic changes within the Greenland Sea using the Mercator Ocean Physical System, an operational ocean model with data-assimilation.
The model shows a large interannual variability, a later start and a decline of convection in the Greenland Sea in recent years. In particular, the depth of the annual maximum mixed layer diminished by 52 % between 2008/2014 and 2015/2020, from 1168 m to 559 m, over the convective area. There, hydrographic changes, especially a temperature increase, have led to isopycnal deepening and stratification strengthening at a larger rate in the north and east of the area (namely the Boreas Basin).
Atlantic Water spreading over the Boreas Basin and the eastern part of the Greenland Basin contributes to the changes of the Greenland Sea hydrography. The model also indicates a decrease in the intensity of the gyre in accordance with the isopycnal deepening while local surface winds and fluxes do not exhibit neither significant trends nor significant interannual variations.

How to cite: abot, L.: Recent Convection Decline in the Greenland Sea - insights from the Mercator Ocean System over 2008-2020, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3170, https://doi.org/10.5194/egusphere-egu23-3170, 2023.

EGU23-3197 | Posters virtual | OS1.5

Restratification Structure and Processes in the Irminger Sea 

Femke de Jong and Miriam Sterl

The Irminger Sea is one of the few regions in the ocean where deep (>1,000 m) convection
occurs. Convection is followed by restratification during summer, when the stratification of the water column
is reestablished and the convectively formed water is exported at depth. There are currently no descriptions
of interannual variability and physical drivers of restratification in the Irminger Sea. We investigate
restratification in the upper 600 m of the central Irminger Sea using reanalysis data for the years 1993–2019.
We find distinctly different restratification processes in the upper 100 m (the upper layer) and the water below
it (the lower layer). In the upper layer, the stratification is dominated by a seasonal cycle that matches the cycle
of the surface heat flux. In 2010 and 2019, there were peaks in upper layer restratification, which could partly
be related to strong atmospheric heat and freshwater fluxes. Greenland runoff likely also contributed to the
high restratification, although this contribution could not be quantified in the present study. In the lower layer
there is strong interannual variability in stratification, caused by variability both in the convection and the
restratification strength. The restratification strength is strongly correlated with the eddy kinetic energy in the
eastern Irminger Sea, suggesting that lower layer restratification is driven by lateral advection of warm, saline
waters through Irminger Current eddies. In the future, surface warming and freshening of the Irminger Sea
due to anthropogenic climate change are expected to increase upper layer stratification, potentially inhibiting
convection.

How to cite: de Jong, F. and Sterl, M.: Restratification Structure and Processes in the Irminger Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3197, https://doi.org/10.5194/egusphere-egu23-3197, 2023.

Subpolar and high-latitude regions of the North Atlantic are subject to changing buoyancy and mechanical forcing, alongside changing heat and freshwater exchanges with subtropical and polar regions. Associated changes in water mass formation and circulation are accompanied by changes in upper ocean stratification, of consequence for the large-scale ocean circulation, air-sea interaction, and ocean biogeochemistry. Changes in water mass volumes, and the associated overturning circulation, have been extensively evaluated with the water mass transformation (WMT) framework. Changes in stratification may be quantified with the Potential Energy Anomaly (PEA) framework, which has been extensively applied to seasonally stratified shelf sea environments. The WMT and PEA frameworks in combination provide complementary and holistic insights, for understanding hydrographic changes in relation to selected drivers. These frameworks are used with high-resolution model ocean datasets, obtained from hindcast and coupled simulations, the latter in control mode and forced by rising greenhouse gas concentrations through the 20th and 21st centuries. For selected sub-regions of the subpolar North Atlantic, bound by OSNAP and neighbouring hydrographic sections, mapped stratification (PEA) anomalies are related to respective changes in surface heat and freshwater fluxes. Residual differences between buoyancy-forced and full PEA tendencies are attributed to vertical mixing and divergences of heat and freshwater transports. Changes in regional stratification are evaluated alongside corresponding rates of water mass transformation and associated volumetric variability, for selected water masses. Eulerian perspectives provided by the WMT and PEA frameworks further complement Lagrangian perspectives provided by particle tracking. In full combination, these diagnostics elucidate multiple drivers of change in the North Atlantic that have potentially far-reaching consequences for the wider Earth System.

How to cite: Marsh, R., Dey, D., and Drijfhout, S.: Evaluating recent and future changes in North Atlantic stratification with complementary energetics and water mass frameworks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3476, https://doi.org/10.5194/egusphere-egu23-3476, 2023.

Winter (December-March) temperatures in most Europe is strongly correlated with the zonal circulation index of the Atlantic sector, North Atlantic Oscillation (NAO), regardless of which from its several available definitions to choose. However, the variability of this index also has a distinct multi-decade component, which makes it difficult to study trends of several decades. In addition, the NAO index itself is also significantly positively correlated with the global anthropogenic forcing and with the global temperature itself. Therefore, using purely statistical methods, it is not easy to distinguish the influence of zonal circulation variability from the trend resulting from the increasing forcing of greenhouse gases when examining the variability of winter temperatures in Europe.

Because of the prevailing western circulation, wintertime temperature in Europe should dpend on the intensity of the western circulation (NAO) as well as the sea surface temperature (SST) of the ocean (indexed by AMO – Atlantic Multidecadal Oscillation). However winter is the only season when there is no statistically significant correlation of AMO and temperatures in Europe. This surprising result had no explanation until the recent discovery of the northern shift of synoptic systems correlated with AMO. This could offer a “Bjerknes compensation” type of effect where the ocean circulation modifies the atmospheric one, making the air masses arriving in winter to Europe sourced in areas of the same SST, regardless of AMO (or general warming of the North Atlantic).

This study uses the data on SST and pressure fields as well as the NAO and AMO, together with an index if temperatures of Poland (as a proxy of Central Europe) in order to throw light on the relationship. The results confirm the existence of a “Bjerknes compensation” mechanism as well as suggest a dependence of wintertime NAO on the greenhouse forcing (visible in their significant correlation), caused most probably by the recently discovered strengthening of wintertime jet stream over the North Atlantic. This relationship can have important impact on future winter temperatures in a large part of Europe and therefore its possible mechanisms should be the point of further research.

 

This work been performed as a part of the SURETY project , funded by Polish National Science Centre (NCN), contract 2021/41/B/ST10/00946.

 

How to cite: Piskozub, J.: Is the North Atlantic modulating wintertime influence of NAO on Europe temperatures?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3850, https://doi.org/10.5194/egusphere-egu23-3850, 2023.

Variability of the Atlantic Meridional Overturning Circulation (MOC) has drawn extensive attention due to the MOC’s impact on global heat and freshwater redistribution. The Overturning in the Subpolar North Atlantic Program (OSNAP) array, consisting of an OSNAP West section covering the Labrador Sea and an OSNAP East section covering the eastern subpolar basins (Irminger and Iceland Basins), has continuously observed the MOC and meridional heat and freshwater transports since 2014. The OSNAP observations have contributed substantially to the understanding of the mean state and sub-seasonal to seasonal variability of the subpolar MOC. In this study, we present the latest OSNAP observational results and investigate interannual variability of the subpolar MOC with respect to water mass transformation and formation in the Labrador Sea and eastern subpolar basins. We detail the differences between formation and transformation in each of these basins and discuss their relationships to overturning on monthly and interannual time scales. Finally, we explore the mechanism(s) responsible for these differences.

How to cite: Fu, Y. and Lozier, M. S.: Interannual variability of the meridional overturning circulation in the subpolar North Atlantic, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4077, https://doi.org/10.5194/egusphere-egu23-4077, 2023.

EGU23-4340 | Orals | OS1.5

Model Biases in the AMOC Stability Indicator 

René van Westen and Henk Dijkstra

The Atlantic Meridional Overturning Circulation (AMOC) is considered to be a multi-stable system with a northward overturning and a southward overturning circulation state. It has been proposed that the stability of the AMOC system can be represented through the net freshwater transport at 34°S (the Atlantic's southern boundary), the so-called Fov index. For example when AMOC transports net freshwater out of the Atlantic sector at 34°S (Fov < 0), freshwater (i.e., salinity) perturbations may grow over time through the salt-advection feedback which eventually can induce a state transition. Present-day observations indicate that Fov is negative and  hence the present-day AMOC is in its multi-stable regime.

AMOC state transitions have regional and global impacts and it is therefore important to study the AMOC stability under climate change. However, most climate models have a tendency of simulating a positive Fov index, implying that the AMOC is too stable in these climate model simulations. Here we analyse Fov-related biases using a high-resolution and a low-resolution model version of the Community Earth System Model (CESM). Under constant pre-industrial conditions, the Fov index drifts from negative values to positive values over a 300-year simulation period. The Fov biases are related to biases in the E-P fluxes, freshwater runoff from Greenland, Agulhas leakage, Southern Ocean deep convection and the (meridional) location of the Antarctic Circumpolar Current front. These numerous processes contributing to Fov are responsible the difficulty in simulating realistic AMOC behaviour in climate model simulations. The implication is that climate models with an inconsistent Fov index are not fit for purpose in making AMOC projections.

How to cite: van Westen, R. and Dijkstra, H.: Model Biases in the AMOC Stability Indicator, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4340, https://doi.org/10.5194/egusphere-egu23-4340, 2023.

EGU23-5053 | ECS | Orals | OS1.5

Meridional Connectivity of a 25-year Observational AMOC Record at 47°N 

Simon Wett, Monika Rhein, Dagmar Kieke, Christian Mertens, and Martin Moritz

The Atlantic Meridional Overturning Circulation (AMOC) plays a key role for the climate system of Europe and the Arctic by redistributing heat and freshwater in the Atlantic. Since climate model studies project a likely decline of the AMOC under climate change in the 21st century, monitoring AMOC changes remains an important task. Several moored arrays in the Atlantic deliver estimates of the AMOC volume transport. The longest of these observational AMOC records is the RAPID array in the subtropical North Atlantic. The depiction of the AMOC as a global ocean conveyor assumes that the AMOC variability is consistent across latitudes. This concept has been questioned by model studies. However, model studies and estimates based on altimetry and Argo data disagree on the regions and timescales of meridional connectivity. From measurements of the North Atlantic Changes (NOAC) array in the subpolar North Atlantic at 47°N we calculate the AMOC volume transport timeseries. Our approach combines data from moored instruments with hydrography (from Argo floats and shipboard measurements) and satellite altimetry. Here, we present this 25-year (1993-2018) purely observational AMOC record in monthly resolution and analyze its meridional connectivity with the subtropical RAPID AMOC.

How to cite: Wett, S., Rhein, M., Kieke, D., Mertens, C., and Moritz, M.: Meridional Connectivity of a 25-year Observational AMOC Record at 47°N, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5053, https://doi.org/10.5194/egusphere-egu23-5053, 2023.

EGU23-5281 | ECS | Orals | OS1.5

Decadal Variability of Transports through Barents Sea Opening: Changing impact of Large-Scale Wind Forcing 

Finn Ole Heukamp, Lars Aue, and Torsten Kanzow

The Barents Sea Opening (BSO) is one of two Atlantic gateways connecting the North Atlantic Ocean to the Arctic Ocean. The ocean transport through the BSO is composed of warm and saline Atlantic Water inflow in the central and southern parts of the section and cold Polar and modified Atlantic Water outflow in the north. The variability of strengths of both inflow and outflow largely controls the evolution of the net ocean heat transport into the Barents Sea, locally impacting e.g., ocean-atmosphere heat fluxes, sea ice extent, and deep-water formation. Moreover, changes in heat fluxes and sea ice extent have been shown to impact remote properties such as wintertime weather in northern Europe and water properties in the central Arctic Ocean.


In this study, we identified and disentangled the contributions of local and remote atmospheric forcing mechanisms of the wintertime volume transport through BSO from 1970-2020. In order to understand the variability and co-variability of the local and remote forcing mechanisms and the linked transport anomalies, we performed dedicated model experiments with the unstructured ocean and sea ice model FESOM2. In addition to a hindcast control simulation using JRA55 reanalysis forcing, we performed two additional model experiments in which we combined JRA55 forcing with CORE1 normal year forcing in a way that the simulations are forced with JRA55 (CORE1) in the Arctic domain and CORE1 (JRA55) outside the Arctic domain. This setup allows the separation of local and upstream forced transport variability. Our experiments show, that both BSO inflow and outflow exhibit strong variability on interannual to decadal timescales. While inflow variability is forced to a similar degree by local alongshore winds and alongshore winds upstream in the Norwegian Sea, the outflow variability is almost entirely forced by wind stress curl anomalies over the northern Barents Sea shelf. Moreover, the inflow anomalies forced upstream are highly correlated with the North Atlantic Oscillation (NAO) while the transport anomalies forced locally exclusively correlate with the NAO during periods of a negative NAO. Furthermore, we observe a drastic drop in the correlation of inflow anomalies forced upstream and the NAO around the year 2000 - the same period in which winters with strongly enhanced outflow anomalies (97/98, 03/04) are found. By expanding our analysis to cyclone activity in the northern North Atlantic, we link the loss of co-variability of NAO and BSO inflow to an anomalous southward deflection of cyclones in these winters, affecting the alongshore winds in the Norwegian Sea as well as the wind stress curl over the northern Barents Sea shelf.


In general, this study aims to improve our understanding of the drivers of volume and heat transport variability in the BSO as a key factor for (sub-)Arctic, ocean, weather, and climate variability. 

How to cite: Heukamp, F. O., Aue, L., and Kanzow, T.: Decadal Variability of Transports through Barents Sea Opening: Changing impact of Large-Scale Wind Forcing, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5281, https://doi.org/10.5194/egusphere-egu23-5281, 2023.

EGU23-5422 | ECS | Posters on site | OS1.5

Argo-based anthropogenic carbon concentration and inventory in the Labrador and Irminger Seas over 2011-2021 

Rémy Asselot, Lidia I. Carracedo, Virginie Thierry, Herlé Mercier, Anton Velo, Raphaël Bajon, and Fiz F. Pérez

The ocean is a net sink for a quater of the carbon dioxide emitted to the atmosphere by human industrial activities and land-use change (Cant). The North Atlantic Ocean encompasses the highest ocean storage capacity of Cant per unit area. In particular, the Labrador and Irminger Seas are two basins storing a high amount of Cant due to the deep convection activity taking place there. The temporal evolution of Cant concentration in these two basins and their Cant inventories in the 0-1800 depth layer are estimated over the period 2011-2021. The Cant values are estimated from Argo floats equipped with oxygen sensors, predictive neural networks (ESPER_NN and CONTENT) and a carbon-based back-calculation method (φCOT method). On average, Cant inventories are similar in the two basins and amount to 75.3 and 75.6 mol/m2 in the Irminger and Labrador seas, respectively. Over the study period, Cant inventories increase in the two basins at a storage rate of 1.01±0.14 mol/m2/yr in the Irminger Sea and 0.94±0.2 mol/m2/yr in the Labrador Sea. The processes involved in Cant evolution in the two basins are then investigated.

How to cite: Asselot, R., Carracedo, L. I., Thierry, V., Mercier, H., Velo, A., Bajon, R., and Pérez, F. F.: Argo-based anthropogenic carbon concentration and inventory in the Labrador and Irminger Seas over 2011-2021, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5422, https://doi.org/10.5194/egusphere-egu23-5422, 2023.

Multicentennial North Atlantic climate variability revealed by paleoclimate reconstruction has been linked to the Atlantic meridional overturning circulation (AMOC) variability. However, mechanisms of multicentennial AMOC variability in coupled models have yet to reach a consensus, reflecting a necessity of more fundamental theoretical studies. To this end, we propose an ocean-only North Atlantic 4-box theoretical model. A self-sustained AMOC oscillation with a typical period of 300-400 years exhibits. The timescale is largely set by rate of AMOC advection but also modulated by thermal processes, while the self-sustained oscillation mechanism can be generalized as a combination of a linear growing oscillation and a nonlinear restraining. The linear growing oscillation is energized by the salinity advection feedback and stabilized by the temperature advection feedback, while the latter is hampered by surface temperature restoring. Nonlinear restraining processes restrict the runaway tendency of the linear growing oscillation and finally turn it into a self-sustained one. We further identify a 300-400-year AMOC oscillation in a CESM1 control simulation, which can be well explained by the self-sustained oscillation mechanism of the theoretical model. Our work demonstrates that internal variability plays a vital role in multicentennial AMOC variability, while the dominating processes primarily lie in the North Atlantic.

How to cite: Yang, K.: A theory for self-sustained multicentennial AMOC oscillation and its evidence in CESM1, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5739, https://doi.org/10.5194/egusphere-egu23-5739, 2023.

Both Greenland Ice Sheet mass loss and Atlantic Meridional Overturning Circulation weakening are considered tipping elements of the climate system under global warming. Ocean and climate models of varying complexity are widely applied to understand and project the future evolution of the two processes and their connection. The results are prone to model uncertainty however. Especially the role of regional mesoscale processes in the subpolar North Atlantic is still being investigated. We ran a systematic set of eight dedicated 60 to 100-year long model experiments with and without atmospheric coupling, with eddy processes parameterized and explicitly simulated, with regular and significantly enlarged Greenland runoff to reconcile findings of the regional ocean and global climate modeling communities.

The most prominent result is a major impact by an interactive atmosphere for limiting the AMOC weakening through enabling a compensating temperature feedback. Coupled experiments yield an AMOC decline of <2Sv to a freshwater perturbation of 0.05Sv whereas the AMOC weakens by >4Sv in the ocean-only runs. In addition to this large-scale effect, we find that the Labrador Sea and the Northwest Corner (off Flemish Cap) are critical regions for the role of mesoscale eddies in redistributing Greenland meltwater and affecting the timing of its impact. We show that an ocean grid at 1/10˚–1/12˚, which is currently used in global high-resolution climate simulations, can already significantly improve the path of the meltwater along the North American coast and into the wider North Atlantic. But the same resolution still falls short in providing sufficient dynamical exchange between the boundary current and the interior Labrador Sea and especially lacks capability in restratifying the Labrador Sea after deep convection. Our experiments demonstrate where an eddy parameterization works quite successfully and where only high resolution (>1/12˚) yields a realistic ocean response. This underlines the necessity to advance scale-aware eddy parameterizations for next-generation climate models.

How to cite: Martin, T. and Biastoch, A.: Ocean response to Greenland melting in a hierarchy of model configurations: Relevance of eddies and an interactive atmosphere, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5909, https://doi.org/10.5194/egusphere-egu23-5909, 2023.

EGU23-5941 | Orals | OS1.5

Realistic freshwater forcing around Greenland in climate models 

Marion Devilliers, Shuting Yang, Steffen Olsen, and Annika Drews
This study summarizes the findings of several realistic freshwater forcing experiments around Greenland and surrounding regions, which were conducted using climate and ocean models over the historical period. The results of the experiments are discussed in terms of their impact on the Atlantic Meridional Overturning Circulation (AMOC) as well as on temperature and salinity changes in the North Atlantic and in the Arctic. It was determined that the addition of freshwater led to a decrease of the AMOC and a reduction of the temperature and salinity biases in the North Atlantic. These results will be of particular interest to researchers interested in the effects of current and future Greenland melting on local and global ocean.

 

How to cite: Devilliers, M., Yang, S., Olsen, S., and Drews, A.: Realistic freshwater forcing around Greenland in climate models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5941, https://doi.org/10.5194/egusphere-egu23-5941, 2023.

EGU23-6169 | ECS | Posters on site | OS1.5

Subpolar Atlantic Meridional Overturning in Community Earth System Model (CESM): setting up the further experiment 

Aleksandr M. Fedorov, M. Femke de Jong, Claudia E. Wieners, and Henk A. Dijkstra

The Atlantic Meridional Overturning Circulation (AMOC) is a three-dimensional system of ocean currents contributing to a relatively mild climate in northern Europe. AMOC transports vary on a range of time scales, from centennial to daily. Despite a wide history of research on AMOC variability from both measurements and modeling perspectives, the role of atmospheric noise in subseasonal and intra-annual AMOC variations remains unclear. In the current study, we describe the modeling experiment planned to reveal the importance of mesoscale winds around the southern tip of Greenland, named Tip Jets, in  AMOC variability. Tip Jet wind events have no regularity in frequency and intensity, they mostly depend on the Icelandic Low location and are partly associated with a positive NAO phase in winter. The experiment design is based on implying the perturbations in the momentum, heat, and freshwater forcing associated with Tip Jet events. Therefore, we constructed the composite Tip Jet forcing using daily ERA5 (25 km, December-March, 1969-2019) wind, surface fluxes, and precipitation/evaporation rates. These composite fields of anomalies are planned to be added to the CESM-derived climatology to describe the possible response of the AMOC system to these types of noise forcing. Setting up the experiment included the model verification based on the comparison between the monthly output from the ~0.1 ° CESM Parallel Ocean Product (POP) simulations and the observational OSNAP array that combines measurements along the line between Labrador, Greenland, and the European shelf. The mean state of the atmosphere from CESM (50 km, monthly) was compared to the ERA5 (25 km, monthly). Generally, the CESM model reproduces the AMOC at OSNAP well. In conclusion, this preliminary research shows that AMOC is well simulated by the Community Earth System Model in the Subpolar North Atlantic. Also, the current research proposes patterns of noise forcing over the North Atlantic Subpolar Gyre that will be used in further modeling experiments.

How to cite: Fedorov, A. M., de Jong, M. F., Wieners, C. E., and Dijkstra, H. A.: Subpolar Atlantic Meridional Overturning in Community Earth System Model (CESM): setting up the further experiment, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6169, https://doi.org/10.5194/egusphere-egu23-6169, 2023.

EGU23-6219 | ECS | Posters on site | OS1.5

Does freshwater content of the East Greenland Current show imprints of increasing meltwater runoff? 

Ilana Schiller-Weiss, Torge Martin, Johannes Karstensen, and Arne Biastoch

Accelerated melting of the Greenland Ice Sheet has been identified as a tipping element in the freshwater balance of the subpolar North Atlantic (SPNA), where the East Greenland Current is a primary pathway for transporting Arctic-sourced freshwater and Greenland glacial meltwater. Understanding the freshwater variability of the East Greenland Current (EGC) and Coastal Current (EGCC) and their interaction is of high importance, as these gather the imprint of ice melt once the meltwater leaves the fjords and enters the open ocean. Using a high-resolution model (VIKING20X, 1/20°) and gridded, observational assimilated reanalysis (GLORYS12, 1/12°), we find the freshest water remains close to the shelf with interannual extremes in freshwater content attributable to the imprint of Greenland melt only in years 2010 and 2012. This signal is only found in the VIKING20X simulation, which in contrast to GLORYS12 uses realistic, interannually varying runoff forcing including estimates of the Greenland Ice Sheet mass balance. We further discuss the role of wind forcing, sea ice melt, and Greenland runoff, which all contribute to variability in freshwater content along the boundary current.

Our results show that slackened alongshore winds reduce onshore Ekman transport allowing for freshwater to spread laterally in the EGC, while stronger alongshore winds constrain freshwater closer to the shelf with saline intrusions from the interior basin into the outer EGC. South of ~65°N sea ice melts year round and retreats northward with melting occurring only in summer. Associated salinity and thus freshwater content anomalies are of similar magnitude as those associated with meltwater runoff and overlap in both seasonal timing and advective time scales. This could explain the challenges to identify freshening events originating from extreme melt events on the Greenland Ice Sheet at currently observed magnitudes. Their detection is critically dependent on synoptic and interannually varying processes. Our findings also suggest that ocean models or model-based reanalysis products aiming to illustrate the processes of meltwater redistribution should feature grid resolutions preferably exceeding 1/12° in order to represent coastal dynamics and fjord-shelf-open ocean exchange. With more observations on the Greenland shelf hopefully becoming available in the future, we anticipate the GLORYS12 assimilation product to show similar variability as higher resolution models.

How to cite: Schiller-Weiss, I., Martin, T., Karstensen, J., and Biastoch, A.: Does freshwater content of the East Greenland Current show imprints of increasing meltwater runoff?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6219, https://doi.org/10.5194/egusphere-egu23-6219, 2023.

EGU23-6539 | Orals | OS1.5

Mechanism of observed North Atlantic multidecadal upper ocean heat content changes 1950-2020 

Ben Moat, Bablu Sinha, Neil Fraser, Leon Hermanson, Simon Josey, Claire MacIntosh, David Berry, Simon Williams, Marilena Oltmanns, Dan Jones, and Rachael Sanders

We synthesize observational datasets and a state of the art forced global ocean model to construct a multidecadal upper ocean heat budget for the North Atlantic for the period 1950 to 2020. Using multiple independent estimates of the variables allows us to provide robust uncertainty estimates for each term. Time-varying ocean heat transport convergence dominates the budget on multidecadal timescales in all regions of the North Atlantic. In the subpolar region (north of 45N) we find that the heat transport convergence is dominated by geostrophic currents whereas in the subtropics (26-45N) advection by ageostrophic currents is also significant. The geostrophic advection is dominated (especially in the subpolar regions) by anomalous geostrophic currents acting on the mean temperature gradient. The timescale and spatial distribution of the anomalous geostrophic currents are consistent with basin scale ‘thermal’ Rossby waves propagating westwards/northwestwards in the subpolar gyre. Multidecadal changes in North Atlantic Changes in ocean heat storage directly affect the climate of the surrounding continents, and hence it is important to understanding the mechanism behind these.

How to cite: Moat, B., Sinha, B., Fraser, N., Hermanson, L., Josey, S., MacIntosh, C., Berry, D., Williams, S., Oltmanns, M., Jones, D., and Sanders, R.: Mechanism of observed North Atlantic multidecadal upper ocean heat content changes 1950-2020, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6539, https://doi.org/10.5194/egusphere-egu23-6539, 2023.

EGU23-7195 | ECS | Posters on site | OS1.5

Quantifying the Contribution of Surface Buoyancy Forcing to Recent Subpolar AMOC Variability 

Charlotte Marris and Robert Marsh

In the Subpolar North Atlantic (SPNA), interannual to multidecadal variability in the Atlantic Meridional Overturning Circulation (AMOC) is primarily attributed to surface buoyancy forcing. Here, warm surface waters arriving via the Gulf Stream and North Atlantic Current undergo an intense loss of heat and freshwater to the atmosphere and are thus transformed to cold and dense waters which subsequently sink and are returned southward at depth. Quantifying the contribution of surface buoyancy forcing to AMOC variability is essential for modelling how the AMOC will respond to predicted warming and freshening at high latitudes. In a water mass transformation framework, fields of surface density and surface density flux from the GODAS ocean reanalysis are used to construct the surface-forced overturning circulation (SFOC) streamfunction for the SPNA (48-65°N) in an operational assimilation over 1980-2020. Computed and plotted in latitude-density space, the SFOC reconstruction compares favourably with the corresponding AMOC, computed from GODAS currents. We thus conclude that subpolar AMOC variability is largely explained by changing air-sea heat and freshwater fluxes controlling water mass transformation across the region. We further highlight the changing relative influences of water mass transformation in the eastern and western subpolar gyre, by partitioning SFOC longitudinally into an East component (5-43 °W) comprising the Irminger and Iceland basins, and a West component (43-60 °W) comprising the Labrador Sea. Our analysis demonstrates that interannual to multidecadal SFOC variability is dominated by changing water mass transformation in the western subpolar gyre. This challenges a shifting consensus that highlights the eastern subpolar gyre as dominant in driving the AMOC across subpolar latitudes.

How to cite: Marris, C. and Marsh, R.: Quantifying the Contribution of Surface Buoyancy Forcing to Recent Subpolar AMOC Variability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7195, https://doi.org/10.5194/egusphere-egu23-7195, 2023.

The Atlantic Meridional Overturning Circulation (AMOC) exports cold, fresh, dense waters formed in the subpolar North Atlantic to equatorward latitudes along the western boundary and interior pathways. The properties of the water formed in the North Atlantic vary from year to year, however the strength and time scale for the downstream communication of this variability is still unclear. While several past studies have focused on tracking specific property anomalies, particularly from the Labrador Sea, we approach our study by investigating property variance downstream of the water mass source region. In effect, we aim to understand the downstream memory of water mass property variability in the North Atlantic along western boundary and interior pathways. To do so, we analyze hydrographic properties on neutral density isopycnal surfaces in the subpolar North Atlantic and along the western boundary and interior pathways with two reanalysis products from the Met Office, the hydrographic dataset (EN4) and ensemble prediction system (GloSea5), over their overlapping time period (1993-2019). Our results show different patterns of downstream variance for the interior compared to the western boundary, which we interpret in terms of known circulation features in the deep North Atlantic and what we have learned from past Lagrangian studies.

How to cite: Fortin, A.-S. and Lozier, S.: Variability of North Atlantic Water Mass Properties along Western Boundary and Interior Pathways, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7535, https://doi.org/10.5194/egusphere-egu23-7535, 2023.

EGU23-7698 | ECS | Orals | OS1.5

Water mass transformation following instability in the mixed layer of the East Greenland Current 

Fraser Goldsworth, Isabela Le Bras, Helen Johnson, and David Marshall

Observations show that strong southerly winds over the Irminger Sea can excite symmetric instability in the East Greenland Current, resulting in the generation of a low potential vorticity layer below the convectively mixed layer (Le Bras et al., 2022). The role of these downfront wind events on the formation of dense waters is not yet well understood.

Using an ensemble of ultra-high resolution models (25 m in the horizontal)  we show that the low potential vorticity layer is virtually indistinguishable from the convectively mixed layer, implying the absence of symmetric instability in coarse models may lead to underestimates in the mixed layer depth and baroclinicity of the East Greenland Current. We explore the hypothesis that symmetric instability acts as the short time-scale response of the current to these southerly wind events and pre-conditions the mixed layer, making it more susceptible to baroclinic instability over longer time-scales. We then investigate whether baroclinic eddy activity is enhanced following these wind events and examine the implications of this on lateral and diapycnal mixing, including by calculating water mass transformation rates.

How to cite: Goldsworth, F., Le Bras, I., Johnson, H., and Marshall, D.: Water mass transformation following instability in the mixed layer of the East Greenland Current, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7698, https://doi.org/10.5194/egusphere-egu23-7698, 2023.

EGU23-7805 | ECS | Orals | OS1.5

Intra-annual variability of carbon signature and transport in the North Atlantic Ocean 

Raphaël Bajon, Lidia Carracedo, Herlé Mercier, Fiz F. Pérez, Anton Velo, Rémy Asselot, and Virginie Thierry

The ocean is the largest carbon reservoir on Earth, and a major sink for the excess of CO2 (anthropogenic carbon) emitted to the atmosphere by human activities. Having removed about a quater of these emissions since the beginning of the industrial era, ocean’s key role in climate is particularty outstanding in the North Atlantic (NA). A combination of physical and biological processes makes the NA a key-role region for the natural and anthropogenic carbon uptake and storage, and hence for the global carbon cycle. Traditionally, the seasonal carbon cycle has been assumed to respond to natural variability, unnafected by the ongoing anthropogenic increase of atmospheric CO2. Recent model projections, however, point otherwise, yet observational evidence to verify these predictions is still missing. Here we examine seasonal cycle in dissolved inorganic carbon (DIC) and its (surface-2000 dbar) transport, estimated using in-situ data and neural networks, across the OVIDE (GO-SHIP A25) section, from 1993 to 2021 at a monthly resolution. Our results highlight that changes in temperature, dissolved oxygen and ocean circulation are key components driving the seasonal DIC variability. DIC concentrations are higher in years with strong winter mixing regimes (which bring more nutrient-rich waters to the surface, favouring photosynthesis, and more (remineralized) carbon back to the surface). Seasonal DIC transport fluctuations are found significant compared to the mean (e.g. +/- 25% in the upper branch of the meridional overturning circulation), putting into relevance that caution is needed if assuming that single-cruise occupations are representative of the annual state. We also observe a yearly variant seasonal imbalance, with a significant reduction over the past two decades in the upper branch of the meridional overturning circulation. These results underscore the importance of considering intra-annual variability in the North Atlantic's carbon cycle when addressing climate change.

 
 
 

How to cite: Bajon, R., Carracedo, L., Mercier, H., Pérez, F. F., Velo, A., Asselot, R., and Thierry, V.: Intra-annual variability of carbon signature and transport in the North Atlantic Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7805, https://doi.org/10.5194/egusphere-egu23-7805, 2023.

EGU23-7844 | ECS | Orals | OS1.5

Linkage between overturning and density anomaly over the subpolar gyre 

Tillys Petit, Jon Robson, David Ferreira, and D. Gwyn Evans

The surface forced water mass transformation (SFWMT) is known to be the main contributor of the Atlantic Meridional Overturning Circulation (AMOC) over the subpolar gyre. Over the eastern part of the subpolar gyre, a recent study revealed the dominant role of surface density changes in driving the SFWMT as opposed to the direct influence of air-sea fluxes. Indeed, the distribution at surface of the isopycnal associated with the maximum overturning streamfunction, Smoc, modulates the area of dense water formation induced by the air-sea fluxes.

The Overturning in the Subpolar North Atlantic Program (OSNAP) showed that the density of Smoc is highly variable in time along each section of the array. However, the drivers of Smoc remain unclear. In our work, we use a combination of atmospheric reanalysis and coupled simulations of HadGEM3-GC3.1 to evaluate the Smoc variability over the subpolar gyre as well as its connection with the overturning strength. At interannual timescale, the variability of Smoc at OSNAP East is strongly related to those at OSNAP West and at 45°N. However, its connection with the overturning strength is more complex. Although Smoc is not well related to the overturning at OSNAP, it is associated with a shift in density of the overturning stream function. The Irminger Sea is identified as being the centre of action driving this variability.

How to cite: Petit, T., Robson, J., Ferreira, D., and Evans, D. G.: Linkage between overturning and density anomaly over the subpolar gyre, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7844, https://doi.org/10.5194/egusphere-egu23-7844, 2023.

EGU23-9765 | ECS | Orals | OS1.5

Cross-shelf exchanges between the East Greenland shelf and interior seas 

Elodie Duyck and Femke De Jong

The Atlantic Meridional Overturning Circulation (AMOC) is predicted to weaken in the 21st century as a result of climate change. One of the proposed drivers for such a weakening is the dampening of deep convection in the Subpolar North Atlantic following an increase in freshwater fluxes from the Greenland ice sheet. However, the fresh waters that flow from Greenland and the Arctic to the Subpolar North Atlantic are primarily found over the Greenland shelf, and it is unclear where and how much freshwater is exported from the shelf to the interior seas where deep convection occurs. While the main export of freshwater off the Greenland shelf is likely to occur west of Greenland, the importance of water mass transformation and overturning east of Greenland in the total subpolar AMOC makes it essential to better understand freshwater exchanges between the east Greenland shelf and deep convection regions of the Irminger and Nordic Sea.

We investigate these exchanges using drifter data from five deployments carried out at different latitudes along the east Greenland shelf in 2019, 2020 and 2021, as well as satellite data and an atmospheric reanalysis. We compute Ekman transport (from winds) and geostrophic velocity (from satellite altimetry) at the shelfbreak and find that the Blosseville Basin, just upstream of Denmark Strait, and Cape Farewell, are particularly favorable to cross-shelf exchanges. We further investigate exchange processes in these regions using drifter data. In the Blosseville Basin, drifters are brought off-shelf towards the Iceland Sea and into the interior of the Basin, possibly joining the separated EGC. As they flow downstream, they re-enter the shelf and most are driven towards the coast. This exchange appears to be mainly driven by the shape of the bathymetry. At Cape Farewell, the wind appears to be the main driver, although occasionally an eddy seems to turn drifters away from the shelf. The drifters brought off-shelf at Cape Farewell mostly continue around Eirik Ridge, where they re-enter the West Greenland Current. How much of the freshwater signature is lost between leaving the East Greenland Current and entering the West Greenland Current is not clear and will need further study.

How to cite: Duyck, E. and De Jong, F.: Cross-shelf exchanges between the East Greenland shelf and interior seas, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9765, https://doi.org/10.5194/egusphere-egu23-9765, 2023.

EGU23-9981 | Orals | OS1.5

Cessation of Labrador Sea Convection by Freshening through (Sub)mesoscale Flows 

Louis Clement, Eleanor Frajka-Williams, Nicolai von Oppeln-Bronikowski, Ilona Goszczko, and Brad de Young

By ventilating the deep ocean, deep convection in the Labrador Sea plays a crucial role in the climate system. Unfortunately, the mechanisms leading to the cessation of convection and, hence, the mechanisms by which a changing climate might affect deep convection remain unclear. In winter 2020, three autonomous underwater gliders sampled the convective region and both its spatial and temporal boundaries. Both boundaries are characterised by higher sub-daily mixed-layer depth variability than the convective region. At the convection boundaries, buoyant intrusions--including eddies and filaments--primarily drive restratification by bringing freshwater, instead of warm warmer, and instead of atmospheric warming. At the edges of these intrusions, submesoscale instabilities, such as symmetric instabilities and mixed-layer baroclinic instabilities, seem to contribute to the decay of the intrusions. In winter, strong destabilising surface heat flux and along-front winds can enhance the lateral stratification, sustaining submesoscale instabilities. Consequently, winter atmospheric conditions and freshwater intrusions participate in halting convection by adding buoyant freshwater into the convective region through submesoscale flows. This study reveals freshwater anomalies in a narrow area offshore of the Labrador Current and near the convective region; this area has received less attention than the more eddy-rich West Greenland Current, but is a potential source of freshwater in closer proximity to the region of deep convection. Freshwater fluxes from the Arctic and Greenland are expected to increase under a changing climate, and our findings suggest that they may play an active role in the restratification of deep convection.

How to cite: Clement, L., Frajka-Williams, E., von Oppeln-Bronikowski, N., Goszczko, I., and de Young, B.: Cessation of Labrador Sea Convection by Freshening through (Sub)mesoscale Flows, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9981, https://doi.org/10.5194/egusphere-egu23-9981, 2023.

EGU23-10227 | ECS | Posters on site | OS1.5

Is the AMOC connected across all latitudes? 

María Jesús Rapanague, Dian Putrasahan, and Jochem Marotzke

The Atlantic Meridional Overturning Circulation (AMOC) is fundamental for the northward transport of heat and the vertical transport of carbon from the surface to the deep ocean in the North Atlantic Ocean, influencing the climate at both local and global scales. However, the mechanisms underlying the AMOC variability are still poorly understood, because of the lack of long-term observations and the challenge of representing key processes in standard climate models. Furthermore, assumptions widely accepted for several decades have re-entered the debate in recent years, such as the AMOC meridional coherence and the role of deep convection in the Labrador Sea in driving the AMOC variability and deepwater formation. New modeling and observational studies suggest that the overturning variability is not coherent between subtropical and subpolar latitudes on interannual to decadal scales and that climate models systematically exaggerate the importance of the Labrador Sea, pointing toward other regions like the Irminger Sea and the Nordic Sea as better candidates for deepwater formation.

In this study, we aim to critically assess the long-held notion of meridional coherence in the AMOC, using output from high- and very-high-resolution model simulations. Specifically, we investigate how the meridional coherence of the AMOC changes when increasing model resolution, via spectral analysis of the MPI-ESM1.2 control simulations with resolutions of 1°, 0.4°, and 0.1°. Preliminary analysis using lead-lag time correlations indicates a high correlation and meridional coherence between the AMOC strength and mixed layer depth variability in the Labrador Sea for the coarsest resolution. However, when increasing the resolution this relationship disappears, and the AMOC is instead better related to overflow changes in the Denmark Strait and in the Nordic Seas. Additionally, the meridional coherence of the AMOC becomes unclear.

How to cite: Rapanague, M. J., Putrasahan, D., and Marotzke, J.: Is the AMOC connected across all latitudes?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10227, https://doi.org/10.5194/egusphere-egu23-10227, 2023.

EGU23-10569 | Orals | OS1.5

Global climate teleconnections into and out of the North Atlantic Ocean 

Matthew H. England, Bryam Orihuela-Pinto, and Andréa Taschetto

The Atlantic Meridional Overturning Circulation (AMOC) has a profound impact on both global and regional climate, yet our understanding of the mechanisms controlling remote teleconnections remains limited. In addition, it is unclear how remote processes impact the North Atlantic and alter the strength of the AMOC.  In this presentation I will show how a slowdown in the AMOC can drive an acceleration of the Pacific trade winds and Walker circulation by leaving an excess of heat in the tropical South Atlantic. This tropical Atlantic warming drives anomalous atmospheric convection, resulting in enhanced subsidence over the east Pacific, and a strengthened Walker circulation and trade winds. Further teleconnections include a shift in the ITCZ, enhanced zonal SST gradients across the tropical Pacific, strengthened convection over the West Pacific Warm Pool, and a deepening of the Amundsen Sea Low off Antarctica.  Teleconnections back to the North Atlantic can in turn be triggered by Southern Hemisphere wind anomalies on a relatively rapid time-scale via propagating planetary waves in the ocean.  There is also evidence that tropical Pacific cooling can feedback and influence the strength of the AMOC.  These findings have implications for understanding both intrinsic decadal climate variability as well as longer-term climate change.

How to cite: England, M. H., Orihuela-Pinto, B., and Taschetto, A.: Global climate teleconnections into and out of the North Atlantic Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10569, https://doi.org/10.5194/egusphere-egu23-10569, 2023.

EGU23-11459 | ECS | Posters on site | OS1.5

Tracing Ocean circulation at the AR7W and OVIDE lines using artificial radionuclides 

Lisa Gerlinde Thekla Leist, Maxi Castrillejo, John N. Smith, Marcus Christl, and Núria Casacuberta

The Subpolar North Atlantic (SPNA) and Labrador Sea are key regions for deep and intermediate water mass formation and contribute to the southward return flow of the lower limb of the Atlantic Meridional Overturning Circulation (AMOC).

The origin and circulation pathways of these water masses can now be studied using the artificial radionuclides 129I and 236U. These tracers are mainly released to the Nordic seas by the European nuclear reprocessing plants of La Hague and Sellafield since the 1960s. This point like source provides a unique fingerprint for Atlantic waters entering the Arctic Ocean and recirculation to the western SPNA.

Here we will present results of the distribution of 129I and 236U in the Labrador Sea (AR7W Line) and the SPNA (OVIDE Line). The 129I concentrations and its temporal evolution is studied at 11 stations on a time series that started in 2014. In addition, first results of 236U  will also be  presented along the AR7W line.

At the timeseries the 129I concentration shows a general increase with time and from east to west, reaching its highest concentration in the deep overflow waters and along the Eastern and Western Greenland current.

The combination of the well-known tracer 129I with 236U allows to study the origin and mixing of different water masses in the SPNA.

How to cite: Leist, L. G. T., Castrillejo, M., Smith, J. N., Christl, M., and Casacuberta, N.: Tracing Ocean circulation at the AR7W and OVIDE lines using artificial radionuclides, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11459, https://doi.org/10.5194/egusphere-egu23-11459, 2023.

EGU23-11576 | ECS | Orals | OS1.5

Assessing the variability of Irminger Water at AR7W between 1993 and 2022 using time-dependent property thresholds 

Kevin Niklas Wiegand, Dagmar Kieke, Paul G. Myers, and Igor Yashayaev

Irminger Water (IW) is a prominent water mass in the subpolar North Atlantic (SPNA). It is warm and saline and originates from the North Atlantic Current and the Irminger Current. The water mass delivers anomalously large amounts of heat and salt to the Labrador Sea. Like any other water mass, IW is subject to temporal and spatial variability, which needs to be adequately identified and tracked.

To separate IW from ambient waters, previous studies identified IW at different times using static thresholds of salinity, temperature, and density (i.e., constant over time within the individual studies). However, given the tremendous variability in the region, such static definitions often do not detect IW sufficiently since these definitions do not account for shifts in the large-scale hydrographic state of the SPNA. To address this issue, this study aims to identify non-static thresholds (i.e., incorporating temporal variability) to analyze IW variability. We refer to the method of identifying IW based on non-static thresholds as the phenomenological approach. To do so, we utilize the observation-based data set ARMOR3D between 1993 and 2022. This new approach allows us to compare estimates of IW properties and volume transports to respective estimates obtained from the static approach.

In the case of the static approach being applied to the AR7W section in the eastern part of the Labrador Sea as a test region, the water column was anomalously saline in years of high IW volume transport. Hence, the static approach identified more IW and thus overestimated its volume transport. In contrast, the water column was anomalously fresh in years when the static approach reveals a low IW volume transport. Hence, applying the static approach, less IW is identified, and thus its volume transport is underestimated. In contrast, the phenomenological approach reveals less pronounced decadal variability of the IW volume transport.

Applying a static IW definition will likely create stronger gradients between IW and ambient water masses when both are fresher. In turn, these gradients may impose or modulate unrealistic changes in the IW volume transport simply because the actual boundary of IW does not coincide with a certain isohaline or isotherm. Any correlated change or shift in IW properties and, for example, Labrador Sea Water will relocate the IW boundary causing the transport to change. The phenomenological approach introduced in our study resolves this issue.

How to cite: Wiegand, K. N., Kieke, D., Myers, P. G., and Yashayaev, I.: Assessing the variability of Irminger Water at AR7W between 1993 and 2022 using time-dependent property thresholds, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11576, https://doi.org/10.5194/egusphere-egu23-11576, 2023.

Over the observed period, North Atlantic Sea surface temperatures have gone through cycles of anomalous warming and cooling relative the global mean. This variability has become known as the Atlantic Multidecadal Variability (AMV), and it has been associated with important regional climate impacts. However, in recent years there has been considerable controversy over the origins of AMV. In particular, there is debate over whether AMV is a natural phenomenon (e.g., an expression of internal variability or natural external forcings), or whether it was caused by human activity through the impact of anthropogenic aerosol forcing.

Here, an analysis of CMIP6 multi-model historical simulations is presented which isolates the internal and externally forced AMV. The analysis shows that, although there is substantial externally forced AMV in the CMIP6 historical simulations, the forced variability is part of a wider hemispheric signal and is not specific to the North Atlantic like in observations. Therefore, the magnitude of the externally forced variability is highly dependent on the definition of the AMV index used. Ocean circulation changes consistently lead the internal AMV across models, but there is no-clear relationship for the external AMV. AMV is also associated with broader changes than just sea surface temperatures, but this multivariate fingerprint of AMV is significantly different between the internal and external components. For example, internal AMV is associated with salinity anomalies and increased turbulent heat loss across the subpolar North Atlantic that agree broadly with observations. However, in contrast, the externally forced AMV is associated with freshening and reduced heat loss across the subpolar North Atlantic and especially in models with the strongest aerosol forcing. Overall, the analysis suggests that internal variability remains a likely hypothesis to explain AMV, but questions remain on whether models adequately simulate the forced response.

How to cite: Robson, J.: Contrasting the internal and external components of Atlantic Multidecadal Variability in CMIP6 historical simulations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11588, https://doi.org/10.5194/egusphere-egu23-11588, 2023.

EGU23-12040 | ECS | Posters on site | OS1.5

Common mechanisms of centennial-scale AMOC variability in CMIP6 models 

Oliver Mehling, Katinka Bellomo, and Jost von Hardenberg

It has been hypothesized that climate variability on centennial timescales – in the North Atlantic region and beyond – is linked to unforced variability of the Atlantic Meridional Overturning Circulation (AMOC). Because of the presence of external forcings, uncertainties in proxy reconstructions of the AMOC and the short observational record, coupled climate models represent a key tool in assessing low-frequency AMOC variability. However, sufficiently long pre-industrial control (piControl) simulations with state-of-the-art climate models have only become widely available during the past decade. While significant centennial-scale AMOC variability has been identified in several single-model studies, proposed physical mechanisms differ considerably.

Here, we assess mechanisms of AMOC variability on centennial timescales in the CMIP6 multi-model piControl ensemble. We find that a relatively large number of models – 11 out of the 15 analyzed – exhibit a statistically significant mode of centennial-scale MOC variability in the Atlantic. We review previously proposed mechanisms for centennial-scale AMOC variability and test whether their key elements are present in the CMIP6 ensemble.

We find that salinity exchanges between the Arctic and North Atlantic basins, which have previously been proposed as drivers of multi-centennial AMOC variability in two CMIP6 models (IPSL-CM6A-LR and EC-Earth3), can also be identified in other CMIP6 models using the same ocean component (NEMO). However, we find only a weak or no signature of this mechanism in models that do not include NEMO. Even among NEMO models, the amplitude and timescale of centennial-scale AMOC variability is model-dependent, and we assess the relative role of deep-water formation sites in shaping these differences. Because AMOC fluctuations are linked to surface temperature anomalies and related impacts over land, our results motivate the need for more paleoclimate evidence at sub-centennial resolution, which would help constrain the CMIP6 inter-model spread in centennial-scale AMOC variability.

How to cite: Mehling, O., Bellomo, K., and von Hardenberg, J.: Common mechanisms of centennial-scale AMOC variability in CMIP6 models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12040, https://doi.org/10.5194/egusphere-egu23-12040, 2023.

EGU23-12097 | ECS | Posters on site | OS1.5

Treasure from trash: Using nuclear waste to trace ocean circulation around Iceland 

Duncan Dale, Marcus Christl, Andreas Macrander, Sólveig Ólafsdóttir, Rob Middag, and Núria Casacuberta

Iceland stands at an important gateway where Arctic and Atlantic waters interact. Atlantic waters pass northward and circulate in the Arctic before returning southward in the East Greenland Current (EGC). Zones of deep water formation in the Nordic Seas contribute to overflows of the Iceland-Scotland Ridge such as Denmark Strait Overflow Water (DSOW). These are key processes in Arctic warming and deep ocean ventilation.

This system has been tagged with anthropogenic radionuclides 129I and 236U by bomb tests in the 1950-60s and point-source nuclear reprocessing plants (NRPs) at Sellafield (UK) and La Hague (FR) since the 1960s providing an opportunity to trace the origins of water masses in the region and their transit timescales. Here we present the results of measurements on samples taken during two cruises around Iceland in 2021 by the Marine and Freshwater Research Institute (MFRI) of Iceland (winter) and the NIOZ MetalGate Cruise of the GEOTRACES program (summer). Models for the origin of waters transiting Denmark Strait and of the evolution of Iceland Scotland Overflow Water (ISOW) are presented that provide a tracer-based perspective for comparison with models based on physical oceanographic tools. This forms a baseline for tracking changes to circulation in the Subpolar North Atlantic using the transient nature of the tracer signals.

How to cite: Dale, D., Christl, M., Macrander, A., Ólafsdóttir, S., Middag, R., and Casacuberta, N.: Treasure from trash: Using nuclear waste to trace ocean circulation around Iceland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12097, https://doi.org/10.5194/egusphere-egu23-12097, 2023.

Given the major role of the Atlantic Ocean meridional heat transport in the climate system, it is essential to characterize its temporal variations at different locations. The 4DATLANTIC-OHC Project (https://eo4society.esa.int/projects/4datlantic-ohc/) aims at developing and testing space geodetic methods based on satellite altimetry and space gravimetry to estimate the local ocean heat content (OHC) changes over the Atlantic Ocean. Combined with independent estimates of the surface heat fluxes this approach holds promise to estimate the Atlantic Meridional Heat Transport (MHT) at any section across the Atlantic basin. 

The official version 1.0 of the 4DAtlantic-OHC product has been released, it provides estimates of local changes in OHC with their uncertainties. This product is accessible with DOI https://doi.org/10.24400/527896/A01-2022.012  and can be downloaded on AVISO portal. At two test sites, OHC changes derived from in situ data (RAPID and OVIDE) are used to evaluate the accuracy and reliability of the new space geodetic based OHC change estimate. Combined with ERA5 estimate of the surface heat fluxes, the Atlantic OHC product will be used to derive an energy budget of the North Atlantic basin and estimate the associated divergence in ocean heat transport. From the divergence field we will derive at the end of the project new estimates of the Atlantic meridional heat transport at different sections in the North Atlantic basin (RAPID and OSNAP sections) and compare it with in situ estimates. 

The V1.0 of 4DAtlantic-OHC products over the Atlantic Ocean, the evaluation results of the OHC against in situ data and preliminary results of MHT estimation will be presented.

How to cite: Fraudeau, R. and the 4DAtlantic-OHC Team: Monitoring local Ocean Heat Content changes with satellite altimetry and space gravimetry to assess the variability of the Meridional Heat Transport in the North Atlantic: the 4DATLANTIC-OHC Project, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12244, https://doi.org/10.5194/egusphere-egu23-12244, 2023.

It has been suggested previously that the long-term warming hole in the subpolar North Atlantic, that is the relative cooling in this region compared to the rest of the globe, is an indicator of a slowdown of the Atlantic Meridional Overturning Circulation (Caesar et al., 2018; Drijfhout et al., 2012; Rahmstorf et al., 2015), yet other drivers like aerosols or a change in the local atmospheric forcing (e.g., the wind stress curl) have been proposed (Li et al., 2021; Piecuch et al., 2017). The still not fully answered question of the driver(s) of the warming hole also raises the question of whether or not ocean temperatures in the subpolar North Atlantic can be used as an indicator for AMOC strength. While several studies suggest that AMOC strength and temperatures in the subpolar North Atlantic are dynamically linked through the AMOC’s northward heart transport (Dima et al., 2022; Latif et al., 2022; Zhang, 2008), a recent model-based study suggests that the correlation between temperature-based AMOC index (Caesar et al., 2018) and AMOC strength depends largely on the subtraction of the global warming signal (Little et al., 2020).

Based on the knowledge that the AMOC transports both heat into and freshwater out of the North Atlantic, we apply a lead-lag correlation analysis to both the North Atlantic’s heat and freshwater content to identify the region and the time lag that give the strongest correlation with the strength of the AMOC (to make use of the available observational data we consider the AMOC strength at 26˚N). We find that an AMOC weakening (strengthening) leads to cooling (warming) and simultaneous freshening (salinification) in the eastern subpolar North Atlantic with the upper ocean (200-1000m) contents showing a higher correlation with AMOC strength than the surface (0-200m) contents. The temporal evolution of heat and freshwater content in the eastern subpolar gyre region are furthermore strongly anticorrelated, with a correlation value of -0.82 (for the annual values) as expected for an AMOC (or otherwise advective) driven signal. On longer time scales this anticorrelation decreases unless the heat content is corrected for a large scale warming signal. This could suggest that it is indeed necessary to look at the relative not the absolute temperature evolution in the subpolar North Atlantic to extract the AMOC signal.

Both the absolute freshening in the eastern subpolar North Atlantic as well as the relative (compared to the rest of the North Atlantic) cooling in this region suggest a linear AMOC trend of about -2 Sv from 1957-2013.

References

Caesar, L., et al. (2018).  https://doi.org/10.1038/s41586-018-0006-5

Dima, M., et al. (2022).  https://doi.org/10.1007/s00382-022-06156-w

Drijfhout, S., et al.  (2012). https://doi.org/10.1175/jcli-d-12-00490.1

Latif, M., et al.  (2022). https://doi.org/10.1038/s41558-022-01342-4

Li, L., et al.  (2021). https://doi.org/10.1007/s00382-021-06003-4

Little, et al.  (2020).  https://doi.org/10.1029/2020gl090888

Piecuch, C. G., et al.  (2017).  https://doi.org/https://doi.org/10.1002/2017JC012845

Rahmstorf, S., et al.  (2015). https://doi.org/10.1038/nclimate2554

Zhang, R. (2008). https://doi.org/10.1029/2008GL035463

How to cite: Caesar, L. and McCarthy, G.: AMOC changed derived from simultaneous (absolute) freshening and (relative) cooling in the subpolar North Atlantic, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13393, https://doi.org/10.5194/egusphere-egu23-13393, 2023.

EGU23-14080 | ECS | Orals | OS1.5

Changes in air-sea fluxes over the North Atlantic during 1950-2019 as derived from ERA5 data 

Johannes Mayer, Leopold Haimberger, and Michael Mayer

Air-sea heat fluxes play a key role for many processes in the North Atlantic Ocean, such as the lateral transport of energy or the formation of storm tracks. Thus, an accurate estimation of air-sea heat flux trends is pivotal and helps to understand implications of climate change. To do so, reanalysis products are attractive candidates due to their excellent spatial coverage over multiple decades. However, trend estimations based on reanalysis data are challenging as changes in the observing system can introduce temporal discontinuities.

In this study, we explore the reliability and temporal stability of net air-sea heat flux trends from ERA5 forecasts in the North Atlantic basin over the period 1950-2019.  The assessment is complemented with an indirect estimate of the net surface flux derived from the atmospheric energy budget. Causes of trends in latent and sensible heat fluxes are identified based on monthly analyzed state quantities from ERA5, such as wind speed, moisture, and temperature. Additionally, the impact of the North Atlantic Oscillation (NAO) and Atlantic Multi-decadal Oscillation (AMO) as well as analysis increments, as introduced by the ERA5 data assimilation, is investigated.  

Our results show a robust increase of latent heat fluxes in the tropical North Atlantic over the past seven decades, which is likely caused by the intensification of the Hadley cell favouring subsidence and advection of drier air masses. In the Norwegian Sea, positive net air-sea heat flux trends (increased ocean heat uptake) are largely dominated by changes in sensible heat fluxes, which are driven by a trend towards more southerly winds and the advection of warmer air. In the Gulf Stream region, the AMO likely drives the multi-decadal variability of net air-sea heat fluxes, while long-term trends over the 1950-2019 period remain insignificantly small. Furthermore, we find significant changes over the North Atlantic Warming Hole and western North Atlantic associated with more frequent positive NAO phases during the past 30 years. From our analysis, we conclude that analysis increments most likely influence the magnitude of these trends, especially at low latitudes where the impact can be as large as ~2 W m-2 dec-1, while the basin-wide trend pattern remains unaffected. The net effect of the found regional changes in fluxes is assessed by the spatial average trend over the whole North Atlantic north of 26°N, which yields a positive but statistically insignificant trend of 0.5 W m-2 dec-1 over the past 70 years. Potential implications for trends in the AMOC are discussed. 

How to cite: Mayer, J., Haimberger, L., and Mayer, M.: Changes in air-sea fluxes over the North Atlantic during 1950-2019 as derived from ERA5 data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14080, https://doi.org/10.5194/egusphere-egu23-14080, 2023.

EGU23-14285 | ECS | Orals | OS1.5

On the inaccuracy of CMIP6 models in capturing the observed long-term variability of the NAO 

Amar Halifa-Marín, Miguel A. Torres-Vázquez, Enrique Pravia-Sarabia, Ricardo Trigo, Sergio M. Vicente-Serrano, Marco Turco, Sonia Jerez, Pedro Jiménez-Guerrero, and Juan Pedro Montavez

This study assesses how the CMIP6 simulations capture the non-stationarity of the main source of winter climate variability in the Euro-Atlantic region, the North Atlantic Oscillation (NAO), observed in the recent past.

For that purpose, we characterise the NAO long-term variability in climate reanalysis, analysing their features in several 30-year periods since 1851; and we evaluate whether CMIP6 historical simulations capture all the observed NAO “types”. Although the literature sometimes assumes that the NAO pattern is stationary, three groups of NAO pattern have been proved in the reanalyses depend on the location of their Action Centres (ACs): 1) the north AC locates over Iceland and the south AC in Azores, 2) the north AC locates over Southern Greenland and the south AC in the Western Mediterranean, and 3) the north AC locates over Northern Scandinavia and the south AC in the Azores.

Our main finding is that the NAO long-term variability is not accurately captured by all CMIP6 models. In particular, the overestimation of the NAO group 3 is remarkable in most simulations. This NAO group mainly represents the last decades, which the literature has addressed with much interest for its exceptional features (e.g. NAO+ strengthening and northeastward shift of its north AC), and which has been generally associated with the anthropogenic warmer climate. We also found underestimation of NAO group 2.

We have also found that each NAO group could be associated with precipitation anomalies in Europe. For example, the NAO group 3 implies drier(wet) conditions in the south(north). While group 2 implies the opposite pattern of anomalies. Therefore, we have reason to suggest that the lack of accuracy of models reproducing the non-stationarity of NAO may explain some of the bias in the expected changes of winter precipitation in Europe for future scenarios.

How to cite: Halifa-Marín, A., Torres-Vázquez, M. A., Pravia-Sarabia, E., Trigo, R., Vicente-Serrano, S. M., Turco, M., Jerez, S., Jiménez-Guerrero, P., and Montavez, J. P.: On the inaccuracy of CMIP6 models in capturing the observed long-term variability of the NAO, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14285, https://doi.org/10.5194/egusphere-egu23-14285, 2023.

EGU23-14337 | ECS | Orals | OS1.5

Impact of interannual chaotic variability on the total interannual variability of the North Atlantic Eighteen Degree Water 

Olivier Narinc, Thierry Penduff, Guillaume Maze, Stéphanie Leroux, and Jean-Marc Molines

Using ensemble ocean simulations, recent studies have shown that non-linear intrinsic oceanic processes are a source of chaotic intrinsic oceanic variability (CIOV). It was found that in eddy-active regions and at interannual timescales, this CIOV can be a significant fraction of total variability, and that as model resolution increases small-scale non-linearities can generate variability at large scales. The Eighteen Degree Water (EDW) is a mode water formed in the winter mixed layer within and south of the Gulf Stream. It is the most abundant T,S class of water in the surface North Atlantic and has been shown to be an important contributor to air-sea exchanges over the entire North Atlantic basin. Observational studies have shown that a significant part of the interannual variability of EDW cannot be explained by atmospheric variability. This motivates the present investigation of the importance of interannual CIOV in the total interannual EDW variability. The present study uses a NEMO-core, 1/4°, 50-member ensemble hindcast of the North Atlantic ocean with a realistic atmospheric forcing. This ensemble simulation is assessed using ARMOR3D, a 3-dimensional gridded observational product obtained using satellite altimetry and ARGO floats. In both datasets, the 3-dimensional structure of EDW is identified using physical criteria. This spatial structure is used to compute timeseries of the EDW’s total volume and average temperature, in each ensemble member and in the observational product. It is found that the ensemble simulation produces a realistic EDW, with a comparable total variability. In the ensemble simulation, the CIOV of integrated EDW properties is estimated from their time-averaged ensemble standard deviation, and is compared to the total variability estimated from the ensemble mean of the temporal standard deviations of all members. In the ensemble, CIOV accounts for 13% of the total interannual variability of EDW volume, and 44% of the total interannual variability of EDW temperature. Notably, this means that CIOV is a source of unquantifiable uncertainty in single-member ocean simulations. This suggests that a significant part of observed interannual variability may also be chaotic intrinsic in nature. This calls for a better parametrisation of chaotic variability in ocean simulations.

How to cite: Narinc, O., Penduff, T., Maze, G., Leroux, S., and Molines, J.-M.: Impact of interannual chaotic variability on the total interannual variability of the North Atlantic Eighteen Degree Water, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14337, https://doi.org/10.5194/egusphere-egu23-14337, 2023.

EGU23-14637 | ECS | Orals | OS1.5

Eight years of continuous Rockall Trough transport observations from moorings and gliders 

Kristin Burmeister, Neil Fraser, Lewis Drysdale, Sam Jones, Stuart Cunningham, Mark Inall, and Alan Fox

The Rockall Trough (RT) is a key pathway for warm and salty water flowing northward, a process which plays a key role in dictating the western European climate. The picture of the mean circulation and variability in the RT is still emerging, as the record of continuous transport observations has only recently been extended to eight years. Here, for the first time, we present the temporally extended record of RT volume, heat and freshwater transports. An important feature of the RT circulation is the European Slope Current (ESC) which is poorly constrained by ship-based, mooring, and satellite observations. To tackle this, we gathered around 150 glider transects over 2.5 years which capture the ESC velocity field in unprecedented detail. The data are sufficient to characterise both the mean state and the emergent seasonal variability of the ESC, and reveal the year-round presence of a southward countercurrent at depth. Variability in the strength and structure of this previously unstudied feature modulates net northward transport in the eastern boundary current system.

We also utilise these observations for monitoring the basin-wide overturning circulation as part of the newly developed OSNAP_I transect. We will present the first results from that programme.

How to cite: Burmeister, K., Fraser, N., Drysdale, L., Jones, S., Cunningham, S., Inall, M., and Fox, A.: Eight years of continuous Rockall Trough transport observations from moorings and gliders, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14637, https://doi.org/10.5194/egusphere-egu23-14637, 2023.

We explore the amplitude and frequency of Atlantic Multi-decadal Variability (AMV) in a 2,000-year pre-industrial control simulation with the FOCI-OpenIFS coupled climate model. We find a statistically significant AMV-like mode on the 20-year and 80-year time scales. We also find a mode of multi-centennial variability where the North Atlantic Ocean shifts a regime of a warm period to/from a cold period of ~400 years. The warm period is characterised by mean states of a stronger and deeper Atlantic Meridional Overturning Circulation (AMOC), less Arctic sea ice, and more deep convection in the Labrador Sea than the cold period. 

 

We find that the AMV has a much higher amplitude in the cold period compared to the warm period, and also that the lead-lag relationship between the AMOC and the AMV is different between the two periods. In the warm period, AMOC leads the AMV; a strong AMOC enhances the oceanic poleward heat transport which warms the North Atlantic Ocean both at the surface and deeper down, producing a positive AMV. In the cold period, however, AMV leads AMOC; a warm surface anomaly reduces the sea ice in the Labrador Sea which enhances local air-sea interactions and deep convection, and later a stronger AMOC. In the cold period, the warm anomaly associated with the AMV does not extend below the mixed layer, suggesting that it is driven by the atmosphere and not ocean dynamics.

How to cite: Kjellsson, J. and Park, W.: Multi-centennial modulation of Atlantic multi-decadal variability in a 2000-year climate integration, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14759, https://doi.org/10.5194/egusphere-egu23-14759, 2023.

EGU23-15366 | ECS | Posters on site | OS1.5

Optimizing simulated oxygen variability, circulation, and export in the subpolar North Atlantic Ocean using BGC-Argo & ship-based observations 

Lauren Moseley, Galen McKinley, Dustin Carroll, Raphael Dussin, Dimitris Menemenlis, and An Nguyen

The subpolar North Atlantic (SPNA) transports surface-ocean properties deep into the interior via deep convection and is one of the most intense regions of air-sea gas exchange globally. Deep convection in the SPNA exports highly-oxygenated water masses to depth, which subsequently ventilate intermediate and deep waters throughout the North Atlantic. The SPNA thus plays a critical role in setting the oxygen inventory of the global ocean. Due to intensifying ocean warming, many climate models predict substantial global-ocean oxygen loss — albeit at magnitudes which vary widely by model. Therefore, there is a need to better understand the impacts of SPNA convective variability on oxygen saturation in intermediate and deep water masses. Here we use a physical-biogeochemical model, ASTE-BGC, which couples the Arctic Subpolar gyre sTate Estimate (ASTE) with the Biogeochemistry with Light, Iron, Nutrients, and Gas (BLING) model to quantify oxygen cycling and deep ventilation in the SPNA. ASTE utilizes the MIT General Circulation Model (MITgcm) and assimilates physical in-situ and satellite data using tools developed by the Estimating the Circulation and Climate of the Ocean (ECCO) consortium. We use a Green’s Functions approach to optimize ASTE-BGC biogeochemistry using BGC-Argo and GLODAPv2 ship-based profiles of O2 and NO3. The Green’s Functions approach allows us to adjust the biogeochemical parameters of the BLING ecosystem towards O(106) in-situ data constraints over the 2002–2017 model period. We then evaluate the optimized simulation against independent data and construct an oxygen budget for the central Labrador Sea to assess the interannual variability of SPNA oxygen.

How to cite: Moseley, L., McKinley, G., Carroll, D., Dussin, R., Menemenlis, D., and Nguyen, A.: Optimizing simulated oxygen variability, circulation, and export in the subpolar North Atlantic Ocean using BGC-Argo & ship-based observations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15366, https://doi.org/10.5194/egusphere-egu23-15366, 2023.

EGU23-15631 | ECS | Orals | OS1.5

Continuity Constraints on the Atlantic Meridional Overturning Circulation 

Neil Fraser, Alan Fox, and Stuart Cunningham

In the subtropics, the Atlantic meridional overturning circulation (MOC) has the same strength and variability whether measured in depth- or density-space. Two different continuity budgets must therefore be satisfied north of the subtropics, one via diapycnal volume transport and the other via downward volume transport. However, as water can get denser without getting deeper (and vice versa), it is unclear why the integrated effect of these processes, the MOC, should have the same strength and variability in both depth- and density-space, provided one integrates these terms sufficiently far south (e.g. to 26 °N). Previous work has investigated the surface buoyancy forcing and mixing processes which drive diapycnal volume transport. Here, we use a suite of observational products and new analyses in a vorticity framework to study the magnitude and distribution of the various terms responsible for vertical volume transport, and gain further insight by also evaluating these terms using VIKING20X model output. We conclude that bottom Ekman transport and advection curl around the boundaries of the subpolar gyre, particularly around Greenland, are dominant drivers of downward vertical transport and hence crucial for closing MOC streamlines in depth-space, with much of the variability also projecting onto the MOC in density-space. As these processes are “spun-up” by the sub-polar gyre yet project onto the overturning, our results offer new insights into the coupling between the overturning and gyre circulations.

How to cite: Fraser, N., Fox, A., and Cunningham, S.: Continuity Constraints on the Atlantic Meridional Overturning Circulation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15631, https://doi.org/10.5194/egusphere-egu23-15631, 2023.

EGU23-15913 | ECS | Orals | OS1.5

The role of surface forcing in driving pathways and time scales of ocean ventilation in the subpolar North Atlantic 

Alice Marzocchi, George Nurser, Louis Clement, and McDonagh Elaine

The ocean takes up 93 % of the excess heat in the climate system and approximately a quarter of the anthropogenic carbon via air–sea fluxes. Ocean ventilation and subduction are key processes that regulate the transport of water from the surface mixed layer to the ocean's interior, which is isolated from the atmosphere for a timescale set by the large-scale circulation. Using numerical simulations (NEMO framework), we assess where the ocean subducts water and takes up properties from the atmosphere, and how ocean currents transport and redistribute these properties. This is achieved by adding a set of simulated seawater vintage dyes (passive tracers) that are released annually from different ocean surface “patches”, representing water masses’ source regions. The dyes’ distribution captures years of strong and weak convection at deep and mode water formation sites in both hemispheres, showing good agreement with observations in the subpolar North Atlantic. We show that interannual variability in subduction rates, driven by changes in surface forcing, is key in setting the different sizes of the long-term inventory of the dyes. The Northern and Southern Hemispheres are characterised by different ventilation pathways and timescales, but our analysis highlights a strong correlation between the strength of ventilation in recently subducted waters and the longer-term dye inventory in each hemisphere. This means that the conditions close to the time of dye injection are driving the amount of seawater being subducted, but also that this signal persists over time and the longer-term tracer inventory is strongly related to the initial surface conditions. The correlation still holds for the different source regions, where it is even stronger, but the slope of the correlation does vary. Export and isolation of subducted waters is shown to be faster in the Northern Hemisphere, defining a stronger ventilation “persistence” – represented by the slope of the correlation between subduction and the longer-term inventory. The highest ventilation persistence is found in the subpolar North Atlantic and specifically in the Labrador and Irminger Seas, which are the dominant regions in retaining tracer on multi-decadal time scales.

How to cite: Marzocchi, A., Nurser, G., Clement, L., and Elaine, M.: The role of surface forcing in driving pathways and time scales of ocean ventilation in the subpolar North Atlantic, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15913, https://doi.org/10.5194/egusphere-egu23-15913, 2023.

EGU23-17084 | Orals | OS1.5

The role of propagating signals in the gyre-scale interannual to decadal sea level variability in the subpolar North Atlantic 

Denis Volkov, Claudia Schmid, Leah Chomiak, Cyril Germineaud, Shenfu Dong, and Marlos Goes

The gyre-scale, dynamic sea surface height (SSH) variability signifies the spatial redistribution of heat and freshwater in the ocean, influencing the ocean circulation, weather, climate, sea level, and ecosystems. It is known that the first empirical orthogonal function (EOF) mode of the interannual SSH variability in the North Atlantic exhibits a tripole gyre pattern, with the subtropical gyre varying out of phase with both the subpolar gyre and the tropics, influenced by the low-frequency North Atlantic Oscillation. We show that the first EOF mode explains the majority (60 %–90 %) of the interannual SSH variance in the Labrador and Irminger Sea, whereas the second EOF mode is more influential in the northeastern part of the subpolar North Atlantic (SPNA), explaining up to 60 %–80% of the regional interannual SSH variability. We find that the two leading modes do not represent physically independent phenomena. On the contrary, they evolve as a quadrature pair associated with a propagation of SSH anomalies from the eastern to the western SPNA. This is confirmed by the complex EOF analysis, which can detect propagating (as opposed to stationary) signals. The analysis shows that it takes about 2 years for sea level signals to propagate from the Iceland Basin to the Labrador Sea, and it takes 7–10 years for the entire cycle of the North Atlantic SSH tripole to complete. We demonstrate that the observed interannual-to-decadal variability of SSH, including the westward propagation of SSH anomalies, is the result of a complex interplay between the local wind and surface buoyancy forcing, and the advection of properties by mean ocean currents. The relative contribution of each forcing term to the variability is space and time dependent. We show that the most recent cooling and freshening observed in the SPNA since about 2010 were mostly driven by advection associated with the North Atlantic Current. The results of this study indicate that signal propagation is an important component of the North Atlantic SSH tripole, as it applies to the SPNA.

 

How to cite: Volkov, D., Schmid, C., Chomiak, L., Germineaud, C., Dong, S., and Goes, M.: The role of propagating signals in the gyre-scale interannual to decadal sea level variability in the subpolar North Atlantic, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17084, https://doi.org/10.5194/egusphere-egu23-17084, 2023.

EGU23-311 | ECS | Orals | OS1.6

On the importance of the atmospheric coupling to the small-scale ocean in the modulation of latent heat flux 

Pablo Fernández, Sabrina Speich, Matteo Borgnino, Agostino Meroni, Fabien Desbiolles, and Claudia Pasquero

In this study, we address the role of the ocean fine scales in north-west tropical Atlantic Ocean air-sea interactions. With this purpose, we use satellite observations of the ocean and the atmosphere, the ERA5 atmospheric reanalysis and a set of regional numerical simulations of the lower atmosphere. In particular, we focus on the coupling between the sea-surface temperature (SST) and the marine atmospheric boundary layer (MABL). We also evaluate the latent heat flux (LHF) sensitivity to SST. The results suggest that the SST-MABL coupling depends on the spatial scale of interest. At scales larger than the ocean mesoscale (larger than 150 km), negative correlations are observed between near-surface wind speed (U10m) and SST and positive correlations between near-surface specific humidity (q2m) and SST. However, when smaller scales (1 – 150km, i.e., encompassing the ocean mesoscale and a portion of the submesoscale) are considered, the U10m-SST and q2m-SST correlate inversely. This is interpreted in terms of an active ocean modifying the near-surface atmospheric state, driving convection, mixing and entrainment of air from the free troposphere into the MABL.

The estimated values of the ocean-atmosphere coupling at the ocean small-scale are then used to develop a linear and SST-based downscaling method aiming to include and further investigate the impact of these fine-scale SST features into an available low-resolution latent heat flux (LHF) data set. The results show that they induce a significant increase of LHF (30% - 40% per °C of SST). We identify two mechanisms causing such a large increase of LHF: (1) the thermodynamic contribution that only includes the increase in LHF with larger SSTs associated with the Clausius-Clapeyron dependence of saturating water vapor pressure on SST and (2) the dynamical contribution related to the change in vertical stratification of the MABL as a consequence of SST anomalies. Using different downscaling setups, we conclude that largest contribution comes from the dynamic mode (28% against 5% for the thermodynamic mode). To validate our approach and results, we have implemented a set of high-resolution WRF numerical simulations forced by high-resolution satellite SST that we have analyzed in terms of LHF using the same algorithm.

To provide further validation to our results we use the high spatio-temporal resolution of in-situ data collected during the EUREC4A-OA/ATOMIC campaigns that go beyond the coarse spatial grid of available satellite observations and include additional variables to the SST such as the impact of ocean currents and the local vertical stratification of the upper ocean.

How to cite: Fernández, P., Speich, S., Borgnino, M., Meroni, A., Desbiolles, F., and Pasquero, C.: On the importance of the atmospheric coupling to the small-scale ocean in the modulation of latent heat flux, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-311, https://doi.org/10.5194/egusphere-egu23-311, 2023.

EGU23-532 | ECS | Posters on site | OS1.6

Tropical Atlantic variability during the Last Millennium 

Laura Sobral Verona, Ilana Wainer, and Myriam Khodri

Climate variability in the Tropical Atlantic is complex with strong ocean-atmosphere coupling, where the sea surface temperature (SST) variability impacts the hydroclimate of the surrounding continents. One of the main modes of SST variability in this region is known as the Atlantic Niño. Its dynamics are dominated by the Bjerknes Feedback, much like the Pacific El Niño. It is characterized by the coupling between SST in the eastern Equatorial Atlantic, zonal wind anomalies, changes in the thermocline depth, and consequent upwelling anomalies. The development of SST anomalies in the Equatorial Atlantic can be explained in terms of an oscillator model of recharge and discharge of heat content. This model is represented by the Bjerknes Feedback Index, which is a set of components representing the mechanisms that enhance (i.e., Thermocline, Zonal Advective, and Ekman feedbacks) or limit (Thermal and Dynamical damping) the growth of the SST anomalies. The pre-industrial millennium is vastly studied with respect to the responses to natural forcing, given the similarity of the climate background with present-day conditions. In addition, this period is known for the occurrence of large volcanic eruptions that were able to change the ocean-atmosphere interaction. Here, we propose to investigate the interannual variability of the Tropical Atlantic during the Last Millennium (LM, 850 to 1849 CE) in terms of the Bjerknes Feedback Index. For that, we rely on results from the Last Millennium period from the Paleoclimate Modeling Intercomparison Project (PMIP4) contribution to Climate Model Intercomparison Project phase 6 (CMIP6).

How to cite: Sobral Verona, L., Wainer, I., and Khodri, M.: Tropical Atlantic variability during the Last Millennium, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-532, https://doi.org/10.5194/egusphere-egu23-532, 2023.

EGU23-535 | ECS | Posters on site | OS1.6

The impact of AMOC weakening on the global monsoon in EC-Earth3 water hosing simulations 

Roberta DAgostino, Katinka Bellomo, and Virna Meccia

Changes in Atlantic Meridional Overturning Circulation (AMOC) affect tropical precipitation through the coupling with the Hadley Circulation and cross-equatorial atmospheric heat transport. Climate model simulations project a possible weakening of the AMOC under global warming. Here, we run model experiments with EC-Earth3 where we artificially weaken the AMOC through the release of a freshwater anomaly at high latitudes. The simulated AMOC collapse of ~57% for 60 model years allows us to investigate atmospheric heat and circulation readjustment to AMOC weakening and impacts on tropical precipitation, including the global monsoon. We find that the Inter Tropical Convergence Zone (ITCZ) shifts equatorward and tropical precipitation decreases over its northern flank while it increases southward due to reduced northward oceanic heat transport. Global monsoon is also impacted by AMOC weakening: Northern/Southern Hemisphere monsoons are weaker/stronger than the control experiment, with different sensitivities according to different regions: monsoons systems in the Atlantic sector are strongly impacted by AMOC decline. We further explore interbasin anomalies in the zonal/meridional atmospheric heat transport and net energy input triggered by the AMOC decline by examining local Hadley and Walker circulation asymmetries. Given that a ~57% reduction in the AMOC strength is within the inter-model range of future projections by the end of the 21st century, our results have important implications for understanding the role of AMOC in future tropical precipitation response. 

How to cite: DAgostino, R., Bellomo, K., and Meccia, V.: The impact of AMOC weakening on the global monsoon in EC-Earth3 water hosing simulations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-535, https://doi.org/10.5194/egusphere-egu23-535, 2023.

EGU23-854 | ECS | Orals | OS1.6

Variability and predictability of surface chlorophyll in the Atlantic upwelling systems 

Elena Calvo Miguélez, Belén Rodríguez-Fonseca, and Iñigo Gómara

Chlorophyll-a surface concentration is partially determined by environmental conditions and its variability, as the highest concentrations are generally found in wind-driven oceanic upwelling regions. These wind regimes that affect upwelling strength can be determined by local and remote drivers, such as sea surface temperature (SST) anomaly patterns (e.g., Pacific and Atlantic Niños/Niñas) that trigger tropical basin interactions.

By performing a Maximum Covariance Analysis (MCA) between chlorophyll-a concentration from Copernicus Satellite data and SST anomalies from OISST (January 1998-December 2019), we here identify the individual SST patterns and the associated atmospheric responses that lead to an increase in chlorophyll concentration in two regions of the tropical Atlantic: the Senegalese coast and the equator during their seasonal maxima (February to May and June to September, respectively). The present study shows how an Atlantic El Niño is capable of promoting a Pacific La Niña, whose atmospheric response affects either the tropical north Atlantic and the equatorial Atlantic, producing an SST cooling in early spring in the former and in summer in the latter, both related to an increase of chlorophyll concentration.

A cross-validated hindcast based on Maximum Covariance Analysis (MCA) is used to assess chlorophyll predictability through these individual SST variability modes.

Key words: chlorophyll-a concentration, SSTs, atmospheric responses, statistical prediction.

How to cite: Calvo Miguélez, E., Rodríguez-Fonseca, B., and Gómara, I.: Variability and predictability of surface chlorophyll in the Atlantic upwelling systems, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-854, https://doi.org/10.5194/egusphere-egu23-854, 2023.

The South American continental slope hosts a variety of topographic waves. We use a 27-year-long global ocean reanalysis (1/12° Spatial resolution) to examine trapped waves (TWs) around South America at periods ranging from 40 to 130 days. The waves propagate from the Equatorial Pacific to the Tropical Atlantic (22°S) with phase velocities between 1.8 and 7 m/s according to the local background characteristics, such as stratification, slope steepness, latitude, mean flow and shelf width. The Madden-Julian Oscillation (MJO) plays a key role in forcing the TWs in two ways (a) through an oceanic connection implying equatorial Kelvin waves reaching the western American Coast and (b) through an atmospheric teleconnection enhancing southerly winds in the south-east Pacific. Furthermore, local winds, not necessarily linked with the MJO, modulate and trigger waves in specific locations, such as the Brazil-Malvinas Confluence. Trapped waves impact the along-shore currents: during the positive phase of the waves the near-surface flow is enhanced by about 0.1 m/s.

How to cite: Poli, L., Artana, C., and Provost, C.: Topographically Trapped Waves Around South America: Oceanic Teleconnections between Equatorial Pacific and Tropical Atlantic, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3178, https://doi.org/10.5194/egusphere-egu23-3178, 2023.

EGU23-3389 | ECS | Posters on site | OS1.6

The Influence of Freshwater Input on the Evolution of the 1995 Benguela Niño 

Leo Costa Aroucha, Joke Lübbecke, Mareike Körner, and Rodrigue Imbol-Koungue

Benguela Niños are events of anomalous Sea Surface Temperature (SST) increase in the Southeastern Tropical Atlantic Ocean. In 1995, the strongest Benguela Niño observed in the satellite era took place. It had a drastic impact on the Angola-Benguela Area (ABA, 8ºS – 20ºS, 8ºE to the coast) ecosystem, including high mortality, poor recruitment, and southward shift of sardine populations, as well as reductions in the number of benthic organisms. Although low Sea Surface Salinity (SSS) values extending as far south as 18ºS have been observed during this event, the role of freshwater input for the SST increase in the 1995 Benguela Niño has not been analyzed yet. In this study, we use satellite data, CTD profiles, and reanalysis products to investigate the impact that freshwater anomalies from anomalously high Congo river discharge (CRD) and precipitation might have had on the evolution of the 1995 Benguela Niño. We find that in the onset phase of the event a freshwater plume from the north was spreading southward towards the Angola-Namibia coastal area, concomitant with signatures of positive Barrier Layer Thickness (BLT) and stratification (N2) anomalies. At the same time, a strong poleward Angola current anomaly was observed. Positive SST anomalies peaked in March when SSS values averaged over the ABA were almost 3 psu lower than normal. Our analysis suggests that the anomalous CRD combined with higher than usual precipitation in November/December 1994 generated a negative SSS plume north of ABA, which was advected into the Angola-Namibia coastal region by the poleward surface current anomaly, increasing ocean stability, and reducing the mixing. A Mixed Layer Heat Budget analysis suggests that both anomalous advection and absence of entrainment contributed to the surface warming while the net surface heat flux provided a damping effect. Thus, the high freshwater input that was advected southwards inhibited the entrainment of cool subsurface waters into the surface mixed layer in the ABA, which contributed to the SST increase in the exceptionally strong 1995 Benguela Niño event.

How to cite: Costa Aroucha, L., Lübbecke, J., Körner, M., and Imbol-Koungue, R.: The Influence of Freshwater Input on the Evolution of the 1995 Benguela Niño, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3389, https://doi.org/10.5194/egusphere-egu23-3389, 2023.

The Atlantic Meridional Mode (AMM) and Atlantic Zonal Mode (AZM) dominate the boreal spring and summer Tropical Atlantic variability (TAV), respectively, at interannual time scales, with pronounced impacts on the climate of adjacent and remote areas. Previous studies demonstrated the existence of an AMM-AZM connection via ocean wave propagation and modulated by the local wind forcing.

Here, we use a novel approach based on Extended Maximum Covariance Analysis (EMCA) to investigate the emergence of evolving spring-to-summer TAV modes in the observational record and its multidecadal modulation. Observational and reanalysis datasets reveal that the first evolving mode corresponds to a basin-wide warming with maximum anomalies over the tropical north Atlantic during boreal spring and equatorial warm conditions in summer season. In contrast, the second evolving mode displays an inter-hemispheric SST gradient during boreal spring that persists until summer months. The first and second evolving modes can be associated with a same-sign and opposite-sign relation between the AMM and AZM, respectively.

The expansion coefficients of the evolving modes are positively and negatively correlated at decadal time scales during the observational record, suggesting the emergence of diverse spatial configurations. This multidecadal modulation coincides with different global ocean background states that resemble the Atlantic Multidecadal Variability (AMV) and Pacific Decadal Variability (PDV).

To corroborate the above-mentioned observational findings, these results will be compared with those from historical and picontrol simulations from the latest state-of-the-art CMIP6 models.

How to cite: Martín-Rey, M. and Pelegri, J. L.: A general view of boreal spring to summer interannual variability: Emergence of evolving tropical Atlantic modes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5391, https://doi.org/10.5194/egusphere-egu23-5391, 2023.

EGU23-6033 | ECS | Orals | OS1.6

The importance of internal climate variability in the multi-decadal trend of the wind-driven upwelling on the west African coasts 

Mohammad Hadi Bordbar, Volker Mohrholz, and Martin Schmidt

Like other Eastern Boundary Upwelling Systems, the upwelling near the southwest African coasts is primarily alongshore-wind-driven, whereas it is controlled mainly by the wind stress curl farther offshore. The surface wind regime across the Benguela Upwelling System (BUS) is strongly related to the South Atlantic Anticyclone (SAA), which is believed to migrate poleward in response to anthropogenic global warming. Here, we investigate multi-decadal changes of the SAA and its impacts on the coastal Ekman transport as a primary driver of coastal upwelling and the wind-stress-curl-driven upwelling across the BUS by using the ERA-5 data sets. Our findings indicate that the SAA plays a significant role in the regional wind-driven upwelling with different impacts on the coastal Ekman transport and the offshore wind-stress-curl-driven upwelling. Further, the upwelling in the equatorward sector is significantly affected by the anticyclone intensity. In contrast, the poleward portion is also influenced by the meridional position of the anticyclone. The multi-decadal trend in the sea level pressure across the South Atlantic renders a considerable heterogeneity in space. However, the trend is broadly associated with a small signal-to-noise ratio, which can be attributed to internal climate variability. This view is further supported by the multi-decadal trend in coastal offshore transport and the wind-stress-curl-driven upwelling in multiple upwelling cells, which hardly depict any significant systematic changes.

How to cite: Bordbar, M. H., Mohrholz, V., and Schmidt, M.: The importance of internal climate variability in the multi-decadal trend of the wind-driven upwelling on the west African coasts, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6033, https://doi.org/10.5194/egusphere-egu23-6033, 2023.

EGU23-7587 | Orals | OS1.6

Ocean circulation underlies the Atlantic meridional mode 

Hyacinth Nnamchi, Riccardo Farneti, Noel Keenlyside, Fred Kucharski, Mojib Latif, Annika Reintges, and Thomas Martin

The Atlantic meridional mode (AMM) is characterized by north-south bands of alternate anomalies in surface-ocean temperatures, and winds from colder bands to the warmer, at a periodicity of 10-15 years. The AMM has been linked to variations in Atlantic hurricanes, global surface-air temperature, and climate variability over the Sahel, South American, North American, and European. Despite these far-reaching impacts, the role of ocean circulation remains uncertain, and the prevailing AMM theories are based on thermodynamic air-sea interactions. Here we we uncover ocean-circulation variability that is linked to the AMM using twentieth century observations. Specifically, sea level-derived index of ocean circulation variabilityleads the AMM pattern by several years, through the interactions of overturning and gyre circulations with Kelvin wave anomalies that propagate from the North Atlantic to the low latitudes and by the thermocline feedback in the Atlantic cold tongue region. The peak of the sea surface temperature variability in the tropical Atlantic in turn drives inter-hemispheric atmospheric teleconnections represented by negative NAO phase over the North Atlantic. These findings imply that, rather than a passive role postulated by the prevailing thermodynamic paradigm, ocean circulation plays an active role in AMM variability.

How to cite: Nnamchi, H., Farneti, R., Keenlyside, N., Kucharski, F., Latif, M., Reintges, A., and Martin, T.: Ocean circulation underlies the Atlantic meridional mode, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7587, https://doi.org/10.5194/egusphere-egu23-7587, 2023.

EGU23-7838 | Posters on site | OS1.6

Atmospheric response to Tropical Instability Waves in high-resolution coupled NextGEMS simulations 

Johann Jungclaus, Dian Putrasahan, Swantje Bastin, and Mia-Sophie Specht

Oceanic Tropical Instability Waves (TIW) are characterized by westward propagating cusp-shaped sea surface temperature (SST) patterns with sharp fronts and strong lateral SST gradients. TIWs impact local winds and ocean atmosphere heat fluxes and these changes consequently feed back onto the ocean.
Previous studies have used stand-alone atmosphere or regional coupled ocean-atmosphere models at moderate to high resolution.  The new global simulations, which are run at km-scale resolution in both ocean and atmosphere in the framework of the H2020 NextGEMS project, offer new opportunities to study local, regional, and remote effects of TIW-related ocean-atmosphere interactions.
Using the coupled ICON-a/ICON-o “Sapphire” simulations (Hohenegger et al., 2023), we compare the ocean-atmosphere coupling in the Pacific and Atlantic basin and investigate the interaction of TIWs with the Intertropical Convergence Zone. 
In the western tropical Atlantic, north of the Equator, TIW induced SST patterns also interact with North Brazil Current eddies and we investigate the effects of pronounced fronts on ocean-atmosphere heat fluxes.


Hohenegger, C. at al., 2023: ICON-Sapphire: simulating the components of the Earth System and their interactions at kilometer and subkilometer scales, Geoscientific Model Development, https://doi.org/10.5194/gmd-2022-171.

How to cite: Jungclaus, J., Putrasahan, D., Bastin, S., and Specht, M.-S.: Atmospheric response to Tropical Instability Waves in high-resolution coupled NextGEMS simulations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7838, https://doi.org/10.5194/egusphere-egu23-7838, 2023.

EGU23-8344 | Posters on site | OS1.6

Dakar Niño variability under global warming investigated by a high-resolution regional coupled model 

Shunya Koseki, Ruben Vazquez, William Cabos, Claudia Guitérrez, and Dmitry Sein

We have investigated an interannual variability of sea surface temperature (SST) along the northwestern African coast, so-called, Dakar Niño, and its change under global warming of highest emission scenario RCP8.5 employing a high-resolution regional coupled model. Our reginal coupled model is capable of reproducing the seasonal cycle of the SST along the northwestern African coast and its interannual variability with respect to amplitude, timing, and position of the maximized variability between 9°N-14°N from March to April. Comparing the Dakar Niño variability between the periods of 1980-2010 and 2069-2099, we found that its variability intensifies under warmer climate without changing its location and timing of maximization. The intensification is more pronounced during the Dakar Niñas (cold SST event) than during Niños (warm SST event) and the variability in ocean temperature is connected more deeply with the Dakar Niño variability (vertical motion is more strongly correlated). The stronger Dakar Niño variability and deeper connection with subsurface variability can be explained by the larger meridional wind stress variability along the northwestern African coast, which can be amplified by more enhanced land-sea thermal contrast anomaly, in the future. In addition, the ocean temperature is warmed more effectively above 40m depth where the temperature anomaly is more dominant, that is, the stratification is reinforced around 40m depth. This enhanced stratification can also cause the reinforcement of Dakar Niño/Niña variability.     

How to cite: Koseki, S., Vazquez, R., Cabos, W., Guitérrez, C., and Sein, D.: Dakar Niño variability under global warming investigated by a high-resolution regional coupled model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8344, https://doi.org/10.5194/egusphere-egu23-8344, 2023.

EGU23-8689 | Posters on site | OS1.6

The southeast tropical Atlantic: improvements and persistent biases in CMIP models 

Riccardo Farneti, Alessandro Stiz, and John B. Ssebandeke

State-of-the-art climate models simulate warmer than observed sea surface temperatures (SST) in eastern boundary upwelling systems (EBUS), generating biases with profound implications for the simulation of present-day climate and its future projections. Amongst all EBUS, the bias is largest in the southeastern tropical Atlantic (SETA). Here, we provide a comprehensive evaluation of the performance in the SETA of the Coupled Model Intercomparison Project phase 6 (CMIP6), including fine resolution (HighResMIP) and ocean-forced (OMIP) models. We show that biases in the SETA remain large in CMIP6 models but are reduced in HighResMIP, with OMIP models giving the best performance. The analysis suggests that, once local forcing errors have been reduced, the major source of the SETA biases lies in the equatorial Atlantic. This study shows that finer model resolution has helped reduce the local origin of the SETA SST bias but further developments of model physics schemes will be required to make progress. 

How to cite: Farneti, R., Stiz, A., and Ssebandeke, J. B.: The southeast tropical Atlantic: improvements and persistent biases in CMIP models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8689, https://doi.org/10.5194/egusphere-egu23-8689, 2023.

EGU23-9943 | Orals | OS1.6

Tropical Atlantic forcing of increased ENSO variability since the 1970’s 

Noel Keenlyside, Hui Ding, Marta Martín del Rey, Irene Polo, Belen Rodriguez-Fonseca, Fred Kucharski, and Ping-Gin Chiu

The El Niño Southern Oscillation (ENSO) underwent a major shift in in the 1970’s, becoming stronger and more predictable. This shift has been attributed to changes in the tropical Pacific mean state. However, around the 1970’s, tropical Atlantic – Pacific variability became coupled, with Atlantic SST leading opposite signed changes in the Pacific by around 6-months. Here we assess the role of the Atlantic in driving the ENSO regime shift using pacemaker experiments with two climate models: ECHAM5/MPIOM and SPEEDY/RGO. In these experiments, model SST is restored to observations in the tropical Atlantic, while elsewhere the models are fully coupled. Both models capture the observed changes in inter-basin interactions and strengthening on ENSO variability after the 1970’s. The warming of the equatorial and south Atlantic and southward shift of the inter-tropical convergence zone causes inter-basin interactions to become active after the 1970’s in the models. In ECHAM5/MPIOM, this leads to Atlantic Niño variability driving increased ENSO activity. In SPEEDY/RGO, the increase in ENSO activity appears more related to induced mean state changes in the Pacific. In addition, experiments with two different versions of the Norwegian Earth System Model and two nudging approaches (anomaly and full field SST) have been performed as part of the CLIVAR RF Tropical Basin Interactions. Initial analysis reveals are rather muted impact of the tropical Atlantic on ENSO. Further analysis is being performed and results will also be presented.

How to cite: Keenlyside, N., Ding, H., Martín del Rey, M., Polo, I., Rodriguez-Fonseca, B., Kucharski, F., and Chiu, P.-G.: Tropical Atlantic forcing of increased ENSO variability since the 1970’s, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9943, https://doi.org/10.5194/egusphere-egu23-9943, 2023.

EGU23-10576 | ECS | Orals | OS1.6

Modulation of Tropical Instability Waves and chlorophyll concentration by equatorial waves during the 2021 Atlantic Niño 

Franz Philip Tuchen, Renellys C. Perez, Gregory R. Foltz, and Peter Brandt

During the boreal summer of 2021, the central and eastern equatorial Atlantic experienced persistent sea surface temperature (SST) anomalies of more than +1°C for several months. These episodic extreme events are referred to as Atlantic Niños, with 2021 marking the strongest event since 1984. Atlantic Niños are known to have far-reaching impacts on, for instance, rainfall over the surrounding continents, and on ocean dynamics through changes in thermohaline gradients and circulation. A pronounced event like the 2021 Atlantic Niño in combination with the steadily expanding coverage by satellite and in-situ observations provides the rare opportunity to study an Atlantic Niño event in unprecedented detail. Here we focus on the influence of the 2021 Atlantic Niño on tropical instability wave (TIW) activity and surface chlorophyll concentration.

The 2021 Atlantic Niño was initiated by a strong downwelling Kelvin wave excited by westerly wind bursts in the western and central equatorial Atlantic. The eastward propagating Kelvin wave induced strong eastward flow anomalies on the equator causing a reduction of the meridional shear of the near-surface zonal flow in the central equatorial Atlantic. At the same time, the Kelvin wave-induced deepening of the thermocline weakened the seasonal development of the equatorial Atlantic cold tongue. The reduction of both the meridional SST gradient and the meridional shear of zonal velocity largely suppressed barotropic and baroclinic instability, which is required for the generation of TIWs. Consistent with these changes, we find that 2021 was one of the least active years in terms of TIW-related temperature, salinity, sea level, and current variability. The overall reduction in TIW activity is characterized by weak TIW activity before and enhanced TIW activity after the climatological TIW peak resulting in a delay of the TIW season. This delayed onset of TIW activity is expected to have considerable consequences for the local heat and freshwater budgets. Low TIW activity and positive SST anomalies also impacted the concentration and distribution of surface chlorophyll as observed by daily gap-free satellite observations. After the initial Kelvin wave, surface chlorophyll concentration dropped to extraordinarily low values and was anti-correlated with the evolution of SST anomalies. The absence of TIWs is also apparent in weaker than normal surface chlorophyll concentration variability on intraseasonal time scales, highlighting the interplay of TIWs and chlorophyll. Our results demonstrate how the 2021 Atlantic Niño impacted oceanic variability, but further analysis is needed to better understand the consequences of such events for regional heat and freshwater budgets as well as for nutrients, productivity, and marine ecosystems.

How to cite: Tuchen, F. P., Perez, R. C., Foltz, G. R., and Brandt, P.: Modulation of Tropical Instability Waves and chlorophyll concentration by equatorial waves during the 2021 Atlantic Niño, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10576, https://doi.org/10.5194/egusphere-egu23-10576, 2023.

EGU23-12326 | ECS | Posters on site | OS1.6

What are the role of mesoscale eddies in air-sea interaction and in sea surface salinity variability in the Tropical Atlantic Ocean? 

Habib Micaël Aguedjou, Alexis Chaigneau, Isabelle Dadou, Yves Morel, Casimir Da-Allada, and Ezinvi Baloẗcha

The ocean kinetic energy is dominated by mesoscale eddies, which are quasi-circular structures with typical horizontal scales of 10 to 100 km and vertical extension of hundreds meters. the mesoscale eddies play significant roles in the transport and redistribution of water masses with their physical and biogeochemical properties throughout ocean. In this study, we used 8 years of satellite altimetry data, combined with sea surface temperature (SST), sea surface salinity (SSS), latent and sensible heat fluxes (LHF and SHF) and precipitation data, to investigate how mesoscale eddies impact on air-sea heat and fresh water exchange in the tropical Atlantic ocean (TAO). We show that an important fraction of eddies exhibit inverse SST anomalies, and that eddy-induced LHF and SHF anomalies are quasi-linearly proportional to SST anomalies. Moreover, eddies contribute to ~10 – 25 % of the total heat flux variability. However, no direct link has been observed between heat flux and precipitation anomalies over eddies in the TAO. Nevertheless, beneath the Intertropical Convergence Zone (ITCZ), significant correlation were found, suggesting that eddies may modulate both heat and freshwater fluxes in this region. Relative to SSS anomalies within eddies, their variability represents up to 30% of the total variability. In addition, beneath the ITCZ, freshwater fluxes would play an important role in their variability. However, oceanic processes such as horizontal and vertical advection and mixing are suspected to play a key role in the SSS variability at mesoscale beneath the ITCZ. To better understand the role of such processes, numerical modeling studies are needed for future investigations.

How to cite: Aguedjou, H. M., Chaigneau, A., Dadou, I., Morel, Y., Da-Allada, C., and Baloẗcha, E.: What are the role of mesoscale eddies in air-sea interaction and in sea surface salinity variability in the Tropical Atlantic Ocean?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12326, https://doi.org/10.5194/egusphere-egu23-12326, 2023.

EGU23-12692 | ECS | Orals | OS1.6

Decadal variability and predictability of Senegalo-mauritanian upwelling system 

Adama Sylla and juliette Mignot

Coastal countries in West Africa heavily rely on the ocean, which is a major source of food and employment. This is mainly due to the presence of coastal upwelling, upward motion of sea water bringing the nutrient-rich deeper waters into the illuminated surface layers in the coastal zone, where they become available for photosynthesis. The resulting phytoplankton production, the base of the food chain, render coastal upwelling the most productive of large marine ecosystems in the world’s oceans. Recently, decadal variability and predictability of coastal upwelling systems has received a lot of attention, since near-term changes of upwelling systems could have a strong impact of living marine resources and hence surrounding countries economy. On this aspect, recent progress has been made in generating near-term (“decadal”) predictions of physical using Earth system models (ESMs). Initialized forecasts have shown significant predictability from 1 to 10 years in advance for climate events showing substantial decadal variability.

Our objective here is two-fold: first we investigate the decadal variability of the Senegalo-mauritanian upwelling system (SMUS) in the reanalysis and historical simulations from eleven climate models using indices based on the SST and wind stress and also identify the processes controlling this variability. Second, we exploit the decadal prediction experiments of CMIP6 (DCPP-A), to investigate this upwelling predictability. Our results show that the SMUS is characterized by a strong decadal variability, in part linked to the Atlantic Multidecadal Variability Consequently, the DCPP- A experiment shows strong and generally significant correlation prediction scores at various lead times for the dynamical indices (Ekman transport and Ekman pumping). However, even though coastal SST are also significantly predictable, non-significant ACC scores are found for the thermal upwelling indices. The analysis concludes on trying to qualify and quantify the predictable components of the SMUS and possible applications.



How to cite: Sylla, A. and Mignot, J.: Decadal variability and predictability of Senegalo-mauritanian upwelling system, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12692, https://doi.org/10.5194/egusphere-egu23-12692, 2023.

EGU23-14070 | ECS | Orals | OS1.6

Sensitivity of tropical Atlantic subsurface currents to different model parameter choices in ICON-O 

Swantje Bastin, Dian Putrasahan, and Johann Jungclaus

The tropical oceans are home to some of the strongest subsurface current systems of the world. Among these are the Equatorial Undercurrent (EUC) which flows eastward below the thermocline, and the Equatorial Intermediate Current System (EICS) consisting of latitudinally alternating zonal jets between 15S and 15N. Ocean and climate models consistently struggle in correctly representing these subsurface currents, but especially the EUC is quite important for tropical Atlantic climate, e.g. the evolution of the Atlantic cold tongue and the associated Atlantic Niño. We use the ocean component of the ICON model to test how the representation of the Atlantic subsurface current systems reacts to different model parameter choices. In general, the EUC is too weak in our ICON configuration. We can show that the EUC is stronger, i.e. better represented with the TKE vertical mixing scheme than with the KPP scheme. Using the TKE scheme, different parameters are tested and it can be shown that the EUC reacts sensitively to the value of the important tuning parameter c_k, being stronger when c_k is smaller. We also test the sensitivity of the EUC strength in the model to the formulation of the Prandtl number in the TKE mixing scheme, which also includes a changeable constant. Apart from changes in the vertical mixing scheme, we also look at the effect of the vertical resolution of the near-surface ocean. We compare a vertical level thickness of 2m in the upper 20m to 10m in the upper 20m, with the same level distribution below 20m depth for both configurations. We can show that for the thinner surface levels, the EUC is much weaker than for the thicker levels. Intriguingly, also the EICS react to the change in near-surface vertical resolution despite being located at much larger depths. The EICS is, like the EUC, generally too weak in ICON, but becomes stronger when the near-surface levels are thinner.

How to cite: Bastin, S., Putrasahan, D., and Jungclaus, J.: Sensitivity of tropical Atlantic subsurface currents to different model parameter choices in ICON-O, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14070, https://doi.org/10.5194/egusphere-egu23-14070, 2023.

EGU23-16159 | Posters virtual | OS1.6

Multidecadal Modulations of ENSO influence on Tropical Atlantic cyclogenesis 

Teresa Losada, Adama Badiane, Belén Rodríguez-Fonseca, Juan Jesús González-Alemán, Abdou Lahat Dieng, and Saidou Moustapha Sall

The impact of ENSO (El Niño Southern Oscillation) events on the development of tropical cyclones in the Eastern Tropical North Atlantic is highlighted, focusing on decadal variations of the interannual relationship at the Senegalese coast, which is the main cyclone development region (MDR). The enhancement of North Atlantic tropical cyclones by the Atlantic Niño and the Pacific El Niño Southern Oscillation (ENSO) is diagnosed. An approach based on  composites of anomalous positive or negative years in terms of cyclone activity is used. Based on 20yr-correlations between the number of cyclones that are born in the MDR and ENSO index, we have selected two different periods of study (period1 (P1): 1954-1973; and period2 (P2): 1986-2005). Results show an increase in the SST impact in cyclone generation from P1 to P2 and intensification of cyclones number over the Senegalese coasts. Likewise, the spatial distribution of the dynamic and thermodynamic parameters used in this composite study shows strong variations between the two periods. Our findings suggest that decadal changes in climatological conditions have a significant effect on the MDR. Additionally, the changes in the interannual signal appear to be related to the concomitant action of interannual SST anomalies over the whole tropical basins.  

How to cite: Losada, T., Badiane, A., Rodríguez-Fonseca, B., González-Alemán, J. J., Dieng, A. L., and Sall, S. M.: Multidecadal Modulations of ENSO influence on Tropical Atlantic cyclogenesis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16159, https://doi.org/10.5194/egusphere-egu23-16159, 2023.

EGU23-16606 | Orals | OS1.6

Influence of atmospheric dust on the equatorial Atlantic Variability 

Ignasi Vallès-Casanova, Ori Adam, and Marta Martín-Rey

Northern tropical and subtropical North Atlantic Ocean is exposed to large plumes of atmospheric dust mostly from Saharan dust outbreaks. The scattering and absorbing radiation in the dust air layer modifies the air column temperature. These temperature changes strongly impact the atmospheric wind forcing and deep atmospheric convection resulting in changes in the latitudinal position of the Intertropical Convergence Zone and the distribution of sea surface temperature anomalies (SSTA). In this study, we analyze the interaction between the interannual variability of Aerosol Optical Depth (AOD) in the North Tropical Atlantic (NTA) and SSTA in the equatorial Atlantic in the period 1996-2020. Observational results show that AOD-induced SSTA in the north tropical Atlantic may impact the equatorial Atlantic variability by different mechanisms such as an intensification of cross-equatorial winds and the excitation of oceanic waves. In particular, the AOD in NTA in boreal winter/spring seems to impact on the onset, intensity and spatial configuration of the Atlantic Zonal Mode, also known as Atlantic Niño. Our findings suggest that atmospheric dust can be a potential precursor for equatorial Atlantic Variability. 

How to cite: Vallès-Casanova, I., Adam, O., and Martín-Rey, M.: Influence of atmospheric dust on the equatorial Atlantic Variability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16606, https://doi.org/10.5194/egusphere-egu23-16606, 2023.

EGU23-17113 | ECS | Posters on site | OS1.6

Impact of the location of tropical convection on climate variability 

Lucía Montoya-Carramolino, Teresa Losada, and Marta Martín-Rey

The Equatorial Mode, characterized by the anomalous warming of the sea surface temperature (SST) in the eastern equatorial Atlantic region, leads the interannual variability of the tropical Atlantic during the boreal summer causing impacts both in tropical and extratropical regions. However, this pattern and its teleconnections, are not stationary; and the origin of such changes continues to be subject of enquiry and debate.

With this premise, in the present work, we evaluate the possible influence of a displacement of the ITCZ (Intertropical Convergence Zone), mediated by a radiative perturbation, on the pattern of the Equatorial Mode and its connection with the Pacific. More particularly, we examine two perturbation experiments that reduce the incident shortwave radiation in two latitude bands: NEXT in the northern extratropics and STRO in the southern tropics. The analysis is carried out from a multi-model perspective, using the data of 8 CMIP5 coupled climate models from the Extratropical-Tropical Interaction Model Intercomparison Project (ETIN-MIP).

Our results suggest that the strengthening of the ITCZ over the Atlantic equatorial band, is capable of generating changes in the mean state of the equatorial Atlantic, as well as conditions of enhanced variability on the interannual scale. In addition, it is found that the westward shift of the SST warm anomalies in the Equatorial Mode and the existence of a more variable mean state in the equatorial Atlantic and Pacific, are key in the intensification of the Atlantic-Pacific connection.

How to cite: Montoya-Carramolino, L., Losada, T., and Martín-Rey, M.: Impact of the location of tropical convection on climate variability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17113, https://doi.org/10.5194/egusphere-egu23-17113, 2023.

In the tropical Atlantic Ocean, extreme climate events with anomalous sea surface temperature, current, and precipitations are often referred to as the Atlantic Niño. It has many similarities with the EI Niño including the analogous mechanism that winds above the western equatorial ocean will excite oceanic waves to amply the temperature anomaly in the east. However, the Atlantic Niño presents more diversity in its intensity and occurrence time, especially in recent years, eg. 2019 and 2021, in which the classic theory becomes insufficient to explain. This study focuses on ocean responses to atmospheric forcing, manipulating the wind forcing in both equatorial and off-equatorial regions to excite linear ocean models for three types of events that occurred in 1999, 2019, and 2021 respectively. This study has found those extraordinary Atlantic Niños may owe to the wind in the off-equatorial region, where the winds can also excite oceanic waves that transfer energy to the western boundary and reflect back to the equatorial Atlantic. The interaction between the energy from the equatorial and the off-equatorial region makes the event less predictable. The participation of off-equatorial wave energy leads to the diversity of the Atlantic Niños. Hence, for the Atlantic Niño forecast,  more concerns about ocean dynamics to cover a wider latitude range should be required.

How to cite: Song, Q., Tang, Y., and Aiki, H.: Participation of off-equatorial wave energy for the Atlantic Niño events identified by wave energy flux in case studies, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17184, https://doi.org/10.5194/egusphere-egu23-17184, 2023.

The Arabian Sea, a productive oceanic region in the North Indian Ocean, is under the direct influence of monsoon winds that impact the surface ocean processes. High biological productivity occurs due to natural nutrient enrichment events via coastal and open ocean upwelling (summer monsoon) and convective mixing (winter monsoon). Ample studies from this basin addressed the diatom community from the surface ocean, yet the key contributing diatom frustules to sedimentary phytodetritus has been overlooked. These microscopic biosilcifiers play an important role in the biological carbon pump by exporting significant organic carbon from the surface waters to the deep sea due to their ballasted silica shell (frustule). Hence, this is imperative to document the diatom genera that are transported efficiently to the sediment. The present study analyzed diatom frustule abundance (valves g-1) and identified the major diatom genera in core top sediments (0.5cm) of 10 locations from the Central (21, 19, 15, 13, and 11 °N along 64 °E) and Eastern Arabian Sea (21, 17, 15, 13, and 11 °N at 200 m isobath).  This is the first of this kind and found a comparable frustule distribution from the surface sediments of both Central (av. 5.16±1.23×104 valves g-1) and Eastern Arabian Sea (av. 5.80±7.14×104 valves g-1). Size-based classification revealed that the contributions of medium-sized (30-60 µm) frustules from both the central (49 %) and eastern (51%) Arabian Sea were quite high. And the contribution of large-sized frustules (>60 µm) was higher in the central Arabian Sea (39%) compared to the eastern part (19%). A total of 40 diatom genera with 18 in common from both locations were detected from the sedimentary phytodetritus with Coscinodiscus and Thalassiosira being the dominating ones. In the north-central (21, 19, 15 °N) Arabian Sea, the prevalence of large-sized diatoms (Coscinodiscus) was attributed to open ocean upwelling as well as convective mixing during summer and winter monsoons, respectively. Such large species can easily escape grazing and sink rapidly due to higher ballasting. Further, the presence of the oxygen minimum zone at the intermediate depth in this region might reduce the remineralization and grazing pressure within the mesopelagic during their transport to the abyss. Whereas relatively smaller diatoms (Thalassiosira, Pseudo-nitzschia, Fragilaria, Nitzschia) were in high abundance towards the south-central (13, 11 °N) that area remains nutrient-poor. In the Eastern Arabian Sea, Thalassiosira was noticed in high abundance towards the southeast (15, 13, 11 °N), whereas the northeast (17, 21 °N) was dominated by Coscinodiscus and mostly due to the prevalence of coastal upwelling and convective mixing, respectively. Likely, these diatoms (Coscinodiscus, Thalassiosira, Pseudo-nitzschia, Fragilaria, Nitzschia) play a key role in transferring the organic matter from the surface to sediments and thus actively contribute to carbon capture, elemental cycling, and supplying food source for the benthic biota. This study highlights for the first time the biogeochemical significance of these diatoms from this highly productive oceanic province.

How to cite: Pandey, M. and Biswas, H.: An account of the key diatom frustules from the surface sediments of the Central and Eastern Arabian Sea and their biogeochemical significance., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-131, https://doi.org/10.5194/egusphere-egu23-131, 2023.

EGU23-264 | ECS | Orals | OS1.7

Seasonality and distribution of Persian Gulf Water and its impact on ventilation: a high resolution view from gliders 

Estel Font, Bastien Y. Queste, Sebastiaan Swart, and Gerd Bruss

The decline in ocean oxygen content is one of the most alarming consequences of anthropogenic-driven climate change. A key challenge is that global climate models do not currently reproduce observed changes in deoxygenation, showing high inter-model variability and uncertainty. This uncertainty is partially due to the models’ inability to resolve features smaller than their computational grid cells resulting in large biases in ventilation. The Persian Gulf Water outflow has been pointed out by several studies as one of the sources of ventilation in the Arabian Sea Oxygen Minimum Zone (OMZ). This oxygenated water mass flows eastward along the shelf edge of the northern Omani coast at 200m depth and is fragmented by the mesoscale eddy field and rough topography, generating small “peddies”. These peddies and their relatively high oxygen concentrations have potential to ventilate the OMZ, yet this has been poorly investigated due to a lack of adequate observations. We use multi-month glider campaigns off the coast of Oman with a SeaExplorer glider equipped with an ADCP to resolve the contribution of the Persian Gulf Water outflow to oxygen supply within the Arabian Sea OMZ. We characterize its properties, seasonality, and spatial distribution and estimate mixing rates from double diffusion, salt-fingering, and shear-driven mixing to understand water mass transformations and oxygen fluxes into the OMZ.

How to cite: Font, E., Y. Queste, B., Swart, S., and Bruss, G.: Seasonality and distribution of Persian Gulf Water and its impact on ventilation: a high resolution view from gliders, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-264, https://doi.org/10.5194/egusphere-egu23-264, 2023.

The biogeochemistry of the Arabian Sea, the northwestern part of the Indian Ocean, is directly impacted by monsoonal wind reversal and is an area of strong ocean-atmospheric interaction. During the summer monsoon, coastal as well as open ocean upwelling occurs in the western, southeastern, and central parts of the Arabian Sea. The highest primary productivity rates are documented in these areas compared to the global oceans. Phytoplankton-derived particulate organic matter (POM) [Particulate organic carbon (POC) and nitrogen (PN)] play a central role in supporting the food chain as well as carbon export flux to the deep sea. Hence understanding the dynamics of POM concentrations and its stable carbon (δ13CPOC) and nitrogen (δ15NPN) isotopic ratios are important in delineating its sources and recycling. However, such studies are scarce from the Indian Ocean region. Here we present the first study describing the POM dynamics during the summer monsoon from the central Arabian Sea, addressing the interannual variability. We studied the monsoonal changes in POM and its isotopic signatures in the central Arabian Sea (21–11°N; 64°E) during August 2017 and 2018. A strong, low-lying atmospheric jet (Findlater Jet) blows across the basin during the southwest (SW) monsoon. Positive wind stress curl resulted in “open ocean upwelling” to the north of the jet’s axis, characterized by substantially shallower Mixed Layers Depths (MLDs) and higher POM contents relative to the jet’s axis and its south. The highest wind speeds were observed in the center of the transect due to the presence of the jet’s axis. And the negative curl to the south of the jet’s axis resulted in downwelling and, consequently, the deepest MLDs. The molar ratio between POC and PN (6.2 ± 1.9 in 2017; 6.4 ± 0.9 in 2018) was close to the canonical Redfield ratio (6.63). The δ13CPOC values (−26.3 ± 1.4‰ in 2017; 25.5 ± 1.4‰ in 2018) exhibited typical marine signature and a noticeable inter-annual difference. Relatively higher δ15NPN values in the north (7.68 ± 2.6‰ in 2017; 9.24 ± 3‰ in 2018) indicated the uptake of regenerated dissolved inorganic nitrogen from the oxygen minimum zone (OMZ). The lower δ15NPN values along the jet’s axis and to its south were attributed to the eastward advection of upwelled waters from the western Arabian Sea. Higher wind speeds and jet-induced wind stress curl in 2018 resulted in lower sea surface temperatures (SST) and higher nutrient concentrations. Despite the higher nutrient availability in 2018, POC contents did not exceed the values in 2017. However, considering the total nitrogen consumption (according to C:N: P = 106:16:1), the potential POC development in 2018 could be double the value in 2017. The interannual differences in SW monsoon onset and wind speeds seemed to directly control the nutrient supply, affecting plankton community structure and POM variability. Thus, any future changes in the physical forcing may directly influence the POC pool and consequent export flux to the mesopelagic.

How to cite: Silori, S., Biswas, H., and Cardinal, D.: Interannual variability in particulate organic matter associated with physical forcing in the central Arabian Sea assessed from (stable) carbon and nitrogen isotopes., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-358, https://doi.org/10.5194/egusphere-egu23-358, 2023.

EGU23-1671 | Orals | OS1.7 | Highlight

New perspective on the overturning dynamics in the Indian Ocean 

Lei Han

The Indian Ocean Meridional Overturning Circulation (IMOC) is well known for its remarkable seasonal variation, which was attributed to Ekman flow plus its barotropic compensation (Lee and Marotzke, 1998). However, by tracking the isopycnal displacement, I defined a  sloshing MOC streamfunction, which was found highly resembling the Eulerian MOC streamfunction (see the attached figure). It was thus concluded that the IMOC is predominantly a sloshing mode, associated with the isopycnal displacement. Recognizing that these isopycnal signals were dominated by the first-baroclinic long Rossby waves, I found the IMOC strength was determined by the zonally-integrated Ekman pumping anomaly. As a result, the deep inflow into the Indian Ocean also had seasonal variation that could be attributed to this sloshing mode of overturning circulation. This could be partly verified by the cross-basin transect survey across 32oS that were fully occupied three times in history. The diffusivity dichotomy problem can be also explained by this new perspective. The importance of the Indian Ocean overturning in the global conveyor-belt was therefore challenged. This result has been published in Han (2021, JPO).

How to cite: Han, L.: New perspective on the overturning dynamics in the Indian Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1671, https://doi.org/10.5194/egusphere-egu23-1671, 2023.

EGU23-2164 * | Posters virtual | OS1.7 | Highlight

Mid-Holocene Monsoon Weakening: A major cause for societal change in the Indian subcontinent 

Hema Achyuthan and Nagasundaram Mohan

A sediment core retrieved from a water depth of 250 m near the Andamans Forearc Basin (AFB), the Landfall Island, North Andaman, reflects a record of sediment provenance and monsoonal shift since the mid to late Holocene. The core represents radiocarbon ages ranging from 6,078 to 1,658 yrs BP (from~ 6,500 yrs BP to the present). The core is dominantly clayey silt with incursions of coarser components that occur around 6,000, 5,400, and 3,400 yrs BP. Grain size variation indicates a cyclic variation of wetter and drier conditions matching changes in the intensity of the Indian Summer Monsoon (ISM), which was at its greatest intensity around 6,400, 5,300, and 3,300-3,000 yrs BP. Geochemical parameters including higher CaCO3 content, εNd, and 18O in Globigerinoides ruber are consistent with the long-term trend from cooler, wetter conditions to warmer, drier conditions at present. Chemical weathering intensity, which lags behind climate changes on land, shows a pulse of highly weathered sediment deposited at about 4,000 BP. Clay minerals represented by smectite, illite, kaolinite, and chlorite in varying amounts indicate high kaolinite content and K/C ratio specify intense Southwest Monsoon (SWM) and stronger bedrock weathering in the hinterland (~6,500–5,400 years BP). Incidence of smectite (48.82 to 25.09 %) and chlorite/illite (C/I) ratio (0.56 to 0.28) indicate an overall weakened southwest monsoon since 6,000 to 2,000 years BP with a brief incursion of extremely reduced SWM around 4,400 to 4,200 years BP. This is corroborated by the oxygen isotope on G. ruber that indicates a significant shift in the isotopic values ~4,300 years BP (−3.39‰), indicating a weak SWM. Fluctuations in the intensity of SWM are also observed for 2,000 years to the present. Sandy sediment was supplied from the Andaman Islands, Irrawaddy, and the Salween sea. Since the Mid Holocene period, longer periods of aridification and shorter periods of wetter conditions increased in the region after approximately 4,300 yrs BP. A correlation of monsoonal events using the Godhavari marine sediment core (Ponton et al.,2012)  and our data is noted that Bronze Age Harappan urbanism flourished since 4,500 yrs BP along the river banks in the western region of the present semi-arid Desert and the Deccan owing to intensified rain-fed agriculture. Since approximately 3,900 yrs ago, the total settled area and many settlement sizes declined, abandoned, and a significant shift in site numbers and density towards the southeast and west is recorded. During the Iron Age, after ca. 3,200 yrs BP, adaptation to semi-arid conditions in western Rajasthan, central and south India appears to have been well established with a significant number of sites in areas receiving <500 mm of rainfall. Weak monsoon precipitation led to conditions adverse to both inundation and rain-based farming and encouraged pastoralism. Monsoonal-fed rivers were active during the short-wet periods and gradually dried or became seasonal, affecting habitability along their courses. 

How to cite: Achyuthan, H. and Mohan, N.: Mid-Holocene Monsoon Weakening: A major cause for societal change in the Indian subcontinent, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2164, https://doi.org/10.5194/egusphere-egu23-2164, 2023.

EGU23-2165 | ECS | Posters on site | OS1.7

Main drivers of Indian Ocean dipole asymmetry revealed by a simple IOD model 

Hyo-Jin Park, Soon-Il An, Soong-Ki Kim, Wenju Cai, Agus Santoso, Daehyun Kim, and Jong-Seong Kug

Indian Ocean Dipole phenomenon (IOD) refers to a dominant zonal contrast pattern of sea surface temperature anomaly (SSTA) over tropical Indian Ocean (TIO) on interannual time scales. Its positive phase, characterized by anomalously warm western TIO and anomalously cold southeastern TIO, is usually stronger than its negative phase, namely a positively skewed IOD. Here, we investigate causes for the IOD asymmetry using a prototype IOD model, of which physical processes include both linear and nonlinear feedback processes, El Nino’s asymmetric impact, and a state-dependent noise. Parameters for the model were empirically obtained using various reanalysis SST data sets. The results reveal that the leading cause of IOD asymmetry without accounting seasonality is a local nonlinear process, and secondly the state-dependent noise, the direct effect by the positively skewed ENSO and its nonlinear teleconnection; the latter two have almost equal contribution. However, the contributions by each process are season dependent. For boreal summer, both local nonlinear feedback process and the state-dependent noise are major drivers of IOD asymmetry with negligible contribution from ENSO. The ENSO impacts become important in boreal fall, along with the other two processes.

 

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-2018R1A5A1024958)

How to cite: Park, H.-J., An, S.-I., Kim, S.-K., Cai, W., Santoso, A., Kim, D., and Kug, J.-S.: Main drivers of Indian Ocean dipole asymmetry revealed by a simple IOD model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2165, https://doi.org/10.5194/egusphere-egu23-2165, 2023.

The middepth zonal velocity resembles a system of eastward/westward jets with a considerably smaller width than the larger-scale ocean surface circulation. Such a phenomenon always occurs in a turbulent ocean that presents eddy or eddy–mean flow interactions. In this study, the upper-ocean absolute geostrophic currents in the southern Indian Ocean are constructed using Argo temperature and salinity data from the middepth (1000 m) zonal velocity derived from the Argo float trajectory. The results reveal alternating quasi-zonal striation-like structures of middepth zonal velocity in the equatorial and southern tropical Indian Ocean, with a meridional scale of 300 km. The triad of baroclinic Rossby wave instability plays a significant role in near-equatorial striations. In the south, the  unstable vertical structure leads to strong baroclinic instability, which increases the eddy kinetic energy in the middepth layer, thus contributing to a turbulent PV gradient. The convergence/divergence of the eddy PV flux generates the quasi-zonal striations. The meridional scale of the striations is controlled by the most unstable wavelength of baroclinic instability, which explains the observations.

How to cite: Xia, Y. and Du, Y.: Middepth Zonal Velocity in the Southern Tropical Indian Ocean: Striation-Like Structures and Their Dynamics, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2472, https://doi.org/10.5194/egusphere-egu23-2472, 2023.

This study analyzed the downwelling Rossby waves in the south Indian Ocean induced spring asymmetric mode and the relationship with the Indian Ocean Dipole (IOD) event based on observations and reanalysis data sets. The westward downwelling Rossby waves favor significant sea surface temperature (SST) warming in the Seychelles thermocline dome that triggers atmosphere response and the asymmetric mode in spring. The zonal sea level pressure gradient causes anomalous easterly winds in the central and eastern equatorial IO, cooling the SST off Sumatra-Java. Meanwhile, the remainder of the downwelling Rossby waves reach the west coast, transform to northward coastal-trapped waves, and then reflect as eastward downwelling Kelvin waves along the equator. The downwelling Kelvin waves reach the Sumatra-Java coast during late spring to early summer, favoring SST warming in the southeastern tropical Indian Ocean. Thus, there are two types of ocean-atmosphere response almost at the same time along the equator. The final SST status depends on which process is stronger, and as a consequence, triggers a negative or a positive phase of the IOD event in the fall season. The results show four positive and three negative IOD events related to the above processes from 1960 to 2019. The strong downwelling Rossby waves are easier to induce intense asymmetric mode and negative IOD event, usually associated with preceding strong El Niño in the Pacific. In contrast, the weak downwelling Rossby waves tend to induce weak asymmetric mode and positive IOD event, usually associated with preceding weak El Niño or anomalous anti-cyclonic atmospheric circulation in the southeastern IO.

How to cite: Zhang, Y. and Du, Y.: Oceanic Rossby waves induced two types of ocean-atmosphere response and opposite Indian Ocean Dipole phases, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2528, https://doi.org/10.5194/egusphere-egu23-2528, 2023.

EGU23-2532 | Posters on site | OS1.7

Effect of mesoscale eddies on the transport of low-salinity water from the Bay of Bengal into the Arabian Sea during winter 

Jiechao Zhu, Yuhong Zhang, Xuhua Cheng, Xiangpeng Wang, Qiwei Sun, and Yan Du

Abstract: The distribution of sea surface salinity (SSS) in the Arabian Sea (AS) and Bay of Bengal (BoB) is in contrast due to differences in air-sea freshwater fluxes and river runoff inputs.The monsoon-induced inter-basin water exchange plays an important role in regional salinity balance and atmosphere-ocean feedback in the North Indian Ocean. The satellite SSS dataset reveals that significant intraseasonal variability of SSS occurs in the region south of the Indian Peninsula with the strongest amplitude in winter. A case study in autumn-winter of 2016 showed that the Northeast Monsoon Current (NMC) and mesoscale eddies play a dominant role in the intraseasonal variability of the SSS in the region south of the Indian peninsula. In November, the East India Coastal Current (EICC) transports the low-salinity water southward to the region east of Sri Lanka. Meanwhile, a cyclonic eddy develops and propagates westward south of the NMC. Both NMC and the cyclonic eddy advects the low-salinity water westward to the region south of the Indian Peninsula. Then, an anticyclonic eddy generates in the north of the NMC. Thus, an eddy pair forms for more than one and a half months, which develops and propagates westward, transporting low-salinity water westward. The perturbation of mesoscale eddies and SSS gradient leads to the significant intraseasonal variability of SSS there.

Key words: Sea Surface Salinity; intraseasonal variability; mesoscale eddies; North Indian Ocean;

How to cite: Zhu, J., Zhang, Y., Cheng, X., Wang, X., Sun, Q., and Du, Y.: Effect of mesoscale eddies on the transport of low-salinity water from the Bay of Bengal into the Arabian Sea during winter, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2532, https://doi.org/10.5194/egusphere-egu23-2532, 2023.

EGU23-3426 | ECS | Posters on site | OS1.7

Eddy activity and its role in barrier layer thickness variability in the southeast Indian Ocean 

Marina Azaneu, Adrian Matthews, Karen Heywood, and Rob Hall

Ocean stratification can modulate the upper ocean response and its feedback to atmospheric forcing. Surface freshwater input by advection and precipitation, for example, can change the upper ocean stratification and produce barrier layers. The existence of a barrier layers can affect SST in several ways, for example by reducing entrainment of cooler water at the base of the mixed layer, and consequently may impact air--sea interactions. In the southeastern Indian Ocean, eddies are abundant and can act on transporting warm and fresh waters westward, thus possibly contributing to the formation of barrier layers. Here we initially evaluate the importance of eddy activity in contributing to barrier layer formation and intraseasonal variability in the southern Indian Ocean. Using 15 years (2005-2019) of ocean reanalysis daily data, we estimate how much of the spatial and time variability of barrier layer thickness is related to eddy activity, which is determined by calculating eddy kinectic energy. With the establishment of a relationship between eddy activity and barrier layer thickness, we then proceed to estimate the relationship between barrier layer thickness and local SST anomalies. This way, we seek to infer the significance of eddy activity in affecting SST through barrier layer formation, and thus its potential impact in air--sea interactions and coupled weather systems such as the MJO.

How to cite: Azaneu, M., Matthews, A., Heywood, K., and Hall, R.: Eddy activity and its role in barrier layer thickness variability in the southeast Indian Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3426, https://doi.org/10.5194/egusphere-egu23-3426, 2023.

EGU23-3967 | ECS | Orals | OS1.7 | Highlight

Ecosystem impacts due to thermocline depression by the 2019 extreme Indian Ocean Dipole event 

Edward Robinson, Philip Hosegood, Vasiliy Vlasenko, Nataliya Stashchuk, Clara Diaz, Nicola Foster, Joanna Harris, Clare Embling, and Kerry Howell

Tropical atoll habitats are often key conservation targets due to being inhabited by several vulnerable species such as reef manta rays and tropical coral species. These atolls are subject to both basin scale forcing through the Indian Ocean Dipole (IOD), monsoonal variation, and local processes. The steep slopes surrounding these atolls support highly dynamic, energetic nearshore ecosystems which vary over sub-kilometre spatial scales that are poorly resolved in general circulation models. Improving our understanding of how physical oceanographic processes control these local ecosystems, through both in-situ observations, and fine scale models, is critical for enabling informed policy decisions and efficient use of conservation resources. Here we summarise the impact of the local fine scale processes, which are heavily modulated by the monsoon and Indian Ocean Dipole (IOD), on a tropical atoll ecosystem in the central Indian Ocean (IO).

The IOD is experiencing increasingly extreme fluctuations with direct impacts on the depth of the thermocline throughout the western IO. In our observations from 2019, the IOD deepened the thermocline to an unprecedented depth of 100 m, subjecting mesophotic corals to temperatures typical of surface waters and causing significant bleaching. High resolution numerical modelling shows that internal waves, rather than alleviating bleaching, further exacerbate the heating effects preferentially advecting high temperature surface water to increased depths. The wave influence is, however, highly localised, necessitating designated studies at individual sites to understand the spatial heterogeneity in internal wave impacts.

At smaller sub-atoll scales, the IOD also influences the feeding behaviour of reef manta rays, which are more frequently detected in the presence of tidally forced surface-to-bottom temperature gradients. The site of most manta ray detections in the study area is a lipped gully, situated at 60-70 m depth, and colloquially named 'Manta Alley'. During deeper thermoclines, the cooling events within Manta Alley, with which increased reef manta presence is associated, are precluded from occurring due to the deep thermocline, impacting feeding behaviour.

Our results highlight the inherent dynamical complexity in these environments, with the impacts of basin scale processes cascading down to local scales. Improving our understanding of how these dynamics cross-interact with each other, as well as the local ecosystem, enhances the value of biological observations, presenting the opportunity for better informed and more effective conservation strategy.

How to cite: Robinson, E., Hosegood, P., Vlasenko, V., Stashchuk, N., Diaz, C., Foster, N., Harris, J., Embling, C., and Howell, K.: Ecosystem impacts due to thermocline depression by the 2019 extreme Indian Ocean Dipole event, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3967, https://doi.org/10.5194/egusphere-egu23-3967, 2023.

Using mooring observations and reanalysis, we show that anomalously strong westward Equatorial Undercurrent (wEUC) developed in June–July in 2016 and 1998 in the Indian Ocean, which coincided with extreme Indian Ocean Dipole (IOD) and El Niño events. Simulations show that equatorial Kelvin and Rossby waves were excited by winds associated with El Niño and positive IOD events during 2015 and 1997, and their negative phases during 2016 and 1998. The constructive relationship between the delayed-time contributions of eastern-boundary-reflected-waves that excited by the easterlies in 2015 and 1997 and the direct contributions of wind-forced-waves that excited by the westerlies in 2016 and 1998 resulted in the intensified wEUC. Slow intermediate-order baroclinic-modes, rather than fast low-order baroclinic-modes, dominated the strong wEUC. The eastern-boundary-reflected-waves dominated in 1997–1998 and directly wind-forced-waves dominated in 2015–2016. Our results emphasize the importance of constructive interactions of the directly-wind-forced and boundary-reflected waves in driving the interannual variability of Indian Ocean wEUC.

How to cite: Huang, K.: Successive Co-occurring IOD and ENSO Unprecedentedly Intensify Indian Ocean Westward Equatorial Undercurrent During the Summers of 1998 and 2016, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5143, https://doi.org/10.5194/egusphere-egu23-5143, 2023.

EGU23-6736 | ECS | Posters on site | OS1.7

How well do CMIP6 models simulate salinity barrier layers in the North Indian Ocean? 

Shanshan Pang, Xidong Wang, and Jérôme Vialard

Previous studies have hypothesized that climatologically thick salinity-stratified Barrier Layers (BL) in the North Indian Ocean (NIO) could influence the upper ocean heat budget, sea surface temperature (SST) and monsoon. Here, we investigate the performance of state-of-the-art climate models from the Coupled Model Intercomparison Project phase 6 (CMIP6) in simulating the barrier layer thickness (BLT) in the NIO. CMIP6 models generally reproduce the main features of the BLT seasonal cycle and spatial distribution, but with a shallow November-February (NDJF) BLT bias in regions with thick observed BLT (eastern equatorial Indian Ocean [EEIO], Bay of Bengal [BoB] and southeastern Arabian Sea [SEAS]). CMIP6 models display an easterly equatorial zonal surface wind bias linked to dry rainfall and cold SST biases in the southern BoB, through the Bjerknes feedback loop. The easterly equatorial bias is also responsible for the shallow isothermal layer depth (ILD) and BLT bias in the EEIO. The underestimated rainfall over the BoB leads to higher sea surface salinity (SSS) and too deep mixed layer depth (MLD), resulting in the BoB BLT bias. The intensity of the easterly equatorial bias also contributes to the inter-model spread in BoB BLT bias, through the propagation of EEIO ILD signals into the coastal waveguide. Finally, the SEAS BLT bias is due to a too deep MLD, which is predominantly controlled by the high SSS related to attenuated monsoonal currents around India and a reduced inflow of BoB low-salinity water. The BL effect on the mixed layer entrainment cooling does not seem to operate in CMIP6 simulations. Rather, deep salinity-related MLD biases in the BoB result in a diminished cooling rate in response to winter negative surface heat fluxes, and hence alleviate cold BoB SST biases. This suggests that salinity effects alleviate the biases that develop through the positive Bejrknes feedback loop between BoB SST, BoB rainfall and equatorial wind stresses in CMIP6.

How to cite: Pang, S., Wang, X., and Vialard, J.: How well do CMIP6 models simulate salinity barrier layers in the North Indian Ocean?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6736, https://doi.org/10.5194/egusphere-egu23-6736, 2023.

EGU23-6781 | ECS | Posters on site | OS1.7

On the influence of the Bay of Bengal’s sea surface temperature gradients on rainfall of the South Asian monsoon 

Peter Sheehan, Adrian Matthews, Benjamin Webber, Alejandra Sanchez-Franks, Nicholas Klingaman, and Pn Vinayachandran

The southwest monsoon delivers over 70% of India’s annual rainfall and is crucial to the success of agriculture across much of South Asia. Monsoon precipitation is known to be sensitive to sea surface temperature (SST) in the Bay of Bengal (BoB). Here, we use a configuration of the Unified Model of the UK Met Office coupled to an ocean mixed layer model to investigate the role of upper-ocean features in the BoB on southwest monsoon precipitation. We focus on the pronounced zonal and meridional SST gradients characteristic of the BoB; the zonal gradient in particular has an as-yet unknown effect on monsoon rainfall. We find that the zonal SST gradient is responsible for a local decrease in rainfall over the southern BoB of approximately 5 mm day−1, and an increase in rainfall over Bangladesh and northern India of approximately 1 mm day−1. This increase is remotely forced by a strengthening of the monsoon Hadley circulation. The meridional SST gradient acts to decrease precipitation over the BoB itself, similarly to the zonal SST gradient, but does not have comparable effects over land. The impacts of barrier layers and high-salinity sub-surface water are also investigated, but neither has significant effects on monsoon precipitation in this model; the influence of barrier layers on precipitation is felt in the months after the southwest monsoon. Models should accurately represent oceanic processes that directly influence BoB SST, such as the BoB cold pool, in order to faithfully represent monsoon rainfall.

How to cite: Sheehan, P., Matthews, A., Webber, B., Sanchez-Franks, A., Klingaman, N., and Vinayachandran, P.: On the influence of the Bay of Bengal’s sea surface temperature gradients on rainfall of the South Asian monsoon, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6781, https://doi.org/10.5194/egusphere-egu23-6781, 2023.

EGU23-6879 | ECS | Orals | OS1.7

Suitability of ocean reanalyses for monitoring of oceanic exchanges through the Indonesian Throughflow 

Magdalena Fritz, Leopold Haimberger, and Michael Mayer

The Indonesian Seas are characterized by numerous narrow channels connecting basins and seas of varying sizes and depths that serve as a transition between the Pacific and the Indian Ocean, known as the Indonesian Throughflow (ITF). The interaction between the ITF and important climate anomalies such as the El Niño Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), or the Australian-Indonesian monsoon indicates the high relevance for monitoring the ITF region. In situ observations of ITF transports are highly valuable but are temporally and spatially limited. Hence, near real-time monitoring is only possible with reanalyses, yet their quality needs to be evaluated. Here we present an assessment of oceanic transports in the ITF diagnosed from the Copernicus Marine Service (CMEMS) Global Reanalysis Ensemble Product (GREP) and the higher-resolution product GLORYS12V1. Validation data comes from several moorings in Makassar strait, Lombok strait, Ombai strait, and Timor passage, obtained as part of the well-known INSTANT (2004-2006) and MITF (2006-2011 and 2013-2017 in Makassar) campaigns. The campaigns provide a total of 11.5 years of in situ observations in Makassar, therefore allowing the assessment of the mean seasonal cycle of ITF transport and a thorough investigation of the shorter sampled outflow passages. The results showcase that reanalysis-based volume transports agree reasonably well with in situ observations, however, some aspects, such as asymmetries in the flow through each strait, are more accurately represented by GLORYS12V1. Also, in terms of mean integrated transports, the increased horizontal resolution of GLORYS12V1 leads to a better performance in the narrower straits of Lombok and Ombai. Furthermore, we draw attention to an apparent one-month lag between reanalyses and observations in Makassar strait transports, which we assess by studying the influence of the monsoon-driven (vertically varying) pressure gradient on the ITF.

How to cite: Fritz, M., Haimberger, L., and Mayer, M.: Suitability of ocean reanalyses for monitoring of oceanic exchanges through the Indonesian Throughflow, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6879, https://doi.org/10.5194/egusphere-egu23-6879, 2023.

EGU23-7577 | ECS | Orals | OS1.7

Nutrient fluxes in the greater Agulhas Current region: signals of local and remote Indian Ocean nitrogen cycling 

Tanya Marshall, Daniel Sigman, Lisa Beal, Alan Foreman, Alfredo Martínez-García, Stéphane Blain, Ethan Campbell, François Fripiat, Robyn Granger, Eesaa Harris, Gerald Haug, Dario Marconi, Sergey Oleynik, Patrick Rafter, Raymond Roman, Kolisa Sinyanya, Sandi Smart, and Sarah Fawcett

The Agulhas Current in the southwest Indian Ocean is the strongest western boundary current on Earth. The major role of the Agulhas Current in driving significant heat and salt fluxes is well known, yet its biogeochemical fluxes remain largely uncharacterised. Here, we use nitrate isotopes (δ15N, δ18O, and Δ(15-18) = δ15N-δ18O) to evaluate nutrient supply mechanisms that ultimately support new production in the southwest Indian Ocean. Across the greater Agulhas region, thermocline nitrate-δ15N is lower (4.9-5.8‰) than the underlying Subantarctic Mode Water source (δ15N of 6.9‰) and the upstream source regions (where nitrate-δ15N ranges from 6.4-7.0‰), which we attribute to local N2 fixation. Using a one-box model to simulate the newly-fixed nitrate flux, we estimate a local N2 fixation rate of 7-25 Tg N.a-1, amounting to ~30-95% of the whole Indian Ocean nitrogen gain estimated by models. Thermocline and mixed-layer nitrate Δ(15-18) is also low, due to both N2 fixation and coupled partial nitrate assimilation and nitrification. This local nitrogen cycling imprints an isotopic signal on Indian Ocean nitrate that persists in Agulhas rings that “leak” into the South Atlantic and are subsequently transported northwards. If this signal is retained in calcifying organisms (e.g., foraminifera) deposited on the seafloor, it could be used to trace past Agulhas leakage, yielding quantitative insights into the strength of the Atlantic Meridional Overturning Circulation over time. In addition to local N2 fixation, the nitrate isotopes reveal three physical mechanisms of subsurface nitrate supply: i) inshore upwelling driven by the current and winds, ii) entrainment at the edges of a mesoscale eddy, and iii) density-driven overturning at the current edge induced by strong horizontal velocity and density shears. All these nitrate supply mechanisms are evident as incidences of relatively high-Δ(15-18) nitrate in the thermocline and surface yet the intensity and subsurface expression of some of them is not apparent in the physical data, highlighting the utility of the nitrate isotopes for exploring physical ocean processes. The high mesoscale variability that likely drives subsurface nitrate supply to Agulhas Current surface waters is common to all western boundary currents, implying that vertical nitrate entrainment is quantitatively significant in all such systems. We posit that along with N2 fixation, physical mechanisms of upward nitrate supply enhance ocean fertility and possibly carbon export in the South Indian Ocean. Higher rates of warming, and thus thermal stratification, are expected to decrease Indian Ocean productivity more rapidly in the future than that of other ocean basins. However, a coincident increase in eddy kinetic energy across boundary currents may enhance the upward nutrient supply, partially offsetting the stratification-driven decline in productivity.

How to cite: Marshall, T., Sigman, D., Beal, L., Foreman, A., Martínez-García, A., Blain, S., Campbell, E., Fripiat, F., Granger, R., Harris, E., Haug, G., Marconi, D., Oleynik, S., Rafter, P., Roman, R., Sinyanya, K., Smart, S., and Fawcett, S.: Nutrient fluxes in the greater Agulhas Current region: signals of local and remote Indian Ocean nitrogen cycling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7577, https://doi.org/10.5194/egusphere-egu23-7577, 2023.

EGU23-8672 | Posters on site | OS1.7

Exploring the Climate-change induced dissolved inorganic carbon trends in the Indonesian Seas and their link to a changing Indonesian Throughflow using a regional downscaling of future climates 

Anna Katavouta, Jeff Polton, Jennifer Jardine, Dale Partridge, Svetlana Jevrejeva, and Jason Holt

The Indonesian Seas act as a main pathway of water transport from the Pacific to the Indian Ocean, known as the Indonesian Throughflow (ITF). Climate-induced changes in the regional water properties within the Indonesian Seas could have extensive impacts on the large-scale ocean budgets, as the ITF will carry these signals from the Indonesian Seas across the Indian Ocean’s upper thermocline. Here, we investigate the impacts of climate change on the Indonesian Seas’ dissolved inorganic carbon (DIC) budget using a regional ocean physics/biogeochemistry model for South East Asia that downscales climate projections from an Earth System Model under the RCP 8.5 scenario. The regional model has a horizontal resolution of about 9 km, uses a hybrid depth-terrain following vertical coordinate system and explicitly includes tides so as to better resolve the shelf-seas processes. A transport-based framework is used to explore the role of climate-induced changes of the ITF on the carbon storage within the Indonesian Seas. Specifically, the DIC trends are separated into: (i) an “added contribution” associated with the uptake of additional carbon from the atmosphere due to carbon emissions, and (ii) a “dynamic redistribution” of the pre-existing ocean DIC associated with changes in the circulation due to climate change. Our analysis reveals that in the next decades, although carbon emissions will lead to an ocean carbon uptake and an increase in the DIC within the Indonesian Seas, a plausible climate-induced weakening in the ITF can lead to either an increase or a decrease in the DIC at different depths associated with different water masses. Hence, the effects of global carbon emissions on the carbon budget within the Indonesian Seas, and particularly whether local waters will experience a lower or higher increase in DIC than the rest of the ocean, are controlled by the dynamical redistribution associated with the response of the ITF to climate change.   

How to cite: Katavouta, A., Polton, J., Jardine, J., Partridge, D., Jevrejeva, S., and Holt, J.: Exploring the Climate-change induced dissolved inorganic carbon trends in the Indonesian Seas and their link to a changing Indonesian Throughflow using a regional downscaling of future climates, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8672, https://doi.org/10.5194/egusphere-egu23-8672, 2023.

EGU23-9682 | ECS | Posters on site | OS1.7

Relative contribution of eddies ant atmospheric forcing to the Bay of Bengal non-seasonal Sea Surface Salinity Variability 

Marie Montero, Clément de Boyer Montégut, Jérôme Vialard, William Llovel, Thierry Penduff, Jean-Marc Molines, Stephanie Leroux, Nicolas Reul, and Jean Tournadre

The Bay of Bengal (BoB) Sea Surface Salinity (SSS) is highly contrasted and variable, in response to the large monsoonal wind and freshwater forcing. In addition to this strong seasonal cycle, previous studies have underlined strong SSS non-seasonal variations associated with the Indian Ocean Dipole (IOD) and mesoscale eddies. In this study, we quantify the relative contributions of externally forced (wind, freshwater) and internally generated (mesoscale eddies) SSS non-seasonal variability in the BoB. To that end, we use Ocean General Circulation Model 10-member ensemble experiments from the IMHOTEP (IMpacts of freshwater discHarge interannual variability on Ocean heaT-salt contents and rEgional sea-level change over the altimetry Period) project.
The model reproduces the large forced interannual SSS signals in the Northernmost part of the BoB and along the east coast of India, associated with the East Indian Coastal Current (EICC) modulation by the IOD. The internal SSS variability is largest in boreal fall in the North-Western BoB and more tightly controlled by the climatological SSS gradient distribution than by that of eddy kinetic energy. The external atmospheric forcing dominates the total variability in the regions of strongest variability, near the Ganges mouth and along the east coast of India in boreal fall and winter. Internal variability, however, contributes to 50-70% of the variability further offshore in boreal fall and winter. This confirms the strong role of eddies in controlling the freshwater extension up to ~700 km away from the coast, through stirring of the intense gradient between the coastal fresh and offshore saltier water. We finally discuss the consequences of these findings for comparing model and observations, in view of the chaotic nature of internal eddy variability.

How to cite: Montero, M., de Boyer Montégut, C., Vialard, J., Llovel, W., Penduff, T., Molines, J.-M., Leroux, S., Reul, N., and Tournadre, J.: Relative contribution of eddies ant atmospheric forcing to the Bay of Bengal non-seasonal Sea Surface Salinity Variability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9682, https://doi.org/10.5194/egusphere-egu23-9682, 2023.

EGU23-11289 | ECS | Posters on site | OS1.7 | Highlight

Characteristics and Drivers of Marine Heatwaves in the Western Equatorial Indian Ocean 

Ruisi Qi, Ying Zhang, Yan Du, and Ming Feng

The spatio-temporal characteristics of the interannual variability and long-term trend of the marine heatwaves (MHWs) and related dynamic mechanisms in the western equatorial Indian Ocean (WEIO) are investigated using satellite observations. A prominent MHW hot spot is found in a region of the WEIO (48°E-54°E, 2°S-2°N), with a mean MHWs' intensity, duration, and frequency of 1.54°C, 13.33 days, and 1.97 times, respectively. MHWs in the hot spot region have significant interannual variability after removing the long-term trend, associated with Indo-Pacific major climate modes. In 1982/1983, 1983/1984, 1987/1988, 1997/1998, 2006/2007, 2009/2010, 2011/2012, 2012/2013, 2014/2015, 2015/2016, and 2019/2020, the MHWs occurred with longer duration, higher frequency, and more total days. These years correspond to a positive Indian Ocean Dipole, or an El Niño event, or both. The occurrence of MHWs accompanied by anomalous positive sea surface height suggests that oceanic planetary wave processes modulate MHWs in the WEIO. Westward-propagating downwelling equatorial Rossby waves triggered by anomalous equatorial easterly winds drive the convergence of warm upper-ocean water and weaken the upwelling of cool subsurface water, which favor anomalously warm sea surface temperature (SST) and the occurrence of MHWs. In addition, the westward-propagating off-equatorial downwelling Rossby waves in the southern tropical Indian Ocean also affect MHWs in the WEIO through the propagation and reflection of waves. The annual MHW frequency, duration, and total days in the hot spot region increase up to 1.56 times, 4.95 days, and 31.72 days per decade, respectively, related to the significant increase in mean SST under global warming.

How to cite: Qi, R., Zhang, Y., Du, Y., and Feng, M.: Characteristics and Drivers of Marine Heatwaves in the Western Equatorial Indian Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11289, https://doi.org/10.5194/egusphere-egu23-11289, 2023.

EGU23-11965 | ECS | Posters on site | OS1.7

118-year hydroclimate reconstruction from Christmas Island (Indian Ocean); an extended record of variability in the Indonesian Throughflow 

Jessica A. Hargreaves, Nerilie Abram, and Jennie Mallela

Future climate trends indicate that changes in temperature and precipitation are likely to influence global supply chains, agricultural productivity, water security, health and well-being; particularly in densely populated nations across the southeast Indian Ocean region. The Indonesian Throughflow is an ocean current that transports low-latitude, warm and relatively fresh water from the western Pacific into the eastern Indian Ocean. It is thought that variability and changes in the Indonesian Throughflow have significant impacts on the climate and oceanography of the Indo-Pacific region. The short coverage of observational records makes assessments of hydrological changes across the region challenging on longer timescales, with changes before the 1970s being particularly unreliable. An extended record of Indonesian Throughflow variability needs to be established to contextualise changes and improve model projections of future variability.

Christmas Island, located in the southeast Indian Ocean (not to be confused with the Pacific Ocean Kiritimati Island), is located along an outflow of the Indonesian Throughflow. This Island is an ideal location to develop new palaeo-reconstructions of sea surface temperature and hydroclimate, extending our understanding of Indonesian Throughflow variability. Here we present a newly developed coral palaeoclimate reconstruction for Christmas Island, covering the last 118 years at approximately monthly-fortnightly resolution. Corals are sensitive recorders of critical environmental variables, including sea surface temperature and hydroclimate through the analysis of paired stable oxygen isotopes (δ18O) and trace element (Sr/Ca) ratios. This reconstruction consists of a composite of four newly developed coral records and one previously published record and provides a newly developed δ18Osw variability record for the region. The newly developed δ18Osw coral reconstruction correlates strongly with salinity variability, however, presents a weak relationship to in-situ precipitation, indicating that coral hydroclimate reconstructions from Christmas Island likely isolate salinity variability associated with changes in the strength of the Indonesian Throughflow. This relationship highlights the importance that ocean advection plays on δ18Osw variability at this site. Comparisons to both observational records of the Indonesian throughflow, and previously published coral δ18Osw records from the Ombai Strait (Timor), a major outflow passage, reveal strong relationships to variability at Christmas Island. The Christmas Island reconstruction provides a unique opportunity to extend current knowledge of the Indonesian Throughflow beyond the observational record. This Christmas Island record also provides an opportunity to evaluate the impact that interannual to multidecadal variability has on the climate across the southeast tropical Indian Ocean.

How to cite: Hargreaves, J. A., Abram, N., and Mallela, J.: 118-year hydroclimate reconstruction from Christmas Island (Indian Ocean); an extended record of variability in the Indonesian Throughflow, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11965, https://doi.org/10.5194/egusphere-egu23-11965, 2023.

EGU23-12048 | ECS | Orals | OS1.7

An asymmetric change in circulation and nitrate transports around the Bay of Bengal 

Jenny Jardine, Sarah Wakelin, Jason Holt, Anna Katavouta, and Dale Partridge

The Bay of Bengal is a dynamic region that experiences intense freshwater runoff, extreme meteorological events, and seasonal reversing surface currents. The region is particularly susceptible to anthropogenic climate change, driven in part by large air-sea fluxes, persistent freshwater stratification, and low overturning rates. Predicting how this ecosystem is likely to change in the future is paramount for planning effective mitigation strategies. Using a relocatable, coupled physics-ecosystem model (NEMO-ERSEM), we investigate the future changes in surface circulation and coastal nitrate pathways in the Bay of Bengal from 1980 to 2060, using a “business-as-usual" (RCP 8.5) climate change scenario. We find that future surface currents during the Summer and Fall Inter-monsoon seasons are reduced in the north/north-eastern Bay and strengthened in the south-western Bay. Coastal nitrate transports around the Bay mirror this asymmetric change, with coastal nitrate transports at 17.5oN decreasing by 185.7 mol N s-1, despite increased riverine runoff from the Ganges and Irrawaddy River systems. This results in a positive feedback loop whereby the northern Bay becomes progressively fresher and more nutrient-rich, strengthening the barrier layer and increasing the risk of toxic algal blooms and eutrophication events. Conversely, in the south-western Bay (12oN), coastal nitrate transports increase by 1317.8 mol N s-1, driven primarily by an intensified Sri Lanka Dome, that promotes localised diatom blooms despite negligible changes in regional river runoff. This work highlights the need for more rigorous ecosystem modelling and future scenario testing. 

How to cite: Jardine, J., Wakelin, S., Holt, J., Katavouta, A., and Partridge, D.: An asymmetric change in circulation and nitrate transports around the Bay of Bengal, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12048, https://doi.org/10.5194/egusphere-egu23-12048, 2023.

EGU23-12052 | Posters on site | OS1.7

The occurrence and distribution of microplastics in epipelagic zone of the western Indian Ocean 

Eun-Ran Baek, Minju Kim, Dong-Jin Kang, and Jung-Hoon Kang

This study investigated the occurrence and distribution of microplastics utilizing zooplankton samples collected in the Western Indian Ocean because there is no information concerning epipelagic zone in the open ocean. We collected microplastics from three water layers [surface mixed layer(SML), middle layer(ML), lower layer(LL)] within 200 m using a Multiple Opening/Closing Net and Environmental Sensing (opening: 1 ㎡) at 22 stations of 1 degree interval between 5°N and 16°S along the 67°E of Western Indian Ocean in 2017. The microplastics were consistently found in almost all samples and the microplastic abundance ranged between 0.00-2.01 particles/㎥ from the 3 layers. And the average microplastic abundance was highest in the lower layer (0.30±0.09 particles/㎥) and lowest in middle layer (0.26±0.08 particles/㎥). The percentage of fiber was highest in the SML (55.7%) and the LL (45.9%), and the percentage of film was highest in ML (46.8%). The microplastic abundance in the size of 1.0-5.0 ㎜ was highest in SML (42.0%), while the abundance in the size of 0.2-0.5 ㎜ was highest in ML(56.8%) and LL(54.5%). The stations can be divided into four sections including upwelling characterized by Seychelles-Chagos Thermal Ridge (SCTR) based on the 20℃-isotherm depth (D20). The average microplastic abundance was the highest in SML (0.23±0.06 particles/㎥) in 1°S~5°S, and in LL (0.50±0.25 particles/㎥) at latitudes of 10°S~16°S and in LL (0.32±0.16 particles/㎥) at latitudes between 5°N~EQ. However, the average microplastic abundance at latitudes of 6°S ~9°S corresponding to the upwelling zone was highest in the ML (0.65±0.38 particles/㎥) with the high percentage of film (68.7%). Cluster analysis by microplastics occurred in each water layers showed that the stations were divided into 3 groups in each layer. Groups in SML and LL were mainly clustered by fiber, whereas groups in ML was mainly clustered by film, which was associated with the upwelled region of Seychelles-Chagos Thermal Ridge (SCTR). Fourier transform infrared spectroscopy analysis showed that the main types of microplastics were dominated by fiber (40.6%) and film (73.2%) characterized by polycarbonate. Present results showed that meridional and vertical distribution of microplastics in the epipelagic zone varied with the physical characteristics of upwelling zone characterized by Seychelles-Chagos Thermal Ridge (SCTR) in the Western Indian Ocean.

How to cite: Baek, E.-R., Kim, M., Kang, D.-J., and Kang, J.-H.: The occurrence and distribution of microplastics in epipelagic zone of the western Indian Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12052, https://doi.org/10.5194/egusphere-egu23-12052, 2023.

EGU23-15102 | ECS | Orals | OS1.7

Indian Ocean mean state biases and IOD behaviour in CMIP6 multimodel ensemble 

Marimel Gler, Andy Turner, Linda Hirons, Caroline Wainwright, and Charline Marzin

The Indian Ocean Dipole (IOD) is the main coupled mode of interannual variability in the equatorial Indian Ocean. The largest IOD event in 2019 is thought to have influenced the strong Indian monsoon precipitation, widespread Australian bushfires, and extreme rainfall and flooding in East Africa during that year. Despite its socio-economic importance, the region suffers large biases in weather and climate models used for seasonal forecasts and climate projections.

In this study, the performance of 42 models from the sixth phase of the Coupled Model Intercomparison Project (CMIP6) in reproducing the observed climate over the Indian Ocean is examined. Model simulations of precipitation and 850 hPa winds in the Atmospheric Model Intercomparison Project (AMIP) experiments for the period 1979-2014 are compared to observational and reanalysis data. Biases in the mean state during boreal summer (JJA) in the AMIP models are analysed to determine whether biases in the seasonal cycle established in JJA impact the IOD behaviour. Skill metrics are calculated to quantify the model performance in reproducing the observed JJA mean state and cluster analysis on the mean state biases is performed to characterise bias patterns in summer that may affect the Indian Ocean seasonal cycle and IOD. Results show that AMIP models simulate varying bias patterns in JJA and that the AMIP multi-model mean outperforms all individual models in reproducing the observed JJA mean state. For comparison, the Indian Ocean mean state biases are investigated in coupled models from the 20th-century all-forcings (CMIP) experiments to determine the impact of ocean-atmosphere coupling and coupled sea surface temperature biases on model performance. The IOD behaviour in the AMIP and CMIP models is assessed and the response of the atmospheric circulation to IOD forcing is examined by performing regression analysis. We investigate whether the ability of a model to capture characteristics of the IOD and simulate IOD teleconnection patterns is related to its representation of the mean state. We expand this work to investigate the variability in the Indian Ocean in the Met Office Global Seasonal Forecasting System version 6, GloSea6, with a focus on examining the systematic errors that develop in the region. The work will contribute to our understanding of Indian Ocean biases in weather and climate models, and their likely sources, and thus the wider implications for predictability of the IOD.  

How to cite: Gler, M., Turner, A., Hirons, L., Wainwright, C., and Marzin, C.: Indian Ocean mean state biases and IOD behaviour in CMIP6 multimodel ensemble, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15102, https://doi.org/10.5194/egusphere-egu23-15102, 2023.

EGU23-15432 | ECS | Orals | OS1.7 | Highlight

Multi-decadal changes in the Indian Ocean heat content from a grand ensemble perspective 

Lukas Fiedler, Vimal Koul, Eduardo Alastrué de Asenjo, Sebastian Brune, and Johanna Baehr

Ocean heat content observations in the Indian Ocean have revealed distinctive periods of significant multi-decadal trends — for example a cooling between 1990 and 1999 followed by an unprecedented warming between 2000 and 2009. However, a systematic assessment of the relative importance of anthropogenic forcings versus natural variability in driving such trends is still missing. Here, we utilise four state-of-the-art Single Model Initial- Condition Large Ensembles with MPI-ESM1.2-LR containing different factual and counterfactual forcing scenarios to address the problem. We are able to robustly attribute the unprecedented warming of the Indian Ocean between 2000 and 2009 to the increasing anthropogenic greenhouse gas emissions. Our results also reveal that the preceding cooling is likely to be intrinsic to Indian Ocean heat content variability, since none of the applied counterfactual scenarios exhibits such an observed decrease in Indian Ocean heat content. Furthermore, we trace the underlying reasons for the observed inherent cooling between 1990 and 1999 to a significant reduction in heat transported into the Indian Ocean from the Pacific Ocean by the Indonesian Throughflow. These results have implications for decadal predictions of Indian Ocean heat content.

How to cite: Fiedler, L., Koul, V., Alastrué de Asenjo, E., Brune, S., and Baehr, J.: Multi-decadal changes in the Indian Ocean heat content from a grand ensemble perspective, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15432, https://doi.org/10.5194/egusphere-egu23-15432, 2023.

EGU23-1051 | Orals | OS1.8

Interannual variability in biogeochemical cycling around the island of South Georgia: insights from a new database of macronutrients from productive regions of the Southern Ocean 

Katharine R Hendry, Sally Thorpe, Emma F Young, Petra ten Hoopen, Geraint A Tarling, and Michael J Whitehouse

A key challenge in understanding carbon cycling in the Southern Ocean is disentangling long-term responses from significant spatial and temporal variability in physical and biogeochemical parameters. As such, there is a critical need for regional long-term observations for the model validation and testing needed for a better mechanistic understanding of primary production drivers. We present a new macronutrient data product for the South Atlantic sector of the Southern Ocean, including depth profiles and underway surface measurements of nitrate, nitrite, ammonium, phosphate, silicic acid, temperature and salinity, collected from 1980 to 2009 and covering most months of the year (https://doi.org/10/h3qr). Using this data product, we explore the differences in shallow and deep-water nutrients around the island of South Georgia that are observed between years. We discuss both the biological and physical driving mechanisms behind this variability, which are interconnected with climate feedbacks. The new data product provides an unprecedented view of biogeochemical cycling in biologically productive regions of the Southern Ocean across a critical period in recent climate history, and illustrates the importance of building these scientifically valuable and FAIR (findability, accessibility, interoperability, and reusability) observational datasets.

How to cite: Hendry, K. R., Thorpe, S., Young, E. F., ten Hoopen, P., Tarling, G. A., and Whitehouse, M. J.: Interannual variability in biogeochemical cycling around the island of South Georgia: insights from a new database of macronutrients from productive regions of the Southern Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1051, https://doi.org/10.5194/egusphere-egu23-1051, 2023.

EGU23-1605 | ECS | Orals | OS1.8

The response of the deep convection to the Antarctic Meltwater 

Jia-Jia Chen, Xuhua Cheng, and Neil C. Swart

Observations indicate that the mass loss from the Antarctic ice sheet has been increasing over the past several decades. This loss is projected to accelerate significantly into the future. Deep convection in the Southern Ocean is expected to bear the brunt of meltwater from a retreating Antarctic Ice Sheet. Here, we present the responses of deep convection and Antarctic Bottom Water (AABW) formation using six coupled climate models with a constant rate of freshwater flux anomaly. Six models all show a significant decrease in the strength of deep convection, albeit the magnitude and location of the changes vary greatly across models. Models that convect more strongly in the base state decrease more in deep convection. We found that the big difference in response between models is surprisingly consistent with their respective base states. With the cessation of deep convection, the AABW becomes warmer and of contraction, and the sea ice concentration and area increase significantly, accompanying surface cooling. However, the link between the responses in deep convection and sea ice area is more complicated than simply meaning more reduction in deep convection corresponds to more increase in sea ice. We suggest that this complexity is partly because some models convect over too large an area and the freshwater forcing is rather strong. Our results suggest that increasing Antarctic meltwater into the ocean will reduce AABW formation, amplifying the warming rate of deep and abyssal waters and reducing the melting rate of sea ice caused by heat input, and reducing vertical exchange due to intensified stratification.

How to cite: Chen, J.-J., Cheng, X., and Swart, N. C.: The response of the deep convection to the Antarctic Meltwater, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1605, https://doi.org/10.5194/egusphere-egu23-1605, 2023.

EGU23-3425 | Orals | OS1.8

Using zonal surface heat flux asymmetry to reveal new features of Southern Ocean air-sea interaction 

Simon Josey, Jeremy Grist, Jennifer Mecking, Ben Moat, and Eric Schulz

New results on Southern Ocean heat exchange and wind forcing are presented with a focus on zonal asymmetry between surface ocean heat gain in the Atlantic/Indian sector and heat loss in the Pacific sector. The asymmetry arises from an intersector variation in the humidity gradient between the sea surface and near surface atmosphere. This gradient increases by 60% in the Pacific sector enabling a 20 Wm-2 stronger latent heat loss compared to the Atlantic/Indian sector. A new zonal asymmetry metric is used for intercomparison of atmospheric reanalyses and CMIP6 climate simulations. CMIP6 has weaker Atlantic/Indian sector heat gain compared to the reanalyses primarily due to Indian Ocean sector differences. The potential for surface flux buoys to provide an observation-based counterpart to the asymmetry metric is explored. Over the past decade, flux buoys have been deployed at two sites (south of Tasmania and upstream of Drake Passage). The data record provided by these moorings is assessed and an argument developed for a third buoy to sample the Atlantic/Indian sector of the asymmetry metric. In addition, we assess evidence that the main westerly wind belt has strengthened and moved southward in recent decades using the ERA5 reanalysis. We find only marginal evidence of a southward broadening of the belt in the Atlantic /Indian sector and northward broadening in the Pacific sector and that the latitude of maximum wind speed remains essentially unchanged.

How to cite: Josey, S., Grist, J., Mecking, J., Moat, B., and Schulz, E.: Using zonal surface heat flux asymmetry to reveal new features of Southern Ocean air-sea interaction, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3425, https://doi.org/10.5194/egusphere-egu23-3425, 2023.

EGU23-3865 | ECS | Posters on site | OS1.8

Zonally Asymmetric Response of Southern Ocean Heat Content to Wind, Heat and Freshwater Forcing at Multi-decadal Time Scales 

Mark Hague, Nicolas Gruber, and Matthias Münnich

Since it was recognised that the Southern Ocean plays a crucial role in global climate, the region has generally been understood in a zonally symmetric framework. While this simplification has provided valuable insight, recent work suggests that significant zonal asymmetries exist in several key parameters such as SST, mixed layer depth and air-sea CO2 flux. This is true both for the mean state and for changes at multiple time scales. 

Of particular interest here are changes in ocean heat content (ΔOHC) over the past three decades. Using an eddy-permitting ocean model forced by ERA5 reanalysis, we find significant asymmetries in ΔOHC both within and north of the ACC, which is robustly reflected in a suite of hindcast and reanalysis models, as well as observation based temperature reconstructions. In our model, asymmetry stems largely from a southward displacement of ΔOHC in the Indian basin, where warming occurs primarily within ACC, as opposed to north of it in the Atlantic and Pacific. However, significant asymmetries are also found within the sea ice zone south of 60o S, where the Ross Sea warms to a much greater degree than other basins. 

In order to better understand the sources of this asymmetric warming, we run several model experiments which decompose the total OHC into components originating from wind, heat and freshwater flux changes. We find roughly equal contributions from wind and surface heat flux north of the ACC, with asymmetric changes in the westerlies driving anomalous convergence of heat. Within and south of the ACC all three forcings play an important role, although this depends strongly on the basin. Overall, we conclude that much of the asymmetries in ΔOHC originate from asymmetries in the surface flux changes, with an important secondary role played by variability in the mean state. These findings have two important implications. First, studies which only consider zonally averaged quantities will likely mask significant variability, and therefore miss important regional and local processes. Second, the impact of multi-decadal climate variability on the Southern Ocean is not manifested in a zonally symmetric fashion, which may have important implications for future changes. 

How to cite: Hague, M., Gruber, N., and Münnich, M.: Zonally Asymmetric Response of Southern Ocean Heat Content to Wind, Heat and Freshwater Forcing at Multi-decadal Time Scales, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3865, https://doi.org/10.5194/egusphere-egu23-3865, 2023.

EGU23-4879 | ECS | Posters on site | OS1.8

Influence of the Wind Stress Curl on the Eddy Saturation Behavior of the ACC in a Barotropic Perspective 

Sima Dogan, Caroline Muller, Louis-Philippe Nadeau, and Antoine Venaille

The size of zonal transport of the Antarctic Circumpolar Current (ACC) is almost independent of the variations in westerly winds over the Southern Ocean; this phenomenon is called eddy saturation. The eddy saturation has been studied in both barotropic and baroclinic contexts in the presence of topography, yet many aspects of its dynamics remain elusive. We focus here on barotropic eddy saturation, which occurs in a narrow band of wind stresses where topographic-barotropic instability takes place. As a result, barotropic eddy saturation is highly sensitive to the specific geometry of bottom topography and to the boundary conditions. Here, we investigate whether the amplitude of the wind stress curl relative to that of a constant background wind stress can also modulate barotropic eddy saturation by modifying the global vorticity budget of a doubly periodic quasigeostrophic flow. We report that the zonal transport and the eddy saturation regime are sensitive to the wind stress curl and explore the underlying dynamics.

How to cite: Dogan, S., Muller, C., Nadeau, L.-P., and Venaille, A.: Influence of the Wind Stress Curl on the Eddy Saturation Behavior of the ACC in a Barotropic Perspective, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4879, https://doi.org/10.5194/egusphere-egu23-4879, 2023.

EGU23-5783 | Posters on site | OS1.8

Acceleration of Antarctic Circumpolar Current at the Drake Passage during the GRACE era 

Chengcheng Yang, Xuhua Cheng, Duotian Huang, Jianhuang Qin, Guidi Zhou, and Jiajia Chen

Previous studies have identified intense climatic change in the Southern Ocean. However, the response of ACC transport to climate change is not fully understood. In this study, by using in-situ ocean bottom pressure (OBP) records and five GRACE products, long-term variations of ACC transport are studied. Our results confirm the reliability of GRACE CSR mascon product in ACC transport estimation at the Drake Passage. Superimposed on interannual variability, ACC transport exhibits an obvious increasing trend (1.32±0.07Sv year-1) during the GRACE era. Based on results of a mass-conservation ocean model simulation, we suggest that the acceleration of ACC is associated with intensified westerly winds and loss of land ice in Antarctica.

How to cite: Yang, C., Cheng, X., Huang, D., Qin, J., Zhou, G., and Chen, J.: Acceleration of Antarctic Circumpolar Current at the Drake Passage during the GRACE era, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5783, https://doi.org/10.5194/egusphere-egu23-5783, 2023.

EGU23-8176 | ECS | Posters on site | OS1.8

The Weddell Gyre: what drives a long-term increase in surface nutrients? 

Krissy Anne Reeve, Mario Hoppema, Torsten Kanzow, Olaf Boebel, Walter Geibert, and Volker Strass

The Weddell Gyre plays a role in connecting the deep ocean to the surface through upwelling, and also in feeding heat towards the Antarctic ice shelves, regulating the density of water masses that feed the deepest limb of the global overturning circulation. Using Argo floats freely drifting throughout the Weddell Gyre, we describe its horizontal circulation as an elongated double-gyre system, with stronger transports in the east than in west, impacting water property distribution. The eastern sub-gyre region is also associated with stronger upwelling rates than in the west, as shown by radionuclide concentrations. To gain insight to long-term changes in the Weddell Gyre, nutrient concentrations can also be investigated as oceanic tracers. We determine long-term trends in surface silicates, a necessary nutrient for silicifying phytoplankton, from ship-based measurements since 1996, and find that the strongest increase is found in the central western sub-gyre region. In association with the eastern sub-gyre, long-term trends along the Prime Meridian are strongest (albeit weaker than in the central western sub-gyre) in the westward flowing southern limb of the gyre, downstream of Maud Rise. We hypothesize that there are different dynamical drivers, such as wind-driven upwelling (west) and turbulent mixing (east), which cause the positive silicate trends in the east versus the west, which are investigated accordingly.

How to cite: Reeve, K. A., Hoppema, M., Kanzow, T., Boebel, O., Geibert, W., and Strass, V.: The Weddell Gyre: what drives a long-term increase in surface nutrients?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8176, https://doi.org/10.5194/egusphere-egu23-8176, 2023.

EGU23-9591 | Posters on site | OS1.8

Exploring the Impact of a Southern Ocean Anticyclonic Eddy on Biogeochemical Structure Using BGC-Argo Float Observations 

Nicola Guisewhite, Don Chambers, Veronica Tamsitt, and Nancy Williams

Recent studies have found that eddies in the Southern Ocean can contribute to both uptake and outgassing of CO2, emphasizing a need to understand the impact of eddies on biogeochemical structure in the Southern Ocean.  Despite having a significant role in climate regulation and global ocean transport, the Southern Ocean and its eddies are largely under-sampled, leaving many unknowns when trying to understand how the Southern Ocean can be impacted by a changing climate.  Whereas CO2 and other biogeochemical properties including oxygen and nitrate (which can be studied as indicators of a changing climate) are historically under-sampled and understudied in the Southern Ocean, the use of autonomous vehicles has allowed for the collection of high-quality data that can be used to analyze the impact of eddies on Southern Ocean biogeochemical structure.  A SOCCOM BGC-Argo Float encountered and sampled an anticyclonic eddy in the area lying between 54° and 50° S and 148° and 143° W in February 2019.  During the encounter, the float collected daily profiles of the biogeochemical structure within the eddy.  Using additional resources for sea surface height, wind, and ocean currents, we conduct a spatial and temporal analysis of the biogeochemical structure of the eddy.  We compare float data to climatologies, examine the physical properties that impact the mixed layer depth within and around the eddy, and understand how these properties influence biogeochemical variability caused by the eddy.  In addition, we pull biogeochemical data from all known eddy encounters by SOCCOM BGC-Argo floats and determine the significance of eddies on biogeochemical structure in the Southern Ocean.

How to cite: Guisewhite, N., Chambers, D., Tamsitt, V., and Williams, N.: Exploring the Impact of a Southern Ocean Anticyclonic Eddy on Biogeochemical Structure Using BGC-Argo Float Observations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9591, https://doi.org/10.5194/egusphere-egu23-9591, 2023.

EGU23-10045 | ECS | Posters on site | OS1.8

On the Western Boundary Current System of the Weddell Gyre: a model intercomparison 

Tania Pereira-Vázquez, Borja Aguiar-González, Ángeles Marrero-Díaz, Francisco Machín, Marta Veny, and Ángel Rodríguez-Santana

The Weddell Sea is located in the Southern Ocean, bounded to the south and west by the Antarctic continent and the Antarctic Peninsula, respectively, and to the north by the Antarctic Circumpolar Current. The cyclonic Weddell Gyre stands out as the dominant feature of the basin circulation, driven by wind and thermohaline forcing as well as topographic steering. Importantly, it is the primary source region of Antarctic Bottom Water (AABW), thus becoming one of the key regions for the global thermohaline circulation. Furthermore, the geographical location of the Western Boundary Current System (WBCS) developed in the gyre allows the leakage of near-freezing subsurface waters into the Bransfield Strait. This cold-water pathway has been recently suggested to maintain regionally low rates of glacier retreat.  In this work, we perform the inter-comparison between NEMO-based and HYCOM-based global ocean circulation models at different resolutions over the WBCS domain. To this aim, we analyse the horizontal and vertical structure of the WBCS and its volume transport along the historical ADELIE transect (SOS-Climate II campaign; https://doi.pangaea.de/10.1594/PANGAEA.864578), which extends oceanward from the northernmost tip of the Antarctic Peninsula and across the WBCS. The choice of this transect is not trivial as it captures the hydrodynamic of the WBCS before water masses either leave the basin or recirculate within the gyre.  

Preliminary results support that both eddy-resolving models are in agreement about the major features of the hydrography and dynamic structure of the WBCS as compared to previous modelling studies. Both reproduce the spatial distribution of the Antarctic Coastal Current (CC), the Antarctic Slope Front (ASF) and the Weddell Front (WF), as reported in Thompson and Heywood (2008). Talking about the NEMO-based model at a lower resolution (¼o), the multi-jet structure of the WBCS is absent, appearing only one major branch. We attribute this mismatch mostly due to a resolution issue. Regarding the volume transport, we find the modelled WBCS displays a seasonal cycle in all cases of study, where minimum values are found in September-December while maximum are in March-July, as also reported Wang et al. [2012]. A major difference occurs towards the interior of the gyre, where the HYCOM-based model exhibits a significantly stronger and wider current branch (~150 km) east of the WF, and whose description is absent in the literature. In previous studies this domain is traditionally excluded and, when volume transport estimates from the NEMO-based model and the HYCOM based model were computed, they both yielded an average transport about 30 Sv, which agrees well with a transport about 24 Sv reported by Wang et al. (2012) and Jullion et al. (2014), also based on modelling estimates across a similar but shorter transect.

These results encourage us to further explore these models in ongoing analyses about the natural variability of the WBCS of the Weddell Gyre and major forcing controlling its variability. We expect that a better understanding of the governing processes will allow us to assess the potential downstream impact of local water masses after their exit from the Gyre. 

How to cite: Pereira-Vázquez, T., Aguiar-González, B., Marrero-Díaz, Á., Machín, F., Veny, M., and Rodríguez-Santana, Á.: On the Western Boundary Current System of the Weddell Gyre: a model intercomparison, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10045, https://doi.org/10.5194/egusphere-egu23-10045, 2023.

EGU23-10301 | ECS | Orals | OS1.8

Identifying the drivers of the Weddell Gyre variability using a barotropic vorticity budget 

Julia Neme, Matthew H. England, and Andrew McC. Hogg
The Weddell Gyre is one of the largest features of the ocean circulation adjacent to the Antarctic margins. The gyre is a dynamically complex region and participates in several processes relevant to global climate. For example, the gyre’s circulation and its strength have been linked to changes in the properties and rates of export of Antarctic Bottom Water into the global abyssal ocean. However, the dynamic controls of the Weddell Gyre’s variability are largely unknown, possibly due to the complexities of the region: the interplay of the Weddell Gyre with an overturning circulation, strong buoyancy fluxes associated with sea ice formation and melt, and open and permeable boundaries which allow for significant inflows and outflows. In this work we analyse the mechanisms controlling the Weddell Gyre’s variability using a barotropic vorticity budget of a MOM6 simulation coupled with SIS2 and forced with a repeat year 1990-91 atmospheric state derived from JRA55-do. Unlike past studies that focus on the stationary state of a control simulation, we focus on the evolution of our simulation and the response to different wind and buoyancy perturbations. Within the gyre we find that a balance is achieved between the curl of surface stress and bottom pressure torque, bottom drag curl and the curl of horizontal viscosity.  

How to cite: Neme, J., England, M. H., and Hogg, A. McC.: Identifying the drivers of the Weddell Gyre variability using a barotropic vorticity budget, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10301, https://doi.org/10.5194/egusphere-egu23-10301, 2023.

EGU23-10304 | ECS | Orals | OS1.8

The Southern Ocean supergyre: a unifyingdynamical framework identified by machinelearning 

Maike Sonnewald, Krissy A Reeve, and Redouane Lguensat

The Southern Ocean closes the global overturning circulation and is key to the regulation of carbon and heat, biological production, and sea level. However, the dynamics of the general circulation and upwelling pathways remain poorly understood. Here, a unifying framework is proposed invoking a semi-circumpolar `supergyre' south of the Antarctic circumpolar current: a massive series of  ‘leaking’ sub-gyres spanning the Weddell and Ross seas that are connected and maintained via rough topography that acts as scaffolding. The supergyre framework challenges the conventional view of having separate circulation structures in the Weddell and Ross seas and suggests a limited utility for climate applications of idealized models and conventional zonal averaged frameworks. Machine learning was used to reveal areas of coherent driving forces within a vorticity-based analysis. Predictions from the supergyre framework are supported by available observations and could aid observational and modelling efforts of the climatically key region undergoing rapid change.

How to cite: Sonnewald, M., Reeve, K. A., and Lguensat, R.: The Southern Ocean supergyre: a unifyingdynamical framework identified by machinelearning, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10304, https://doi.org/10.5194/egusphere-egu23-10304, 2023.

EGU23-10379 | Orals | OS1.8

Decadal reorganization of Subantarctic Mode Water 

Ivana Cerovecki and F. Alexander Haumann

Subantarctic Mode Water (SAMW) is one of the most important water masses globally in taking up anthropogenic heat and carbon dioxide. However, its long-term changes in response to varying climatic conditions are not well understood. Here we use data from the ”Estimating the Circulation and Climate of the Ocean” (version 4, release 4, ECCOv4r4) state estimate to calculate SAMW volume budgets for the period 1992 to 2017. They reveal a SAMW volume reorganization on decadal timescales in the Indian and on multidecadal timescales in the Pacific Ocean. In the Pacific, this multidecadal variability exceeds the long-term trend and is governed by an accumulation of signals from the Interdecadal Pacific Oscillation. This implies that SAMW volume trends observed during the shorter Argo period largely arise from the multidecadal variability. In both ocean sectors, the SAMW reorganization exhibits a two-layer density structure, with nearly compensating volume changes of lighter and denser SAMW. They are caused by heat flux changes in the Indian Ocean, freshwater flux changes in the southeast Pacific, and both heat and freshwater flux changes in the central Pacific Ocean. Our results indicate that the recently observed SAMW changes have to be interpreted in the context of the strong long-term variability, which imposes challenges to detecting and attributing climate change signals in SAMW.

How to cite: Cerovecki, I. and Haumann, F. A.: Decadal reorganization of Subantarctic Mode Water, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10379, https://doi.org/10.5194/egusphere-egu23-10379, 2023.

As insolation increases during the Antarctic summer, sea ice melts, and the iron in the sea ice is released into the ocean Iron leads to phytoplankton blooms at sea ice margins and polynya in Antarctica, a high nutrient and low chlorophyll region. However, it is difficult to investigate the direct effect of sea ice on chlorophyll a changes because the observed regions are physically difficult to access. Therefore, this study intends to investigate the sea ice effect on chlorophyll a variation by using the decreasing salinity when sea ice melts in the Ross Sea, Antarctica. Using Random Forest, an ensemble bagging tree method of machine learning, the relationship was analyzed by 11 variables, including physical variables (sea surface temperature, photosynthetically available radiation, atmospheric temperature, wind, and salinity) as input data and chlorophyll a data observed from satellites as output data. As a result, the square of the correlation coefficient (R2) of the test data set was 0.97, and the root mean square error (RMSE) was 0.41 mg m-3, showing high accuracy. In addition, the importance of salinity was identified by calculating the variable importance in the model. These results provide the importance of salinity in predicted future chlorophyll a changes in the Antarctic Ocean due to climate change.

How to cite: Yang, H.-J., Baek, J.-Y., and Jo, Y.-H.: Interrelationship of sea ice-salinity-chlorophyll a changes using multi-satellite based on machine learning analysis in Ross Sea, Antarctica, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10869, https://doi.org/10.5194/egusphere-egu23-10869, 2023.

EGU23-11655 | ECS | Orals | OS1.8

The Influence of Surface Buoyancy Flux and Ekman transport on Upper Ocean Pycnocline Stratification in the Southern Ocean. 

Romain Caneill, Fabien Roquet, Gurvan Madec, and Jonas Nycander

In this study, we examine the factors that influence upper ocean pycnocline (UOP) stratification in the Southern Ocean. The UOP is a layer located just below the mixed layer and its stratification controls the rates of exchange of heat, carbon, and nutrients between the ocean interior and the atmosphere. We classify regions of the UOP based on the relative roles of temperature and salinity in stabilizing the layer, resulting in alpha (temperature-stabilized), beta (salinity-stabilized), or transition (temperature and salinity-stabilized) zones. Our analysis uses observation profiles from the EN4.2 database and calculates annual mean buoyancy fluxes by combining existing heat and freshwater flux products and accounting for the effect of Ekman transport. Our results show that the polar transition zone has a complex structure, with interlocking beta pools and local intrusions into alpha zones. Deep mixed layers are found in the southernmost flank of the alpha region, with the exception of the southeast Pacific sector where they are located in the polar transition zone. Regions of negative buoyancy flux show mixed layer deepening along the water path, but deep mixed layers only form when the buoyancy flux is negative throughout the path. Ekman transport contributes also to the formation of deeper mixed layers throughout the Southern Ocean by bringing cold water northward. Overall, our findings reveal that boundaries between alpha, transition and beta regions are generally consistent with more traditional frontal definitions and provide a comprehensive view of upper ocean pycnocline stratification in the Southern Ocean.

How to cite: Caneill, R., Roquet, F., Madec, G., and Nycander, J.: The Influence of Surface Buoyancy Flux and Ekman transport on Upper Ocean Pycnocline Stratification in the Southern Ocean., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11655, https://doi.org/10.5194/egusphere-egu23-11655, 2023.

EGU23-12422 | ECS | Posters on site | OS1.8

On the implications of a warm bias in modelling an eddying Southern Ocean 

Mathias Zeller and Torge Martin

Mesoscale eddies are considered to have a major impact on the horizontal and vertical redistribution of heat, freshwater, carbon and other passive tracers across the Southern Ocean (SO). A way to investigate the role of mesoscale dynamics in a region where observations are sparse is running a high-resolution model. Here, we apply 2-way nesting to the ocean model NEMO3.6 using its AGRIF module to simulate an eddying SO embedded in the fully coupled climate model FOCI. The nest enhances the horizontal ocean grid resolution from 1/2˚ to 1/10˚ everywhere south of 28˚S. Since the nested model, called FOCI-ORION10X, is computationally relatively expensive, our goal was to gain a spun up climate state with just the non-eddying resolution model (without nest). This would open the opportunity to efficiently run a coarse climate model into different climate states under which the role of mesoscale eddies could then be studied with the nested setup. Here, we demonstrate that there are limits to such an approach arising from the mean state of the climate model.

The non-eddying standard FOCI model features a significant warm bias in the SO similar to many CMIP-class climate models. To test the implications of the warm bias on the nested model configuration, we compare two such simulations branching off from coarse FOCI pre-industrial control simulations and contrast these to a nested run starting from rest initialized with Levitus (WOA13) temperature and salinity fields. The two FOCI control runs differ in warm bias intensity due to a shorter coupling frequency with the atmosphere and modified ocean mixing parameters. Further, one nested run is started already after 500 years from the weakly biased run and the other after 1500 years of the strongly bias run yielding a difference of ~50% in the temperature bias. In both cases, Weddell Gyre stratification becomes unstable within the first decade of the nested runs initiating open ocean deep convection and releasing the excess heat to the atmosphere. While the spurious deep convection results in a widely reduced heat bias in the nested runs after a few decades, it directly increases the meridional density gradient to the mid latitudes and enhances the strength of the Antarctic Circumpolar Current. Besides these positive effects, we also find unusually strong production of bottom water yielding a too strong bottom cell in the meridional overturning circulation. Especially because of this lasting deep ocean impact, we see no advantage in branching off from a biased mean state compared to the nested run starting from rest, which reaches a quasi-equilibrium after 100 years. We conclude, the presence of a typical warm bias and the SO’s sensitivity to stratification hinder the combination of eddying and eddy-parameterized model configurations to facilitate cost-efficient long spinup procedures.

How to cite: Zeller, M. and Martin, T.: On the implications of a warm bias in modelling an eddying Southern Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12422, https://doi.org/10.5194/egusphere-egu23-12422, 2023.

EGU23-13417 | ECS | Posters on site | OS1.8

Spatio-temporal variability of the Peninsula Front and the surface chlorophyll-a bloom in the Bransfield Strait 

Marta Veny, Borja Aguiar-González, Ángeles Marrero-Díaz, Tania Pereira-Vázquez, and Ángel Rodríguez-Santana

The Bransfield Strait (BS) is a relatively narrow region located between the South Shetland Islands (SSI) and the Antarctic Peninsula (AP), where the dominant cyclonic circulation is composed by two major inflows, which appear to influence the development of the seasonal chlorophyll bloom. On the one hand, the Bransfield Current transports Transitional Zonal Water with Bellingshausen influence (TBW) northeastwards along the SSI slope. TBW is characterised by well-stratified, relatively warm (Ɵ > -0.4ºC) and fresh (<34.45) waters. On the other hand, the Antarctic Coastal Current (CC) transports Transitional Zonal Water with Weddell influence (TWW) southwestwards along the AP coastline, being distinguished by homogeneous, colder (Ɵ < -0.4ºC) and saltier (>34.45) waters (Sangrà et al., 2017). These two water masses confront each other forming the Peninsula Front (PF; García et al., 1994; López et al., 1999). Interestingly, the chlorophyll-a (chl-a) spatial distribution in the BS has already been linked in the past to the spatial distribution of both water masses and their water column vertical stability, among other factors (Lipski and Rakusa-Suszczewski, 1990; Basterretxea and Arístegui, 1999). Thus, higher chl-a concentrations have been reported around the SSI and Gerlache Strait where TBW flows, while lower concentrations have been traditionally found north off the SSI and closer to the AP coastline, where more homogeneous surface waters prevail (AASW and TWW, respectively) (Corzo et al., 2005).

In this work we aim to provide a further understanding on the bio-physical coupling occurring in the Bransfield Strait, focused on the physical drivers controlling the surface distribution of the chl-a bloom and the location of the PF at seasonal and interannual scales. To do this we use various remotely-sensed observations over the period 1998-2018: Sea Surface Temperature (SST), Sea-Ice Coverage (SIC), chlorophyll-a, wind stress and Photosynthetically Active Radiation (PAR). Preliminary results confirm that the spatial distribution of the surface chl-a bloom in the Bransfield Strait is strongly influenced by the location of the PF, both seasonally and interannually. Also, a shift in the strength of the chl-a bloom has been identified, where significantly stronger events are found from 2005 onwards; when mean chl-a bloom values are slightly greater, and about 0.61 mg m-3, than in previous years, when they averaged about 0.49 mg m-3. We hypothesize this shift might be linked to observed changes in the seasonal evolution of the SIC and SST over the same period. Ongoing analyses attempt to elucidate the major mechanisms accounting for this apparent variability of the bio-physical coupling controlling the chl-a blooms in the Bransfield Strait.

How to cite: Veny, M., Aguiar-González, B., Marrero-Díaz, Á., Pereira-Vázquez, T., and Rodríguez-Santana, Á.: Spatio-temporal variability of the Peninsula Front and the surface chlorophyll-a bloom in the Bransfield Strait, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13417, https://doi.org/10.5194/egusphere-egu23-13417, 2023.

EGU23-15324 | Orals | OS1.8

Constraining phytoplankton response to climate change in the Southern Ocean using observed mixed layer depth seasonality 

Tianfei Xue, Ivy Frenger, Jens Terhaar, Wolfgang Koeve, Thomas L. Frölicher, A.E. Friederike Prowe, and Andreas Oschlies

Phytoplankton, as the base of the marine food web, has great importance for marine ecosystems and the global carbon cycle. However, Earth system models indicate considerable uncertainty of our knowledge about the underlying processes that determine phytoplankton evolution under climate change. Particularly large differences between models can be found in the Southern Ocean, a region notorious for its difficulty in modeling. The objective of this study is to analyze the potential phytoplankton response to climate change from both a "bottom-up" and a "top-down" perspective. Within the Southern Ocean, we determine a relationship between surface phytoplankton and mixed layer depth under present-day seasonality and apply it to climate change on a longer timescale. Applying this present-day constraint, we confirm the trend of increasing surface phytoplankton by the end of the 21st century under a 'high emissions no mitigation scenario' with further reduction in phytoplankton projection uncertainty. The increase of surface phytoplankton is due to weakening bottom-up control as a result of improving light conditions with shoaling mixed layers. At the same time, due to shoaling mixed layers, total phytoplankton biomass integrated over the water column slightly decreases. Zooplankton follows the trend of surface phytoplankton and shows an increase. This is mainly caused by improved zooplankton grazing conditions with shoaling mixed layers that result in enhanced efficiency of trophic energy transfer. In comparison with the changes in bottom-up conditions, top-down control appears to become increasingly important under climate change in the Southern Ocean. 

 

 

How to cite: Xue, T., Frenger, I., Terhaar, J., Koeve, W., L. Frölicher, T., Prowe, A. E. F., and Oschlies, A.: Constraining phytoplankton response to climate change in the Southern Ocean using observed mixed layer depth seasonality, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15324, https://doi.org/10.5194/egusphere-egu23-15324, 2023.

EGU23-16070 | Posters on site | OS1.8

Interannual variability in the physical and biological drivers of carbon sequestration in the southeast Pacific Subantarctic Mode Water Formation region 

Pete Brown, Pablo Trucco Pignata, Sophy Oliver, Maribel García-Ibañez, Paula Conde Pardo, Dorothee Bakker, and Adrian Martin

The uptake of carbon by the Southern Ocean plays a critical role in mitigating atmospheric CO2 increases, but its magnitude and temporal and spatial variability are still subject to large uncertainties due to the scarcity of observations, and model disagreements. The Southeast Pacific is one such region where deep, nutrient and carbon-rich circumpolar deep waters upwell, but also where Subantarctic Mode Water and Antarctic Intermediate Water are formed and subducted, carrying with them high loadings of anthropogenic carbon dioxide into the ocean interior. The processes driving the upper ocean carbon levels are a balance of biological activity and heat-flux driven solubility effects in response to changing physical dynamics. While the former is thought to drive the region being a net annual carbon sink, the depth at which exported organic matter is remineralised will have a large effect on whether it remains there on climatically-important timescales. Here we present a multi-year biogeochemical timeseries from the OOI mooring located in the region, combined with observations from profiling BGC-Argo floats, a 6-week process cruise in austral summer 2019-2020 (as part of the UK CUSTARD programme), and outputs from data assimilation models to investigate the effects of interannual variability in mixed layer dynamics on primary production, carbon export and long-term carbon sequestration. We find a strong relationship between winter mixed layer depths and densities, biological activity the following summer, and impacts on the magnitude and distribution of subsurface remineralisation, providing insight into the controls on carbon uptake in a region of global significance for climate regulation

How to cite: Brown, P., Trucco Pignata, P., Oliver, S., García-Ibañez, M., Conde Pardo, P., Bakker, D., and Martin, A.: Interannual variability in the physical and biological drivers of carbon sequestration in the southeast Pacific Subantarctic Mode Water Formation region, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16070, https://doi.org/10.5194/egusphere-egu23-16070, 2023.

A fraction of the deep-water plume that flows along the slope in the NW Weddell Sea eventually leaks from the Weddell Gyre through deep passages on its northern margin. This provides source waters for the Antarctic Bottom Water that ultimately fills the ocean abyss as the lower branch of the Meridional Overturning Circulation (MOC). Despite the importance of this supply, uncertainties still remained associated to its interannual variability. Here we investigate the role played by the combined effect of two natural climate modes in the interannual variability of the densest  water mass found within this plume, the Weddell Sea Bottom Water (WSBW). Previous studies found that both the Southern Annular Mode (SAM) and the El Niño-Southern Oscillation (ENSO) influence the winds around Antarctica, and suggested that their overlapping effects on the along-shore winds are reinforced when they occur at opposite phases (i.e. a positive SAM with a La Niña or a negative SAM with an El Niño). We prepared a combined SAM-ENSO climate index (SEI) that takes into account their overlapping effects on the winds and performed a lagged cross-correlation analysis with a 2005-2022 timeseries of WSBW thermohaline properties measured at the bottom instrument of a mooring redeployed in the NW Weddell Sea (AWI207). The significant correlations found suggest that a positive SAM occurring in summer, reinforced by a La Niña event, can influence the WSBW properties measured in the NW Weddell Sea at two different time scales. First, it would produce a warming of the WSBW reaching our mooring in the NW Weddell Sea between 4 and 5 months later. We propose that this warming is caused by the entrainment of a less modified WDW during the formation of WSBW. This is enabled by the weaker along-shore winds induced by a positive SAM and a La Niña event. Second, it would induce a freshening  of the WSBW that can be measured in the NW Weddell Sea between 13 and 14 months later. This freshening is probably related to the first mechanism proposed by McKee et al. (2011), i.e. negative anomalies in the meridional winds in the eastern side of the Antarctic Peninsula in summer would induce a reduced HSSW formation during the next winter and a decrease in the export of dense shelf waters during the next summer. However, the freshening mechanism proposed by Gordon et al., (2020), i.e. the wind-driven deepening of the V-shaped double front located at the shelf break in the western Weddell Sea, might also contribute to this freshening by enabling the injection of fresh shelf waters into the WSBW plume.

How to cite: Llanillo, P., Kanzow, T., and Janout, M.: The influence of natural climate modes on the interannual variability of the deep-water plume in the northwestern Weddell Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16370, https://doi.org/10.5194/egusphere-egu23-16370, 2023.

EGU23-419 | ECS | Orals | OS1.9 | Highlight

Contributions of atmospheric forcing and ocean preconditioning in the 2016 Antarctic sea ice extent drop 

Bianca Mezzina, Hugues Goosse, Pierre-Vincent Huot, Sylvain Marchi, and Nicole Van Lipzig

The observed evolution of Antarctic sea ice extent is marked by an abrupt decrease in 2016/2017. After several years of gradual increase culminated in an all-time record high in 2014/2015, a rapid decline in 2016 led to an unprecedented minimum, and unusual low extents have been observed since then. Even though this record has now been beaten, the sudden drop from extreme high values to a minimum in less than two years is unique to this event, whose dynamics are still uncertain. While it was likely triggered by anomalous atmospheric conditions in the prior months, the contribution of the ocean conditions, as a preconditioning which amplified the response of the sea ice or helped to maintain the anomalies for a longer period, still needs to be quantified. 

To evaluate the respective influences of the atmosphere and ocean on this 2016 event, we have performed sensitivity experiments using the circum-Antarctic fully coupled model (ice-sheet–ocean–sea-ice–atmosphere) PARASO. First, a control experiment with the model forced by lateral boundary conditions derived from observations (ERA5 in the atmosphere, ORAS5 in the ocean) is performed over the period 1985-2018. In such a set-up, the model correlates well with the observations and is able to capture the 2016 drop. Then, the model is integrated again between 2016 and 2018 with the same atmospheric boundary forcing, but with different initial conditions in the ocean: namely, ocean conditions from previous years in the control run are used as initial state in 2016 in the sensitivity experiments, producing an ensemble of 5 members.

Preliminary results indicate that the 2016 drop is captured by all members, suggesting the atmospheric boundary forcing as the dominant driver and confirming that the event is induced by large-scale atmospheric dynamics. However, some variability is present in the amplitude and timing of the drop, as well as in the evolution and recovery of the sea ice in the following months, which may be influenced by the different states of the ocean. Related processes are further investigated by examining different oceanic and atmospheric fields, focussing on the role of ocean preconditioning by identifying the differences between the members and their impact. 

How to cite: Mezzina, B., Goosse, H., Huot, P.-V., Marchi, S., and Van Lipzig, N.: Contributions of atmospheric forcing and ocean preconditioning in the 2016 Antarctic sea ice extent drop, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-419, https://doi.org/10.5194/egusphere-egu23-419, 2023.

EGU23-480 | ECS | Orals | OS1.9

Spatial variations in the sea ice-mixed layer depth relationship in the West Antarctic Peninsula 

Milo Bischof, Daniel Goldberg, Sian Henley, and Neil Fraser

The impacts of upper-ocean mixing on primary productivity are complex and range from an entrainment of nutrients to modulating light limitations. Sea ice in turn plays an important role in determining mixing conditions through its cycles of formation and melt, and by moderating wind forcing. With sea ice conditions in the Southern Ocean projected to undergo large changes over the course of the century, understanding the relationship between sea ice and upper-ocean mixing is crucial for understanding the impacts of climate change on biological production in this region. Due to the inaccessibility of sea ice-covered waters however, mixed layer depth observations are often not available at a high temporal and spatial resolution. Here we present an analysis of sea ice-mixed layer depth relationships during a 40-year regional ocean-sea ice simulation of the  West Antarctic Peninsula (WAP) and Bellingshausen Sea, a highly biologically productive region of global importance. The relationship between winter sea ice and spring mixed layer depth shows clear differences on and off the WAP continental shelf, with decadal variations in the location of the boundary between negative and positive correlations. Potential mechanisms causing this effect are considered in detail, including the nonlinear relationship between sea ice cover and turbulent mixing, the transport of sea ice within the region, and a difference in the timing of the sea ice seasonal cycle between the two regions. The transport of warm Circumpolar Deep Water onto the shelf is also discussed. The presented findings have implications for the spatial distribution of primary producers in a more ice-free future WAP.

How to cite: Bischof, M., Goldberg, D., Henley, S., and Fraser, N.: Spatial variations in the sea ice-mixed layer depth relationship in the West Antarctic Peninsula, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-480, https://doi.org/10.5194/egusphere-egu23-480, 2023.

EGU23-3111 | Posters on site | OS1.9

On the 2018-2020 Ice Shelf Water outflow event in the southeastern Weddell Sea 

Markus Janout, Mathias van Caspel, Elin Darelius, Tore Hattermann, Svein Østerhus, Jean-Baptiste Sallée, and Nadine Steiger

The southern Weddell Sea features a vast perennially ice-covered continental shelf with polynyas, strong sea ice formation, first- and multi-year ice. Sea ice and the general ocean circulation maintain predominantly near-freezing waters on the shelf, which help to maintain the comparatively moderate basal melt rates of the Filchner-Ronne Ice Shelf (FRIS), Antarctica’s largest ice shelf by volume. In contrast to FRIS, other West Antarctic ice shelves show strong basal melt rates, caused by warm intruding ocean waters. In the southern Weddell Sea, however, warm water inflows occur episodically and spatially limited, when modified warm deep water enters the continental shelf through incisions in the shelf break and flows southward towards the FRIS front. Overall, the majority of the shelf is dominated by dense and cold water masses such as High Salinity Shelf Water (HSSW) and Ice Shelf Water (ISW), which are precursors of Antarctic Bottom Water and thus relevant for the global ocean circulation. In 2018, a comprehensive CTD survey found unprecedented (in the available observations) volumes of ISW in Filchner Trough. The ISW was exported from underneath the Filchner Ice Shelf (FIS) following a shift to enhanced cavity circulation due to strong sea ice formation in front of the Ronne Ice Shelf. These Filchner Trough conditions are summarized as the “Ronne-mode”, which is in contrast to the “Berkner-mode”, characterized by a greater influence of locally-formed waters. In this presentation, we introduce new multi-year time series from an international mooring network from various Southeast Weddell Sea locations (sub-FIS, Filchner Trough and Sill), to highlight the temporal and spatial extent of the recent Ronne-mode event, which lasted from 2018-2020, before shifting back into a Berkner-mode. The dominance of either circulation mode is controlled by large-scale atmospheric forcing and has implications on ice shelf basal melt and dense water export into the Weddell Sea. 

How to cite: Janout, M., van Caspel, M., Darelius, E., Hattermann, T., Østerhus, S., Sallée, J.-B., and Steiger, N.: On the 2018-2020 Ice Shelf Water outflow event in the southeastern Weddell Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3111, https://doi.org/10.5194/egusphere-egu23-3111, 2023.

EGU23-3627 | ECS | Posters on site | OS1.9

Sub-ice shelf circulation and melt rate variability in the Energy Exascale Earth System Model 

Irena Vankova, Xylar Asay-Davis, and Stephen Price

In-situ observations from the Filchner-Ronne Ice Shelf (FRIS) have uncovered dominant time scales of variability in basal melting and circulation beneath this extensive ice shelf. In particular, the data characterize mechanisms of seasonal and inter-annual variability in sub-ice shelf properties, and show that the amplitude of the variability over the past thirty years is very modest.
Because accurate representation of variability under present-day climate is an obvious prerequisite for earth system models that aim to project climate under a future change, this new observational understanding presents an opportunity for critical evaluation and improvement of existing models. We focus on the Energy Exascale Earth System Model (E3SM) and through a series of simulations we investigate the impact of ocean mixing parameterizations on the variability in the FRIS cavity.

How to cite: Vankova, I., Asay-Davis, X., and Price, S.: Sub-ice shelf circulation and melt rate variability in the Energy Exascale Earth System Model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3627, https://doi.org/10.5194/egusphere-egu23-3627, 2023.

EGU23-4468 | ECS | Posters on site | OS1.9

The role of the Pacific-Antarctic Ridge in establishing the northward extent of Antarctic sea-ice 

Antonino Ian Ferola, Yuri Cotroneo, Peter Wadhams, Giannetta Fusco, Pierpaolo Falco, Giorgio Budillon, and Giuseppe Aulicino

Monitoring the Antarctic sea-ice is essential for improving our knowledge of the Southern Ocean. We used satellite sea-ice concentration data for the 2002-2020 period to retrieve the sea-ice extent (SIE) and analyze its variability in the Pacific sector of the Southern Ocean. Results provide observational evidence of the recurring formation of a sea-ice protrusion that extends to 60° S at 150° W during the winter season. These activities are carried on in the framework of the ACCESS and SWIMMING projects of the PNRA.
Our findings show that the northward deflection of the southern Antarctic Circumpolar Current front is driven by the Pacific Antarctic Ridge (PAR) and is associated with the enhanced sea-ice advance. The PAR also constrains anticyclonic and cyclonic eddy trajectories, limiting their interaction with the sea-ice edge. These factors, within the 160° W - 135° W sector, determine an average SIE increase of 61,000 km2 and 46,293 km2 per year more than the upstream and downstream areas, respectively.

How to cite: Ferola, A. I., Cotroneo, Y., Wadhams, P., Fusco, G., Falco, P., Budillon, G., and Aulicino, G.: The role of the Pacific-Antarctic Ridge in establishing the northward extent of Antarctic sea-ice, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4468, https://doi.org/10.5194/egusphere-egu23-4468, 2023.

EGU23-5080 | ECS | Orals | OS1.9 | Highlight

Multi-decadal trends in Antarctic deep convection from satellite-derived steric height 

Jennifer Cocks, Alessandro Silvano, Alice Marzocchi, Alberto Naveira-Garabato, and Anna Hogg

Deep convection from dense water formation in the Southern Ocean drives the lower limb of the global overturning circulation, sequesters anthropogenic heat and carbon from the atmosphere and ventilates the abyssal ocean. The rate and location of dense water formation and its trajectory to the deep ocean is determined by changes in ocean density and stratification and influenced by ocean-ice-atmosphere interactions such as polynya openings (both open-ocean and coastal), sea ice formation and ice shelf collapse.

Signatures of deep convection are logistically difficult to measure. The highest-quality observations of water column density are currently provided by in-situ moorings and profiles from Argo floats or CTDs mounted on elephant seals (MEOP data[1]), but these data are spatially and temporally sparse. Satellite products providing complete coverage of high latitudes at regular repeat periods are becoming more readily available and offer an alternative method for capturing changes the extent and variability of deep-water formation in polar regions.

 

We compute steric height anomalies in the Southern Ocean from 2002-2018 using a novel method combining satellite altimetry and gravimetry data. We use these to explore density changes, focussing on deep water formation regions including the Weddell and Ross seas, the Adelie coastline and Amery shelf region, and infer multi-decadal changes in deep convective processes. Long term changes in the steric height anomalies can be linked to recorded ocean-ice events, such as the 2010 collapse of the Mertz glacier, the 2017 Maud Rise polynya and recent recovery of Ross Sea Bottom Water. The satellite-derived steric height anomalies have been validated against in-situ Argo and MEOP profiles and show good agreement in regions with a high data density.


[1]https://meop.net/meop-portal/

How to cite: Cocks, J., Silvano, A., Marzocchi, A., Naveira-Garabato, A., and Hogg, A.: Multi-decadal trends in Antarctic deep convection from satellite-derived steric height, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5080, https://doi.org/10.5194/egusphere-egu23-5080, 2023.

EGU23-6731 | ECS | Orals | OS1.9

The ice-cavity feedback in an Earth system model 

Pengyang Song, Patrick Scholz, Gregor Knorr, Dmitry Sidorenko, Ralph Timmermann, and Gerrit Lohmann

The melting of the Antarctic ice shelves becomes critical in a warming climate. However, the ocean component of climate models do not consider the effect of the Antarctic ice-shelf cavities. Here, we implement ice-shelf cavity features into the new AWI Earth system model (AWI-ESM2) based on unstructured meshes allowing for varying resolution in a multi-scale approach. We create a global mesh explicitly resolving the Antarctic ice-shelf cavities and evaluate the effect of the cavities under global warming scenarios. The new mesh provides a more realistic freshwater input into the Antarctic coast and the Southern Ocean. In an extreme warming climate scenario, the melting of the Antarctic ice shelves gets stronger by a factor of ~3, affecting the North Atlantic salinity and the overturning circulation. We conclude that the incorporation of ice-cavity feedback is essential to study the past, present, and future. Our approach might be seen as a prototype for the next phase of the Coupled Model Intercomparison Project.

How to cite: Song, P., Scholz, P., Knorr, G., Sidorenko, D., Timmermann, R., and Lohmann, G.: The ice-cavity feedback in an Earth system model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6731, https://doi.org/10.5194/egusphere-egu23-6731, 2023.

EGU23-9657 | Orals | OS1.9 | Highlight

Spatial and temporal variability of water masses in the Southern Ross Sea 

Karen J. Heywood, Esther Portela, Walker Smith, Gillian Damerell, Peter Sheehan, and Meredith Meyer

Relatively warm modified Circumpolar Deep Water accesses the southern Ross Sea steered by bathymetric troughs. There it provides nutrients to support phytoplankton blooms in spring, and heat to melt the Ross Ice Shelf.  Here we present new observations collected by two ocean gliders during December 2022 and January 2023, in the Ross Sea polynya adjacent to the Ross Ice Shelf.  The gliders surveyed the full depth of the water column (about 700 m depth) carrying sensors measuring temperature, salinity, dissolved oxygen, chlorophyll fluorescence and optical backscatter, and also yielded estimates of the dive-average-current which we use to reference geostrophic shear.  Repeated quasi-meridional high resolution (profiles approximately every 1.5 km) sections along the sea ice edge allow analysis of the spatial and temporal variability, as well capturing the dynamic field of eddies, tides and coastal current. We discuss the influence of the sea ice and the atmospheric forcing on the water properties. One glider made an unauthorised foray beneath the Ross Ice Shelf, surveying the upper 200 m of the water column in high resolution beneath an ice shelf base at about 80 m depth. We observe solar-warmed water penetrating beneath the ice shelf with significant signatures of elevated chlorophyll fluorescence and optical backscatter, and low oxygen and salinity. We discuss the likely mechanisms involved in advecting this water beneath the ice shelf and its importance for physical and biogeochemical processes of ocean-ice interaction.



How to cite: Heywood, K. J., Portela, E., Smith, W., Damerell, G., Sheehan, P., and Meyer, M.: Spatial and temporal variability of water masses in the Southern Ross Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9657, https://doi.org/10.5194/egusphere-egu23-9657, 2023.

EGU23-10081 | ECS | Orals | OS1.9

Exploring oceanic heat pathways along the George V Land continental shelf 

Eliza Dawson and Earle Wilson

Ocean circulation patterns along the continental shelf in the Australian Antarctic Basin remain poorly understood due to the scarcity of in-situ observations and limited modeling studies. In this dynamically complex and climatically important region, the Ross Gyre, Antarctic Slope Current, and Antarctic Circumpolar Current converge just offshore of the George V Land continental shelf. If warm deep water could access the continental shelf and increase basal melt rates along the George V Land coastline, marine-terminating glaciers in the region could retreat and threaten the stability of the vast Wilkes Subglacial Basin. Here, we explore potential pathways for warm deep water to access the shelf along the George V Land coastline using output from the Southern Ocean State Estimate (SOSE) model. We use the SOSE output to map bottom temperatures and identify where warm bottom water could come close to the grounding line due to bathymetric steering. While SOSE provides observationally constrained hydrographic estimates along the George V Land continental shelf, there are substantial discrepancies between the model’s estimates and observations. Most notably, SOSE does not reproduce the dense, high salinity shelf waters observed in the region. SOSE is a model-generated best fit to Southern Ocean observations, so biases could be present in sparsely sampled regions like this one. To further examine the dynamics of this region, we also present preliminary results from an idealized ocean circulation model that explores the sensitivity of cross-shelf heat transport to changes in local heat and wind forcing.

How to cite: Dawson, E. and Wilson, E.: Exploring oceanic heat pathways along the George V Land continental shelf, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10081, https://doi.org/10.5194/egusphere-egu23-10081, 2023.

EGU23-11464 | ECS | Posters on site | OS1.9

Identification of ventilated and submarine glacial meltwaters in the Amundsen Sea, Antarctica, using noble gases 

DongYoub Shin, Doshik Hahm, Tae-Wan Kim, Tae Siek Rhee, SangHoon Lee, Keyhong Park, Jisoo Park, Young Shin Kwon, Mi Seon Kim, and Tongsup Lee

To estimate the glacial meltwater distribution, we used five noble gases as tracers for optimum multiparameter analysis (OMPA) of the water masses in the Amundsen Sea, Antarctic. The increased number of tracers allowed us to define additional source waters at the surface, which have not been possible with a limited number of tracers. The highest fraction of submarine meltwater (SMW, ~0.6%) was present at the depth of 400 -- 500 m near the Dotson Ice Shelf. The SMW appeared to travel along an isopycnal layer to the continental shelf break >300 km away from the ice shelf. Ventilated SMW (VMW) and surface melts (up to 1.5%) were present in the surface layer <100 m. The distribution of SMW indicates that upwelled SMW, known as an important carrier of iron to the upper layer, amounts for 29% of the SMW in the Dotson Trough. The distinction between SMW and VMW made it possible to clearly distinguish the locally-produced SMW since the previous Winter Water formation from the fresh water (VMW) originated from the upstream; the production rate of the former was estimated as 53-94 G ton yr-1. The Meteoric Water fractions, consisted of SMW and VMW, comprised 24% of those derived from oxygen isotopes. This indicates that the annual input from basal melting is far less than the inventory of meteoric water derived from oxygen istopes.

How to cite: Shin, D., Hahm, D., Kim, T.-W., Rhee, T. S., Lee, S., Park, K., Park, J., Kwon, Y. S., Kim, M. S., and Lee, T.: Identification of ventilated and submarine glacial meltwaters in the Amundsen Sea, Antarctica, using noble gases, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11464, https://doi.org/10.5194/egusphere-egu23-11464, 2023.

EGU23-11864 | ECS | Orals | OS1.9

Evolution of warm water intrusions in the Filchner Trough, Antarctica 

Vanessa Teske, Ralph Timmermann, and Tido Semmler

The Filchner Trough on the continental shelf in the southern Weddell Sea is a region of great importance for the water mass exchange between the open ocean and the Filchner Ronne Ice Shelf cavity. Observations of the last 20 years and modelling studies show seasonal variations and longer lasting pulses of warm water intruding into the trough and reaching the Filchner Ice Shelf front. In this study, we evaluate the evolution of these intrusions in four climate scenarios defined for CMIP6 and simulated with the AWI Climate Model. We show that a warming climate will lead to more frequent pulses in the mitigation scenarios SSP1-2.6 and SSP2-4.5. For the high emission scenarios SSP3-7.0 and SSP5-8.5, hydrography in Filchner Trough will shift to a substantially warmer state during the second half of the 21st century with a temperature rise of 2°C in the trough until 2100. We demonstrate that the system‘s tipping into a warmer state is primarily caused by changes in the local sea ice formation and the depth of the Antarctic Slope Front. Our results show that a regime shift can be avoided by reaching the 2°C climate goal.

How to cite: Teske, V., Timmermann, R., and Semmler, T.: Evolution of warm water intrusions in the Filchner Trough, Antarctica, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11864, https://doi.org/10.5194/egusphere-egu23-11864, 2023.

The local temperarturecannot explain the inter-annual variation in δ18Oprecip in the coastal Antarctic in past few decades. To understand this enigmatic variation, we have used long-term modern δ18Oprecip value of three coastal Antarctic sites. Using the δ18O-d-excess relationship and modelled δ18O value of vapor at source, we have shown that δ18Oprecip inherits the signature of moisture source parameters (MSPs). Furthermore, the wavelet analysis suggests that the variation in the MSPs impacts the seasonal cycle of δ18Oprecipwhich lead to disparity in the seasonal isotope-temperature relationship. The Southern Ocean surface stratification, due to increase in the freshwater flux by glacier melting, led to alignment of MSPs in such a manner that altogether significantly lowered the isotopic composition of initially formed vapor, which is reflected in δ18Oprecip at inter-annual scale.Our observations suggest that the palaeothermometry will underestimate the Antarctic temperature change for the periods characterized by warming and high glacier-melt.

How to cite: Sanyal, P. and ajay, A.: The Imprint of Southern Ocean Stratification on the Isotopic Composition of Antarctic Precipitation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12482, https://doi.org/10.5194/egusphere-egu23-12482, 2023.

EGU23-12791 | ECS | Orals | OS1.9

Observed Seasonal Evolution of the Antarctic Slope Current System at the Coast of Dronning Maud Land, East Antarctica 

Julius Lauber, Laura de Steur, Tore Hattermann, and Elin Darelius

The Antarctic Slope Front and the associated Antarctic Slope Current shield the continental shelves in East Antarctica from offshore warm water that holds the potential for considerable ice shelf melting and, consequently, sea level rise. Here, we present two-year-long records of temperature, salinity, and velocity (2019-2020), obtained from two oceanographic moorings located within the slope front/current over bathymetries of around 1000m and 2000m slightly east of the prime meridian. The two-year data record reveals clear differences in the seasonality of the thermocline depth and the baroclinicity of the current between the deep and shallow mooring locations. In combination with climatologies of hydrography and satellite-derived surface geostrophic currents, we use the new data to refine the baroclinic seasonality of the ASF. The results highlight the role of surface buoyancy fluxes via seasonal sea ice melt and freeze. Finally, the slope current is shown to control flow into and out of the cavity of the close-by Fimbulisen Ice Shelf on seasonal time scales depending on the orientation of the entrances of the cavity. Our findings contribute to a better understanding of the processes controlling the slope front/current seasonality and resulting inflow into the East-Antarctic ice shelf cavities.

How to cite: Lauber, J., de Steur, L., Hattermann, T., and Darelius, E.: Observed Seasonal Evolution of the Antarctic Slope Current System at the Coast of Dronning Maud Land, East Antarctica, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12791, https://doi.org/10.5194/egusphere-egu23-12791, 2023.

EGU23-13222 | Posters on site | OS1.9

The role of WDW density for a regime shift in the FRIS cavity 

Verena Haid, Ralph Timmermann, Simon Schöll, Torsten Albrecht, and Hartmut H. Hellmer

A potential tipping point on the Antarctic continental shelves, in which cold shelf water is replaced by (modified) Circumpolar Deep Water (CDW) / Warm Deep Water (WDW), is currently the subject of many studies. Such a regime shift entails a drastic increase of basal melt for the fringing ice shelves and could ultimately destabilize large portions of the Antarctic ice sheet.

From the results of a large suite of experiments conducted with the Finite Element Sea ice-Ocean Model (FESOM), we identified for the Weddell Sea the density balance between the densest shelf water produced on the continental shelf and the WDW present on the continental slope at sill depth (shallowest depth of deepest connection to the cavity) as the crucial criterion for a shift in on-shelf circulation leading to a substantially increased heat flux into the cavity. This finding holds true for model runs using both z-level and sigma vertical coordinates as well as ocean-ice sheet (with the Parallel Ice Sheet Model, PISM) coupled model runs. We also find evidence that the same principle is valid in other Antarctic regions with a backward-sloping continental shelf.

Apart from the shelf water characteristics that largely depend on sea ice formation, the development of CDW/WDW characteristics  is crucial, but often neglected, in this context, especially in regional model studies. If under the influence of the globally warming climate the continental slope current becomes warmer and fresher, the associated density decrease could keep the continental shelf stable. Even if none of the on-shelf water classifies as High Salinity Shelf Water any more, as long as it is denser than the off-shelf CDW/WDW, it will block access to the cavity and prevent a regime shift.

How to cite: Haid, V., Timmermann, R., Schöll, S., Albrecht, T., and Hellmer, H. H.: The role of WDW density for a regime shift in the FRIS cavity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13222, https://doi.org/10.5194/egusphere-egu23-13222, 2023.

EGU23-13262 | Posters on site | OS1.9

Ice sheet-ocean coupling in an Earth System Model 

Xylar Asay-Davis, Carolyn Begeman, Darren Engwirda, Holly Han, Matthew Hoffman, and Stephen Price

We present our approach to coupling an ocean component (MPAS-Ocean, Model for Prediction Across Scales-Ocean) to an ice sheet component (MALI, MPAS-Albany Land Ice) within an Earth System Model (E3SM, the Energy Exascale Earth System Model) developed by the US Department of Energy.  First, we present an extrapolation technique, similar to the ISMIP6 (Ice Sheet Modeling for CMIP6) protocol, that can be used in the absence of evolving grounding lines in the ocean component.  This technique, while crude, can be used in both Greenland fjords and ice-shelf cavities as a stop-gap in situations where the ocean component cannot capture the topographic evolution (e.g. because the ocean grid is too coarse or full coupling has not yet been completed).  Second, we demonstrate progress on a fully conservative wetting-and-drying technique using the idealized MISOMIP1 (Marine Ice Sheet-Ocean Intercomparison Project, phase 1) experiments within E3SM.

How to cite: Asay-Davis, X., Begeman, C., Engwirda, D., Han, H., Hoffman, M., and Price, S.: Ice sheet-ocean coupling in an Earth System Model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13262, https://doi.org/10.5194/egusphere-egu23-13262, 2023.

EGU23-15622 | ECS | Orals | OS1.9 | Highlight

Ice shelf-ocean interaction at shallow depths needs more attention 

Ole Richter, Ben Galton-Fenzi, Kaitlin Naugthen, and Ralph Timmermann

Understanding the processes involved in basal melting of Antarctic ice shelves is important to quantify the rate at which Antarctica will lose mass. Current research of ice shelf-ocean interaction highlights deep warm water intrusions and melting along narrow grounding lines. The majority of the ice, however, lies in much shallower waters. Here we analyse the vertical structure of previously published Antarctic-wide estimates of ice shelf basal melting derived from satellites and ice shelf buttressing derived from ice sheet flow modelling. The results show that ice shelf regions with a draft shallower than 500 m account for more than 60 % of the total basal mass loss and more than 30 % of the total buttressing flux response. The oceanic processes that drive melting in shallow regions might be very different compared to the ones at depth and how well these are represented in large-scale models of Antarctic ice shelf-ocean interaction is not clear. This gap should be addressed for more accurate predictions of the Antarctic response to climate change.

 

How to cite: Richter, O., Galton-Fenzi, B., Naugthen, K., and Timmermann, R.: Ice shelf-ocean interaction at shallow depths needs more attention, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15622, https://doi.org/10.5194/egusphere-egu23-15622, 2023.

EGU23-16106 | Posters on site | OS1.9

Weddell Watch 

Svein Østerhus

Long term observations of the flow of dense waters from their area of formation to the abyss of the World Ocean, and the return flow of warm waters, are central to climate research. For the Weddell Sea an important component of such a system entail monitoring the formation of High Salinity Shelf Water (HSSW) on the continental shelf north of Ronne Ice Front, the transformation to Ice Shelf Water (ISW) beneath the floating Filchner-Ronne ice shelf, and the flux of ISW overflowing the shelf break to the deep Weddell Sea. Equally important is the return flow of warm water toward the Filchner-Ronne Ice Shelf system.

We operate several monitoring stations in the southern Weddell Sea. The systems build upon techniques and methods developed over several decades and have a proven record of high data return. Here we present plans for extending, integrating, and operating the existing long-term observatories to increase our knowledge of the natural variability of the ocean-ice shelf system, and to allow early identification of possible changes of regional or global importance.

How to cite: Østerhus, S.: Weddell Watch, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16106, https://doi.org/10.5194/egusphere-egu23-16106, 2023.

Long-term and abrupt changes in precipitation (P) patterns remain ambiguous in a warmer climate. Modern studies project that a warmer climate will cause intensification of the hydrological cycle. However, paleoclimate evidence from the warm period, i.e., the Medieval Climate Anomaly (MCA; 800-1400 AD), contradicts this because, during MCA, some regions were humid (wet), while others had arid (dry) climates. Here, we investigated the P response to variations in the temperature (T) and Atlantic Meridional Overturning Circulation (AMOC) variation throughout the Northern Hemisphere (NH) using 75 for P, 17 for the AMOC, and 48 records for T from NOAA and PAGES paleoclimate databases.

Our results show a continuous weakening trend in AMOC from the 9th to 13th centuries. The weakened AMOC has probably altered the atmospheric heat and water vapor distribution, and consequently the hydroclimate around the NH. The hydroclimate over the eastern North America and the Western Europe looks more vulnerable to weak AMOC as it shifted from warm-humid to cold-arid climates. Weak AMOC induces motion in Inter-Tropical Convergence Zone (ITCZ) southwards. Our results show signals of an ITCZ shift over equatorial Africa and southern Asia with the warm and humid response. Although warm (cold) climates are not always associated with increased (decreased) P, they may also lead to arid (humid) climates. Overall, we found that when T is higher than their average, the hydrological conditions are arid, but when T is similar or close to the average level, the conditions are humid. However, these hydroclimate responses may vary according to the regionally available water resources. Therefore, an improved understanding of long-term T variability and AMOC trend changes, specifically during warmer periods, could provide relevant insights into the present and future climates.

How to cite: Pratap, S., Markonis, Y., and R. Blöcher, J.: Understanding Atlantic Meridional Overturning Circulation and linked variations in precipitation and temperature distribution during the warmer climate, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-129, https://doi.org/10.5194/egusphere-egu23-129, 2023.

EGU23-551 | ECS | Orals | SSP2.2

Eocene seismicity and paleogeography of the Central Crimea 

Ekaterina Chizhova, Ekaterina A. Lygina, Natalia V. Pravikova, Tatiana Yu. Tveritinova, and Elizaveta A. Krasnova

The nature of Cretaceous-Eocene boundary is one of the outstanding questions of Crimea Geology. The new data are presented to show that the Cretaceous-Eocene boundary can be established in the Central Crimea very accurately by using the method of quantitative genetic analyses including the Isotope Geochemistry. Integrated lithostratigraphic investigations and Isotope composition of Carbon/Oxygen were conducted on the Cretaceous -Eocene section of the western slope of Ak-Kaya mount (Belogorsk, Crimea). Four layers of different types of rocks were investigated, where the layer 1 and 2 belong to the Maastrichtian, 3 and 4 to the Eocene.

The top of the Maastrichtian layer is characterized by a differently oriented fracture system, including large paleoseismic dislocations or a seismogenic trench. The fracture networks are connected and filled with material similar to the Eocene basal horizon including fragments of various sizes of Maastrichtian rocks.

Five microfacial types of the collected rock samples were distinguished as a result of microscopic examination. Also X-ray phase analysis, δ13С and δ18О isotopic analysis and X-ray fluorescence analysis were made to specify and compare the mineral composition of Maastrichtian and Eocene rocks. These analyzes allowed to specify paleogeographic conditions. In addition, measurements of fractures in the Cretaceous–Eocene boundary deposits were made to determine the stages of deformation of the whole structure.

As a result of the research, it was obtained:

1) throughout the entire studied geological interval, sedimentation occurred in a shallow sea of normal salinity. However, conditions were probably more humid in the Eocene, based on lower salinity values.

2) Three major stages of deformation were identified: pre-Eocene, Eocene, and post-Eocene.

3) The average temperature of the formation of Maastrichtian rocks is 19-22°C, and Eocene rocks is 24-27°C. The increase in temperature up to 38°C during the formation of the Eocene basal horizon may be associated with the global climatic event EECO (Early Eocene Climate Optimum). The synchronicity of the formation of steep submeridional fractures and the basal horizon of the Eocene has been proved. It is shown that the Eocene deformation stage corresponds to the formation of paleoseismic dislocations during the main phase of tectonic activity in the Pontids (Eastern Turkey).

How to cite: Chizhova, E., Lygina, E. A., Pravikova, N. V., Tveritinova, T. Yu., and Krasnova, E. A.: Eocene seismicity and paleogeography of the Central Crimea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-551, https://doi.org/10.5194/egusphere-egu23-551, 2023.

EGU23-2714 | Orals | SSP2.2 | Highlight

Exploring links between the North Atlantic Igneous Province and Paleocene–Eocene climate change using sedimentary mercury 

Joost Frieling, Tamsin Mather, Morgan Jones, Isabel Fendley, Weimu Xu, Christian Berndt, Sverre Planke, and Carlos Alvarez Zarikian and the IODP Expedition 396 scientists

The North Atlantic Igneous Province (NAIP), a large igneous province (LIP), was emplaced between ~62 and 50 million years ago (Ma), with a voluminous burst of volcanic activity centred around 56-54 Ma. Global paleoclimate reconstructions from this Paleocene and Early Eocene interval indicate progressively warmer conditions, with several superimposed warming events or ‘hyperthermals’, such as the PaleoceneEocene Thermal Maximum (PETM; 56 Ma). These hyperthermals represent transient massive perturbations to the carbon cycle, marked by substantial global warming, ocean acidification and negative stable carbon isotope excursions. International Ocean Discovery Program Expedition 396 to the Mid-Norwegian continental margin recovered a suite of PaleoceneEocene sedimentary and igneous materials. This notably includes a unique and extremely expanded succession comprising of up to ~80m of PETM (ash-rich) sediments and volcanic ash layers infilling a hydrothermal vent crater. The craters on the Mid-Norwegian margin and similar structures associated with other LIPs were previously identified as surface expressions of a potent carbon release mechanism: the venting of thermogenic carbon generated in the thermal aureoles around volcanic dikes and sills intruded into the underlying sedimentary basins.

In recent years, much progress has been made towards understanding the role of deep earth processes and particularly LIP volcanism on paleoclimate through the application and refinement of proxies as sedimentary mercury (Hg) content. Large scale and especially LIP volcanism are considered important Hg emitters that may result in increased sedimentary Hg content. Here, we present high-resolution bulk sedimentary Hg content data from the sedimentary strata within the hydrothermal crater, spanning the PETM. We use our new data with biostratigraphic, stable carbon isotope, and lithological constraints, to shed light on the timing of hydrothermal crater formation, duration and re-activation of hydrothermal activity within the crater after formation. Finally, these new findings are placed in a global Hg and carbon cycle framework to assess the timing, characteristics, and impact of NAIP activity during the PETM.

How to cite: Frieling, J., Mather, T., Jones, M., Fendley, I., Xu, W., Berndt, C., Planke, S., and Alvarez Zarikian, C. and the IODP Expedition 396 scientists: Exploring links between the North Atlantic Igneous Province and Paleocene–Eocene climate change using sedimentary mercury, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2714, https://doi.org/10.5194/egusphere-egu23-2714, 2023.

EGU23-5618 | ECS | Posters on site | SSP2.2

A Novel Approach to Constraining Carboniferous Tidal Currents using Bedforms in Tidal Rhythmites 

Jennifer Hewitt, Jaco Baas, Justyna Bulawa, Amy Ewing, Brennan O'Connell, and Mattias Green

A novel methodology shows that the dimensions of current ripples within tidal rhythmites can be used as a proxy for tidal current velocity, allowing us to contribute to the validation of numerical tidal model simulations. Our understanding of changing tides through geological history is facilitated by tidal simulations, which are generally poorly constrained due to the limited availability of proxy data. We aim to rectify this by developing a new type of geological proxy for tides based on sedimentary textures and structures, as bedforms are widely reported but uncommonly measured in the literature. The Carboniferous is a particularly data-rich time period with globally abundant tidal lithofacies including tidal rhythmites; successions of rhythmically alternating coarser and finer layers which can be used to describe tidal cyclicity, changes in the Earth – Moon system, and palaeoenvironmental conditions. Using data collected from a previously unstudied succession of Late Carboniferous (318 Ma) tidal rhythmites in Pembrokeshire, South Wales, UK, and empirical relationships identified through a series of flume studies in the literature, we deducted that the current ripples in our studied outcrop were formed at tidal flow velocities ranging between 0.28 and 0.34 m s-1. The latest palaeogeographical reconstructions depict South Wales as entirely continental, however the studied section revealed evidence of deposition in a shallow-marine palaeoenvironment. Identifying these palaeoenvironmental inaccuracies such as these allows us to rectify the palaeogeographical reconstructions; once tuned, the numerical tidal model simulation matched well with our proxy results. These promising findings demonstrate proof-of-concept of utilising bedforms as a proxy for palaeotides as well as its feasibility to validate tidal model simulations of other geological time periods and areas.  

How to cite: Hewitt, J., Baas, J., Bulawa, J., Ewing, A., O'Connell, B., and Green, M.: A Novel Approach to Constraining Carboniferous Tidal Currents using Bedforms in Tidal Rhythmites, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5618, https://doi.org/10.5194/egusphere-egu23-5618, 2023.

EGU23-5657 | ECS | Orals | SSP2.2

Planktonic Foraminiferal δ13Corg as a novel proxy for Carbon Cycling 

Tommaso Paoloni, Babette Hoogakker, Helen Grant, Patrick Keenan, and Helliot Hamilton

It has been hypothesized that lower atmospheric CO2 concentrations and lower temperatures during glacial times caused the enrichment of carbon isotopes of particulate organic material (δ13Corg-POM) produced in the surface ocean. Some downcore measurements of organic carbon isotopes of bulk sediments show such a trend, however, others do not. The lack of a coherent picture could be due to issues relating to the bulk sediments, including diagenetic alteration, the nature of the organic material, input of allochthonous material, and sediment redistribution.

Recent work by Hoogakker et al. (2022) shows that planktonic foraminifera-bound organic carbon δ13C values (δ13CFBOM) are remarkably similar to those of δ13Corg-POM. Here we present the first down-core organic carbon isotope record of planktonic foraminifera-bound organic carbon (δ13CFBOM) from the Southern Ocean (ODP Site 1088), to test for a glacial enrichment in δ13Corg-POM. The samples (Globigerina bulloides, Globorotalia truncatulinoides, and G. inflata) cover the last 20,000 years.

Our δ13CFBOM results show a slight positive trend toward the Last Glacial Maximum (LGM), in accordance with the hypothesized δ13Corg-POM trend, but not to the extent as shown in some bulk sediments from more tropical latitudes. We discuss our results in the context of predicted past δ13Corg-POM using ice core atmospheric pCO2 concentrations, G. bulloides calcification DIC (from inorganic carbon isotopes), and temperature (using Mg/Ca). 

How to cite: Paoloni, T., Hoogakker, B., Grant, H., Keenan, P., and Hamilton, H.: Planktonic Foraminiferal δ13Corg as a novel proxy for Carbon Cycling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5657, https://doi.org/10.5194/egusphere-egu23-5657, 2023.

EGU23-5912 | Posters on site | SSP2.2

A comparison study of Mg/Ca-, alkenone- and TEX86-derived temperatures for the Brazilian Margin during Marine Isotope Stages 6–5 

André Bahr, Andrea Jaeschke, Alicia Hou, Christiano M. Chiessi, Ana Luiza Spadano Albuquerque, Janet Rethemeyer, and Oliver Friedrich

The reconstruction of accurate sea-surface temperatures (SST) is of utmost importance due to the central role of the ocean in the global climate system. Yet SST-proxies might be influenced by a number of environmental processes that may potentially bias the accurate reconstruction of the target variable. Here, we investigate the fidelity of SST reconstructions for the Western Tropical South Atlantic (WTSA) for Marine Isotope Stages (MIS) 6–5, utilizing a core collected off eastern Brazil at ~20°S. This interval was selected as previous SST estimates based on Mg/Ca ratios of planktic foraminifera suggested a peculiar pooling of warm surface waters in the WTSA during MIS 6 despite glacial boundary conditions. To ground-truth the Mg/Ca-based SST data we generated SST reconstructions from the same core using both, alkenone and TEX86 paleothermometers. Comparison with alkenone-based temperature estimates corroborate the previous Mg/Ca-based SST reconstructions, supporting the presumed warm-water anomaly during MIS 6. In contrast, while core top samples indicate that TEX86-derived temperatures represent annual mean SST, the TEX86-derived paleo-temperatures are up to 6°C colder than Mg/Ca- and alkenone-based SST reconstructions. We interpret the periods of anomalously cold TEX86-temperatures as a result of a vertical migration of the TEX86 producers (heterotrophic marine Thaumarchaeota) to deeper water depths in response to an increase in food availability during phases of enhanced fluvial suspension input.

How to cite: Bahr, A., Jaeschke, A., Hou, A., Chiessi, C. M., Spadano Albuquerque, A. L., Rethemeyer, J., and Friedrich, O.: A comparison study of Mg/Ca-, alkenone- and TEX86-derived temperatures for the Brazilian Margin during Marine Isotope Stages 6–5, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5912, https://doi.org/10.5194/egusphere-egu23-5912, 2023.

The ~1800–800 Ma period is known as the 'Boring Billion (BB)' because of the relative stasis of the carbon isotope record during this time. However, geochemical data from the Paleo-Mesoproterozoic strata deposited in different areas indicate heterogeneity and complexity of the oxygen contents in the oceans, which hampers paleoenvironmental reconstructions from this period. In addition, very little research has been carried out on the Palaeoproterozoic strata of the North China Craton (NCC). In this study, we report analyses of U-Pb isotopes, elemental abundances, Fe speciation, and molecular markers from the Huangqikou formation in the northwestern part of the Ordos Basin (OB), NCC. The Huangqikou formation was deposited in the rift valley at about 1736 Ma. Our new data, combined with previous analyses, suggest that the warm and humid depositional environment of the Huangqikou formation in the Helanshan area evolved from a marine foreshore setting to a marine backshore setting, with increasing degree of seawater hypoxia. But a relatively oxygenated environment corresponded to the lower part. On the other hand, the Huangqikou formation in the Zhuozishan area evolved from a terrestrial deltaic environment to a marine foreshore environment, with cumulatively reducing conditions. This study points out that the late Paleoproterozoic strata deposited in the western part of the NCC might mainly formed in reduced seawater. But some degree of oxidation had occurred in the surface water during this period, which proves the oxygenation of the surface environment during the early period of Earth evolution.

How to cite: Ma, Q., Zhou, Y., and Zerkle, A.: Sea water chemistry in the late Paleoproterozoic: Insight from the Huangqikou formation, western part of the North China Craton, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5965, https://doi.org/10.5194/egusphere-egu23-5965, 2023.

EGU23-6265 | ECS | Posters virtual | SSP2.2

Geochemical and palynological analyses of the Shivee Ovoo coal deposit (Choir-Nyalga basin, Central Mongolia)-palaeoclimatic implications 

Nyamsambuu Odgerel, Niiden Ichinnorov, Hitoshi Hasegawa, Bat Orshikh Erdenetsogt, Luvsanchultem Jargal, and Sukhbat Purevsuren

The Shivee Ovoo is one of the big industrial mine of continental Choir-Nyalga basin in central Mongolia. The depositional environment and petroleum source rock potential of major coal-bearing strata in the Choir-Nyalga basin has been studied (Erdenetsogt et al., 2009, 2022), and age of the deposits (Khukhteeg Formation) has been assigned to Aptian-Albian on the basis of radiometiric age of intercalated tuff  (Hasegawa et al. 2018). We carried out a geochemical and palynological study on 10 samples (47 m mine wall) collected from Shivee Ovoo.

Geochemical analysis completed for major, trace, and rare earth elements (REE) in the SGS laboratory in Mongolia. Palynological study was carried out at the Basic Research Laboratory of National University of Mongolia. Fossil palynomorphs were investigated by LM using single grain technique (Hesse et al., 2009). As a result of geochemical analysis of major oxides, SiO2   hasthe highest content with 44.2%-66.9%. Following this Al2O3 (16.24%-19.14%), K2O (1.03%-4.09%) and TFe2O3 (total iron) (1.75%-3.36%) are the second most abundant oxides. The rest of the oxides (MgO, Na2O, P2 O5, MnO, CaO and TiO2) have concentration of less than 2.31%. The Al/Si ratio was between 0.26-0.41, SiO2 is related with quartz. The chemical weathering parameter CIA varies 71.3-81.6, with an average of 78.97, showing intermediate chemical weathering. Also, the Zr/Rb ratio 0.93 it can be seen the hydrodynamic force was weak. Generally, V/Cr:1.18, U/Th: 0.4, δU:1.68 implies oxidation environment. All weathering parameters show oxidation environment during sedimentation indicating that the paleoclimate is a warm and humid.

Palynological data,  6 of the 10 samples contain rich palynological fossils providing important information on the paleovegetation and paleoclimates. Sporomorph plants in the Khukhteeg formation contain 23 genera, 32 species. The palynological percentages of plants Cyathidites 32%, Baculatisporites 20%, Osmundacidites 11.1%, Gingkocycadopites 11%. Dominant plants mainly belong to the Filicales of the ferns represented by Osmundacidites and Dicksoniaceae. The plants 63.1% grow swamps, wet valleys, subtropical temperate zones. This palynological and geochemical data indicate that the at 47m depth Khukhteeg formation had a warm subtropical climate was at that time.

REFERENCES

Erdenetsogt, B. O., Lee, I., Bat-Erdene, D., & Jargal, L. (2009). Mongolian coal-bearing basins: geological settings, coal characteristics, distribution, and resources. International Journal of Coal Geology80(2), 87-104.

Erdenetsogt, B. O., Hong, S. K., Choi, J., & Lee, I. (2022). Depositional environment and petroleum source rock potential of Mesozoic lacustrine sedimentary rocks in central Mongolia. Marine and Petroleum Geology140, 105646.

Hasegawa, H., Ando, H., Hasebe, N., Ichinnorov, N., Ohta, T., Hasegawa, T., Yamamoto, M., Li, G.,  Erdenetsogt, B-O., Ulrich, H., Murata, T.,  Shinya, H.,  Enerel, G., Oyunjargal, G., Munkhtsetseg, O., Suzuki,N., Irino, T.,  Yamamoto, K., (2018). Depositional ages and characteristics of Middle–Upper Jurassic and Lower Cretaceous lacustrine deposits in southeastern Mongolia. Island Arc. 2018; e12243. 17 https://doi.org/10.1111/iar.12243

Hesse, M., Halbritter.H., Zetter, R., Weber, M., Buchner, R., Frosch-Radivo,A. & Ulrich,S. (2009). Pollen terminology-an illustrated handbook. Wein: Springer.

 

How to cite: Odgerel, N., Ichinnorov, N., Hasegawa, H., Erdenetsogt, B. O., Jargal, L., and Purevsuren, S.: Geochemical and palynological analyses of the Shivee Ovoo coal deposit (Choir-Nyalga basin, Central Mongolia)-palaeoclimatic implications, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6265, https://doi.org/10.5194/egusphere-egu23-6265, 2023.

EGU23-6929 | Posters on site | SSP2.2

Aspects of the geomorphology of the Late Palaeozoic glaciated landscape of Namibia as revealed by photogrammetry 

Daniel Le Heron, Christoph Kettler, Pierre Dietrich, Neil Griffis, Isabel Montañez, and Ricarda Wohlschlägl

The geometry of unconformities carved by deep time ice sheets is often obscured and restricted by discontinuous exposure, or outcrop conditions that do not readily permit the examination of glacial unconformities (for example, steeply dipping strata). Here, we present new uncrewed aerial vehicle (UAV) data from selected outcrops across northern, central and southern Namibia to shed new light on the nature of the basal Dwyka unconformity. This includes the onlap relationship of basal diamictites onto the Gomatum palaeo-fjord system in northern Namibia, highly complex mapped ice flow orientations elsewhere in the northern Kaokoveld, previously undiscovered grooves along the Fish River area, and a spectacular set of subglacial grooves along the border with South Africa along the Orange River. In the latter two cases, photogrammetric methods integrating orthophotos and digital elevation models reveal the presence of subglacial grooves for the first time, since the features are too subtle to observed using conventional approaches at outcrop. Furthermore, subglacial grooves often show different orientations to striations and fabrics measured in overlying diamictites, raising fresh questions about the nature of small-scale flow variations beneath Late Palaeozoic ice sheets.

How to cite: Le Heron, D., Kettler, C., Dietrich, P., Griffis, N., Montañez, I., and Wohlschlägl, R.: Aspects of the geomorphology of the Late Palaeozoic glaciated landscape of Namibia as revealed by photogrammetry, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6929, https://doi.org/10.5194/egusphere-egu23-6929, 2023.

EGU23-7230 | ECS | Posters on site | SSP2.2

Late Paleozoic glaciated landscape in northern Africa as an outstandingly well-preserved analogue to Quaternary deglaciated areas 

Ricarda Wohlschlägl, Christoph Kettler, Daniel Le Heron, and András Zboray

The Ennedi sandstone plateau in Chad in north-central Africa exposes an outstanding example of an ice stream paleo-landscape that is of Paleozoic age. This assemblage of paleo-glacial structures is of comparable quality to that found in Quaternary deglaciated landscapes. A wide range of exceptionally well-preserved proglacial, ice-marginal and subglacial features are visible due to the absence of vegetation in the desert environment. Paleo-ice stream pathways contain swarms of large-scale glacial lineations distributed over the whole plateau that tell the story of a dying ice sheet during the late Paleozoic. A putative grounding zone wedge within a paleo-ice stream pathway allows the position of the former coastline to be reconstructed as it is assumed that ice streams terminated into a former ocean basin. Based on the convex topography and its position orthogonal to the large-scale glacial lineations, we present the first geomorphological interpretation of a grounding zone wedge in the Paleozoic record. Additionally, a unique system of inverted channel sediments in close proximity to glacial structures might record different phases of meltwater release during ice retreat. In summary, the Ennedi paleo-glacial landscape provides an excellent natural laboratory to understand the spatial relationship between subglacial, ice-marginal and proglacial components of a former ice sheet, with emphasis on exceptional outcrop quality that can be used to further our understanding of some Quaternary glaciated landscapes.

How to cite: Wohlschlägl, R., Kettler, C., Le Heron, D., and Zboray, A.: Late Paleozoic glaciated landscape in northern Africa as an outstandingly well-preserved analogue to Quaternary deglaciated areas, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7230, https://doi.org/10.5194/egusphere-egu23-7230, 2023.

EGU23-7618 | ECS | Posters virtual | SSP2.2

Disentangling regional and global signatures from benthic foraminifera records during the Late Miocene-Early Pliocene Biogenic Bloom (IODP Site U1506 and ODP Site 1085) 

Maria Elena Gastaldello, Claudia Agnini, Thomas Westerhold, Anna Joy Drury, Rupert Sutherland, Michelle K. Drake, Adriane R. Lam, Gerald R. Dickens, Edoardo Dallanave, Stephen Burns, and Laia Alegret

The Late Miocene-Early Pliocene Biogenic Bloom (~ 9-3.5 Ma) is a paleoceanographic event defined by anomalously high marine biological productivity and associated with changes in the marine carbon cycle. Marine sedimentary records in the Indian, Pacific, and Atlantic oceans, point to a significant increase in primary productivity across low-latitude oceanic regions maintained for several millions of years. Surface primary productivity is typically limited by the availability of nutrients; whose residence times are fairly short in the global ocean. Therefore, the global nature and the multimillion years duration of the Biogenic Bloom make this event a paleoceanographic puzzle. Two main explanations for these anomalously high productivity conditions have been proposed: a major redistribution of nutrients triggering an intensification of regional upwelling; or an absolute increase of nutrients delivery to the oceans. We investigated the Biogenic Bloom at IODP Site U1506 (Tasman Sea, southwest Pacific Ocean, 1505 m water depth) and at ODP Site 1085 (Cape Basin, southeast Atlantic Ocean, 1713 m water depth). For these sites we generated implemented age models and quantitative benthic foraminiferal records across an interval spanning from the Tortonian (Late Miocene) to the Zanclean (Early Pliocene). The benthic foraminiferal assemblage analysis shows that the Biogenic Bloom was a complex, multiphase event rather than a single uniform period of sustained high marine water productivity. Both sites record changes that can be interpreted in terms of modification of productivity conditions. Intervals with low diversity and abundant opportunistic and phytodetritus exploiting taxa (PET) are indicative of transient pulsed food supply, high oxygen levels, and oligotrophic conditions. Intervals characterized by increased diversity, higher relative abundance of uvigerinids and buliminids, and relative lower abundance of PET instead suggest lower oxygen and /or more eutrophic conditions. However, the two sites show a different taxonomic composition of the benthic foraminiferal assemblages. The dominating PET comprise distinct species at different the study sites, with Globocassidulina crassa and Globocassidulina subglobosa displaying high abundance at Site U1506, and Epistominella exigua and Alabaminella weddellensis at Site 1085. While showing common features, the Biogenic Bloom is also characterized by unique regional responses at different study sites which highlight the need for further high-resolution records to provide global mechanisms and dynamics for the Biogenic Bloom event.

Acknowledgments

The authors acknowledge funding from University of Padova DOR grant, CARIPARO Foundation Ph.D. scholarship, Fondazione Ing. Aldo Gini scholarship, and Spanish Ministry of Economy and Competitiveness and FEDER funds (PID2019-105537RB-I00).

How to cite: Gastaldello, M. E., Agnini, C., Westerhold, T., Drury, A. J., Sutherland, R., Drake, M. K., Lam, A. R., Dickens, G. R., Dallanave, E., Burns, S., and Alegret, L.: Disentangling regional and global signatures from benthic foraminifera records during the Late Miocene-Early Pliocene Biogenic Bloom (IODP Site U1506 and ODP Site 1085), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7618, https://doi.org/10.5194/egusphere-egu23-7618, 2023.

EGU23-7830 | ECS | Posters on site | SSP2.2

Carbon and nitrogen isotope stratigraphy of the Cambrian SPICE record in the UK 

Francesca Warren, Darren R. Gröcke, Martin Smith, and Matthias Sinnesael

Carbon isotope fluctuations have been determined globally within the late Cambrian with particular focus on the Steptoean Positive Carbon Isotope Excursion (SPICE) and the negative Hellnmaria-Red Tops Boundary/Top of the Cambrian Excursion (HERB/TOCE). These events correspond to global anoxia/euxinia, increased global weathering of organic rich material and a shift in dissolved inorganic carbon availability. We have extended our knowledge of SPICE and HERB/TOCE in the UK by conducting coupled carbon and nitrogen isotope analysis of cores (Merevale 1, 3) and quarry samples from Warwickshire (Oldbury Quarry). Our organic δ13C record replicates the changes previously published for SPICE in other global records. The bulk sediment δ15N record reveals a rapid positive excursion at the start of SPICE followed by a gradual decline through the remaining SPICE interval. We interpret the δ15N record as reflecting expansion of the oxygen minimum zone into the upper water column and replacing nitrification with denitrification processes. Denitrification is also supported during the SPICE interval from previously published iron-speciation data from the same cores. The negative δ13C HERB/TOCE record is coupled with a more subtle δ15N positive excursion. There is a paucity of organic carbon isotope records through this time interval, and hence a lack of global comparability is possible. The shift in δ13C and δ15N, coupled with changes in redox conditions in Cambrian oceans may also reflect biological shifts between red and green phytoplankton superfamilies making up the upper water column community. Additional research on organic carbon, nitrogen and redox proxies are required to ascertain the link between phytoplankton superfamily dominance, species richness, diversity and/or the onset of the Phytoplankton Revolution and the Great Ordovician Biodiversity Event.

How to cite: Warren, F., Gröcke, D. R., Smith, M., and Sinnesael, M.: Carbon and nitrogen isotope stratigraphy of the Cambrian SPICE record in the UK, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7830, https://doi.org/10.5194/egusphere-egu23-7830, 2023.

EGU23-8260 | ECS | Posters on site | SSP2.2

Climatic differences between Estonia and Svalbard during the second half of the Holocene 

Katre Luik and Hannes Tõnisson

This overview compares various environmental publications to find out the contrasts and similarities in climatic conditions in the last 6000 years in Estonia and Svalbard.

Both regions with their geographical differences are sensitive to climate change, Estonia on the meeting borderline with maritime and continental air masses and Svalbard at the end of the North Atlantic Cyclone track with very changeable climatic conditions. The study aims to find out how the colder and warmer periods differ in the larger time scale such as the Middle and Late Holocene.

The Holocene in Estonia and Svalbard experienced dramatic climate changes including several cold and warm episodes.  A variation of paleoclimatic records was compared with other geological proxies (lake sediments, glaciers, pollen, coastal and dune belt formation data presented in scientific publications) and a good correspondence between cold and warm climate periods was found in both areas. 

The climate conditions were warm and dry during the Middle Holocene with step wise cooling, no glacigenic input in Svalbard, water level in Estonian lakes extremely low; abrupt decrease in temperature appeared around 4000 BP and 2500 BP in both areas. Approximately 4500 years BP, North Atlantic Oscillation (NAO) changed its phase from primarily positive NAO conditions to weakly positive NAO roughly for the next 2500 years. Around 4000 BP dry conditions changed to humid in Estonia and remained so for a thousand years (broad-leaved trees declined and pine forests became dominant approximately 3000 BP; stormy period ∼3300 - 3000 BP recorded in ancient beach formations), the climate likely shifted towards maritime; in Svalbard more intense precipitation stages were recorded in lakes runoff ∼3150 – 3000 BP. The next 2000 years the temperature appeared stabilised, Estonia mostly dry (more continental climate again) with a strong storm period characterised by large beach ridges  in the NW of the country formed ∼2300 - 2000 BP, Svalbard cool and moist with possible glacier advance around 2000 BP and a 400-year humid phase in 1600 - 1350 BP. The Little Ice Age (LIA) occurred around 600 - 100 BP in Svalbard and 500 - 200 BP in Estonia. During the LIA, precipitation and storminess increased in Svalbard whereas the Estonian climate turned more continental (dry and cool) with prevailing northern storms, clearly reflecting in the morphology and shape of dunes formed during this period.

Despite the distinct climatic conditions between Estonia and Svalbard there's no major differences in climate in the last 6000 years, still some noticeable shifts occur. Several detectable changes taking place in both areas were noticed around 3300 - 3000 BP: weaker NAO+ phase, humid conditions in Svalbard, exceptionally stormy period in Estonia followed by explicit changes in dominant tree species. During LIA more continental climate was dominating in Estonia while maritime influence was increasing in Svalbard. Similar opposite  shifts in the past cannot be ruled out and need further investigations and more precise dating information. 

How to cite: Luik, K. and Tõnisson, H.: Climatic differences between Estonia and Svalbard during the second half of the Holocene, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8260, https://doi.org/10.5194/egusphere-egu23-8260, 2023.

EGU23-8719 | ECS | Orals | SSP2.2

Extraterrestrial 3He-based reconstruction of sedimentation rates across the Paleocene-Eocene transition at ODP Site 1209 (North Pacific) 

Nicolas Pige, Guillaume Suan, Pierre Henri Blard, and Emanuela Mattioli

Numerous hyperthermal events have been documented through the Paleocene-Eocene transition. The best known hyperthermal event is the Paleocene-Eocene Thermal Maximum (PETM; around 56Ma), a period that led to surface and bottom water warming of about 5°C within a few millennia at tropical latitudes. It is therefore considered as one of the best analogues of current global warming. The PETM is also characterized by an abrupt 3-4 per mil negative δ13C excursion in deep marine core sediments and by a thin clay-rich layer associated with the PETM onset, most often interpreted as carbonate dissolution due to the shoaling of the CCD. The duration represented by these clays and carbonates is of peculiar interest to constrain the exported carbonate production dynamics of surface ocean and its dissolution throughout the water column. This is key to produce realistic carbon budgets across hyperthermal events.

To this end, we generated a new 4 Ma (57.5-53.5) record of extraterrestrial 3He-derived sedimentation rates from pelagic sediments recording at least 10 hyperthermal events at ODP Site 1209 (North Pacific). Our main results indicate that carbonate sedimentation dropped drastically during the PETM onset (minimum of 0.02 cm/ka) and recovered rapidly during the recovery phase of the event (around 0.7 cm/ka). Surprisingly, the sedimentation rate is low (0.3 cm/ka) after the recovery until the Eocene Thermal Maximum 2 (ETM2; around 54Ma). After this major event, the sedimentation rate increased abruptly (0.7 cm/ka) over the last 500 ka of the studied interval due to the overabundance of Zygrhablithus bijugatus a large rod-shaped nannofossil whose ecology is poorly understood yet.

Comparisons between the new record of extraterrestrial 3He-derived sedimentation rate and dissolution proxies from this and previous studies lead us to challenge the widely accepted model previously proposed for hyperthermal events, which assumes that the CaCO3 accumulation is mainly controlled by dissolution.

How to cite: Pige, N., Suan, G., Blard, P. H., and Mattioli, E.: Extraterrestrial 3He-based reconstruction of sedimentation rates across the Paleocene-Eocene transition at ODP Site 1209 (North Pacific), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8719, https://doi.org/10.5194/egusphere-egu23-8719, 2023.

EGU23-8831 | Orals | SSP2.2

Sea surface temperature evolution of the North Atlantic Ocean across the Eocene-Oligocene Transition 

Kasia K. Sliwinska, David K. Hutchinson, Devika Varma, Tirza Weitkamp, Emma Sheldon, Diederik Liebrand, Helen K. Coxall, Agatha M. de Boer, and Stefan Schouten

When a permanent ice cap developed on Antarctica during the Eocene–Oligocene transition (EOT; ~34.44 to 33.65 million years ago (Ma)), Earth witnessed a transition from a greenhouse towards a glacially driven climate. Evidence of high-latitude cooling and increased latitudinal temperature gradients across the EOT has been found in both marine and terrestrial environments. However, the timing and magnitude of temperature change in the North Atlantic remains poorly constrained.

Here, we used two independent organic geochemical palaeothermometers derived from (i) alkenones and (ii) Glycerol Dialkyl Glycerol Tetraether (GDGT) lipids, to reconstruct sea surface temperature (SST) evolution across the EOT from the southern Labrador Sea (Sites: ODP 647 and DSDP 112). In the Labrador Sea alkenones do not appear until the earliest Oligocene (both sites) while GDGT lipids (analysed in Site 647 only) provides a well-constrained temperature record across the EOT.  

Our SST records provide the most detailed record for the northern North Atlantic through the 1 Myr leading up to the EOT onset, and reveals a distinctive cooling step of ~3 ºC (from 27 to 24 ºC), between 34.9 and 34.3 Ma, ~500 kyr prior to Antarctic glaciation. This cooling step, when compared visually to other SST records, is asynchronous across North and South Atlantic sites. This illustrates a considerable spatiotemporal variability in SST evolution in the northern sector of the North Atlantic and the Norwegian-Greenland Sea. Overall, the cooling step fits within a phase of general SST cooling recorded across sites in the North Atlantic in the 5 Myr interval bracketing the EOT.

We used a modelling study (GFDL CM2.1) to try and reconcile the observation of pre-EOT cooling with the hypothesis that Atlantic Meridional Overturning Circulation (AMOC) switched on or intensified on the lead up to the EOT, which would be expected to have warmed the North Atlantic region. Results suggest that a reduction in atmospheric CO2 from 800 to 400 ppm may be sufficient to counter warming from an AMOC start-up. In the model, the AMOC start-up is initiated during closure of the Arctic–Atlantic gateway.

While the model simulations applied here are not yet in full equilibrium, and the experiments are idealized, the results, together with the proxy data, highlight the heterogeneity of basin-scale surface ocean responses to the EOT thermohaline changes, with sharp temperature contrasts expected across the northern North Atlantic as positions of the subtropical and subpolar gyre systems shift in response to climatic and oceanic adjustments.

How to cite: Sliwinska, K. K., Hutchinson, D. K., Varma, D., Weitkamp, T., Sheldon, E., Liebrand, D., Coxall, H. K., de Boer, A. M., and Schouten, S.: Sea surface temperature evolution of the North Atlantic Ocean across the Eocene-Oligocene Transition, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8831, https://doi.org/10.5194/egusphere-egu23-8831, 2023.

EGU23-10010 | ECS | Orals | SSP2.2

Alkenones confirmed in sediments from high southern latitudes during the Cretaceous and Paleocene: results from the Transkei Basin (IODP Site U1581) 

Kelsey Doiron, Simon Brassell, Peter Bijl, Thomas Wager, Jens Herrle, Gabriele Uenzelmann-Neben, Steven Bohaty, and Laurel Childress and the Expedition 392 Science Party

Preliminary examination of the biomarker composition of Paleocene to Campanian (~63-74 Ma) organic-rich sediments recovered from the Transkei Basin (Hole U1581B; 35° 41’S, 29° 39’E), offshore South Africa, during IODP Expedition 392 reveals suites of alkenones and alkyl alkenoates derived from haptophyte algae. This discovery augments evidence for the temporal continuity of their occurrence since the early Aptian and expands their paleogeographic range to high southern latitudes (~60°S) during the Cretaceous and Paleocene. In addition, the similarity of alkenone distributions between Maastrichtian and Danian samples suggests a conformity in the biosynthetic pathways for their production across the K/Pg boundary likely attesting to the survival of their source haptophytes and recovery after the extinction event. Alkenone distributions in the Transkei Basin sediments are dominated by series of C37 to C40 diunsaturated components and remain broadly consistent throughout the Cretaceous to Paleocene stratigraphic  succession. The presence of both the C38 alkadien-2-one and C39 alkadien-3-one represents the earliest recognition of these compounds thereby extending the advent for biosynthesis of both methyl and ethyl alkenones to the Campanian (~74 Ma). These sediments also contain C37 methyl and both C38 and C40 ethyl alkadienoates. No C37, C38 or C39 triunsaturated alkenones were detected in the Paleocene through Campanian succession but minor amounts of a C40 alkatrien-3-one were confirmed in Cretaceous samples based on its elution time and diagnostic mass spectrum. This finding raises the question why only the C40 triunsaturated component is observed, coupled with pervasive evidence that C37 to C39 triunsaturated alkenones emerge after the Early Eocene Climatic Optimum (EECO). Among extant haptophytes, C40 alkenones occur in species within phylogenic Group II, notably Isochrysis, but are absent in extant marine species comprising phylogenic Group III. These observed distributions of alkenones in the marine realm can be best explained as evidence for contributions from both Isochrysidaceae and Noelaerhabdaceae following their divergence in the early Cretaceous.  

How to cite: Doiron, K., Brassell, S., Bijl, P., Wager, T., Herrle, J., Uenzelmann-Neben, G., Bohaty, S., and Childress, L. and the Expedition 392 Science Party: Alkenones confirmed in sediments from high southern latitudes during the Cretaceous and Paleocene: results from the Transkei Basin (IODP Site U1581), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10010, https://doi.org/10.5194/egusphere-egu23-10010, 2023.

EGU23-10905 | Posters on site | SSP2.2

Tracking climate changes in the Gulf of California and the Eastern Tropical Pacific Ocean during the past 18,000 yr 

Ligia Perez-Cruz, Mauricio Velázquez-Aguilar, Andrea Lefranc-Flores, Abdel Siffedine, and Jaime Urrutia-Fucugauchi

The location, sedimentology, and oceanographic characteristics of the southern Gulf of California make it suitable for investigating the Quaternary climate changes of the Eastern Tropical Pacific Ocean (ETPO). We investigate changes in precipitation, ocean patterns and variations in paleoproductivity in the Eastern Tropical Pacific Ocean related to insolation, migrations, and dynamics of the of Intertropical Convergence Zone (ITCZ), the North America Monsoon (NAM), and inter-hemispheric teleconnections. Proxy records are obtained from sediments in the marginal Alfonso Basin, situated in the southwestern sector of the Gulf of California near its junction with the Pacific Ocean. The age model was based on eleven radiocarbon dates, the MARINE 20 calibration curve, and a reservoir age of 253 + 18 years. 
High-resolution records of elemental geochemistry, magnetic properties, and radiolarian assemblages are used to track climate changes in the tropical climate system at millennial and centennial time scales over the past 18,500 yr. Geochemical and magnetic proxies revealed an increase of precipitation at  ~17,500 and 16,536 yr, in the Bolling Allerod (from ~14,988 to 14,057 yr), and during the early Holocene. Humid conditions predominated between ~7,404 and 5,200 cal yr BP. Records indicate a climatic shift at ~4,860 cal yr BP, suggesting increased aridity and the strength of winds to continue through the late Holocene. Roughly 4000 cal yr BP the productivity increased as a result of the intensification of the winds. Paleoprecipitation changes are associated with ITCZ latitudinal migration and the NAM responding to insolation changes during the Holocene. Aeolian and fluvial inputs, marked by variations in Ti, K, Fe, Zr/Ti and magnetic properties, indicate that precipitation-controlled changes in summer monsoon rainfall primarily forced terrigenous supply throughout the mid-Holocene. We propose that these conditions arise from the northern hemisphere's high insolation at low latitudes, with the average position of the ITCZ migrating northward. Development of the NAM amplifies the seasonality and promotes increased precipitation during summer seasons. 
During the late Holocene, terrigenous input appears mainly controlled by the intensification of the NW winds. The record indicates a drop-in precipitation and abrupt enhancement of Aeolian activity. 
Radiolarian assemblages reveal the upper layers of two water masses (TSW and GCW), suggesting that the advection of coastal currents and mesoscale features controlled these conditions. The dominance of  Phormostichoartus corbula, Lithomelissa thoracites, and Arachnocorallium calvata, surface dwellers species reveal the Gulf of California Water and relatively high productivity during the BA, and in the transition to the middle to late Holocene, Botryostrobus aquilonaris suggests that during the deglaciation, (~17,468 to 15,426 yr), and at ~12,604 yr the occurrence of the California Current in the Alfonso Basin. Tetrapyle octacantha group represents the dominance of Superficial Tropical Water in the Alfonso basin, associated with conditions of marked stratification in the water column and oligotrophic conditions in the superficial layer during the Holocene Climatic Optimum and the Medieval Warm Period. which fluctuated due to variations in mesoscale gyres and also coastal upwellings off the western coast could contribute.

How to cite: Perez-Cruz, L., Velázquez-Aguilar, M., Lefranc-Flores, A., Siffedine, A., and Urrutia-Fucugauchi, J.: Tracking climate changes in the Gulf of California and the Eastern Tropical Pacific Ocean during the past 18,000 yr, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10905, https://doi.org/10.5194/egusphere-egu23-10905, 2023.

EGU23-11475 | Posters on site | SSP2.2

Effects of the Indian Ocean Monsoon oscillation during the Pleistocene-Holocene transition on the palinomorphic records in the NW Arabian Sea. 

Patricia Rodrigues, Hermann Behling, Gösta Hoffmann, and Wilfried Bauer

The Indian Ocean Monsoon is one of the largest land-ocean coupled events on Earth. Its occurrence is not only of climatic importance but also has a considerable economic impact on the livelihood of people/countries within its coverage zone. The monsoon winds travelling over the Arabian Sea (AS) carry moisture and bring rainfall to the southern part of the Sultanate of Oman and over a broad area of the Indian continent. In addition to rainfall, the monsoon also causes an intense and extensive deep-water upwelling along the coast and offshore of East Africa and the southern Arabian Peninsula. This intense and pronounced upwelling increases the productivity turning the western Arabian Sea into one of the most productive regions in the world.  In this poster we display partial results of a high-resolution study aiming at identifying monsoonal climatic changes recorded in marine sediments from the northwestern Arabian Sea during the late Pleistocene-Mid Holocene. It was carried out on 11 samples taken from an offshore core IODP Leg 117-721A-1H-1-W. An interval from 80 to 30 cm has been selected and samples have been taken every 3 cm.  We show here results obtained from 6 radiocarbon dating together with the study of palynomorphs. The main objective is to qualitatively identify and characterize pollen grains and spores, as well as the non-pollen palynomorphs (NPP) present in the samples, correlating them with other study sites in the AS. In addition, we evaluate their potential as paleoenvironmental indicators. Samples have presented a low number of pollen grains and spore, which has ranged from 3 to 27 identified specimens. The deeper/older samples have presented a higher concentration of pollen grains. However, due to the low content of specimens, quantitative paleoenvironmental conclusion could not be drawn. Nonetheless, non-pollen palynomorphs are relatively abundant throughout samples. Dinocysts represent the most abundant type of NPP, followed by fungi, microscopic remains of algae and others still not identified. Palynological studies carried on the NW Arabian Sea are scarce and NPP identification and characterization have not been done at the study site yet. Therefore, our work presents novelty on recognizing palinomorphic imprints left by Indian Ocean Monsoon oscillation during the transition Pleistocene-Holocene off the Omani coast.

How to cite: Rodrigues, P., Behling, H., Hoffmann, G., and Bauer, W.: Effects of the Indian Ocean Monsoon oscillation during the Pleistocene-Holocene transition on the palinomorphic records in the NW Arabian Sea., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11475, https://doi.org/10.5194/egusphere-egu23-11475, 2023.

EGU23-12410 | Posters on site | SSP2.2

Late Quaternary climate variability in Madagascar and its connection to South-East Africa hydroclimate changes and atmospheric circulation patterns 

Elin Norström, Rienk Smittenberg, Anneli Ekblom, Simon Haberle, and Christos Katrantsiotis

Madagascar is characterized by high climatic heterogeneity and its topography plays a key role in modulating the regional hydroclimate variability in South and East Africa. However, knowledge on past climate of Madagascar very limited, in line with the general scarcity of paleoclimate records from the southern tropics and subtropics. We generated a 26 kyr paleoclimate record from Madagascar, located in the southwestern Indian Ocean spanning the Last Glacial Maximum (LGM) to the late Holocene. In particular, we present a deuterium/hydrogen isotopic ratio of terrestrial leaf waxes (δ2Hwax) from a sediment core taken from the central eastern part of the island near the capital Antananarivo. The δ2H records of both the aquatic and terrestrial plant derived n-alkanes exhibit similar long-term trends implying that they all record changes in the isotopic composition of source water, namely meteoric water that recharges soil and lake waters. In this tropical region, the δ2H variability of precipitation recorded by n-alkanes δ2H is mainly influenced by the amount effect resulting in lower values for periods with high rainfall. We observe five long-term trends: (i) stable and relatively dry conditions during the Last Glacial Maximum (LGM) (ii) gradually wetter conditions from 17.5 ka to 11.5 ka, especially during the Heinrich stadial 1 (HS1) and the Younger Dryas (YD) (iii) an arid interval from 11.5 ka to 8.5 ka, and (iv) a general trend to more humid climate until 3.0 ka, followed by (v) a drier interval until 1.0 ka. The Madagascar climatic signal is opposite to other records from South Africa and East Africa records especially during the YD and early to middle Holocene period. This regional dipole mode is consistent with the modern rainfall anomaly pattern associated with the variability of Mozambique Channel Trough and the migration of austral summer Intertropical Convergence Zone (ITCZ) position as a response to changes in local summer insolation orbital and/or Northern Hemisphere cold events, such as the YD and HS1.

How to cite: Norström, E., Smittenberg, R., Ekblom, A., Haberle, S., and Katrantsiotis, C.: Late Quaternary climate variability in Madagascar and its connection to South-East Africa hydroclimate changes and atmospheric circulation patterns, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12410, https://doi.org/10.5194/egusphere-egu23-12410, 2023.

EGU23-12950 | Orals | SSP2.2

Sulphur isotopes in Permian–Triassic evaporites: an 80‐million‐year record of pyrite burial 

Jack Salisbury, Darren Gröcke, H.D.R. Ashleigh Cheung, Lee Kump, Tom McKie, and Alastair Ruffell

The Permian–Triassic time interval is associated with major perturbations in the biogeochemical cycling of several redox-sensitive elements. In particular, sulphur isotope ratios (δ34S) reveal substantial perturbations in sedimentary sulphates. Despite this, few studies utilise this δ34S variability for long-term high-resolution correlation. Through the sulphur isotope analysis of sedimentary evaporites of the Staithes S-20 borehole (northeast England), we have generated the most stratigraphically complete evaporite sulphur isotope (δ34Sevap) curve from a single stratigraphic section for the late Permian to Late Triassic. The Staithes S-20 record and its comparison with the global δ34Sevap curve demonstrate the utility of sulphur isotope data for stratigraphic correlation and dating, especially evaporite bearing sequences. The δ34Sevap data for the late Permian to Late Triassic were incorporated into a biogeochemical box model to yield estimates for the pyrite burial flux with time. We propose three significant pyrite burial events (i.e. PBEs) throughout the Triassic. Our model outputs predict a major increase in pyrite burial over the Permian/Triassic boundary, possibly driven by Siberian Traps volcanism. After ~10 million years, the pyrite burial flux achieves relative stability until the latest Triassic.  

How to cite: Salisbury, J., Gröcke, D., Cheung, H. D. R. A., Kump, L., McKie, T., and Ruffell, A.: Sulphur isotopes in Permian–Triassic evaporites: an 80‐million‐year record of pyrite burial, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12950, https://doi.org/10.5194/egusphere-egu23-12950, 2023.

EGU23-13034 | ECS | Posters on site | SSP2.2

A Siderian Snowball Earth? Multiscale and interdisciplinary Analyses of the Makganyene Formation, South Africa 

Sabine Wimmer, Daniel P. Le Heron, Marie E. Busfield, and Albertus J.B. Smith

Snowball Earth events, or at least intense glaciations, belong to one of the most important types of events in Earth’s Deep Time climate record. The Siderian (2.45–2.22 Ga) contained several such events, during which a diamictite-dominated succession named the Makganyene Formation was deposited in the Griqualand West Basin, South Africa. By comparison to their younger cousins in the Cryogenian, Siderian diamictites have been subject to comparatively less sedimentological investigation, although they have much potential in terms of reconstructing aspects of paleoclimate and former ice-sheet behaviour. In this study, multiscale and interdisciplinary analyses of both field and core data provide new insights into the sedimentology and deposition of the Makganyene and thereby aspects of its associated glaciation in the Siderian. Outcrop and core descriptions were supplemented by polarised light microscopic and scanning electron microscopic analyses, including element distribution maps for Al, Ca, Fe, Mg, Si and Ti. We propose that the deposits are the record of grounding zone wedge (GZW) deposition at the ice margin, with a contribution of iceberg-rain out, subglacial deposition and localised mass flow deposition playing a role. We show how interdisciplinary perspectives enrich the overall picture and allow a more accurate interpretation of the Makganyene Formation as a glacigenic sediment. 

How to cite: Wimmer, S., Le Heron, D. P., Busfield, M. E., and Smith, A. J. B.: A Siderian Snowball Earth? Multiscale and interdisciplinary Analyses of the Makganyene Formation, South Africa, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13034, https://doi.org/10.5194/egusphere-egu23-13034, 2023.

EGU23-13268 | Posters on site | SSP2.2

Devonian mass extinctions: cumulative or cataclysmic? 

David Bond, Sarah Greene, Jason Hilton, Gilda Lopes, Jing Lu, John Marshall, Ye Wang, Charles Wellman, and Runsheng Yin

The Late Devonian Mass Extinction is the least understood of the ‘Big 5’ extinctions in virtually every aspect: timing, effects and causes - and there is little knowledge of the coupling of events on land and in the ocean. At one extreme, the marine crisis is viewed as a rapid, cataclysmic event at the Frasnian/Famennian boundary (the “Kellwasser Event”) followed by another crisis 13 Myr later (the “Hangenberg Event”). Alternatively, these Late and end-Devonian extinctions are viewed as a cumulative series of minor events, drawn out over the entire Devonian. Our project aims to resolve these through study of the spectacular Devonian sedimentary succession in northern Spain that is both remarkably complete and laterally extensive, providing a transect across an entire Devonian marine shelf from deep marine to near terrestrial environments. We present initial results from Piedrasecha, north of Léon. We analysed 47 samples spanning the Frasnian Nocedo Formation, and the Famennian-Tournasian (Carboniferous) Fueyo, Ermita and Baleas Formations. Combined geochemical and palynological analyses reveal:

1) δ13Corg values are stable around -26‰ through the Frasnian and Famennian prior to a 2‰ negative shift associated with the onset of black mudstones at the base of the Baleas Formation (latest Famennian). This is likely a muted expression of the Hangenberg Event negative δ13Corg excursion.

2) Redox proxies (Th/U, Mo/Al, V/Al and U/Al) indicate bottom waters remained oxygenated until the latest Famennian, when weakly dysoxic (at worst) conditions developed. There is no obvious expression of Kellwasser Event anoxia in this offshore setting, and only a weak manifestation of Hangenberg oxygen restriction.

3) An order of magnitude shift in productivity proxy values (Ba/Al, Ni/Al, Zn/Al and P/Al) in the latest Famennian suggests that the Hangenberg Event is associated with increased primary productivity.

4) Mercury is enriched in the upper Frasnian Nocedo Formation where it withstands normalisation to TOC (Hg/TOC values reach 388 ppb/wt%, similar to those reported for the Upper Kellwasser Horizon elsewhere). This mercury might derive from large igneous province volcanism and is potentially a chemostratigraphic marker for the Kellwasser Event, though we require better stratigraphic control to evaluate this. Significant Hg enrichments (up to 160 ppb) in the latest Famennian Baleas Formation do not withstand normalisation, as TOC reaches 4.7 wt% at this level. The succession is thermally mature and since TOC drops with thermal maturity, Hg/TOC values might be elevated in comparison to original values.

5) Palynomorph assemblages are dominated by simple spores and Geminospora. The latter derives from the Mid-Late Devonian forest tree Archaeopteris. This suggests a rather homogenous vegetation typical of Late Devonian settings where successive extinctions stripped out diversity from terrestrial floras. However, it may be that in this distal section we are sampling spores that have been winnowed during transport. Work on other sections will enable us to test this.

We have sampled 14 further sections providing a complete Devonian succession and with >500 samples in preparation we hope to resolve whether the Late and end-Devonian crises were the result of cumulative stresses, or were indeed cataclysmic events.

How to cite: Bond, D., Greene, S., Hilton, J., Lopes, G., Lu, J., Marshall, J., Wang, Y., Wellman, C., and Yin, R.: Devonian mass extinctions: cumulative or cataclysmic?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13268, https://doi.org/10.5194/egusphere-egu23-13268, 2023.

The distribution of the bryozoans in the shallow-marine-estuarine sediments of the late Early–Late Eocene La Meseta Formation, Seymour Island shows a sharp decline in bryozoan biodiversity between the lower, basal transgressive facies of Telm1 and upper part of the formation (Telm6-7) at the end of Eocene (Hara 2001). In the lowermost part of LMF (Telm1) the cheilostome bryozoans, preserved as internal moulds systematically belonging to buguloids and catenicelloideans, at the present day are widely distributed in the tropical-warm temperate latitudes and deposited in the shallow-water settings (Hara 2015). Within a 2 meters thick interval of the basal transgressive facies of Telm1 unit, the most common are multilamellar colonies, showing a great variety of shapes dominated by celleporiforms and cerioporids.

The middle part of (LMF, Telm4-5) reveal a presence of the microporoideans and disc-shaped lunulitiform - warm-loving, free-living bryozoans. Environmentally, Recent, lunulitids are known to occur in warm, shallow-shelf conditions, at temperatures of 10-29˚C, on coarse, sandy to muddy bottom, what suggest the shallow-water setting for the middle part of the LMF.

10 million years older, the Cape Melville Formation on King George Island dated as Early Miocene is dominated by the infaunal bivalves, which provide a unique fossil record in the Antarctic Peninsula region during the latest Oligocene to earliest Miocene interglacial to glacial transition. Only one bryozoan was described identified as Aspidostoma melvillensis (Hara and Crame, 2004).

The shallow-marine, pectinid-rich biofacies of the Pecten Conglomerate of CIF, Cockburn Island, taxonomically shows the mosaic pattern in occurrence of bryozoan taxa, which are known from the Middle and Late Cretaceous, another originated in the Paleogene, as well as those which are solely common in the Neogene. Exclusively encrusting colony growth-form of the Pliocene biota suggests sedimentation in the shallow-water environment and indicates an interglacial palaeoenvironment of the CIF Formation (Hara & Crame, in revision).

The cold-water geographical distribution of the Recent bryozoans with dominant Neocheilostomatina of Buguloidea and the ascophoran lepraliomorphs of Smittinoidea and Schizoporelloidea, shows a dynamic history of this highly endemic fauna, which evolved over long period of time.

Hara, U. 2001. Bryozoa from the Eocene of Seymour Island, Antarctic Peninsula. Palaeontologia Polonica, In: Palaeontological Results of the Polish Antarctic Expeditions, Part III, 60, 33-156.

Hara U., 2015. Bryozoan internal moulds from the La Meseta Formation (Eocene) of Seymour Island, Antarctic Peninsula. Polish Polar Research, 36: 25-49.

  • Hara and J. A. Crame 2004. A new aspidostomatid bryozoan from the Cape Melville Formation (lower Miocene) of King George Island, West Antarctica. Antarctic Sciences, 16, 319-327.

 

 

How to cite: Hara, U.: Cenozoic bryozoan biota: their palaeoecology and climatic environmental significance  in Antarctic ecosystems , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14043, https://doi.org/10.5194/egusphere-egu23-14043, 2023.

EGU23-14508 | Posters virtual | SSP2.2

Bathyal bivalve assemblages of the eastern Mediterranean record the Early-Middle Pleistocene transition 

Efterpi Koskeridou, Danae Thivaiou, Konstantina Agiadi, Frédéric Quillévéré, Pierre Moissette, and Jean-Jacques Cornée

Molluscs, and among them bivalves, are organisms known for their ability to precisely record paleoenvironmental changes, both in shallow and deep marine settings. When looking into the recent geological past, bivalve assemblages offer information on the climatic changes that have impacted their taxonomic compositions. In the eastern Mediterranean, assemblages of bathyal bivalves are scarce. In order to investigate the impact of climatic changes on deep-water bivalve communities during the Early-Middle Pleistocene Transition, we focus here on two well-dated sections on Rhodes Island (Greece) corresponding to the Lindos Bay Formation. The sections of Lindos and Lardos present a continuous sedimentation of fine, marly sediments, and cover the Marine Isotopic Stages (MIS) 32 to 18. A total of 15 samples were analysed, resulting in the recovery of 31 species of bathyal bivalves. The depositional depths of these samples are estimated to be between 150 and 500 m. All samples are dominated by Protobranch bivalves, with the larger diversity found in families Nuculanidae and Yoldiidae. Three species, found only in cool intervals, are now extinct: Ledella nicotrae, Katadesmia confusa, and Pseudoneilonella pusio. Differences in sample composition are thought to be due mainly to climatic rather than bathymetric conditions. Although the associations in most MIS are similar to those found in the Italian Pleistocene deposits, those of the MIS 21 interglacial (Nucula nucleusSaccella commutataCyclopecten hoskynsiLimea crassa) and the MIS 20 glacial (Saccella commutataBathyspinula excisaYoldiella curtaBathyarca spp.) are new for the Mediterranean region. These results imply that there were significant changes in bathyal bivalve associations during the climatic transitions of the Early-Middle Pleistocene and that modern bathyal associations of bivalves have been stabilized after the Middle Pleistocene.

How to cite: Koskeridou, E., Thivaiou, D., Agiadi, K., Quillévéré, F., Moissette, P., and Cornée, J.-J.: Bathyal bivalve assemblages of the eastern Mediterranean record the Early-Middle Pleistocene transition, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14508, https://doi.org/10.5194/egusphere-egu23-14508, 2023.

EGU23-14596 | ECS | Orals | SSP2.2

Multiproxy constraints on recovery processes during the hyperthermal Toarcian Oceanic Anoxic Event 

Alicia Fantasia, Thierry Adatte, Jorge E. Spangenberg, Emanuela Mattioli, Marcel Regelous, Christian Salazar, Romain Millot, Stéphane Bodin, Thomas Letulle, Mikhail Rogov, and Guillaume Suan

Extreme and rapid climatic and environmental perturbations have punctuated Earth history. The causes and consequences of these past global-change events are relatively well constrained, but how the system can naturally recover through feedbacks remain largely unconstrained. The Toarcian in the Early Jurassic is an ideal time interval to understand the response of Earth system to rapid climate change. Indeed, it was marked by one of the most extreme hyperthermal events of the Phanerozoic accompanied by major environmental changes, named the Toarcian Oceanic Anoxic Event (T-OAE, ca. 183 Ma). Most studies have focused on the triggering mechanisms and the palaeoenvironmental response, whereas the recovery phase has been less studied. Increased chemical weathering of silicate rocks and burial of organic carbon are the two primary natural mechanisms generally proposed as negative feedbacks controlling the recovery. However, to date, the response of these feedbacks, their efficiency, and their timing are still uncertain, hampering an accurate view of the carbon cycle-climate dynamics. This study aims to tackle this lack of empirical data by providing a multi-proxy dataset combining sedimentological observations, mineralogical and geochemical analyses. Four worldwide distributed sites have been selected for this study: Fontaneilles in France (Grand Causses Basin), Vilyui in Siberia (Siberian Basin), Agua de la Falda in Chile (Andean Basin), and Ait Athmane in Morocco (High Atlas Basin). Our high-resolution carbon isotope records allow us to correlate the studied sites to trace the global carbon cycle dynamics in the aftermath of the Toarcian event. Lithium isotope ratios are used to trace global weathering rates and to understand processes that control the long-term carbon cycle. Our results indicate that higher silicate weathering rates during the Toarcian hyperthermal likely helped the climate system recover and return to cooler climatic conditions. High mercury and tellurium concentrations recorded after the T-OAE interval suggest that protracted Karoo-Ferrar volcanic activity may have played a role in the recovery.

How to cite: Fantasia, A., Adatte, T., Spangenberg, J. E., Mattioli, E., Regelous, M., Salazar, C., Millot, R., Bodin, S., Letulle, T., Rogov, M., and Suan, G.: Multiproxy constraints on recovery processes during the hyperthermal Toarcian Oceanic Anoxic Event, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14596, https://doi.org/10.5194/egusphere-egu23-14596, 2023.

EGU23-15207 | Posters on site | SSP2.2

A climate perturbation at the Middle –Late Jurassic Transition? Evaluating the isotopic evidence 

Gregory Price, Bernát Heszler, Lauren-Marie Tansley Charlton, and Jade Cox

The Jurassic greenhouse is punctuated by short cooling intervals with at times postulated polar ice-sheet development. For example, oxygen isotope records of belemnite rostra and fish teeth from the Russian Platform, eastern France and western Switzerland have been interpreted to reveal a prominent decrease in seawater temperature during the Late Callovian–Early Oxfordian. This is in part the basis for a proposed an ice age at the Middle-Late Jurassic Transition. In contrast relatively constant oxygen isotope records and therefore seawater temperatures and carbon isotope values characterized by significant scatter but showing more positive values during the middle and late Callovian have been reported from elsewhere. The aim of this research has been to determine a stable isotope stratigraphy (from belemnites and oysters) principally from the Callovian-Oxfordian interval (from southern England) and integrate these data with existing data to assess the pattern of carbon and oxygen isotopic change.  Our marine macrofossil record reveals isotopic patterns that are generally comparable with other European basins. Carbon isotopic trends are consistent with bulk carbonate carbon isotope records displaying systematic fluctuations, the largest of which (Middle Callovian, Calloviense/Jason Zones to Early Oxfordian, Mariae Zone) corresponds to previously identified phases of environmental perturbation. Such a trend may have resulted from enhanced burial and preservation of organic matter, leaving the seawater more positive in terms of carbon. Cooling post-dates this positive carbon isotope excursion. Inferred cooling, derived from our oxygen isotope data from southern England, occurs within the Late Callovian and Oxfordian (Athleta to Mariae zones). Enhanced carbon burial and atmospheric carbon dioxide draw down may have induced cooling. In this study the analysis of a single region (southern England) allows some constraints on potential variable that may influence isotope records.

 

How to cite: Price, G., Heszler, B., Tansley Charlton, L.-M., and Cox, J.: A climate perturbation at the Middle –Late Jurassic Transition? Evaluating the isotopic evidence, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15207, https://doi.org/10.5194/egusphere-egu23-15207, 2023.

EGU23-17352 | Orals | SSP2.2 | Highlight

Tracing ocean circulation using neodymium isotopes – promises and limitations 

Katharina Pahnke, Torben Struve, Mika Sutorius, Henning Waltemathe, and Martin Zander

Neodymium (Nd) isotopes have been applied for decades now to trace ocean circulation both in the present and past oceans. Their tracer utility stems from the characteristic Nd isotope signature of different rocks and their imprint on seawater as well as the biological inactivity of Nd and its appropriate residence time in the ocean, allowing for the determination of water mass provenance and flow paths. However, the application of this tracer, especially for the reconstruction of past ocean circulation changes, has been challenged based on uncertainties e. g. in the magnitude of the benthic flux of Nd to deep waters, Nd isotope exchange and input at ocean margins, and diagenetic alterations of the original bottom water Nd isotope signature in sediments.

Based on recent studies of dissolved Nd isotope distributions in surface to deep waters we show the power of Nd isotopes for tracing the provenance of currents and water masses particularly within restricted geographic regions. Using additional trace metal and isotope data from marine sediments analyzed alongside authigenic Nd isotopes, we explore the validity and limits of Nd isotopes as tracer of past ocean circulation changes.

How to cite: Pahnke, K., Struve, T., Sutorius, M., Waltemathe, H., and Zander, M.: Tracing ocean circulation using neodymium isotopes – promises and limitations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17352, https://doi.org/10.5194/egusphere-egu23-17352, 2023.

EGU23-17389 | Orals | SSP2.2

A millennial-scale record of mean annual air temperatures spanning 70 ka over the Cretaceous-Paleogene boundary 

Lauren O'Connor, Rhodri Jerrett, Gregory Price, Bart van Dongen, Emily Crampton-Flood, and Sabine Lengger

The Cretaceous-Paleogene (K-Pg) boundary experienced major environmental perturbations due to volcanism and bolide impact, as well as the most famous mass extinction in geologic history. However, the response of the climate system to these drivers at different timescales, and thus their relationship to the mass extinction is highly debated. In particular, the role of climate change in biodiversity patterns immediately preceding the boundary is poorly understood. 


Lipids from fossil peats (coals) provide an opportunity to reconstruct terrestrial temperatures across the Cretaceous–Paleogene boundary at a millennial-scale resolution. Here we present mean annual air temperature records spanning ~70 ka over the K-Pg boundary, from sites across North America (palaeolatitudes 45–55 degrees N). Our data show that temperatures ranged from 16–29 degrees C, more than 10 degrees C higher modern temperatures at equivalent latitudes in North America.


Using 5-ka temporal bins, our data show that MAATs peaked at ~26 degrees C in the last millennia of the Cretaceous, following 35 ka of warming from ~23 degrees C. Peak warmth was followed by ~5 degrees C cooling over the following 30 ka. We observe no “impact winter” nor a spike in temperature immediately following the boundary. If such phenomena occurred, their duration was below the resolution of our record: ~1 ka. Our record also shows a previously unrecognised brief interval of cooling from 10 to 5 ka pre-boundary.


Our study places new bounds on millennial-scale trends in MAAT change in the terrestrial realm and demonstrates large and rapid temperature swings across the K-Pg interval. These data allow for improved understanding of the role of climate in the decline of Cretaceous flora and fauna and may help elucidate the relative influence of volcanism and bolide impact on terrestrial temperatures.

How to cite: O'Connor, L., Jerrett, R., Price, G., van Dongen, B., Crampton-Flood, E., and Lengger, S.: A millennial-scale record of mean annual air temperatures spanning 70 ka over the Cretaceous-Paleogene boundary, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17389, https://doi.org/10.5194/egusphere-egu23-17389, 2023.

EGU23-41 | Orals | CL1.1.3 | Highlight

Variability of the Indonesian Throughflow and Australian monsoon dynamism across the Mid Pleistocene Transition (IODP 363, Site U1483) 

Kenji Matsuzaki, Ann Holbourn, Wolfgang Kuhnt, Li Gong, and Masayuki Ikeda

The Mid-Pleistocene Transition (MPT) between ~1200 and ~800 ka was associated with a major shift in global climate and was marked by a change in glacial/interglacial periodicity from ~41 to ~100 kyr that resulted in higher-amplitude sea-level variations and intensified glacial cooling. The Indonesian Throughflow (ITF), which controls the exchange of heat between the Pacific and Indian Oceans, is a major component of the global climate system. On the other hand, Asian-Australian Monsoon dynamics play a key role in regional primary productivity. Therefore, reconstruction of ITF and Asian-Australian Monsoon variability during the MPT could potentially clarify the impact of the glacio-eustatic sea level changes on the climate and ecosystem of Northwest Australia. The International Discovery Program (IODP) Expedition 363 retrieved an extended, continuous hemipelagic sediment succession spanning the past two million years at Site U1483 on the Scott Plateau off Northwestern Australia.

In this study, we analyzed radiolarian assemblages in core top samples retrieved during the RV Sonne Expedition 257 and downcore samples from IODP Site U1483 to estimate the variability in regional sea surface temperatures (SSTs) during the MPT, and to explore ITF dynamics in relation to glacio-eustatic sea-level variations and tropical monsoon strength. We suggest that glacio-eustatic sea-level variations have been a key factor affecting changes in SSTs at Site U1483, primarily because the shallow and hydrogeographically complex nature of the sea means that SSTs are highly sensitive to glacio-eustatic sea-level variation. Based on comparisons with SST data from the mid latitudes off Northwest Australia and the South China Sea, we suggest that the SSTs at Site U1483 are highly dependent on prevailing climate changes in the northern hemisphere rather than changes in the climate of the Southern hemisphere. In addition, comparisons of radiolarian total abundances with X-ray fluorescence-scanning elemental data suggested that, until the onset of the MPT (~1200 ka), radiolarian productivity was higher during strong summer monsoons during interglacial periods, probably because of the high riverine runoff generated by heavy summer monsoonal precipitation. However, since ~900 ka, there appears to have been a shift in the mode of radiolarian productivity that has resulted in increased radiolarian productivity during glacial periods when the delivery of nutrients is increased due to the enhanced mixing of the upper water column in the shallow sea caused by strong trade winds. 

How to cite: Matsuzaki, K., Holbourn, A., Kuhnt, W., Gong, L., and Ikeda, M.: Variability of the Indonesian Throughflow and Australian monsoon dynamism across the Mid Pleistocene Transition (IODP 363, Site U1483), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-41, https://doi.org/10.5194/egusphere-egu23-41, 2023.

EGU23-464 | ECS | Posters on site | CL1.1.3

Effects of the Pacific Antarctic Circumpolar Current on the extant coccolithophore Emiliania huxleyi 

Ellis Morgan, Mariem Saavedra-Pellitero, and Elisa Malinverno

During the last decades, the Southern Ocean (SO) has been experiencing physical and chemical drastic changes which are affecting the distribution and composition of pelagic plankton communities. Coccolithophores (small-sized haptophyte algae) are the most prolific carbonate-producing phytoplankton group, playing a key role in biogeochemical cycles at high latitudes.

In this work we investigated the biogeographical distribution and calcification patterns of the ecologically dominant species Emiliania huxleyi across a latitudinal transect in the Pacific sector of the SO (from ~40°S to ~54°S). We aimed to assess the response of E. huxleyi to steep environmental gradients across the frontal system of the Antarctic Circumpolar Current.

The plankton samples were collected during International Ocean Discovery Program Expedition 383: Dynamics of Pacific Antarctic Circumpolar Current (DYNAPACC, May-July, 2019) onboard the R/V JOIDES Resolution (https://iodp.tamu.edu/scienceops/expeditions/dynamics_of_pacific_ACC.html). In situ environmental data (such as sea surface temperature, total alkalinity and pH) were measured at each sampling location.

The samples were prepared and analysed at the University of Portsmouth using a combination of electron backscatter diffraction (EBSD), Scanning Electron Microscope (SEM) and light microscopy techniques.

How to cite: Morgan, E., Saavedra-Pellitero, M., and Malinverno, E.: Effects of the Pacific Antarctic Circumpolar Current on the extant coccolithophore Emiliania huxleyi, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-464, https://doi.org/10.5194/egusphere-egu23-464, 2023.

EGU23-1105 | Posters on site | CL1.1.3

The start of the Great Barrier Reef is a result of the increased stability of Temperatures in the Mid to Late Pleistocene. 

Benjamin Petrick, Lars Reuning, Alexandra Auderset, Miriam Pfeiffer, and Lorenz Schwark

The Great Barrier Reef is a unique environmental resource threatened by future climate change. However, it has always been unclear how this ecosystem developed in the Mid to Late Pleistocene. Work has shown that the reef developed between ~ 600-500 ka during MIS 15-13, although some records suggest a start at MIS 11 at 400 ka. There is a lack of Sea Surface Temperature (SST) records for this time for the area around the Great Barrier Reef. Furthermore, the few existing SST records do not show temperature changes during these key periods, leading researchers to suggest that factors other than temperature, such as sea-level change or sediment transport, explain the start of the reef. We used the TEX86 proxy to produce a new SST record starting at 900 ka from ODP Site 820. This core is located next to the northern Great Barrier Reef. In this new record, there are SST changes that seem to match both dates for the start of the Great Barrier Reef. First, there is a period of stable SST between 700-500 ka, with no glacial cooling during this time. This could promote the development of a reef system during this time, allowing the reef more time to evolve from isolated smaller reefs to a continuous barrier reef. However, there is some suggestion based on facies analyses that even though the barrier system developed around MIS 15, the modern coral reef system was not yet fully established. Our records show that glacial temperatures during MIS 14 still are similar to SSTs from records further south. However, this trend shifts around MIS 11 when glacials became warmer. In fact, while before MIS 11, SST at ODP 820 was colder than records from the Western Pacific Warm Pool, afterwards SST was either the same or sometimes warmer than at these sites. Also, unlike other nearby records, the difference in SSTs between glacials and interglacials is reduced after MIS 11. This suggests that the northern Coral Sea might have been protected from the extremes of glacial temperature changes after the MPT. This process might have allowed the development of a continuous coral reef system by encouraging the growth of reefs even during glacials. Therefore, our research suggests that major steps in the development of the Great Barrier Reef system are linked to changes in the SSTs. Our SST record suggests that SST changes are the primary driver of reef development and other non-SST factors are less important.

How to cite: Petrick, B., Reuning, L., Auderset, A., Pfeiffer, M., and Schwark, L.: The start of the Great Barrier Reef is a result of the increased stability of Temperatures in the Mid to Late Pleistocene., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1105, https://doi.org/10.5194/egusphere-egu23-1105, 2023.

EGU23-1997 | Orals | CL1.1.3

Chemical Weathering in New Guinea since the Mid Miocene 

Peter Clift and Mahyar Mohtadi

Chemical weathering of silicate rocks is a well recognized method by which carbon dioxide is removed from the atmosphere and fixed as calcium carbonates in the sedimentary record. For many years the long term cooling of the Earth during the Cenozoic has been linked to uplift, erosion and weathering of the Himalayas and Tibetan Plateau, however following scientific ocean drilling of the submarine fans in the Asian marginal seas it now seems that this region could not be responsible for cooling, at least during the Neogene. Although other factors such as burial of organic carbon and the rates of degassing during seafloor spreading may also be important, erosion and weathering of other regions may also be important in controlling global CO2 concentrations. In this study we focus on the role of New Guinea, the large (>2500 km long) orogen formed as Australia collided with Indonesia since the Mid Miocene. New Guinea comprises slices of arc and ophiolite rocks that are susceptible to weathering, and is located in the tropics where warm, wet conditions favor rapid weathering. Rainfall exceeds >4 m annually in the island center. Analyses of sediment from Deep Sea Drilling Project Sites 210 and 287 in the Gulf of Papua now allow the weathering and erosion history of the island to be reconstructed. A trend to more continental erosion since 15 Ma reflects uplift and erosion of tectonics slices of the Australian plate. At the same time chemical weathering shows increasing intensity, especially since 5 Ma, as proxied by major element ratios (K/Rb, K/Al) and clay minerals. Greater proportions of kaolinite point to more tropical weathering since the Mid Miocene. Trends to more weathering contrast with Himalayan records that show the reverse, and suggest that New Guinea may be an important component in controlling global climate in the past 15 Ma.

How to cite: Clift, P. and Mohtadi, M.: Chemical Weathering in New Guinea since the Mid Miocene, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1997, https://doi.org/10.5194/egusphere-egu23-1997, 2023.

EGU23-2105 | Orals | CL1.1.3

Changes in intermediate circulation waters along the tropical eastern Indian Ocean during quaternary climatic oscillations 

Sandrine Le Houedec, Maxime Tremblin, Amaury Champion, and Elias Samankassou

The Indo-Pacific Warm Pool (IPWP) is the warmest and most dynamic ocean-atmosphere-climate system on Earth and has undergone significant climatic changes during the Pleistocene glacial periods (De Deckker et al., 2012; Lea et al., 2000; Russell et al., 2014). During the Last Glacial Maximum, the latitudinal position of the Southern Ocean fronts, both south of Africa and Australia, was shown to be critical in controlling the outflow of warm water of the Agulhas Current from the Indian Ocean and the IPWP area. Yet, there is no direct evidence for such oceanic change on the scale of the Late Pleistocene glacial-interglacial transitions.

Here, we combine sea surface temperature proxies (d18O and Mg/Ca) with the neodymium (Nd) isotopic signature to reconstruct changes in climate and oceanic circulation in the eastern tropical Indian Ocean over the last 500 ka. The most striking feature of our dataset is the oscillating Nd signal that mimics the glacial-interglacial cycles. While interglacial periods are characterized by a more significant contribution from the less radiogenic Antarctic intermediate water mass (AAIW, ~ -7 εNd), glacial periods are marked by more radiogenic water mass of Pacific origin (~ -5 εNd). We argue that under global cooling, the northward penetration of the AAIW has weakened due to the general slowdown of the global thermohaline circulation. Furthermore, the oscillating pattern is also recorded in the sea surface temperature and salinity, indicating the settlement of cooler and more saline surface water masses probably linked to a less expanded IPWP and weaker Leeuwin Current during glacial intervals.

We suggest that under low AAIW a less intense advective mixing occurred, allowing a deepening of both halocline and thermocline in the tropical eastern Indian Ocean. Our new proxy-derived dataset confirms results from models (DiNiezo et al., 2018), suggesting that these ocean conditions could amplify the externally forced climate changes resulting from drier atmospheric conditions and weaken the monsoon during glacial periods in the Indonesian region.

How to cite: Le Houedec, S., Tremblin, M., Champion, A., and Samankassou, E.: Changes in intermediate circulation waters along the tropical eastern Indian Ocean during quaternary climatic oscillations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2105, https://doi.org/10.5194/egusphere-egu23-2105, 2023.

EGU23-2802 | Orals | CL1.1.3 | Highlight

The Cenozoic sea surface temperature evolution offshore Tasmania 

Peter Bijl, Frida Hoem, Suning Hou, Lena Thöle, Isabel Sauermilch, and Francesca sangiorgi

During the Cenozoic (66–0 Ma) Tasmania has continuously been at a crucial geographic location. It represented the final tectonic connection between Australia and Antarctica before complete separation of both continents in the late Eocene, and therefore a barrier for circumpolar flow. Since the Eocene-Oligocene transition, the northward drifting Tasmania was bathed by the throughflow of the subtropical front, but remained an obstacle of the ideal flow path of strengthening ocean currents. The sedimentary record around Tasmania thus represents a perfect archive to record the oceanographic consequences of this regional tectonic change. We here present a new TEX86 and UK37-based SST compilation from 4 sediment cores: ODP Site 1172 (East Tasman Plateau), Site 1170 and 1171 (South Tasman Rise) and Site 1168 (western Tasman margin). We paired these reconstructions with microplankton (dinoflagellate cyst) assemblage data which reflect qualitatively the surface water conditions: nutrients, temperature, salinity. Together, the >1.300 samples portray the SST evolution around the island, from the time it was still connected to the Antarctic continent in the Paleocene to its near-subtropical location today. Trends in the SST compilation broadly follow those in benthic foraminiferal stable isotope compilations, but with some interesting deviations. Differences in SSTs on either side of the Tasmanian Gateway are small in the early Paleogene (66–34 Ma), even when the Tasmanian Gateway is considered closed. Widening of the Tasmanian Gateway around the Eocene-Oligocene transition (34Ma) immediately allows throughflow of what later becomes the Leeuwin Current, which warms the sw Pacific. Oligocene and Neogene SST trends follow those of the benthic d18O, and with continuous influence of the proto-subtropical front. While the SST evolution of Tasmania is remarkably stable in most of the Oligocene, prominent cooling steps are inferred in the Late Oligocene (26 Ma), at the MMCT (~14 Ma), in the mid-to-late Miocene (9 Ma, 7 Ma and 5.3 Ma) and in the Pliocene (2.8 Ma). The remarkably strong Neogene cooling of the subtropical front implies expansion of subpolar temperate conditions and probably gradual strengthening of the Antarctic circumpolar current. Pliocene-Pleistocene SST variability is strong over glacial-interglacial cycles. Taken together, the sites portray a complete overview of local environmental change of the subtropical front area, and provides crucial context to the history of Southern Ocean heat transport and regional climate.

How to cite: Bijl, P., Hoem, F., Hou, S., Thöle, L., Sauermilch, I., and sangiorgi, F.: The Cenozoic sea surface temperature evolution offshore Tasmania, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2802, https://doi.org/10.5194/egusphere-egu23-2802, 2023.

Late Quaternary clay mineral assemblages, radiogenic isotope, and siliciclastic grain size records collected from high sedimentation Site U1483 of the International Ocean Discovery Program (IODP), beneath the path of the modern-day Indonesian Throughflow (ITF) and Leeuwin Current of northwest Australia are studied to reconstruct sediments provenance, transport processes and ocean current behavior, and to evaluate the Australian summer monsoon over the last 500 kyr. Clay minerals are primarily composed of smectite (41–70 %), followed by kaolinite (10–28 %), illite (13.5–25 %), and minor chlorite (3–14 %). Our reconstructed model based on the clay minerals source comparison and radiogenic isotope (Sr-Nd-Pb) records suggest the Victoria and Ord rivers of the Kimberley region as the source over the past 500 kyr for Site U1483. Smectite is mainly derived from the mafic volcanic and smectite-rich Bonaparte Gulf, whereas kaolinite and illite are primarily derived from felsic igneous and metamorphic rocks, respectively, found in the drainage areas of these rivers. Chlorite is primarily contributed by the Indonesian Throughflow (ITF), with a minor contribution from the northwest Australian rivers. Variations in the clay mineral assemblages and grain size records indicate strong glacial-interglacial cyclicity, with small grain size, high smectite, and low kaolinite and illite during glacial periods, while interglacial intervals are marked by a relative increase in kaolinite and illite, mean grain size, and decrease in smectite content. (Kaolinite+illite+chlorite)/smectite and kaolinite/smectite ratios are adopted as proxies for the ITF strength and Australian summer monsoon, respectively. High values of kaolinite/smectite and (kaolinite+illite+chlorite)/smectite ratios during the interglacial intervals indicate a wet summer monsoon with high river discharge and a strong ITF and Leeuwin Current, which has the capacity to transport a relatively high percentage of large-size kaolinite and illite sediments to Site U1483. In contrast, during glacials, the low values of kaolinite/smectite and (kaolinite+illite+chlorite)/smectite ratios imply a dry summer monsoon with low sediment discharge and weak ITF and Leeuwin Current, which can majorly carry the small smectite size particles in its suspension. The mean grain size and clay/silt ratio also indicate that the strength of ITF and Leeuwin Current was weak during glacials and gained high strength during the interglacials. The proxy records’ spectral analysis indicates a strong eccentricity period of 100-kyr, an obliquity period of 41-kyr, and a precession period of 23-kyr, implying that the clay mineral input along the northwest Australian margin is influenced by both high-latitude ice sheet forcing and low-latitude tropical processes.

How to cite: Sarim, M. and Xu, J.: Late Quaternary glacial-interglacial variability of the Indonesian Throughflow and Australian summer monsoon: Evidences from clay mineral and grain size records at IODP Site U1483 of northwest Australia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3430, https://doi.org/10.5194/egusphere-egu23-3430, 2023.

EGU23-5655 | ECS | Orals | CL1.1.3

Astronomically-paced changes in paleoproductivity, winnowing, and mineral flux over Broken Ridge (Indian Ocean) since the Early Miocene 

Jing Lyu, Sofía Bar­ra­gán-Mon­til­la, Gerald Auer, Or Bialik, Beth Christensen, and David De Vleeschouwer

Earth’s climate during the Neogene period changed in several steps from a planet with unipolar ice sheets to today’s bipolar configuration. Yet, time-continuous and well-preserved sedimentary archives from this time interval are scarce. This is especially true for those records that can be used for tracing the role of astronomical climate forcing. Ocean Drilling Program (ODP) Site 752 was drilled on Broken Ridge (Indian Ocean) and provides a time-continuous sedimentation history since the early Miocene in its upper portion.  To date, no astronomical-scale paleoclimate research has been conducted on this legacy ODP site. Here, we use X-ray fluorescence (XRF) core scanning data and benthic foraminifera (BF) taxonomic and quantitative analyses to reconstruct the paleoceanographic changes in the Indian Ocean since 23 Ma. Productivity-related elements from the XRF dataset, show higher productivity during the early Miocene and late Pliocene/early Pleistocene. Moreover, we found strong 405-kyr and ~110-kyr eccentricity imprints in the spectral analysis result of this XRF-derived paleoproductivity proxy. Although the precession signal is also quite remarkable in the spectral analysis results, the 4-cm resolution may not be adequate to further test the precession contribution. Bottom water oxygenation reconstructed using BF, suggest no oxygen minimum zone conditions for the late Miocene on site 752. Dissolved oxygen concentrations (DOC) indicate low oxic conditions (⁓ 2 ml/L) during this time, and relatively low stress species distribution (< 32%) along with abundant oxic species like H. boueana, C. mundulus, L. pauperata and Gyroidinoides spp. suggest predominantly high oxic conditions during the late Miocene (DOC > 2 ml/L). Meanwhile, the grain size (> 425µm) record shows an increasing trend at ~5 Ma, which indicates more current winnowing. Therefore, we argue that the drop in Mn is the result of the increase in the current winnowing, instead of the OMZ expansion. On the other hand, high-amplitude changes in Fe content from the lower Miocene to the middle Miocene, cannot be explained by eolian input, suggesting the source might be the neighbor-distanced Amsterdam-St. Paul hot spot. The source of Fe might be the neighbor-distanced Amsterdam-St. Paul hot spot. We conclude that the legacy ODP Site 752 constitutes an excellent paleoceanographic archive that allows us to reconstruct Indian Ocean dynamics since the early Miocene. New drillings on Broken Ridge with state-of-the-art scientific ocean drilling techniques will provide more detailed information and be highly beneficial for paleoclimatic and paleoceanographic research.

How to cite: Lyu, J., Bar­ra­gán-Mon­til­la, S., Auer, G., Bialik, O., Christensen, B., and De Vleeschouwer, D.: Astronomically-paced changes in paleoproductivity, winnowing, and mineral flux over Broken Ridge (Indian Ocean) since the Early Miocene, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5655, https://doi.org/10.5194/egusphere-egu23-5655, 2023.

The Early Middle Pleistocene Transition (EMPT) represents a fundamental reorganization in Earth’s climate system as the obliquity-dominated glacial/interglacial rhythmicity characterizing the Quaternary got progressively replaced by a high-amplitude, quasi-periodic 100 kyr cyclicity. This critical change in the climatic response to orbital cycles occurred without proportional modifications in the orbital-forcing parameters before or during the EMPT, implying a substantial change internal to the climate system. The EMPT had a severe impact on marine ecosystems. However, the trigger mechanisms and the components of the climate system involved in this global reorganization are still under debate, and high-resolution studies from the equatorial to mid-latitude shelf regions are at present rarely available.

In this study, we analyze the benthic foraminifera assemblage of an expanded section from Site U1460 (eastern Indian Ocean, 27°22.4949′S, 112°55.4296′E, 214.5 meters water depth), collected during International Ocean Discovery Program (IODP) Expedition 356 on the southwestern Australian shelf covering the EMPT. At this site, we provide a new benthic and planktonic foraminifera dataset to better define the response of the Leeuwin Current System during the EMPT on low to mid latitude shelf regions that are strongly sensitive to glacial/interglacial sea-level oscillations. Specifically, benthic foraminifera assemblage and the plankton/benthos (P/B) ratio are used to understand the bottom water community and its reaction to the Leeuwin Current System variations during the EMPT. Additionally, these data will untangle the local impact of eustatic sea-level changes in a highly dynamic setting.

Preliminary data of the microfossil content revealed a polyspecific benthic foraminifera assemblage with high diversity. The most abundant taxa are trochospiral forms (e.g., Cibicides, Cibicidoides, Heterolepa, Nuttallides, Eponides). Triserial and biserial taxa are abundant (e.g., Textularia, Spirotextularia, Gaudryina, Bolivina, Uvigerina). Planispiral tests such as Melonis and Lenticulina are also commonly present, as well as uniserial ones such as Siphogenerina, Lagena, and Cerebrina. Preservation varies significantly between glacial and interglacial intervals. Particularly, benthic foraminifera are poorly to moderately preserved during glacial stages while exhibiting moderate to good preservation in the interglacials. The variations in the P/B ratio allowed to constrain the sea-level changes along the Australian shelf. Specifically, higher and lower values of this ratio indicate highstand and lowstand phases, respectively. In this regard, foraminifera data will be integrated in a multiproxy dataset available for Site U1460 to obtain new insights on sea-level-driven environmental changes in the area during the EMPT. This, in turn, will allow to resolve the impact of local versus global climatic change across the studied interval.

How to cite: Arrigoni, A., Auer, G., and Piller, W. E.: The Leeuwin Current System during the Early Middle Pleistocene Transition (EMPT): foraminiferal assemblage and sea level reconstruction, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5891, https://doi.org/10.5194/egusphere-egu23-5891, 2023.

EGU23-7477 | Orals | CL1.1.3 | Highlight

Pliocene-Pleistocene evolution of the Agulhas leakage to the Atlantic Ocean 

Erin McClymont, Thibaut Caley, Christopher Charles, Aidan Starr, Maria Luisa Sanchez Montes, Martin West, Linda Rossignol, Ian Hall, and Sidney Hemming

The Agulhas leakage is an important contributor to the global thermohaline conveyor system, adding warm and saline subtropical waters from the Indian Ocean to the South-east Atlantic Ocean. It has been proposed that weaker Agulhas leakage occurred during glacial stages, but that leakage was reinvigorated during deglaciations and was, in turn, potentially important for the development of interglacial warmth.

Little is known about the longer-term evolution of Agulhas leakage during the Pliocene and Pleistocene (the last 5.3 Ma). In the Pliocene, the continental ice sheets were smaller in size, and the position and strength of key ocean and atmosphere circulation systems in the South Atlantic region were different. The Pliocene is also characterised by a series of gateway changes which are argued to have affected North Atlantic climate, but the response of the Agulhas leakage system remains unclear. It is also unclear whether the ‘early deglaciation’ signal is a specific component of the late Pleistocene 100 kyr cycles. Identifying how and when this signal developed could have important implications for understanding the impact of ocean circulation changes on the development of the mid-Pleistocene climate transition (MPT) ~1.2-0.6 Ma, when the period of the glacial-interglacial cycles shifted from ~41 kyr to ~100 kyr.

Here we present initial results from a new Cape Basin site (Site U1479, 35°03.53′S; 17°24.06′E), which was recovered by IODP Expedition 361 in 2016 from the western slope of the Agulhas Bank (Hall et al., 2016). We combine reconstructions of sea surface temperatures (using the alkenone-derived UK37’ index) and sea surface salinity (from alkenone dD analysis) with details of planktonic foraminifera assemblages, to identify and understand variability in Agulhas leakage operating across both orbital and longer timescales. There is an overall cooling of ~4°C since the Pliocene, but it is focussed around ~2 Ma and from 1.2 Ma. Orbital scale and longer-term variability in SST, sea surface salinity and Agulhas leakage fauna are also determined, demonstrating that the Agulhas leakage system has evolved across a range of timescales through the Plio-Pleistocene, especially in association with the MPT.

References

Hall, I.R., Hemming, S.R., LeVay, L.J., and the Expedition 361 Scientists, 2016. Expedition 361 Preliminary Report: South African Climates (Agulhas LGM Density Profile). International Ocean Discovery Program. http://dx.doi.org/10.14379/iodp.pr.361.2016

How to cite: McClymont, E., Caley, T., Charles, C., Starr, A., Sanchez Montes, M. L., West, M., Rossignol, L., Hall, I., and Hemming, S.: Pliocene-Pleistocene evolution of the Agulhas leakage to the Atlantic Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7477, https://doi.org/10.5194/egusphere-egu23-7477, 2023.

EGU23-7924 | ECS | Orals | CL1.1.3

Mid-Pliocene subtropical front variability in the Southern Ocean 

Suning Hou, Malte Stockhausen, Leonie Toebrock, Francesca Sangiorgi, Aidan Starr, Melissa Berke, Martin Ziegler, and Peter Bijl

The mid-Pliocene (3.3-3.0 Ma) is a time when the Earth's climate fluctuated between cold glacial conditions (marine isotope stage M2; 3.3 Ma) and periods when global temperatures were ~3°C warmer than the pre-industrial (Mid-Pliocene warm period; 3.3-3.025 Ma) when CO2 concentrations reached ~400 ppm. Thus, the paleoclimate reconstruction of this time interval provides an analogue of the present-day and near-future climate change in a moderate pCO2 increase scenario. Although fluctuations in benthic δ18O in the mid-Pliocene were predominantly associated with Northern Hemisphere glacial dynamics, the contribution of Antarctic ice to mid-Pliocene glacial-interglacial cyclicity is unknown. Moreover, the surface oceanographic response of the Southern Ocean to Pliocene glacial-interglacial climate change is poorly documented

We studied 2 sedimentary records from offshore west Tasmania (ODP Site 1168) and the Agulhas Plateau (IODP Site U1475), both located close to the modern position of the subtropical front (STF) in the Southern Ocean and encompassing the mid-Pliocene. The STF is a crucial surface water mass boundary separating cold, fresher subantarctic waters and warm, more saline subtropical waters and plays a key role in global ocean circulation, ocean-atmosphere CO2 exchange and meridional heat transport.

We use lipid biomarkers, dinoflagellate cyst assemblages and benthic foraminiferal clumped isotopes to reconstruct surface and bottom oceanographic conditions over the mid-Pliocene including the M2 glaciation. We identify similar sea surface temperature (SST) changes at the two sites. Site 1168 SST cools from 18°C to 12°C and at Site U1475 from 21°C to 18°C across the M2 glaciation. Dinoflagellate cyst assemblages suggest strong latitudinal shifts of the subtropical front associated to Pliocene glacial-interglacial climate changes. However, the most profound assemblage shift occurs at the M2 deglaciation stage at both sites, suggesting strong and unprecedented surface water freshening. Preliminary clumped isotope results suggest bottom water temperatures at Site 1168 are stable around 9°C between M2 and mid-Piacenzian warm period, indicating that the enrichment in δ18O across the M2 is mainly contributed by large ice volume changes. We interpret the surface water freshening of the subantarctic zone as signaling major iceberg calving following the M2 glaciation, suggesting that the Antarctic contribution to the M2 glaciation was profound.

How to cite: Hou, S., Stockhausen, M., Toebrock, L., Sangiorgi, F., Starr, A., Berke, M., Ziegler, M., and Bijl, P.: Mid-Pliocene subtropical front variability in the Southern Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7924, https://doi.org/10.5194/egusphere-egu23-7924, 2023.

EGU23-9653 | Posters on site | CL1.1.3

Using Legacy Data to Explore the Onset and Development of the Southern Hemisphere Supergyre 

Beth Christensen, Anna Joy Drury, Gerald Auer, David DeVleeschouwer, and Jing Lyu

The Southern Hemisphere Supergyre refers to the strong connections and intertwining of the southern subtropical gyres. Tasman Leakage is a fundamental part of the supergyre, as well as of the  global thermohaline circulation, as it provides a return flow from the Pacific and Indian Oceans to the North Atlantic at intermediate depths.   However, both are only relatively recently documented, and the timing and conditions of onset are not well understood.

This study characterizes the newly identified onset of Tasman Leakage in sedimentary records in and around the Indian Ocean using core descriptions and data derived from sediments.  Since much of this is legacy core material, core photographs were used to develop complementary and more continuous records to help refine the timing of onset.  These newly constructed time series based on core photographs are compared with X-ray Fluorescence time series based on core scanning provide both insight into onset of Tasman Leakage and a first test of the utility of time series based on core photos.

This effort will focus on the intermediate water pathway associated with Tasman Leakage and identify conditions at critical around the basin from at least 8 Ma at Broken Ridge and Mascarene Plateau to understand the role of Indian Ocean intermediate waters in the Southern Hemisphere Supergyre in major climate events of the late Miocene. 

This proposed work provides the first synoptic view of SHS onset using intermediate depth cores, which in turn will provide an important framework for basin-wide synthesis of Indian Ocean drilling, much of which is outside of the main pathway of the SHS.  It will also serve as a test of the utility of legacy material as primary data.

How to cite: Christensen, B., Drury, A. J., Auer, G., DeVleeschouwer, D., and Lyu, J.: Using Legacy Data to Explore the Onset and Development of the Southern Hemisphere Supergyre, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9653, https://doi.org/10.5194/egusphere-egu23-9653, 2023.

A collapse of the Atlantic Meridional Overturning Circulation (AMOC) could drive widespread changes in tropical rainfall, but the underlying physical mechanisms are poorly understood. Numerical simulations validated against hydroclimate changes during Heinrich Stadial 1(HS1) – the most recent, best-documented AMOC collapse – show a global response driven by cooling over the tropical North Atlantic. This pattern of ocean cooling is key to link changes in rainfall across the tropics with the reductions in AMOC strength. Cooling over the tropical North Atlantic drives changes over the Pacific and Indian oceans that uniquely explain the paleoclimatic evidence. A similar response is active in simulations of future greenhouse warming, but model disagreement regarding the pattern of AMOC-induced tropical cooling produces divergent rainfall predictions across the tropics. Models with responses consistent with the paleodata predict more pronounced rainfall reductions across the tropics, revealing a heightened risk of drought over vulnerable societies and ecosystems worldwide.

How to cite: DiNezio, P.: The tropical response to a collapse of the Atlantic Meridional Overturning Circulation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10410, https://doi.org/10.5194/egusphere-egu23-10410, 2023.

EGU23-11089 | Posters on site | CL1.1.3

Understanding the Changes in the Post-Glacial Depositional Environments through High-resolution Geochemical Proxies in the Central Yellow Sea 

Jin Hyung Cho, Byung-Cheol Kum, Seok Jang, Cheolku Lee, Seunghun Lee, Young Baek Son, and Seom-Kyu Jung

Sediment cores (A10 and I06) were analyzed using a high-resolution X-ray fluorescence (XRF) core scanner to understand changes in paleo-sedimentary environments of the study area. Age dating reflects environmental changes from interglacial marine isotope stage 3 (MIS 3) through the last glacial maximum (LGM; MIS 2) to the Holocene. Three layers were identified in the seismic profiles as follows: unit 1 (thickness = ca. 5 m) in a homogeneous sedimentary phase; unit 2 formed by erosion; unit 3, which is parallel and continuous. XRF elemental proxy data indicate anomalous distributions of Ca/Fe, Ca/K, and Fe/Ti caused by organic substances that appear at several depths in the A10 core. Results show that the seafloor was exposed to air during the LGM. The I06 core shows characteristic anomalies at depths of 0.8, 1.5, and 2.5 m, which were caused by sediments supplied from surrounding rivers.

How to cite: Cho, J. H., Kum, B.-C., Jang, S., Lee, C., Lee, S., Son, Y. B., and Jung, S.-K.: Understanding the Changes in the Post-Glacial Depositional Environments through High-resolution Geochemical Proxies in the Central Yellow Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11089, https://doi.org/10.5194/egusphere-egu23-11089, 2023.

EGU23-11804 | ECS | Posters on site | CL1.1.3

Late Cenozoic oxygenation of the Pacific Ocean, a perspective from planktic foraminiferal I/Ca 

Katrina Nilsson-Kerr, Babette Hoogakker, Dharma Andrea Reyes Macaya, and Helge Arne Winkelbauer

The Pacific Ocean hosts one of the most extensive areas of oxygen deficient waters at present with well-defined areas of oxygen minima existing both north and south of the equator along the eastern basin. This deficiency in oceanic O2 concentrations is mainly due to a combination of upwelling induced high primary productivity and poorly ventilated intermediate waters. Across the Miocene-Pliocene the Pacific Ocean is thought to have been distinctly different with an elevated water column temperature profile, reduced Walker circulation, active deep-water formation in the north Pacific, high primary productivity, and differences in its fundamental configuration with gateway changes occurring at the eastern and western margins. Collectively, and individually, these different factors will have had implications on Pacific Ocean O2 distribution. To better understand the past oxygenation of Pacific waters amidst this backdrop of climatic and geographical changes we reconstruct iodine/calcium ratios from planktic foraminifera across multiple Pacific Ocean sites. Our I/Ca records extending from the mid-late Miocene through to Pleistocene show the progressive reduction in oceanic O2 content across the Pacific. We place these records in the context of changes in the Central American Seaway and the resultant changes in oceanic circulation.

How to cite: Nilsson-Kerr, K., Hoogakker, B., Reyes Macaya, D. A., and Winkelbauer, H. A.: Late Cenozoic oxygenation of the Pacific Ocean, a perspective from planktic foraminiferal I/Ca, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11804, https://doi.org/10.5194/egusphere-egu23-11804, 2023.

EGU23-12026 | Posters on site | CL1.1.3

Microfossil-based reconstruction of latitudinal thermal gradients in the Southern Ocean during MIS11c 

Iván Hernández-Almeida, Janik Hirt, and Johan Renaudie

The Southern Ocean (SO) is a region particularly sensitive to the anthropogenic global warming because of the raising ocean temperatures, leading to latitudinal shifts of oceanographic fronts which govern the position of the South Westerly Winds (SWW) in the SO. Sediments represent a natural climate archive that allows to observe changes in Earth’s systems only affected by natural forcing. In this sense, Marine Isotope Stage (MIS) 11c (∼426–396 ka) is the most similar climate state to the ongoing climate warming that we are facing today, but quantiative climate reconstructions in the SO for this period are scarce. Radiolarians (zooplankton) live in a wide range of depths in the water column and are very abundant in sediments throughout the Neogene in the SO.  Recent radiolarian databases and transfer functions for the SO (Lawler et al. 2021; Civel-Mazens et al. 2022) enable reconstructing quantitatively past climate. For this, three sediment cores, drilled during IODP Expedition 382 and located along latitudinal gradient in the Atlantic sector of the SO (between 53.2°S and 59.4°S), were studied for their fossil radiolarian assemblage composition for the interval corresponding to MIS 11c. Application of the newly developed radiolarian transfer functions to the fossil radiolarian assemblages in these three cores enabled the reconstruction of ocean temperatures and thermal gradients in the SO during MIS 11c. These reconstructions will be used also to infer the position of the oceanographic frontal zones and the position of the SWW in this sector of the SO in the past, which are important for promoting upwelling nutrient rich bottom waters and degassing of deeply sequestered CO2 during the interglacial maxima.

References:

Civel-Mazens, M., Cortese, G., Crosta, X., Lawler, K. A., Lowe, V., Ikehara, M., & Itaki, T. (2022). New Southern Ocean transfer function for subsurface temperature prediction using radiolarian assemblages. Marine Micropaleontology, 102198.

Lawler, K. A., Cortese, G., Civel-Mazens, M., Bostock, H., Crosta, X., Leventer, A., & Armand, L. K. (2021). The Southern Ocean Radiolarian (SO-RAD) dataset: a new compilation of modern radiolarian census data. Earth System Science Data, 13(11), 5441-5453.

How to cite: Hernández-Almeida, I., Hirt, J., and Renaudie, J.: Microfossil-based reconstruction of latitudinal thermal gradients in the Southern Ocean during MIS11c, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12026, https://doi.org/10.5194/egusphere-egu23-12026, 2023.

EGU23-12214 | Posters on site | CL1.1.3

Middle to Late Miocene responses of primary producers to monsoonal upwelling in the western Arabian Sea 

Gerald Auer, Or M Bialik, Mary-Elizabeth Antoulas, and Werner E Piller

Today, the western Arabian Sea represents one of the most productive marine areas in the world. The high productivity in this region is governed by upwelling related to the intensity of the South Asian Monsoon (SAM). Previous studies show that high productivity has prevailed since the late Early Miocene (~15 Ma) after establishing a favorable tectonic configuration in the region. Existing productivity records have further demonstrated that upwelling intensity varied in the western Arabian Sea over different time scales. This variability has been attributed mainly to changing monsoonal upwelling intensity linked to global climatic changes. However, the abundance and contribution of individual primary producers (calcareous nannoplankton and diatoms) have never been studied in the context of upwelling and SAM changes. To fully disentangle the variability in the context of local upwelling changes and nutrient availability at ODP Site 722B, we link assemblage-based primary productivity records to the established multi-proxy framework in the region. Quantitative nannofossil assemblage records and absolute diatom abundances are examined in conjunction with existing and new planktonic foraminifer data to better constrain the temporal variation in productivity in the western Arabian Sea.

In our record, the first increase in cool and eutrophic nannofossil taxa (i.e., Coccolithus pelagicus and Reticulofenestra pseudoumbilicus) corresponds to the initial phase of sea surface temperatures (SST) cooling ~13.4 Ma. By ~12 Ma, rare occurrences of diatoms frustules correspond to the maximum abundances of Reticulofenestra haqii and Reticulofenestra antarctica, indicating higher upwelling derived nutrient levels. However, these changes ~12 Ma occur in the absence of coeval high latitude cooling, as shown by deep-sea benthic oxygen isotope records. By 11 Ma, diatom abundance increases significantly, leading to alternating blooms of upwelling sensitive diatom species (Thalassionema spp.) and eutrophic nannoplankton species (e.g., R. pseudoumbilicus). These changes in primary producers are also well reflected in geochemical proxies with increasing δ15Norg. values (> 6‰) and high C/N ratios also confirming high productivity and beginning denitrification at the same time.

Our multi-proxy-based evaluation of Site 722B primary producers thus indicates a stepwise evolution of productivity in the western Arabian Sea related to the intensity of upwelling and forcing SAM dynamics throughout the Middle to Late Miocene. The absence of full correspondence with existing deep marine climate records also suggests that local processes, such as lateral nutrient transport, likely played an important role in modulating productivity in the western Arabian Sea. We show that using a multi-proxy record provides novel insights into how fossil primary producers responded to changing nutrient conditions through time in a monsoon-wind-driven upwelling zone.

How to cite: Auer, G., Bialik, O. M., Antoulas, M.-E., and Piller, W. E.: Middle to Late Miocene responses of primary producers to monsoonal upwelling in the western Arabian Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12214, https://doi.org/10.5194/egusphere-egu23-12214, 2023.

EGU23-13273 | Posters on site | CL1.1.3

Late Pleistocene-Holocene coccolithophore variations in the Subantarctic South Pacific 

Elisa Malinverno, Mariem Saavedra-Pellitero, Amy Jones, Sofia Cerri, and Tom Dunkley Jones and the IODP-383 Scientific Party

International Ocean Discovery program (IODP) Expedition 383 Dynamics of the Pacific Antarctic Circumpolar Current (DYNAPACC) (Lamy et al., 2019; 2021) drilled a series of cores from the Pacific sector of the Southern Ocean in order to explore atmosphere-ocean-cryosphere glacial-interglacial dynamics their implications for regional and global climate changes. IODP Expedition 383 sites constitute the first continuous drill cores at key locations of the Subantarctic Pacific Southern Ocean extending through the Pleistocene and back into the Pliocene.

Here we focus on coccolith relative and absolute abundance as well as productivity variations for the last 0.5 Million year, in order to understand the nannofloral response to glacial-interglacial cycles and related changes in carbonate production and export. Our data has been generated at IODP Sites U1539 (56°09.0655′S, 115°08.038′W, ~1600 nmi west of the Strait of Magellan at 4070 m water depth) and U1540 (55°08.467′S, 114°50.515′W, ~1600 nmi west of the Strait of Magellan at 3580 m water depth), drilled at a southern and northern location in the central Pacific within the ACC, respectively. Coccolithophore diversity and coccolith numbers change dramatically in the studied cores, ranging from high values during interglacials (up to ca. 1011 coccoliths per gram of sediment, as in MIS11, Saavedra-Pellitero et al., 2017) to low values during the glacials, where they are outcompeted by siliceous microfossils, mostly diatoms.

References

Lamy, F., Winckler, G., Alvarez Zarikian, C.A., and the Expedition 383 Scientists, 2019. Expedition 383 Preliminary Report: Dynamics of the Pacific Antarctic Circumpolar Current. International Ocean Discovery Program. https://doi.org/10.14379/iodp.pr.383.2019

Lamy, F., Winckler, G., Alvarez Zarikian, C.A., and the Expedition 383 Scientists, 2021. Dynamics of the Pacific Antarctic Circumpolar Current. Proceedings of the International Ocean Discovery Program, 383: College Station, TX (International Ocean Discovery Program). https://doi.org/10.14379/iodp.proc.383.2021

Saavedra-Pellitero M., Baumann K.-H., Lamy F., and Köhler P., 2017. Coccolithophore variability across Marine Isotope Stage 11 in the Pacific sector of the Southern Ocean and its potential impact on the carbon cycle. Paleoceanography, 32, 864–880, doi:10.1002/2017PA003156.

How to cite: Malinverno, E., Saavedra-Pellitero, M., Jones, A., Cerri, S., and Dunkley Jones, T. and the IODP-383 Scientific Party: Late Pleistocene-Holocene coccolithophore variations in the Subantarctic South Pacific, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13273, https://doi.org/10.5194/egusphere-egu23-13273, 2023.

EGU23-17081 | ECS | Posters on site | CL1.1.3

The early to mid-Pliocene latitudinal migration of the Southern Ocean subtropical front (IODP Site U1475, Agulhas Plateau) 

Deborah Tangunan, Ian Hall, Luc Beaufort, Melissa Berke, Leah LeVay, Luz Maria Mejia, Heiko Palike, Aidan Starr, and Jose Abel Flores

The latitudinal migration of the Southern Ocean hydrographic fronts has been suggested to influence oceanographic conditions within the Indian-Atlantic Ocean gateway by restricting the amount of warm, saline water from the Indo-Pacific, transported by Agulhas Current, feeding into the South Atlantic via the Agulhas leakage. The Agulhas Current is an integral part of the global thermohaline circulation system as it acts as potential modulator of the Atlantic Meridional Overturning Circulation, which drives changes in regional and global climate, over at least the last 1.4 million years. However, the dynamics of this frontal system and associated changes in surface ocean biogeochemistry have not been explored beyond this time period due to absence of long continuous records spanning the Pliocene. Using International Ocean Discovery Program Site U1475 located on the southwestern flank of the Agulhas Plateau (41°25.61′S; 25°15.64′E; 2669 m water depth), we present high-resolution palaeoclimate records spanning the early to mid-Pliocene (~2.8 to ~5 Ma), from assemblage composition and morphometry of coccoliths, combined with oxygen and carbon stable isotopes from the bulk coccolith fraction. Our new Pliocene reconstructions offer evidence of the changing position of the subtropical front in the Southern Indian Ocean, driving variations in surface ocean conditions (e.g., nutrients, temperature, stratification), and thus biological productivity. We also explore expressions of coccolith δ13C vital effects from size-separated coccolith fractions together with planktic foraminifer carbon and oxygen stable isotopes from co-registered samples, that have been linked to cell size, growth rate, and calcification degree, providing empirical correlation with aqueous and atmospheric CO2 concentrations.

How to cite: Tangunan, D., Hall, I., Beaufort, L., Berke, M., LeVay, L., Mejia, L. M., Palike, H., Starr, A., and Flores, J. A.: The early to mid-Pliocene latitudinal migration of the Southern Ocean subtropical front (IODP Site U1475, Agulhas Plateau), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17081, https://doi.org/10.5194/egusphere-egu23-17081, 2023.

Indian Ocean Dipole (IOD) is an air-sea coupled variability in the Tropical Indian Ocean (TIO), which strongly impacts climate variability over the Indian Ocean rim countries. Though many positive IODs co-occurred with El Niño Southern Oscillation (ENSO), IODs do evolve independently, suggesting the possible role of internal dynamics of the Indian Ocean. In this study, the subtropical IOD (SIOD) is reported as one of the triggers for non-ENSO IODs. The study highlights the existence of cyclic feedback between IOD and SIOD through tropical subtropical interaction, a possible mechanism for the biennial tendency of both IOD and SIOD modes. The positive SIOD induce warming in the southwest of the Subtropical South Indian Ocean (SSIO) during April-May months and creates a meridional cell with subsidence over the southwestern TIO region (10oS). The subsidence expands the existing anticyclonic circulation over SSIO towards the equator and develops easterlies along the equator, warming the western TIO region. A zonal-vertical cell with convection over the western TIO and subsidence over the eastern TIO originates during June-July, which subsequently generates positive IOD in the following months. The positive IOD triggers negative SIOD by developing a stationary Rossby wave train in the midlatitudes. The southeastern anticyclonic circulation develops during the IOD peak season as Gill’s response initiates warm SST anomalies in the northeastern subtropics. As a result of the warming, the evolution of upper-level divergence and high absolute vorticity gradient over the subtropics generate an equivalent barotropic Rossby wave number 3 pattern in the extratropics. The cyclonic circulation over the southwest SSIO related to this Rossby wave pattern creates cold SST anomalies there. The cooling in the southwest and the warming in the northeast SSIO persisted from the IOD peak season, which strengthened the cyclonic circulation over SSIO, reinforcing the existing negative and positive SST anomalies through a positive feedback mechanism and generating negative SIOD, which peaks in the following January-March months.

How to cite: Sebastian, A. and Gnanaseelan, C.: Coupled feedback between the tropics and subtropics of the Indian Ocean with emphasis on the coupled interaction between IOD and SIOD, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-373, https://doi.org/10.5194/egusphere-egu23-373, 2023.

Tropical cyclone (TC) activity varies substantially yearly, and tropical cyclone-related damage also changes. Longer-term prediction of tropical cyclones plays an important role in reducing the wear and human loss caused by TCs. In this study, we have used a Causal-network-based algorithm to find the main development regions and precursors responsible for TC genesis and intensification. However, all the extreme events are interconnected through various global links. Therefore, analysis of the teleconnection and correlation of Tropical Cyclones with El Nino Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), and North Atlantic Oscillation (NAO) during the satellite era (1980-2020) over the North Indian Ocean (NIO) basins using this Causal Effect Network (CEN) based algorithms is checked. The most appropriate metric for cyclone energy is Accumulated Cyclone Energy (ACE); its correlation with the various factors are investigated. We examined the variation in TCs activity during all three phases (positive, negative, and neutral phases).

The results show an increasing trend in ACE over the NIO region during that specific period. The duration of most intense cyclones is increased, but their frequency decreases in this period. A shift in ACE starts after 1997 and still rises significantly. Analysis of Sea Surface Temperature (SST), Vertical Wind Shear (VWS) between 850 and 250 hPa, mid-tropospheric (800 hPa) Relative Humidity (RH), low level (850 hPa) Relative Vorticity (RV), and Tropical Cyclone Heat Potential (TCHP) is done, and it shows positive changes and variability of ACE. These results may help get better knowledge about the atmospheric or oceanic teleconnections between the events, and improved tropical cyclone prediction can help reduce the loss caused by the TCs.        

How to cite: kumar sagar, A.: identification of robust predictors of tropical cyclones using causal effect network over the north indian ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-502, https://doi.org/10.5194/egusphere-egu23-502, 2023.

EGU23-595 | ECS | Posters on site | CL4.6

Intra-decadal variability of the Indian Ocean shallow meridional overturning circulation during boreal winter 

Rahul Pai, Anant Parekh, Jasti S Chowdary, and Gnanaseelan Chellappan

The variability of Indian Ocean shallow meridional overturning circulation (SMOC) is studied using the century-long ocean reanalysis simple ocean data assimilation (SODA) data. Though SMOC exhibits stronger southward transport during boreal summer, it displays stronger variability during boreal winter. The spectrum analysis of the winter SMOC index reveals the presence of the highest amplitude between 5 to 7 years at 95% confidence level, suggesting the dominance of intra-decadal SMOC variability. The robustness of intra-decadal SMOC variability is also confirmed in different ocean reanalysis data sets. Composite analysis of filtered upper Ocean Heat Content, sea level, thermocline depth, and Sea Surface Temperature anomalies for strong (weak) SMOC years show negative (positive) anomalies over north and East of Madagascar. Correlation analysis, of filtered SMOC index and sea level pressure (zonal winds) over the Indian Ocean, found a significant negative (positive) correlation coefficient north of 40 °S (around 10 °S) and a significantly positive (negative) correlation coefficient over the 45 °S to 70 °S (20 °S to 50 °S and north of 5 °S). This meridional pattern of the correlation coefficient for sea level pressure, manifesting the out-of-phase relationship between sub-tropics and high latitude mean sea level pressure, resembles Southern Annular Mode (SAM). We conclude that the intra-decadal variability of mean sea level pressure leads to zonal wind variation around 10 °S modulating SMOC, which in turn affects the upper ocean thermal properties in the east and north of Madagascar. This study for the first time brought out coherent intra-decadal evolution of SAM and SMOC during boreal winter.

How to cite: Pai, R., Parekh, A., Chowdary, J. S., and Chellappan, G.: Intra-decadal variability of the Indian Ocean shallow meridional overturning circulation during boreal winter, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-595, https://doi.org/10.5194/egusphere-egu23-595, 2023.

EGU23-864 | ECS | Posters on site | CL4.6

The Teleconnection of Indian Summer Monsoon Clouds with Global Predictors: An Unexplored Measure for Coupled Model development 

Ushnanshu Dutta, Anupam Hazra, Hemantkumar S Chaudhari, Subodh Kumar Saha, Samir Pokhrel, and Utkarsh Verma

The teleconnection studies regarding Indian summer monsoon (ISM) clouds are not focused on detail from both observational and modeling aspects. This is despite the fact that clouds play a seminal role in governing rainfall variability through the modulation of heating and induced circulation. Therefore, we find it essential to explore whether the inter-annual variability of ISM clouds is also remotely influenced by the slowly varying predictable component e.g. Sea Surface Temperature (SST). 

The findings reveal the linkage of observed TCF (and rainfall) over the ISM region with slowly varying forcing (e.g., global SST). The observed/reanalysis teleconnection pattern of TCF-SST is almost similar to that of rainfall-SST.In the long-term period, TCF and SST show a strong and positive correlation with Extra-Tropics (R ~ 0.41), NAO (R ~ 0.51), and AMO (R ~ 0.41) SST regions, in addition to canonical ENSO teleconnection (R ~ −0.39). This is better captured in CMIP6-MME than in CMIP5-MME. The representation of the global teleconnection pattern has been significantly improved in participating models from CMIP5 to CMIP6. The teleconnection with extra-tropics and north Atlantic mode of variability is markedly enhanced in CMIP6-MME compared to CMIP5-MME. The present study has also shown the lag correlations in the teleconnection analysis, i.e., the correlation of June–September (JJAS) mean of rainfall/TCF with October–December (OND) SST from observation/reanalysis, CMIP5-MME, and CMIP6-MME. The CMIP6-MME performs better than CMIP5-MME as compared to observation/reanalysis. 

Thus, the improved understanding of the teleconnection of cloud variables with ENSO and other predictors (ET, NAO, and AMO) will help researchers take up the challenges of improving the ISMR skill far ahead using the new generation coupled climate models. This may facilitate reliable seasonal ISM forecasting.

Keywords: Indian Summer Monsoon, Clouds, Teleconnection, CMIP5, CMIP6

How to cite: Dutta, U., Hazra, A., Chaudhari, H. S., Saha, S. K., Pokhrel, S., and Verma, U.: The Teleconnection of Indian Summer Monsoon Clouds with Global Predictors: An Unexplored Measure for Coupled Model development, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-864, https://doi.org/10.5194/egusphere-egu23-864, 2023.

EGU23-1944 | ECS | Orals | CL4.6

ENSO-driven abrupt phase shift in North Atlantic Oscillation in early January 

Xin Geng, Jiuwei Zhao, and Jong-Seong Kug

El Niño-Southern Oscillation (ENSO) teleconnections exhibit a strong dependency on seasonally and intraseasonally varying mean states, which leads to impactful short-term variations in regional climate. The North Atlantic Oscillation (NAO)-ENSO relation is a typical example, in that its phase relationship reverses systematically between the early and late winter. However, the details and underlying mechanisms of this relationship transition are not well understood yet.

Here based on observations and an ensemble of atmosphere-only climate model simulations, we first reveal that this NAO phase reversal occurs synchronously in early January, which indicates strong abruptness. We demonstrate that this abrupt NAO phase reversal is caused by the change in ENSO-induced Rossby wave-propagating direction from northeastward to southeastward over the northeastern North American region, which is largely governed by a climatological alteration of the local jet meridional shear. We also provide evidence that the North Atlantic intrinsic eddy–low-frequency flow feedback further facilitates and amplifies the NAO responses. This abrupt NAO phase reversal signal is strong enough during the ENSO winter to be useful for intraseasonal climate forecasting in the Euro-Atlantic region.

How to cite: Geng, X., Zhao, J., and Kug, J.-S.: ENSO-driven abrupt phase shift in North Atlantic Oscillation in early January, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1944, https://doi.org/10.5194/egusphere-egu23-1944, 2023.

Weather type classification is a well-established and thoroughly researched field of study in atmospheric sciences. One of its applications is the analysis of occurrence of and transitions between large scale synoptic types. This is typically done by calculating the moving average of, or estimating linear or polynomial fits to relative frequencies. The presented work points out the theoretical inconsistencies implied by such approaches and, instead, employs binomial and multinomial logistic regression for consistent estimation of long-term trends in occurrence and transition probabilities between synoptic types, while assuming first-order Markovian behaviour throughout. The methodological framework's functioning is demonstrated using two prominent examples of weather type classification schemes with regional focus on Germany and central Europe. Temporal refinement to seasonal and monthly level and aggregation into combined groups of classes allows for tracing of observed trends, providing a more comprehensive understanding of the systems investigated. The results, by and large, fit in well with expectations about circulatory changes suggested by research about global warming induced climate change and can be verified by existing research in some cases. Inspection of transition probability changes allows for differentiation between changes in occurrence probability caused by changes in the mean vs. changes in circulatory dynamics. Limitations and favourable implementational details of the approach are determined and the Wald Null test is recommended for assessing statistical significance.

How to cite: Schoeller, H.: Occurrence and Transition Probabilities for two Weather Classification Systems over Germany, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2004, https://doi.org/10.5194/egusphere-egu23-2004, 2023.

EGU23-2125 | Orals | CL4.6

Resolving weather fronts increases the large-scale circulation response to Gulf Stream SST anomalies 

Robert Jnglin Wills, Adam Herrington, Isla Simpson, and David Battisti

Canonical understanding based on general circulation models (GCMs) is that the large-scale circulation responds only weakly to extratropical sea-surface temperature (SST) anomalies, compared to the larger influence of tropical SST anomalies. However, the horizontal resolution of modern GCMs, which ranges from roughly 200 km to 25 km, is too coarse to fully resolve mesoscale atmospheric processes such as weather fronts. Here, we investigate the large-scale atmospheric circulation response to idealized Gulf Stream SST anomalies in a variable resolution version of the Community Atmospheric Model (CAM6), with regional grid refinement of 14 km over the North Atlantic, and compare it to versions with 28-km regional grid refinement and global 111-km resolution. The high-resolution simulations show a large positive response of the wintertime North Atlantic Oscillation (NAO) to positive SST anomalies in the Gulf Stream, a 1-standard-deviation NAO anomaly for 2°C SST anomalies. The lower-resolution simulations show a much weaker response, and in some cases, a different spatial structure of the response. The enhanced large-scale circulation response at high resolution results from an increase in resolved vertical motions, which enables SST forcing to have a larger influence on transient-eddy heat and momentum fluxes. In response to positive SST anomalies, these processes contribute to a stronger North Atlantic jet that varies less in latitude, as is characteristic of the positive phase of the NAO. Our results suggest that the atmospheric circulation response to extratropical SST anomalies is fundamentally different at higher resolution. Regional refinement in key regions offers a potential pathway towards improving simulation of the atmospheric response to extratropical SST anomalies and thereby improving multi-year regional climate predictions.

How to cite: Jnglin Wills, R., Herrington, A., Simpson, I., and Battisti, D.: Resolving weather fronts increases the large-scale circulation response to Gulf Stream SST anomalies, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2125, https://doi.org/10.5194/egusphere-egu23-2125, 2023.

EGU23-5034 | ECS | Orals | CL4.6

Impact of tropical eastern Pacific warming bias on Caribbean climate 

Marta Brotons Blanes, Rein Haarsma, and Nadie Bloemendaal

During the last decades, CMIP5 models simulate a warming trend in the tropical eastern Pacific that has not been present in observations (Seager et al., 2019). Associated with this, the Walker circulation has experienced a westward migration while CMIP5 models simulate an eastward migration. This mismatch is still present in CMIP6 models and might affect climate projections worldwide. In the Caribbean region, CMIP6 models project a strong drying at the end of the 21st century. El Niño-like changes in the Walker circulation are the dominant teleconnections driving the Caribbean drying. The models that project a strong Caribbean drying also simulate generally a strong equatorial eastern Pacific warming trend over the recent decades. Thus, the mismatch between observed and simulated warming trends over the equatorial eastern Pacific questions the reliability of the Caribbean precipitation projections. The warming bias might also have implications for tropical cyclones’ projections in the Atlantic and Pacific through the effect of vertical wind shear, which is related to shifts in the Walker circulation. In addition, the double Intertropical Convergence Zone (ITCZ) bias might be influenced by the mismatching trends. The strong influence of El Niño-Southern Oscillation (ENSO) dynamics on the world’s climate demands more in-depth studies addressing the drivers of the Walker circulation and the equatorial Pacific warming bias.

How to cite: Brotons Blanes, M., Haarsma, R., and Bloemendaal, N.: Impact of tropical eastern Pacific warming bias on Caribbean climate, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5034, https://doi.org/10.5194/egusphere-egu23-5034, 2023.

Climate change affects the hydrological cycle and induces extreme weather events, such as storms, floods and droughts. Adaptation to climate change needs to be based on assessments of future impacts. The new generation of Coupled Model Inter-comparison Project Phase 6 (CMIP6) is widely used in future flood prediction and drought risk assessment. However, many studies have found that CMIP6 global climate models for simulating land surface water and energy fluxes have significant biases, which poses a problem for using CMIP6 as input data for hydrological impact studies. Therefore, the output of CMIP6 cannot be directly used in hydrological models to project the impacts of future climate change. To overcome this problem, the correction of model output towards observations for its subsequent application in climate change impact studies has now become a standard procedure. And hydrological simulations generally use bias corrected output. But bias correction methods cannot really correct bias. The commonly used bias correction approaches only force the model outputs to match observations, and does not consider the mechanisms within the model and the interaction between variables. This study systematically evaluates water and energy fluxes of CMIP6 model over the Tibetan Plateau. Results show that the inter-model variability is substantial in temperature simulations. Snow that the largest component of the cryosphere responds significantly to changes in temperature. In the study, we study snow depth simulations corresponding to temperature simulations of different models over the Tibetan Plateau. Based on the water balance formula, analysis of how water balance fluxes respond to temperature changes in CMIP6, and determine the sources of error and ultimately lead to improved predictions.

How to cite: Liu, S., Liu, Z., and Duan, Q.: Evaluation of CMIP6 models for water and energy fluxes and analysis of source of errors over the Tibetan Plateau, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5066, https://doi.org/10.5194/egusphere-egu23-5066, 2023.

EGU23-5176 | ECS | Orals | CL4.6

Global warming induces more internally generated extremes of North Atlantic Oscillation and East Atlantic pattern 

Quan Liu, Johann Jungclaus, Daniela Matei, and Juergen Bader

Increased weather and climate extreme events are often attributed solely to either human-induced climate change or internal variability, under the assumption that external forcing does not influence the internal variability. However, with the development of single-model initial-condition large ensembles, recent research shows the impact of global warming on internal variability. This study investigates how global warming influences the North Atlantic Oscillation (NAO) and the East Atlantic (EA) pattern, which are the dominant large-scale circulation/teleconnection modes in the North Atlantic sector.

The study analyzes the geopotential height data of the Max Planck Institute Grand Ensemble (MPI-GE)  with 100 ensemble members. The internal variability is quantified as the deviation from the ensemble mean. The influence of global warming on the internal variability is checked with a 1pcCO2 experiment, where the  concertation is increased by 1% every year. This experiment provides a scenario for relatively strong global warming based on increasing greenhouse gas concentration alone. The extreme NAO and EA are defined as those years where the indexes are above (positive extremes) or below (negative extremes) 2 standard deviations.

The results show increases in extreme events, especially negative extremes, for both NAO and EA during wintertime, in a warmer climate. While NAO extremes increase consistently across the whole troposphere, EA extremes increase more at higher altitudes (500hpa-200hpa) than at lower altitudes. The warming effect of positive extreme NAO over northern Eurasia gets weaker, while the cooling effect of negative extreme NAO over northern Eurasia gets stronger. The effects of both, positive and negative extremes of EA, extend eastward till Eastern Asia. Overall, this study underlines the impact of global warming onto the internal variability of NAO and EA.

How to cite: Liu, Q., Jungclaus, J., Matei, D., and Bader, J.: Global warming induces more internally generated extremes of North Atlantic Oscillation and East Atlantic pattern, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5176, https://doi.org/10.5194/egusphere-egu23-5176, 2023.

EGU23-5279 | Orals | CL4.6

Reconciling conflicting evidence for the cause of the observed early 21st century Eurasian Cooling 

Stefan Sobolowski, Stephen Outten, and Camille Li

Arctic amplification of global warming is accompanied by a dramatic decline in sea ice. This, in turn, has been linked to cooling over the Eurasian subcontinent over recent decades, most dramatically during the period 1998-2012. Such a coherent and pronounced cooling is a counterintuitive impact under global warming. Some studies have proposed a causal teleconnection from Arctic sea ice retreat to Eurasian wintertime cooling; others argue that Eurasian cooling is mainly driven by internal variability. Overall, there is an impression of strong disagreement between those holding the “ice-driven” versus “internal variability” viewpoints. We offer an alternative framing that shows that the sea ice and internal variability views can be compatible. Key to this is viewing Eurasian cooling through the dual lens of dynamics (linked primarily to internal variability with a small contribution from sea ice; cools Eurasia) and thermodynamics (linked to sea ice retreat; warms Eurasia). This framing, combined with recognition that there is uncertainty in the hypothesized mechanisms themselves, allows both viewpoints (and others) to co-exist and contribute to our understanding of Eurasian cooling. A simple autoregressive model shows that strong Eurasian cooling is consistent with internal variability, with some periods being more susceptible to strong cooling than others. Rather than posit a “yes-or-no” causal relationship between sea ice and Eurasian cooling, a more constructive way forward is to consider whether the cooling trend was more likely given the observed sea ice loss, as well as other sources of low-frequency variability. Taken in this way both sea ice and internal variability are factors that affect the likelihood of strong regional cooling in the presence of ongoing global warming. Improving our understanding of the underlying mechanisms is critical for quantifying regional responses and impacts as well as producing reliable near-term climate predictions. 

How to cite: Sobolowski, S., Outten, S., and Li, C.: Reconciling conflicting evidence for the cause of the observed early 21st century Eurasian Cooling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5279, https://doi.org/10.5194/egusphere-egu23-5279, 2023.

EGU23-5775 | ECS | Orals | CL4.6 | Highlight

Impacts of a weakened AMOC on the European climate 

Katinka Bellomo, Virna Meccia, Roberta D'Agostino, Federico Fabiano, Sarah Larson, Jost von Hardenberg, and Susanna Corti

Previous studies have shown that the response of the Atlantic Meridional Overturning Circulation (AMOC) to increasing greenhouse gas forcing is a key driver of inter-model uncertainties. While all models project an AMOC decline, the inter-model spread in the decline rate drives very different climate change impacts, including temperature, precipitation, and large-scale atmospheric circulation patterns. Here we investigate the impacts of a weakened AMOC by performing idealized climate model experiments using EC-Earth3, a state-of-the-art GCM participating in CMIP6. We compare results from a control experiment run under preindustrial forcing, with an experiment in which we force a weakened AMOC by applying a virtual salinity flux in the North Atlantic/Arctic basin. Here we analyze previously unexplored aspects of the climate response to a weakened AMOC, focusing on impacts on wintertime daily timescales in the Euro-Atlantic region.

We find that a weakened AMOC forces an overall drier climate over most of Europe; however, some regions especially in northwestern Europe experience an increase in the number of very wet days. We investigate drivers of precipitation changes by performing a moisture budget and analyzing the association with changes in weather regimes at daily timescales. We find that an increase in the occurrence of the NAO+ days (going from a frequency of ~26% of occurrence to above 42%) together with an enhanced and more central jet, favors drier conditions over southern Europe and wetter conditions over northwestern Europe. Further, enhanced but drier storms cause dryness over Europe while thermodynamic processes per se, namely the Clausius-Clapeyron constraint on temperature, play a second role. Finally, we explore these relationships in additional experiments in which we keep the AMOC constant in a forced 4xCO2 experiment by applying a reversed virtual salinity flux, which allows us to separate the effects of 4xCO2 forcing from the weakened AMOC on climate change impacts. Our results have broader implications for understanding the role of the AMOC response on future climate change, allowing us to separate the impacts of the AMOC from those of the CO2 increase.

How to cite: Bellomo, K., Meccia, V., D'Agostino, R., Fabiano, F., Larson, S., von Hardenberg, J., and Corti, S.: Impacts of a weakened AMOC on the European climate, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5775, https://doi.org/10.5194/egusphere-egu23-5775, 2023.

The central role of tropical sea surface temperature (SST) variability in modulating Northern Hemisphere (NH) extratropical climate has long been known. However, the prevailing pathways of teleconnections in observations and the ability of climate models to replicate these observed linkages remain elusive. Here, we apply maximum covariance analysis between atmospheric circulation and tropical SST to reveal two co-existing tropical-extratropical teleconnections albeit with distinctive spatiotemporal characteristics. The first mode, resembling the Pacific-North American (PNA) pattern, favors a Tropical-Arctic in-phase (warm-Pacific-warm-Arctic) teleconnection in boreal spring and winter. The second mode, predominant in summer and autumn, is manifested as an elongated Rossby-wave train emanating from the tropical eastern Pacific that features an out-of-phase relationship (cold-Pacific-warm-Arctic) between tropical Pacific SST and temperature variability over the Arctic. This Pacific-Arctic teleconnection (PARC) mode partially explains the observed summertime warming around the Arctic. The reliability of climate models to replicate these leading teleconnections is of primary interest in this study to improve decadal prediction on regional climate. While climate models participating in CMIP6 appear to successfully simulate the PNA mode and its temporal characteristics, the majority of models’ skill in reproducing the PARC mode is obstructed by apparent biases in simulating low-frequency SST and rainfall variability over the tropical eastern Pacific and the summer climatological mean flow over the North Pacific. Considering the contribution of the PARC mode in shaping low frequency climate variations over the recent decades from the tropics to the Arctic, improving models’ capability to capture the PARC mode is essential to reduce uncertainties associated with decadal prediction and climate change projection over the NH.

How to cite: Feng, X.: Possible causes of model biases in simulating Tropical-Arctic teleconnections in CMIP6, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6017, https://doi.org/10.5194/egusphere-egu23-6017, 2023.

EGU23-6966 | ECS | Orals | CL4.6

Opposite Impacts of Interannual and Decadal Pacific Variability in the Extratropics 

Melissa Seabrook, Doug Smith, Nick Dunstone, Rosie Eade, Leon Hermanson, Adam Scaife, and Steven Hardiman

It is well established that the positive phase of El Niño Southern Oscillation (ENSO) tends to weaken the Northern Hemisphere stratospheric polar vortex (SPV), promoting a negative North Atlantic Oscillation (NAO). Pacific Decadal Variability (PDV) is characterised by a pattern of sea surface temperatures similar to ENSO, but its impacts are more uncertain: some studies suggest similar impacts of ENSO and PDV on the SPV and NAO, while others find the opposite. We use climate model experiments and reanalysis to find further evidence supporting opposite interannual and decadal impacts of Pacific variability on the extratropics. We propose that the decadal strengthening of the SPV in response to positive PDV is caused by a build-up of stratospheric water vapour leading to enhanced cooling at the poles, an increased meridional temperature gradient and a strengthened extratropical jet. Our results are important for understanding decadal variability, seasonal to decadal forecasts and climate projections.

How to cite: Seabrook, M., Smith, D., Dunstone, N., Eade, R., Hermanson, L., Scaife, A., and Hardiman, S.: Opposite Impacts of Interannual and Decadal Pacific Variability in the Extratropics, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6966, https://doi.org/10.5194/egusphere-egu23-6966, 2023.

EGU23-7011 | ECS | Orals | CL4.6

AMOC variations modulated by Tropical Indio-Atlantic SST Gradient 

Brady Ferster, Leonard Borchert, Juliette Mignot, and Alexey Fedorov

A potential future slowdown or acceleration of the Atlantic Meridional Overturning Circulation (AMOC) would have profound impacts on global and regional climate. Recent studies have shown that AMOC responds, among many other processes, to anthropogenic changes in tropical Indian ocean (TIO) temperature. However, internal unforced co-variations between these two basins are largely unexplored as of yet. Here, we use the ERSST v5, HadISST v1, and COBE v2 gridded observational products for the period 1870-2014, as well as dedicated simulations with coupled climate models, and show that internal changes in sea surface temperature gradients between the Indian and Atlantic Ocean (SSTgrad) can drive teleconnections that influence internal variations of North Atlantic climate and AMOC.

We separate the unforced observed component (i.e., internal signal) from the forced signal following the residuals method presented by Smith et al. (2019). In the absence of direct AMOC observation we estimate AMOC variability from an SST index (SSTAMOC; Caesar et al., 2018). We find a robust observed relationship between the unforced tropical SSTgrad and SSTAMOC when TIO leads by ~25 years. This time-lag is in line with a recently described mechanism of anomalous tropical Atlantic rainfall patterns that originate from TIO warming and cause anomalously saline tropical Atlantic surface water which slowly propagate northward into the subpolar North Atlantic, ultimately altering oceanic deep convection and AMOC (Hu and Fedorov, 2019; Ferster et al. 2021). Our study now suggests that it is the tropical SSTgrad that drives those AMOC changes, with a limited role for the western tropical Pacific. Pre-industrial control simulations with the IPSL-CM6A-LR model confirm this relationship, indicating a time lag of ~25 years between SSTgrad and SSTAMOC variations. These simulations also confirm that the SSTAMOC is representative of unforced AMOC variations when SSTAMOC leads by 5 years. This work therefore indicates that an unforced pathway between tropical ocean temperature and AMOC exists with a ~20 year lag, which opens the potential for using SSTgrad as precursor to predict future AMOC changes.

 

Caesar, L., Rahmstorf, S., Robinson, A., Feulner, G., & Saba, V. (2018). Observed fingerprint of a weakening Atlantic Ocean overturning circulation. Nature, 556(7700), 191-196.

Ferster, B. S., Fedorov, A. V., Mignot, J., & Guilyardi, E. (2021). Sensitivity of the Atlantic meridional overturning circulation and climate to tropical Indian Ocean warming. Climate Dynamics, 1-19.

Hu, S., & Fedorov, A. V. (2019). Indian Ocean warming can strengthen the Atlantic meridional overturning circulation. Nature climate change, 9(10), 747-751.

Smith, D. M., Eade, R., Scaife, A. A., Caron, L. P., Danabasoglu, G., DelSole, T. M., ... & Yang, X. (2019). Robust skill of decadal climate predictions. Npj Climate and Atmospheric Science, 2(1), 1-10.

How to cite: Ferster, B., Borchert, L., Mignot, J., and Fedorov, A.: AMOC variations modulated by Tropical Indio-Atlantic SST Gradient, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7011, https://doi.org/10.5194/egusphere-egu23-7011, 2023.

A detailed assessment of climate variability of the Baltic Sea area for the period 1958-2009 (Lehmann et al. 2011) revealed that recent changes in the warming trend since the mid-1980s, were associated with changes in the large-scale atmospheric circulation over the North Atlantic. The analysis of winter sea level pressure (SLP) data highlighted considerable changes in intensification and location of storm tracks, in parallel with the eastward shift of the North Atlantic Oscillation (NAO) centres of action. Additionally, a seasonal shift of strong wind events from autumn to winter and early spring existed for the Baltic area. Lehmann et al. (2002) showed that different atmospheric circulation regimes force different circulation patterns in the Baltic Sea. Furthermore, as atmospheric circulation, to a large extent, controls patterns of water circulation and biophysical aspects relevant for biological production, such as the vertical distribution of temperature and salinity, alterations in weather regimes may severely impact the trophic structure and functioning of marine food webs (Hinrichsen et al. 2007). To understand the processes linking changes in the marine environment and climate variability, it is essential to investigate all components of the climate system which of course include also the large-scale atmospheric circulation. Here we focus on the link between changes/shifts in the large scale atmospheric conditions and their impact on the regional scale variability over the Baltic Sea area for the period 1950-2021. This work is mostly an extension of previous studies which focused on the response of the Baltic Sea circulation to climate variability for the period 1958-2008 (Lehmann et al. 2011, Lehmann et al. 2014). Now extended time series ECMWF ERA 5 reanalysis for 7 decades are available, highlighting recent changes in atmospheric conditions over the Baltic Sea. The main focus of this work is to identify predominant large scale atmospheric circulation patterns (climate regimes) on a monthly/seasonal time scale influencing the regional atmospheric circulation over the Baltic Sea area. Furthermore, long-term changes on the annual to decadal time scale will also be investigated.

How to cite: Lehmann, A., Post, P., and Myrberg, K.: Changing impact of the large-scale atmospheric circulation on the regional climate variability of the Baltic Sea for the period 1950-2022, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8461, https://doi.org/10.5194/egusphere-egu23-8461, 2023.

EGU23-10582 | Orals | CL4.6 | Highlight

Impacts of oceanic warming patterns versus CO2 radiative forcing on the Hadley Circulation 

Yong Sun, Gilles Ramstein, Alexey V. Fedorov, Lin Ding, and Bo Liu

The Hadley circulation (hereafter HC) is one of the most prominent meridional overturning circulations in the climate system. In addition to maintaining energy balance and momentum exchange in tropics and extratropics, it can also shape the Intertropical Convergence Zone (ITCZ) and subtropical dry arid zones by regulating the hydrological cycle in tropical and extratropical regions. Weakening and expanding HC and narrowing of the ITCZ are projected with human greenhouse gas emissions. However, no consensus has been achieved regarding the relative importance of direct CO2 radiative effect and indirect effects via SST changes in shaping the future HC changes. This limits our deep understanding of the climate impacts imposed by changes in the HC. Here we analyze a broad range of CMIP5 experiments and show that future changes in SST patterns play the leading role in the determining the future changes in HC and ITCZ. In addition, a series of individual basin perturbation experiments were conducted at 1.5°C, 2°C, and 3°C temperature thresholds to identify key basins that determine HC strength, edges, and ITCZ locations. Our work highlights the overwhelming role of future tropical Indian Ocean warming on the HC and ITCZ changes.

How to cite: Sun, Y., Ramstein, G., Fedorov, A. V., Ding, L., and Liu, B.: Impacts of oceanic warming patterns versus CO2 radiative forcing on the Hadley Circulation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10582, https://doi.org/10.5194/egusphere-egu23-10582, 2023.

EGU23-11826 | ECS | Posters on site | CL4.6

Precipitation weather typing over the South Pacific: application to the TRMM satellite product calibration 

Oscar Mirones, Joaquín Bedia, Juan A. Fernández-Granja, Sixto Herrera, Sara O. Van Vloten, Andrea Pozo, Laura Cagigal, and Fernando J. Méndez

In the South Pacific region, the precipitation patterns are mostly driven by a number of processes operating at spatial and temporal scales. One of the most important features is the South Pacific Convergence Zone (SPCZ).

Five Daily Weather Types (WT) of precipitation are presented, based on Principal Component Analysis (PCA) and k-means clustering using ERA5 precipitation and atmospheric circulation variables such as mean sea-level pressure (SLP), day-to-day difference of mean daily SLP or northward and eastward 10-m wind component fields, able to capture distinct precipitation spatio-temporal patterns, interpretable in terms of salient regional climate features such as the SPCZ state and tropical cyclone activity. We then undertake a weather-type conditioned calibration of the TRMM (Tropical Rainfall Measuring Mission) product using in-situ rain gauge records from the PACRAIN database as reference. “Conditioning” is here based on applying separate statistical corrections for each of the generated WTs, since biases might be dependent on specific atmospheric situations that can be partially captured by the clustering procedure, thus adapting the correction factors to specific synoptic conditions. 

Our results indicate that the WT-conditioned calibration provides an overall marginal added value over the unconditioned approach, although it makes a significant difference for a better correction of extreme rainfall events, critical in many impact studies. The approach can be extended to compound extreme events, in which several variables are involved (e.g. precipitation, sea level, wind, etc.), in order to better preserve multi-variable consistency.

How to cite: Mirones, O., Bedia, J., Fernández-Granja, J. A., Herrera, S., Van Vloten, S. O., Pozo, A., Cagigal, L., and Méndez, F. J.: Precipitation weather typing over the South Pacific: application to the TRMM satellite product calibration, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11826, https://doi.org/10.5194/egusphere-egu23-11826, 2023.

EGU23-11931 | ECS | Posters virtual | CL4.6

The Large-Scale Climate of Alaska - The Effects of the Pacific Decadal Oscillation on the Climate of Alaska 

Jasper Heuer, Martin Stuefer, and Lea Hartl

The climate of the state of Alaska is influenced not only by regional anthropogenic climate change, but also by the effects of large-scale ocean atmosphere systems like the Pacific Decadal Oscillation (PDO). Whereas positive anomalies in the PDO index coincide with warmer (sea surface) temperatures in the Gulf of Alaska and across the state, negative anomalies have the opposite effect. After analyzing the strength and direction of the correlation between the PDO index and the average temperatures in each of the 13 climate divisions of Alaska – both annually, as well as seasonally (DJF, MAM, JJA, and SON) – it becomes apparent, that the PDO affects the southern coastal and Panhandle regions much stronger than the Interior and North Slope. Over the course of a year, the correlations are strongest during the winter months, decrease during the spring and summer, only to increase again in the fall. Since the effects of large-scale circulations such as the PDO are changing under the influence of natural and anthropogenic climate change, reliable predictions on the future of the Alaskan climate are extremely complicated. In the future, further analysis is needed to support policy makers in their efforts to help adept the state’s ecosystems and economies to the changing climate.

How to cite: Heuer, J., Stuefer, M., and Hartl, L.: The Large-Scale Climate of Alaska - The Effects of the Pacific Decadal Oscillation on the Climate of Alaska, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11931, https://doi.org/10.5194/egusphere-egu23-11931, 2023.

EGU23-12678 | ECS | Orals | CL4.6

Classification of Atmospheric Circulation Patterns That Trigger Rainfall Extremes in the Sudan-Sahel Region 

Manuel Rauch, Jan Bliefernicht, Patrick Laux, and Harald Kunstmann

A better understanding of the rainfall variability and extremes in tropical regions is crucial for the development of improved statistical and numerical approaches used for climate research and weather prediction. In this study, we present a novel fuzzy rule-based method for classifying atmospheric circulation patterns relevant to heavy rainfall in the Sudan-Sahel region over West Africa. In the first step, we determine large-scale atmospheric patterns to describe important seasonal features of the West African Monsoon like the movement of Saharan Heat Low over the African continent. In the second step, meso-scale monsoon patterns are classified to better describe rainfall variability and extremes during the monsoon period. In addition to a comprehensive predictor screening using more than 30 variables at different atmospheric levels, a detailed sensitivity analysis is performed, which aims to improve the transferability of the classification approach to an independent dataset. Furthermore, crucial aspects of the methodological development of fully automatic classification approaches are addressed. Using mean sea level pressure and stream function fields (700hPa) as final predictor variables, we identified 23 circulation patterns as robust solution to represent key atmospheric processes and rainfall variability in the study region. The two wettest patterns are distinguished by an enhanced Saharan Heat Low and cyclonic rotation near the study region, suggesting the presence of a tropical wave trough and triggering about 50% of the rainfall extremes on 6.5% of the days. The identified atmospheric circulation patterns are currently used to develop a variety of improved statistical approaches for this challenging region, such as pattern-dependent bias correction, geostatistical interpolation, and simulation. 

How to cite: Rauch, M., Bliefernicht, J., Laux, P., and Kunstmann, H.: Classification of Atmospheric Circulation Patterns That Trigger Rainfall Extremes in the Sudan-Sahel Region, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12678, https://doi.org/10.5194/egusphere-egu23-12678, 2023.

EGU23-13294 | ECS | Orals | CL4.6

Regime-oriented causal model evaluation of Atlantic-Pacific teleconnections in CMIP6 

Soufiane Karmouche, Evgenia Galytska, Jakob Runge, Gerald Meehl, Adam Phillips, Katja Weigel, and Veronika Eyring

Regime-oriented causal model evaluation of Atlantic-Pacific teleconnections in CMIP6

Abstract:

The Pacific Decadal Variability (PDV) and the Atlantic Multidecadal Variability (AMV) are two important modes of long-term internal variability that significantly impact the climate system and its spatio-temporal changes. In this study, we use a regime-oriented causal discovery method (Karmouche et al, 2022) to examine the changing interactions between the PDV and AMV. The results of this analysis are used to evaluate the ability of models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6) to represent the observed changing interactions between the PDV, AMV, and their extra-tropical teleconnections.

Applying the regime-oriented causal discovery method to reanalysis time series revealed that the interactions between AMV and PDV differ from one regime to the other. The results also show that there are both direct and indirect connections between the Atlantic and Pacific oceans, which are established through various teleconnection patterns.

In order to evaluate the ability of climate models to represent these observed interactions, we applied the same regime-oriented causal discovery method to the CMIP6 Large Ensemble historical simulations. We show that several models performed well in simulating the observed causal patterns when AMV and PDV are "out-of-phase", and that the two models with the largest number of members generally outperformed other models in simulating observed causal patterns during longer regimes. This work shows how causal discovery on LEs complements the available diagnostics and statistics metrics of climate variability to provide a powerful tool for climate model evaluation.

Karmouche, S., Galytska, E., Runge, J., Meehl, G. A., Phillips, A. S., Weigel, K., and Eyring, V.: Regime-oriented causal model evaluation of Atlantic-Pacific teleconnections in CMIP6, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2022-1013, 2022.

How to cite: Karmouche, S., Galytska, E., Runge, J., Meehl, G., Phillips, A., Weigel, K., and Eyring, V.: Regime-oriented causal model evaluation of Atlantic-Pacific teleconnections in CMIP6, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13294, https://doi.org/10.5194/egusphere-egu23-13294, 2023.

EGU23-13299 | Orals | CL4.6

Drivers of the Annual Cycle of Rainfall over Central Africa: The Role of Water Vapor and the Mid-Tropospheric Meridional Circulation 

Georges-Noel T. Longandjo, Bellinda Mashoene Monyela, and Mathieu Rouault

The Intertropical Convergence Zone (ITCZ), with its twice-annual passage over central Africa, is considered as the main driver of the rainfall seasonality. But recently, this paradigm was challenged. To find out what are the main drivers of the annual cycle of rainfall over central Africa, we present a simple comprehensive paradigm with both local forcings and regional-scale processes playing crucial role. Due to the local evaporative cooling effect, the foot of the ascending branch of Hadley cells occurs where the temperature is the warmest, indicating a thermal low. This distorts the southern Hadley cell by developing its bottom-heavy structure. As result, both shallow and deep Hadley cells coexist over central Africa year–round. The deep mode is associated with poleward branches at upper levels that transport the atmospheric energy. The shallow mode is characterized by a meridional return flow in the mid-troposphere that transports the water vapour instead of lower branches as widely reported. This favours the building-up of the mid-tropospheric moisture flux convergence with a limited contribution of the midlevel easterly jet, conducive to deep convection. Embedded in this strong rising branch of Hadley cells at midlevels, the intense convective rainfall, and with it the rainfall maximum position, is seasonally controlled by the dynamics of the midlevel shallow meridional return flow. This highlights the interhemispheric rainfall contrast over central Africa and outlines its unimodal seasonality. On the other hand, forced by the Congo basin cell, the precipitable water regulates the deep convection from the vegetated surface of Congo basin, acting as a continental sea. This nonlinear mechanism separates the rainfall into three distinct regimes – (i) the moisture-convergence-controlled regime, with convective rainfall exclusively occurring in the rainy season and (ii) the local evaporation-controlled regime with drizzle and (iii) the precipitable-water-controlled regime, with exponential increase of rainfall that both occur during the dry season.

How to cite: Longandjo, G.-N. T., Monyela, B. M., and Rouault, M.: Drivers of the Annual Cycle of Rainfall over Central Africa: The Role of Water Vapor and the Mid-Tropospheric Meridional Circulation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13299, https://doi.org/10.5194/egusphere-egu23-13299, 2023.

In this communication, we will present results from an analysis of the variability of the vertically averaged (i.e., barotropic) atmospheric circulation simulated by climate models, which integrate the current Coupled Model Intercomparison Project (CMIP6). The variabilities in two ensembles of Atmospheric Model Intercomparison Project (AMIP) simulations were compared with the variabilities in two ensembles of fully coupled simulation counterparts of the current CMIP6 (Castanheira and Marques, 2022).

The atmospheric models simulate less variability of the barotropic atmospheric circulation over the Northern Atlantic and more variability over the North Pacific when compared with the corresponding variabilities in the ERA5 reanalysis (“observations”), at intraseasonal and interannual scales. When integrated over the whole globe, the variability in the coupled climate simulations is smaller than the variability in the corresponding AMIP simulations. The smaller global variability of the coupled simulations results in no mean overestimation of the subtropical jet variability in the North Pacific, but further underestimation of the jet stream variability in the Northern Atlantic. The results suggest that the reduction of the biases in the barotropic atmospheric variability over the North Pacific, in the coupled climate simulations, is achieved through compensating biases in the mean Sea Surface Temperatures (SSTs). Moreover, the reduction of the positive biases in the North Pacific seems to be associated with a reduction of the excitation of the most unstable barotropic mode of the atmospheric circulation, which contributes also to a reduction of the barotropic atmospheric variability in the North Atlantic region.

 Acknowledgements: The CESAM is supported by FCT/MCTES through the project UIDP/50017/2020+ UIDB/ 50017/20201+LA/P/0094/2020.

References

Castanheira, J. M., Marques, C. A. F. (2022). Biases of the Barotropic Atmospheric Circulation Variability in CMIP6 Models. Journal of Climate,  Vol. 35, 5071–5085 DOI: 10.1175/JCLI-D-21-0581.1. 

How to cite: Castanheira, J. M. and Marques, C. A. F.: How can the most unstable barotropic mode of atmospheric models contribute for the explanation of atmospheric variability biases of climate models in the North Atlantic?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13716, https://doi.org/10.5194/egusphere-egu23-13716, 2023.

EGU23-13726 | ECS | Posters on site | CL4.6

Causal drivers of central Amazon precipitation variability during austral summer 

Emily Henningsen, Giorgia Di Capua, and Reik V. Donner

The South American monsoon system is one key component of the regional climate of South America, and its interannual as well as intraseasonal variability is of great relevance for water availability over vast parts of the continent. To further develop advanced prediction systems for hydro-meteorological conditions, a better understanding of the underlying atmospheric as well as coupled ocean-atmosphere and land-atmosphere processes governing the intraseasonal variations of rainfall is of paramount importance.

 

In this work, we focus on rainfall variability over the central Amazon basin (CAB) as a particularly vulnerable region during the peak season of the monsoon (December to February). In order to identify causal precursors of CAB rainfall variability and their mutual causal interdependence structure, we employ a causal discovery tool called Peter and Clark Momentary Conditional Information (PCMCI) algorithm to monthly average sea surface temperature (SST), mean sea level pressure (MSLP) and precipitation fields from reanalysis data sets for two different time periods, 1950-2020 and 1979-2020. As a first step, anomaly maps and correlation maps are used to identify potential candidate drivers of the precipitation variability in the CAB at lead times of up to three months. The causal effect networks resulting from the subsequent application of the PCMCI algorithm unveil the causal dependencies of different climate phenomena with CAB rainfall variability during austral summer, confirming previous results based on standard correlation analyses and allowing for a quantitative assessment of the different effects.

 

Among others, we find that SST changes in the tropical Pacific Nino1+2 region close to the South American west coast have a causal effect on CAB precipitation, with lower SSTs promoting more rainfall with a lag of one to two months. Notably, we do not find any similar statistically significant causal impact of SST variations in the Nino3.4 region in the central tropical Pacific, which is commonly most closely associated with the El Niño Southern Oscillation. Additionally, the obtained causal effect networks demonstrate that the Southern Annular Mode (SAM) causally influences the Amundsen Sea Low (ASL), which in turn causally affects the CAB rainfall. Both links are negative, i.e. a positive SAM mode leads to a deeper ASL with a lag of one month, and a deeper ASL supports higher precipitation in central Amazonia with a lag of three months. Finally, SST variability in the tropical North Atlantic as well the Madden-Julian Oscillation do not show a significant causal relationship with CAB rainfall. Our obtained findings are qualitatively consistent among the two different time periods. However, when analyzing data starting only after 1979, some links increase in strength while generally less causal links show up in the networks.

How to cite: Henningsen, E., Di Capua, G., and Donner, R. V.: Causal drivers of central Amazon precipitation variability during austral summer, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13726, https://doi.org/10.5194/egusphere-egu23-13726, 2023.

EGU23-14053 | ECS | Posters virtual | CL4.6

Cyclonic development in the Mediterranean Basin in CMIP6 models using a neural network approach 

George Blougouras, Kostas Philippopoulos, Chris G. Tzanis, and Constantinos Cartalis

The Mediterranean basin is located between the subtropical high-pressure belt and the mid-latitude westerlies and is characterized by complex topography. Its orography, the relatively warm Mediterranean Sea, which is a source of energy and moisture, and the land-sea interactions result in significant cyclonic behavior in the synoptic and sub-synoptic scales. Due to its high sensitivity to climate change forcings, the Mediterranean region is considered a climate change hot spot, with impacts, such as the decline in the projected precipitation, leading to increasing aridification in an already water-stressed area. The above highlight the importance of examining the cyclonic development in the area and assessing the respective changes under different climate change scenarios. In this research, unsupervised machine learning algorithms are used in order to objectively identify cyclonic development in the Mediterranean basin using CMIP6 data for a subset of the different shared socio-economic pathways (SSP) that explore a wide range of possible future outcomes. In more detail, Sea Level Pressure from selected CMIP6 models is used as an input in a Self-Organizing Map (SOM) which is trained to identify the cyclone activity in the Mediterranean Basin for the 1981-2010 reference period. The ability of the network in terms of identifying effectively cyclogenesis regions and the transition probabilities is evaluated. The trained SOM is used to classify CMIP6 mid-century (2031-2060) projections and changes in the frequencies of occurrence of cyclonic development. These are evaluated in terms of physical drivers and regionally specific mechanisms. Examining the responses of cyclonic development to different forcing scenarios will not only shed light on the physical and dynamical processes that govern these circulations but will also allow identifying high–risk regions with potential socio-economic impacts.

How to cite: Blougouras, G., Philippopoulos, K., Tzanis, C. G., and Cartalis, C.: Cyclonic development in the Mediterranean Basin in CMIP6 models using a neural network approach, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14053, https://doi.org/10.5194/egusphere-egu23-14053, 2023.

EGU23-15027 | ECS | Orals | CL4.6

Representation of relationship between PDO and global precipitation in CMIP6 models 

Jivesh Dixit, Vikram M. Mehta, and Krishna M. AchutaRao

Decadal Climate Variability (DCV) modes perturb regional climatic parameters across the globe at multi-year timescales. Precipitation is one such climatic parameter of socio-economic importance.

Our study examines the ability of Coupled Model Intercomparison Project Phase 6 (CMIP6) models in representing the observed teleconnection of DCV modes; Pacific Decadal Oscillation (PDO) and Tropical Atlantic SST Gradient with the global precipitation. We chose the subset of CMIP6 models that participate in both historical and hindcast experiments.

In this study we examine the relationship between the model's ability to simulate the long-term DCV pattern and its ability to simulate the teleconnection between DCV mode and global precipitation.

HadGEM3-GC31-MM and MPI-ESM-1-2-HR, which simulate the observed global SST anomaly pattern in the warm phase of PDO considerably well, also simulate observed global precipitation patterns during the warm phase of PDO quite well in regions like  central India, Europe, North- and South-America, Eastern Africa, Eastern Australia etc. However, BCC-CSM2-MR and NorCPM1 fail to effectively simulate observed precipitation patterns in the warm phase of PDO in regions like, North- and South-America, Africa etc. 

Hence, we found that models that are able to simulate the PDO pattern of SST are also able to represent the teleconnection between PDO modes and precipitation across the globe. We also examined the regression pattern of wind circulation, and the regression pattern of converging and diverging parts of the wind with PDO index. Models that better represent the observed warm phase of PDO pattern, also well represent the observed circulation pattern in respective phases of PDO. Similar analysis is also performed for TAG.


Keywords: Decadal Climate Variability (DCV), CMIP6, historical experiments, teleconnection, precipitation.

How to cite: Dixit, J., Mehta, V. M., and AchutaRao, K. M.: Representation of relationship between PDO and global precipitation in CMIP6 models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15027, https://doi.org/10.5194/egusphere-egu23-15027, 2023.

EGU23-15646 | ECS | Posters on site | CL4.6

Characterization of Mediterranean large-scale atmospheric circulation based on Jenkinson-Collison Weather Type classification. 

Juan Antonio Fernández-Granja, Ana Casanueva, Joaquín Bedia, Swen Brands, and Jesús Fernández

The evaluation of new generations of global climate models (GCMs) with respect to their large-scale circulation features is crucial for model development and has recently been brought into focus by the downscaling community, interested in the suitability of GCMs for downscaling purposes. In such evaluation experiments, additional uncertainties emerge from differences among the reference datasets used for evaluation, typically reanalyses. In this context, weather typing techniques are a useful tool for the classification of the full diversity of data into a few recurrent patterns that can serve as objective characterizations of either global or regional atmospheric circulation. A well-known weather typing classification algorithm is the Jenkinson-Collison Weather Type (JC-WT, Jenkinson and Collison 1977) approach. Although the methodology was originally developed for the British Isles (Lamb, 1972), the JC-WT approach can in principle be applied to any mid-to-high latitude region (Jones et al, 2013). Fernandez-Granja et al (2023) extended the limits of applicability from 23.5º to 80º latitude on both hemispheres, but the suitability of the method is questionable for certain seasons over some areas of the globe, such as the Mediterranean region in summer.

In this study, we first explore the applicability of the JC classification over the Mediterranean by linking the JC-WTs with main northern hemisphere teleconnection indices and blocking conditions. Further, the diversity of JC-WTs and occurrence of the unclassified type are used to examine the suitability of the method. Results show that the application of the JC-WT classification is physically meaningful in large parts of the domain. Secondly, fundamental characteristics of the JC-WTs such as transition probabilities between consecutive types and persistence of the dominant JC-WTs (number of time-steps staying in the same type) obtained for five different reanalyses are compared. Important differences among reanalyses are found, especially in summer, which may bring additional uncertainties when the method is used in model evaluation experiments. 

References:

Fernández-Granja, J. A., Brands, S., Bedia, J., et al (2023) Exploring the limits of the Jenkinson–Collison weather types classification scheme: a global assessment based on various reanalyses. Climate Dynamics. DOI: 10.1007/s00382-022-06658-7

Jenkinson A., Collison F. (1977) An initial climatology of gales over the north sea. synoptic climatology branch memorandum. Meteorological Office, 62

Jones P.D., Harpham C., Briffa K.R. (2013) Lamb weather types derived from reanalysis products. International Journal of Climatology 33(5):1129–1139. DOI: 10.1002/joc.3498

Lamb H. (1972) British isles weather types and a register of daily sequence of circulation patterns 1861-1971. Meteorological Office, Geophysical Memoir 116:1–85

How to cite: Fernández-Granja, J. A., Casanueva, A., Bedia, J., Brands, S., and Fernández, J.: Characterization of Mediterranean large-scale atmospheric circulation based on Jenkinson-Collison Weather Type classification., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15646, https://doi.org/10.5194/egusphere-egu23-15646, 2023.

EGU23-15694 | Posters on site | CL4.6

Heat Waves over Eastern Balkans: A statistical analysis, possible causes and physical drivers. 

Hristo Popov and Oleg Stepanyuk

Heat wave is a period of prolonged abnormally high surface temperatures relative to those normally expected. Heat waves may form when high pressure system strengthens and remains over a region from several days up to several weeks. Severe and exceptional heat waves, such as those that occurred over the Balkans (2007), France (2003), or Russia (2010), are associated with increased mortality, health hazards, reduced personal work productivity and have significant economic impacts by compromising agricultural harvest. Extremely high air temperature values in the Balkan Region are associated with anticyclones formed at the Azores maximum or high-pressure ridges and advections of hot air from the south and southwest.

In our study we perform statistical analysis of the occurrence, durability and intensity of the heat waves over the Balkan Peninsula for the period 1980-2020 based on historical satellite and reanalysis datasets. We analyse correlation between heat waves occurrence and North Atlantic Oscillation Index and certain historical meteorological data for Atlantic and Mediterranean regions aiming to figure out possible causes and physical drivers of this phenomena. One of the mid-term goals of the project is to develop a CNN based predictive system for short and long-time forecasting of extreme weather conditions over the Balkans.

 

How to cite: Popov, H. and Stepanyuk, O.: Heat Waves over Eastern Balkans: A statistical analysis, possible causes and physical drivers., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15694, https://doi.org/10.5194/egusphere-egu23-15694, 2023.

EGU23-16668 | ECS | Orals | CL4.6

Emergence of Low-Frequency Temperature Variability in Instrumental Data and Model Simulations 

Raphael Hébert and Thomas Laepple

The amplitude and spatial distribution of low-frequency natural variability is determinant for regional climate projections, but it is still poorly understood. 

 

In a previous study, pollen-based temperature reconstructions were used to quantify spatial patterns of millennial temperature variability. This showed an inverse relationship across timescales with sub-decadal variability from instrumental data in extra-tropical regions over land (Hébert et al., 2022, under review). We concluded that due to varying marine influence, regions characterized by stable oceanic climate at sub-decadal timescales experience stronger long-term variability while continental regions with higher sub-decadal variability show weaker long-term variability. Indications of this relationship could also be inferred from instrumental data alone as regions of low sub-decadal variability were more likely to exhibit a steeper increase of variability over multi-decadal timescales and vice versa. 

 

In the current work, the relationship found in the instrumental data was further investigated using different instrumental products. In addition, a large multi-model ensemble of CMIP6 models, as well as single-model ensembles, were considered for analysis and it was found that they do not systematically reproduce the relationship found in the instrumental data. This indicates a fundamental deficiency in the model simulations with regard to the mechanism driving the emergence of low-frequency climate variability. This characteristic being related to multi-decadal variability thus has important significance for multi-decadal regional climate projections and might be used as an emergent constraint in model evaluation and inter-comparison.

How to cite: Hébert, R. and Laepple, T.: Emergence of Low-Frequency Temperature Variability in Instrumental Data and Model Simulations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16668, https://doi.org/10.5194/egusphere-egu23-16668, 2023.

EGU23-16755 | Orals | CL4.6

An Atlantic interhemispheric teleconnection established by South American summer monsoon 

Wan-Ling Tseng, Yi-Chi Wang, Yu-chi Lee, Huang-Hsiung Hsu, and Noel Keenlyside

This paper reports the structure of an interhemispheric atmosphere–ocean coupling pattern, which occurs over the Atlantic Ocean from January to February, and refers to it as the Atlantic symmetric pattern (ASP). The ASP occurs in the middle–upper troposphere, with two trains of cyclonic–anticyclonic–cyclonic anomalous circulations aligned meridionally over the Atlantic Ocean. The sea surface temperature (SST) signature of the ASP, which is composed of a distinct SST dipole, is the leading mode of the interannual SST of the Southwest Atlantic Ocean. Experiments with the linear baroclinic model shows that the interhemispheric wave trains of the ASP can be excited as a Gill-type response to convection in the South American monsoon system and the South Atlantic convergence zone. Further studies are warranted to elucidate other aspects of the ASP, including teleconnection in the Northern Hemisphere and interactions with other climatic modes.

How to cite: Tseng, W.-L., Wang, Y.-C., Lee, Y., Hsu, H.-H., and Keenlyside, N.: An Atlantic interhemispheric teleconnection established by South American summer monsoon, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16755, https://doi.org/10.5194/egusphere-egu23-16755, 2023.

EGU23-16955 | Posters on site | CL4.6

Tropical to extratropical interactions in the Southern Hemisphere 

Julie Arblaster

Tropical variability has long been identified as having an important influence on climate variability and change in the Southern Hemisphere (SH). In all three ocean basins, heating from tropical convection can generate stationary Rossby waves that propagate polewards and eastwards towards Antarctica, influencing temperature and rainfall patterns along the way. Recent studies have also highlighted the reverse – an influence of the polar regions on changes further north, for example, the stratospheric weakening of the SH polar vortex that contributed to the prolonged drought and extreme fire weather in Australia in the spring and summer of 2019. On longer timescales, the climate of the Southern Hemisphere has undergone significant changes over the past 30-50 years. The extratropical atmosphere has seen a shift to a more positive phase of the Southern Annular Mode and a stronger and more poleward eddy-driven jet, particularly in austral summer. While the influence of anthropogenic forcing such as ozone depletion and increasing greenhouse gases on these changes is well-established, the importance of tropical to extratropical interactions in shaping some recent events is becoming more evident. Examples include the deepening of the Amundsen Sea Low which has been associated with tropical Pacific decadal variability and the rapid decline in Antarctic sea ice in 2016 which was linked to a record positive Indian Ocean Dipole event. Recent insights into tropical to extratropical interactions, including the mechanisms through which they operate and links to observed changes on interannual to interdecadal timescales will be discussed.

How to cite: Arblaster, J.: Tropical to extratropical interactions in the Southern Hemisphere, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16955, https://doi.org/10.5194/egusphere-egu23-16955, 2023.

EGU23-408 | ECS | Posters on site | CL2.2

El Niño Southern Oscillation influence over the Orinoco low-level jet variability 

Alejandro Builes, Johanna Yepes, and Hernán D. Salas

We studied the most active season of the Orinoco Low-Level jet (OLLJ), December-January-February (DJF), during the El Niño-Southern Oscillation canonical phases, El Niño and La Niña. In particular, we studied the occurrence days of the jet in each month, wind speed, moisture transport and precipitation over northern south America. In terms of the occurrence of the OLLJ, during El Niño in January, the jet exhibits its highest reduction with changes up to 24% in the eastern Colombian plains. On the contrary, during La Niña, the jet exhibits an increase between 6–16% in the frequency of occurrence mainly located in the eastern Colombian plains and the border between Colombia, Ecuador and Peru. Although the diurnal cycle of the OLLJ windspeed remains unaltered during the ENSO phases the maximum decrease (increase) up to -2m/s (up to 1 m/s) during El Niño (La Niña). Regarding moisture transport there is a gradual reduction during the season in both ENSO phases reaching up to 18 gm-1 kgm-1 during El Niño, and the precipitation also shows a reduction of around 5 mm/day. In conclusion, during DJF at the ENSO canonical phases the OLLJ shows changes in its occurrence along the jet corridor, and the region experiences changes in both moisture transport and precipitation.

How to cite: Builes, A., Yepes, J., and Salas, H. D.: El Niño Southern Oscillation influence over the Orinoco low-level jet variability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-408, https://doi.org/10.5194/egusphere-egu23-408, 2023.

EGU23-410 | Orals | CL2.2

Phase-Locking between precipitation and El Niño-Southern Oscillation over northern South America 

Hernán D. Salas, Germán Poveda, Óscar J. Mesa, Alejandro Builes-Jaramillo, Niklas Boers, and Jürgen Kurths

We study phase-locking between the El Niño - Southern Oscillation (ENSO) and precipitation at inter-annual time scales over northern South America. To this end, we characterize the seasonality of the regional patterns of sea surface temperature, surface pressure levels, and precipitation anomalies associated with the states of the canonical ENSO. We find that the positive (negative) precipitation anomalies experienced in northern South America differ from those previously reported in the literature in some continental regions. In particular, the Orinoco Low-level Jet corridor separates two regions with negative (positive) rainfall anomalies during El Niño (La Niña), which are located in the Guianas (northeastern Amazon) and the Caribbean. Moreover, we show that the ENSO signal is phase-locked with the inter-annual rainfall variability in most of the study regions although some areas exhibit phase-locking without a significant change in the anomalies of precipitation. This suggests that ENSO could induce changes only in terms of phases and not so in terms of magnitude. This work provides new insights into the non-linear interactions between ENSO and hydro-climatic processes over the tropical Americas.

How to cite: Salas, H. D., Poveda, G., Mesa, Ó. J., Builes-Jaramillo, A., Boers, N., and Kurths, J.: Phase-Locking between precipitation and El Niño-Southern Oscillation over northern South America, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-410, https://doi.org/10.5194/egusphere-egu23-410, 2023.

EGU23-1522 | ECS | Orals | CL2.2

Future climate response to observed extreme El Niño analogues 

Paloma Trascasa-Castro, Yohan Ruprich-Robert, and Amanda Maycock

Model simulations show a robust increase in ENSO-related precipitation variability in a warmer climate, but there remains uncertainty in whether the characteristics of ENSO events themselves may change in the future. Our study aims to disentangle these effects by addressing how the global impacts of observed large El Niño events would change in different background climate states covering the preindustrial, present and future periods.

Pacemaker simulations with the EC-Earth3-CC model were performed replaying the 3 strongest observed El Niño events from the historical record (1982/83, 1997/98, 2015/16). Model tropical Pacific sea surface temperature (SST) anomalies were restored towards observations, while imposing different background states, mimicking past, present and future climate conditions (following the SSP2-4.5). All variables outside the restoring region evolve freely in a coupled-atmosphere ocean transient simulation. For each start date, 30 ensemble members with different initial conditions were run for 2 years. Using this approach we ask ‘what impacts would arise if the observed El Niño occurred in the past or future’?

In response to the same imposed El Niño SST anomalies, precipitation anomalies are shifted towards the Eastern equatorial Pacific in the future compared to the present day, leading to changes in the extratropical response to El Niño. Some examples are an amplification of the surface temperature response over north-eastern North America, northern South America and Australia in boreal winter. We link the changes of El Niño related tropical Pacific precipitation to a decrease in the climatological zonal SST gradient in the equatorial Pacific, as we move from past to future climatologies, which potentially leads to a higher convection sensitivity to SST anomalies over the Central and Eastern equatorial Pacific in the future. Interestingly, the simulations indicate there has already been an intensification of El Niño impacts between present day and preindustrial, which is comparable to the differences found between future and present. This nonlinear behaviour highlights the need to understand potential changes to convection thresholds in the tropical Pacific to be able to explain El Niño teleconnections under climate change scenarios. Ongoing work is exploring the changes in atmospheric circulation that lead to the overall intensification of El Niño impacts that we show in our study.

How to cite: Trascasa-Castro, P., Ruprich-Robert, Y., and Maycock, A.: Future climate response to observed extreme El Niño analogues, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1522, https://doi.org/10.5194/egusphere-egu23-1522, 2023.

EGU23-1960 | Posters on site | CL2.2

Two regimes of inter-basin interactions between the Atlantic and Pacific Oceans on interannual timescales 

Jae-Heung Park, Sang-Wook Yeh, Jong-Seong Kug, Young-Mean Yang, Hyun-Su Jo, Hyo-Jeong Kim, and Soon-Il An

Understanding the inter-basin interactions between the Atlantic and Pacific Oceans is of great concern due to their substantial global climatic implications. By analyzing observational reanalysis datasets (1948-2020), we found that there are two regimes in Atlantic–Pacific inter-basin interactions: (i) the Pacific-driven regime, and (ii) the Atlantic-driven regime. In the Pacific-driven regime before the mid-1980s, the El Niño-Southern Oscillation (ENSO) in winter effectively drives the primary mode of sea surface temperature anomaly (SSTA) in the tropical Atlantic (i.e., NTA mode) in boreal spring. The NTA mode has a meridional contrast of SSTA along the Atlantic Intertropical convergence zone due to the ENSO effect, similar to the Atlantic Meridional Mode. Whereas, in the Atlantic-driven regime after the mid-1980s, the ENSO effect on the NTA becomes remarkably weaker, so that the NTA mode is featured with a SSTA monopole. Notably, the NTA mode without the meridional contrast of SSTA is capable of modulating an ENSO event. Our analyses of the latest climate models participating in the Coupled Model Intercomparison Project (CMIP) phases 6 support the hypothesis that the two regimes engendered by the Atlantic-Pacific inter-basin interactions are likely due to natural variability.

How to cite: Park, J.-H., Yeh, S.-W., Kug, J.-S., Yang, Y.-M., Jo, H.-S., Kim, H.-J., and An, S.-I.: Two regimes of inter-basin interactions between the Atlantic and Pacific Oceans on interannual timescales, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1960, https://doi.org/10.5194/egusphere-egu23-1960, 2023.

EGU23-2136 | ECS | Posters on site | CL2.2

A multi-modal representation of El-Niño Southern Oscillation Diversity 

Jakob Schlör, Antonietta Capotondi, and Bedartha Goswami

Sea surface temperature anomalies (SSTA) associated with the El-Niño Southern Oscillation (ENSO) show strong event-to-event variability, known as ENSO diversity. El Niño and La Niña events are typically divided into Eastern Pacific (EP) and Central Pacific (CP) types based on the zonal location of peak SSTA. The separation of these types is usually based on temperature differences between pairs of predefined indices, such as averages over boxes in the Eastern and Central Pacific or the two leading Principal Components of tropical SSTA. 
Using results from unsupervised learning of SSTA data, we argue that ENSO diversity is not well described by distinctly separate classes but rather forms a continuum with events grouping into "soft'' clusters. We apply a Gaussian mixture model (GMM) to a low-dimensional projection of tropical SSTA to describe the multi-modal distribution of ENSO events. We find that El-Niño events are best described by three overlapping clusters while La-Niña events only show two "soft'' clusters. The three El-Niño clusters are described by i) maximum SSTA in the CP, ii) maximum SSTA in the EP, and iii) strong basin-wide warming of SSTA which we refer to as the "super El-Niño'' cluster. The "soft'' clusters of La-Niña correspond to i) anomalous cool SST in the CP and ii) anomalously cool SST in the EP. We estimate the probability that a given ENSO event belongs to a chosen cluster and use these probabilities as weights for estimating averages of atmospheric variables corresponding to each cluster. These weighted composites show qualitatively similar patterns to the typically used averages over EP and CP events. However, the weighted composites show a higher signal-to-noise ratio in the mid-latitudes for the "super El-Niño'' events. 
We further apply our approach to CESM2 model data and discuss the potential of GMM clustering for evaluating how well ENSO diversity is captured in Global Circulation models.

How to cite: Schlör, J., Capotondi, A., and Goswami, B.: A multi-modal representation of El-Niño Southern Oscillation Diversity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2136, https://doi.org/10.5194/egusphere-egu23-2136, 2023.

An information theory based framework is developed to assess the predictability of the ENSO complexity, which includes different types of the ENSO events with diverse characteristics in spatial patterns, peak intensities and temporal evolutions. The information theory advances a unique way to quantify the forecast uncertainty and allows to distinguish the predictability limit of each type of event. With the assistance of a recently developed multiscale stochastic conceptual model that succeeds in capturing both the large-scale dynamics and many crucial statistical properties of the observed ENSO complexity, it is shown that different ENSO events possess very distinct predictability limits. Beyond the ensemble mean value, the spread of the ensemble members also has remarkable contributions to the predictability. Specifically, while the result indicates that predicting the onset of the eastern Pacific (EP) El Ninos is challenging, it reveals a universal tendency to convert strong predictability to skillful forecast for predicting many central Pacific (CP) El Ninos about two years in advance. In addition, strong predictability is found for the La Nina events, corresponding to the effectiveness of the El Nino to La Nina transitions. In the climate change scenario with the strengthening of the background Walker circulation, the predictability of sea surface temperature in the CP region has a significant response with a notable increase in summer and fall. Finally, the Gaussian approximation exhibits to be accurate in computing the information gain, which facilitates the use of more sophisticated models to study the ENSO predictability.

How to cite: Fang, X. and Chen, N.: Quantifying the Predictability of ENSO Complexity Using a Statistically Accurate Multiscale Stochastic Model and Information Theory, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2209, https://doi.org/10.5194/egusphere-egu23-2209, 2023.

EGU23-2470 | ECS | Orals | CL2.2

The Dynamics of the El-Niño Southern Oscillation (ENSO) Diversity 

Priyamvada Priya, Dietmar Dommenget, and Shayne McGregor

This study investigates the observed El-Niño Southern Oscillation (ENSO) dynamics for the eastern Pacific (EP) and central Pacific (CP) events. Here we use the recharge oscillator (ReOsc) model concept to describe the ENSO phase space, based on the interaction of sea surface temperatures in the eastern equatorial Pacific (T) and thermocline depth (h), for the different types of ENSO events. We further look at some important statistical characteristics, such as power spectrum and cross-correlation, as essential parameters for understanding the dynamics of ENSO. The results show that the CP and EP events are very different in the ENSO phase space and less well described by the ReOsc model than a T index-based model. The EP events are closer to the idealised ReOsc model, with clear propagation through all phases of the ENSO cycle and strongly skewed towards the El-Niño and subsurface ocean heat discharge states. The CP events, in turn, do not have a clear propagation through all phases and are strongly skewed towards the La-Nina state. Also, the CP events have a slower cycle (67 months) than the EP events (50 months). Further, the CP events collapse after the La-Nina phase, whereas the EP events appear to collapse after the discharging phase. The characteristics out-of-phase cross-correlation between T and h is nearly absent for the CP events, suggesting that the interaction between T and h is not as important as for the EP or the canonical ENSO events. Furthermore, the coupling factor of T and h is smaller for the CP events than the EP events, implying that the CP events are not influenced much by T and h interactions. This study will provide new insight to understand these events by developing a dynamical approach to explain the observed ENSO dynamics for the EP and CP events in the ReOsc model framework.

How to cite: Priya, P., Dommenget, D., and McGregor, S.: The Dynamics of the El-Niño Southern Oscillation (ENSO) Diversity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2470, https://doi.org/10.5194/egusphere-egu23-2470, 2023.

EGU23-2477 | Posters on site | CL2.2

ENSO phase space dynamics with an improved estimate of the thermocline depth 

Dietmar Dommenget and Priyamvada Priya

The recharge oscillator model of the El Niño Southern Oscillation (ENSO) describes the ENSO dynamics as an interaction and oscillation between the eastern tropical Pacific sea surface temperatures (T) and subsurface heat content (thermocline depth; h), describing a cycle of ENSO phases. h is often approximated on the basis of the depth of the 20oC isotherm (Z20). In this study we will address how the estimation of h affects the representation of ENSO dynamics. We will compare the ENSO phase space with h estimated based on Z20 and based on the maximum gradient in the temperature profile (Zmxg). The results illustrate that the ENSO phase space is much closer to the idealised recharge oscillator model if based on Zmxg than if based on Z20. Using linear and non-linear recharge oscillator models fitted to the observed data illustrates that the Z20 estimate leads to artificial phase dependent structures in the ENSO phase space, which result from an in-phase correlation between h and T. Based on the Zmxg estimate the ENSO phase space diagram show very clear non-linear aspects in growth rates and phase speeds. Based on this estimate we can describe the ENSO cycle dynamics as a non-linear cycle that grows during the recharge and El Nino state, and decays during the La Nina states. The most extreme ENSO states are during the El Nino and discharge states, while the La Nina and recharge states do not have extreme states. It further has faster phase speeds after the El Nino state and slower phase speeds during and after the La Nina states. The analysis suggests that the ENSO phase speed is significantly positive in all phases, suggesting that ENSO is indeed a cycle. However, the phase speeds are closest to zero during and after the La Nina state, indicating that the ENSO cycle is most likely to stall in these states.

How to cite: Dommenget, D. and Priya, P.: ENSO phase space dynamics with an improved estimate of the thermocline depth, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2477, https://doi.org/10.5194/egusphere-egu23-2477, 2023.

EGU23-3263 | ECS | Orals | CL2.2

Model Resolution Effects on ENSO and its Teleconnections 

Ned Williams, Adam Scaife, and James Screen

The El Niño-Southern Oscillation (ENSO) influences climate on a global scale and is a source of long-range predictability. Accurate modelling of the impact of ENSO requires accurate representation of teleconnections as well as of ENSO itself. We consider a set of CMIP6 models and assess the effect of increasing model resolution on ENSO and its boreal winter teleconnections. The spatial structure, strength and asymmetry of both ENSO and its teleconnection to the extratropical North Pacific are considered. We find evidence of an improved El Niño teleconnection in high resolution models, but this effect is weaker for La Niña. We aim to establish whether ocean or atmospheric resolution is the primary driver of resolution-based trends, and we evaluate the relevance of mean state biases on these trends. 

How to cite: Williams, N., Scaife, A., and Screen, J.: Model Resolution Effects on ENSO and its Teleconnections, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3263, https://doi.org/10.5194/egusphere-egu23-3263, 2023.

EGU23-3278 | ECS | Posters on site | CL2.2

Oceanic and Atmospheric Feedbacks Associated with the Spreading of Pacific Coastal Niño Events 

Daniel Rudloff and Joke Lübbecke

In early 2017 a very strong coastal warming occurred off the coast of Peru. This event, which caused heavy rainfalls and flooding over land, marked the strongest so called ‘Pacific Coastal Niño Event’ observed. Most intriguing about this event was the fact that the central Pacific was not showing any significant anomalies during that time. Since then several studies have investigated Pacific Coastal Niños but the exact mechanisms of how such events behave are still not clear. While most studies focus on their onset mechanisms, we here analyze their evolution and decay and in particular their connection to the central equatorial Pacific.

To address those questions, we are using the coupled climate model FOCI (Flexible Ocean Climate Infrastructure). Starting from a long control simulation with pre-industrial conditions we perform sets of 2-year long sensitivity experiments in which a coastal warming is generated by a local wind stress anomaly utilizing a partial coupling approach. Once the warming is initiated by reduced upwelling the wind forcing is switched off and the model can evolve freely, which enables us to investigate the evolution and decay of the warming. The approach allows to vary the forcing in strength, location and timing. By starting from different conditions in terms of equatorial heat content and applying the forcing during different months, the influences of both the background state of the equatorial Pacific during the Coastal Niño and the seasonality of the coastal warming are investigated. To understand which factors influence the spreading of the warm anomaly we analyze both local coastal feedbacks, which lead to an alongshore extension of the anomaly, and equatorial feedbacks that are crucial for a spreading along the equator.

How to cite: Rudloff, D. and Lübbecke, J.: Oceanic and Atmospheric Feedbacks Associated with the Spreading of Pacific Coastal Niño Events, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3278, https://doi.org/10.5194/egusphere-egu23-3278, 2023.

EGU23-3440 | ECS | Posters on site | CL2.2

New insight into multi-year La Niña dynamics from the perspective of a near-annual ocean process 

Fangyu Liu, Wenjun Zhang, Fei-Fei Jin, Feng Jiang, Julien Boucharel, and Suqiong Hu

The El Niño-Southern Oscillation (ENSO) exhibits highly asymmetric temporal evolutions between its warm and cold phases. While El Niño events usually terminate rapidly after their mature phase and show an already established transition into the cold phase by the following summer, many La Niña events tend to persist throughout the second year and even re-intensify in the ensuing winter. While many mechanisms were proposed, no consensus has been reached yet and the essential physical processes responsible for the multi-year behavior of La Niña remain to be illustrated. Observations show that a unique ocean physical process operates during multi-year La Niña events. It is characterized by rapid double reversals of zonal ocean current anomalies in the equatorial Pacific which exhibits a fairly regular near-annual periodicity. Analyses of mixed-layer heat budget reveal comparable contributions of the thermocline and zonal advective feedbacks to the SST anomaly growth for the first year of multi-year La Niña events; however, the zonal advective feedback plays a dominant role in the re-intensification of La Niña events. Furthermore, the unique ocean process is identified to be closely associated with the preconditioning heat content state in the central to eastern equatorial Pacific before the first year of La Niña, which sets the stage for the future re-intensification of La Niña. The above-mentioned oceanic process can be largely reproduced by state-of-the-art climate models despite systematic underestimation, providing a potential predictability source for the multi-year La Niña events.

How to cite: Liu, F., Zhang, W., Jin, F.-F., Jiang, F., Boucharel, J., and Hu, S.: New insight into multi-year La Niña dynamics from the perspective of a near-annual ocean process, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3440, https://doi.org/10.5194/egusphere-egu23-3440, 2023.

EGU23-3598 | Orals | CL2.2 | Highlight

Prediction Challenges from Errors in Tropical Pacific Sea Surface Temperature Trends 

Michelle L'Heureux, Michael Tippett, and Wanqiu Wang

Initialized, monthly mean predictions of the tropical Pacific Ocean are made against the backdrop of a warming climate, and it is unclear to what extent these predictions are impacted by trends.  Here, we analyze the forecast models that comprise the North American Multi-Model Ensemble (NMME) and uncover significant linear trend errors that have consequences for the tropical Pacific basin and ENSO variability.  All models show positive trend errors over the eastern equatorial Pacific over the 1982-2020 hindcast and real-time period.  These positive trend errors interact with the mean bias of each respective model, reducing, over time, the bias of models that are too cold and increasing the bias of models that are too warm.  These trend errors lead to a tropical Pacific that is too warm and too wet over the basin, and is significantly correlated with an increase in El Niño false alarms.  Finally, we explore the consequences of these tropical Pacific Ocean trend errors on predictions of global precipitation anomalies. 

How to cite: L'Heureux, M., Tippett, M., and Wang, W.: Prediction Challenges from Errors in Tropical Pacific Sea Surface Temperature Trends, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3598, https://doi.org/10.5194/egusphere-egu23-3598, 2023.

EGU23-3631 | Posters on site | CL2.2

Multiyear ENSO dynamics as revealed in observations, CMIP6 models, and linear theory 

Tomoki Iwakiri and Masahiro Watanabe

El Niño–Southern Oscillation (ENSO) events occasionally recur one after the other in the same polarity, called multiyear ENSO. However, the dynamical processes are not well understood. This study aims to elucidate the unified mechanisms of multiyear ENSO using observations, CMIP6 models, and the theoretical linear recharge oscillator (RO) model. We found that multiyear El Niño and La Niña events are roughly symmetric except in some cases. The composite multiyear ENSO reveals that anomalous ocean heat content (OHC) in the equatorial Pacific persists beyond the first peak, stimulating another event. This prolonged OHC anomaly is caused by meridional Ekman heat transport counteracting geostrophic transport induced recharge–discharge process that otherwise acts to change the OHC anomaly. A meridionally wide pattern of sea surface temperature observed during multiyear event is responsible for the Ekman heat transport. CMIP6 multi-model ensemble shows a significant correlation between the ENSO meridional width and the occurrence ratio of multiyear ENSO. A multiyear ENSO-like oscillation was simulated using the linear RO model that incorporates a seasonally varying Bjerknes growth rate and a weak recharge efficiency representing the effect of Ekman transport. When the recharge efficiency parameter was estimated using reanalysis data based on geostrophic transport alone, a multiyear ENSO rarely occurred, confirming the importance of Ekman transport in retarding the recharge–discharge process.

How to cite: Iwakiri, T. and Watanabe, M.: Multiyear ENSO dynamics as revealed in observations, CMIP6 models, and linear theory, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3631, https://doi.org/10.5194/egusphere-egu23-3631, 2023.

EGU23-3637 | ECS | Posters on site | CL2.2

Is a Preceding Strong El Niño Required to Generate Multi-year La Niña? 

Ji-Won Kim, Jin-Yi Yu, and Baijun Tian

By analyzing observational data covering the period from 1900 to 2021, we show that the known mechanism linking multi-year La Niña with a preceding strong El Niño has been overemphasized. A majority of multi-year La Niña (64%; 7 out of 11 events) do not require a preceding strong El Niño to generate their 2nd-year La Niña. We find that the negative phase of the Pacific Meridional Mode (PMM) during 1st-year La Niña’s decaying spring, rather than the preceding strong El Niño, offers the key mechanism to produce 2nd-year La Niña, resulting in a multi-year La Niña. It is further found that the westward extension of the 1st-year La Niña cold sea surface temperature anomalies, which interacts with the eastern edge of the western Pacific warm pool, is a key factor inducing the negative PMM. The negative PMM mechanism to generate multi-year La Niña is also applied to the 3rd-year La Niña of multi-year La Niña, giving rise to a triple-dip event. The possible reason(s) how and why a multi-year La Niña can become either a double-dip or a triple-dip event will be discussed.

How to cite: Kim, J.-W., Yu, J.-Y., and Tian, B.: Is a Preceding Strong El Niño Required to Generate Multi-year La Niña?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3637, https://doi.org/10.5194/egusphere-egu23-3637, 2023.

EGU23-4180 | Orals | CL2.2

Why is El Nino warm? 

Stephan Fueglistaler, Laure Resplandy, and Allison Hogikyan

El Nino years stand out in the global average temperature time series as record-warm years. The coupled atmosphere-ocean dynamics leading to warming in the climatologically cold equatorial Eastern Pacific are well understood, but cannot be the cause for the very strong signal in global average temperarture. The latter must be caused by an increase in subcloud Moist Static Energy (MSE) in the domain of highest subcloud MSE where atmospheric deep convection couples the surface, boundary layer and free troposphere. Transformation of the data from geographical space to sea-surface temperature (SST) percentiles eliminates the large spatial see-saws in all variables arising from the geographic reorganization of the general circulation, and brings to light the mechanism: While in the Eastern Pacific region oceanic heat uptake is reduced (corresponding to a heat flux out of the ocean), the deep convective domain sees a heat flux from the atmosphere into the ocean. We show that this heat flux into the ocean at the high end of SSTs - the opposite of the canonical perspective of a warming due to a heat flux from the ocean to the atmosphere - is mechanically forced: surface wind speeds are lower in regions of active deep convection than in ENSO neutral (and La Nina) years. The resulting reduced evaporation leads to the increase in subcloud MSE that causes the global temperature signal.

How to cite: Fueglistaler, S., Resplandy, L., and Hogikyan, A.: Why is El Nino warm?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4180, https://doi.org/10.5194/egusphere-egu23-4180, 2023.

The equatorial Atlantic zonal sea surface temperature (SST) gradient, which has significant climatic and biogeochemical effects, is closely associated with the equatorial Pacific zonal SST gradient through Walker circulation on seasonal and interannual time scales. However, discrepancies in current SST datasets mean that its long-term trend is not well understood. Here, using multiple datasets, we find a robust weakening long-term trend (i.e., greater warming in the east than west) in the equatorial Atlantic zonal SST gradient over the period 1900–2010 in all datasets. We also find that this weakening trend is closely linked to the tropical Pacific cold tongue mode (CTM), which corresponds to a strong increasing long-term trend of zonal SST gradient along the equatorial Pacific (i.e., warming in the west and cooling in the east). Specifically, the long-term cooling SST anomalies associated with the CTM modify the Walker circulation, and leads to weaker trade winds over the western equatorial Atlantic. These in turn deepen the thermocline in the eastern equatorial Atlantic, and cause the weakening long-term trend of SST gradient along the equatorial Atlantic. The long-term trend of the CTM is induced by ocean dynamical feedback in response to global warming, suggesting that global warming could affect the equatorial Atlantic zonal SST gradient via the CTM. Our results provide a novel explanation of the linkages between the long-term trend of equatorial Atlantic zonal SST gradient and the CTM under global warming, which carries important implications for the relationship between global warming and the equatorial Atlantic zonal SST gradient.

How to cite: Li, Y.: Long-term trend of equatorial Atlantic zonal SST gradient linked to the tropical Pacific cold tongue mode under global warming, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4360, https://doi.org/10.5194/egusphere-egu23-4360, 2023.

EGU23-4971 | ECS | Orals | CL2.2

Indo-Pacific teleconnection changes during the Holocene: model-proxy comparison 

Isma Abdelkader Di Carlo, Pascale Braconnot, Mary Elliot, and Olivier Marti

The teleconnections between the Indian and Pacific Oceans are very complex, involving multiple modes of variability and phenomena such as the El Niño-Southern Oscillation, Indian Ocean Dipole, Indian Ocean Basin mode, and the Asian monsoon. Their interactions are complex because changes in one of these phenomena affect the others. Insufficient agreement exists on the predicted evolution of mean states of both basins and the impacts of climate variability in this region in response to increasing CO2 emissions. To better constrain Indo-Pacific interactions, we have studied the Holocene period. We consider four transient simulations from three General Circulation Models (GCM) and a collection of paleo-archives from the Holocene in the Indo-Pacific region. Our study allows us to put into perspective the links between long-term changes in variability and in the mean state. The main driver is insolation and trace gases (CO2) that have increased the mean sea surface temperature of the tropical ocean over the last 6,000 years. Our first results show that modeled trends in the regional long-term variability are in agreement, but differences are observed when we analyze the data at shorter interannual timescales. We also explain why the simulations differ or agree with the paleoclimate reconstructions. One way is to look at the relative role of temperature and salinity in determining the changes in δ18O recorded by the various climate archives. 

How to cite: Abdelkader Di Carlo, I., Braconnot, P., Elliot, M., and Marti, O.: Indo-Pacific teleconnection changes during the Holocene: model-proxy comparison, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4971, https://doi.org/10.5194/egusphere-egu23-4971, 2023.

EGU23-5205 | ECS | Posters on site | CL2.2

Impact of tropical SSTs on the monthly signal over the North Atlantic-European region 

Sara Ivasić, Ivana Herceg Bulić, and Margareta Popović

Targeted numerical simulations were designed to test the potential impact of tropical sea surface temperatures (SSTs) on the geopotential heights at 200 hPa (GH200) signal over the North Atlantic-European region. Five experiments with SST anomalies prescribed in different areas, acting as lower boundary forcing, were created with an intermediately complex atmospheric general circulation model (ICTP AGCM). In the AGCM experiments, the SST forcing was prescribed globally, in the tropical zone of all oceans, only in the tropical Atlantic, tropical Indian Ocean and limited to the tropical Pacific. All of the simulations covered a 156-year-long period.

The monthly GH200 signal was calculated based on the difference between the ensemble mean of each experiment and the climatological mean for the considered period. In addition, to inspect the impact of the El Niño-Southern Oscillation (ENSO), the signal was calculated for ENSO and non-ENSO years, respectively. Here, the ENSO years were classified according to the value of the late-winter Niño3.4 index.

Additionally, each experiment’s monthly signal was averaged over the signal maximum over the North Atlantic-European region. The characteristics of the spatially averaged signal were compared to the signal averaged over a similar signal maximum observed over the Pacific North American region.

Results have shown that the GH200 signal is the strongest in the late-winter months in all experiments. The AGCM experiment with SST boundary forcing prescribed only in the tropical Atlantic consistently had the smallest signal amplitude. The strongest signal linked to ENSO events was found in the experiment with the SST forcing prescribed only in the tropical Pacific. The signal averaged over the NAE maximum generally yields smaller values than the PNA maximum average. Also, the differences between the (non) ENSO signal and the signal for all years are less pronounced in the case of the NAE maximum average.

How to cite: Ivasić, S., Herceg Bulić, I., and Popović, M.: Impact of tropical SSTs on the monthly signal over the North Atlantic-European region, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5205, https://doi.org/10.5194/egusphere-egu23-5205, 2023.

EGU23-5310 | ECS | Orals | CL2.2

Distinct and reproductible northem hemisphere winter teleconnection pattern during strong El Niño events : relative roles of Sea Surface Temperature forcing and atmospheric nonlinearities 

Margot Beniche, Jérôme Vialard, Matthieu Lengaigne, Aurore Voldoire, Srinivas Gangiredla, and Nicholas Hall

The strengthening and north-eastward shift of El Niño Northern hemisphere winter teleconnections relative to those of La Niña is a well-known asymmetry of ENSO (El Niño Southern Oscillation). It is generally attributed to atmospheric nonlinearities associated with the Sea Surface Temperature (SST) threshold for tropical deep convection. Here, we re-examine these teleconnection asymmetries in the context of ENSO SST pattern diversity. We find that the asymmetries are mainly attributable to strong El Niño events (eg. 1982-83, 1997-98, 2015-16), both in observations and in ensemble simulations with the atmospheric component of the CNRM-CM6 model. This strong El Niño teleconnection pattern also results in specific impacts, characterized by enhanced rainfall along the United States (US) west coast and warm anomalies over Canada and the Northern US. Our ensemble simulations further indicate that moderate “Eastern Pacific” El Niño events exhibit teleconnection patterns that are similar to those of “Central Pacific” El Niño, or to the opposite of La Niña events. We also find that the teleconnection spread between ensemble members or events is reduced for strong El Niño relative to moderate El Niño or La Niña events, with important implications for predictability. Sensitivity experiments in which the atmospheric model is forced by the opposite of observed SST anomalies are used to assess the mechanisms inducing the strong El Niño teleconnection pattern. In addition to the well-known influence of atmospheric nonlinearities, these experiments reveal an important contribution from the Eastward-shifted SST pattern during strong El Niño events.

 

How to cite: Beniche, M., Vialard, J., Lengaigne, M., Voldoire, A., Gangiredla, S., and Hall, N.: Distinct and reproductible northem hemisphere winter teleconnection pattern during strong El Niño events : relative roles of Sea Surface Temperature forcing and atmospheric nonlinearities, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5310, https://doi.org/10.5194/egusphere-egu23-5310, 2023.

The amplitude of El Niño/Southern Oscillation (ENSO) varied considerably over the last 140 years, for which we have relatively reliable Sea Surface Temperature (SST) observations over the tropical Pacific. The difference between periods of high and low ENSO amplitude results mainly from the number of strong Eastern Pacific (EP) El Niños, while the amplitude of Central Pacific (CP) El Niños is comparable in both periods. Further, the asymmetry of ENSO, i.e. that the SST anomalies during El Niño are on average stronger and located further to the east than during La Niña, covaries with ENSO amplitude in observations, indicating that the number of strong EP El Niño events dominates both ENSO amplitude and asymmetry variations.

We find similar relations in the 40 historical runs of the Large Ensemble with the CESM1-CAM5-BGC model that can simulate the ENSO asymmetry quite realistically.  Further, there is a strong relation between the ENSO amplitude and the tropical Pacific mean state, indicating that a warmer eastern equatorial Pacific favors more EP El Niños due to a lower convective threshold in that area. We also analyze the spatial asymmetry and amplitude asymmetry of the atmospheric and oceanic feedbacks and show that the spatial asymmetry is more pronounced in the atmospheric feedbacks, while the amplitude asymmetry is more pronounced in the oceanic feedbacks, and that both together form the observed asymmetry of ENSO.  A comparison with 360 years-long CESM1 experiments with a -4.0 K colder and +3.7 K warmer mean state indicates that the present-day ENSO may be in a transition zone between a CP El Niño dominated ENSO state and an EP El Niño dominated ENSO state and that ENSO may lock-in into the EP El Niño dominated state under global warming.

Finally, our analysis of ENSO-amplitude variability in preindustrial control simulations of the CMIP6 database supports a strong relation of ENSO amplitude and asymmetry with the number of strong EP El Niño events.

How to cite: Bayr, T., Lübbecke, J. F., and Latif, M.: The role of strong Eastern Pacific El Nino events in ENSO-amplitude variability in Observations and Climate Models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6109, https://doi.org/10.5194/egusphere-egu23-6109, 2023.

Using observational analysis and numerical experiments, we identify that the dipole mode of 
spring surface wind speed (SWS) over the Tibetan Plateau (TP) could act as a trigger for subsequent winter El 
Niño–Southern Oscillation events. During the positive phase of spring SWS dipole mode (south-positive and 
north-negative), a self-sustaining “negative sensible heating–baroclinic structure” prevails over the western TP, 
which is characterized by negative surface sensible heating anomalies, anomalous low-level anticyclones, and 
mid–high-level cyclones. The “negative sensible heating–baroclinic structure” stimulates the surface westerly 
wind anomalies over the tropical western Pacific in May through two pathways, favoring the occurrence of 
subsequent El Niño events. One is through weakening the zonal monsoon circulation over the tropical Indian 
Ocean and the Walker circulation over the tropical western Pacific. The other is modulating the air–sea 
interaction over the North Pacific through triggering Rossby waves. The negative SWS dipole mode tends to 
induce La Niña events.

How to cite: Yu, W.: Potential Impact of Spring Thermal Forcing Over the Tibetan Plateau on the Following Winter El Niño–Southern Oscillation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6401, https://doi.org/10.5194/egusphere-egu23-6401, 2023.

EGU23-7693 | Orals | CL2.2

Atmospheric nonlinearities strong contribution to the skewed ENSO amplitude and phase transition 

Jérôme Vialard, Srinivas Gangiredla, Matthieu Lengaigne, Aurore Voldoire, Takeshi Izumo, and Eric Huilyardi

ENSO features prominent asymmetries, in terms of amplitude, spatial pattern and phase-transition between warm and cold events. Here we examine the contribution of atmospheric nonlinearities to ENSO asymmetries through a set of forced experiments with the CNRM-CM6 AGCM and the NEMO OGCM. Control experiments can reproduce the major atmospheric and oceanic asymmetries of ENSO, with stronger signals east of the dateline for strong El Niño events, and west of it for strong La Niñas. Ensemble atmospheric experiments forced by observed ENSO SST anomalies and their opposites allow diagnosing asymmetries in air-sea heat and momentum fluxes directly attributable to atmospheric nonlinearities. They indicate that atmospheric nonlinearities are largely attributable to nonlinearities in the rainfall-SST relation and act to enhance El Niño atmospheric signals east of the dateline and those of La Niña west of it. An ocean simulation where the non-linear signature of air-sea fluxes is removed from the forcing reveals that asymmetries in the ENSO SST pattern are primarily due to atmospheric nonlinearities, and result in a doubling of eastern Pacific warming during the peak of strong El Niño events and a 33% reduction during that of strong La Niña events. Atmospheric nonlinearities also explain most of the observed prolonged eastern Pacific warming into boreal summer after the peak of strong El Niño events. Overall, these results imply that properly simulating the nonlinear relationship between SST and rainfall in CGCMs is essential to accurately simulate asymmetries in ENSO amplitude, spatial pattern and phase transition. Finally, we discuss the inherent limitations to our two-tier forced approach.

How to cite: Vialard, J., Gangiredla, S., Lengaigne, M., Voldoire, A., Izumo, T., and Huilyardi, E.: Atmospheric nonlinearities strong contribution to the skewed ENSO amplitude and phase transition, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7693, https://doi.org/10.5194/egusphere-egu23-7693, 2023.

EGU23-7791 | Posters on site | CL2.2

The multiverse future of ENSO diversity in large ensembles of climate models 

Bastien Dieppois, Nicola Maher, Antonietta Capotondi, and John O'Brien

El Niño Southern Oscillation (ENSO) shows large differences from one event to another in terms of its intensity, spatial pattern, and temporal evolution, which are typically referred to as “ENSO diversity”. While such differences in ENSO patterns are associated with different regional climate impacts throughout the world, influencing the skill of impact prediction systems, large uncertainties remain concerning its potential future evolution and trends. The location and intensity of ENSO events are indeed strongly influenced by internal/natural climate variations, limiting the detection of forced changes.

Here, we exploit the power of single model initial-condition large ensembles (SMILEs) from 13 fully coupled climate models from both CMIP5 and CMIP6 (totalling 580 realizations in historical and SSP-RCP scenarios) to first examine the ability of climate models to simulate realistic diversity of ENSO events compared to multiple observational datasets, and then use those models to characterize future trajectories in the location and intensity of El Niño and La Niña events. We define the location of ENSO events as the longitude of the absolute maximum (the intensity) of sea-surface temperature anomalies (SSTa) during boreal Winter (December-February) in the equatorial Pacific. Future projections of ENSO diversity are assessed in terms of joint probability distributions of ENSO events’ location and intensity.

While some models show a degree of diversity in the location and intensity of events that are comparable with observed statistics, other models tend to favour the occurrence of eastern or central Pacific events. Such contrasting performances during the historical period are found to be associated with different future trajectories of ENSO diversity: i) models favouring the occurrence of eastern Pacific events (e.g., ACCESS-ESM1-5, CanESM2, and 5) show a westward shift in event location over the 21st century; ii) models simulating ENSO events anomalously westward tend to show an eastward shift in event locations and an increased intensity in the 21st century (e.g., CESM1 and 2, CSIRO-MK3-6-0, GFDL-CM3, GFDL-ESM2M, MIROC-ES2L, MIROC6). Nevertheless, we note that models showing the closest match to observed statistics during the historical period also present a westward shift in ENSO locations and a slight increase in intensity in the 21st century (e.g., GFDL-SPEAR and IPSL-CM6-LR).

Although the physical cause of model discrepancies remains unclear, this study provides a broader perspective on expected ENSO changes over the 21st century in different models and highlights the spread of projections among models.

How to cite: Dieppois, B., Maher, N., Capotondi, A., and O'Brien, J.: The multiverse future of ENSO diversity in large ensembles of climate models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7791, https://doi.org/10.5194/egusphere-egu23-7791, 2023.

EGU23-8299 | ECS | Orals | CL2.2

Effect of Indian Ocean Dipole on ocean meridional heat transport depends on ENSO 

Kay McMonigal and Sarah Larson

Meridional heat transport within the Indian Ocean can drive climate and ecosystem impacts, by changing ocean temperature. Previous studies have linked variability in meridional heat transport to Indian Ocean Dipole (IOD) and El Niño-Southern Oscillation (ENSO). Recent studies have shown that some IOD events are caused by ENSO (termed “ENSO forced IOD”), while other events occur without ENSO (termed “internal IOD”). It is unclear whether these different kinds of IOD have different effects on the ocean. By comparing a climate model that includes ENSO to the same climate model but with ENSO dynamically removed, we show that internal IOD does not lead to variability in Indian Ocean meridional heat transport. However, ENSO forced IOD does lead to meridional heat transport variability. This is due to differing wind patterns associated with each kind of IOD event. These results suggest that the ecosystem and climate effects of IOD likely depend upon whether the IOD occurs with or without ENSO. 

How to cite: McMonigal, K. and Larson, S.: Effect of Indian Ocean Dipole on ocean meridional heat transport depends on ENSO, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8299, https://doi.org/10.5194/egusphere-egu23-8299, 2023.

EGU23-8733 | ECS | Orals | CL2.2

Stochastic perturbations of El Nino Southern Oscillations (ENSO) : a Wiener chaos approach 

Yusuf Aydogdu, Peter Baxendale, and N. Sri Namachchivaya

The phenomena of El Nino Southern Oscillations (ENSO) is modeled by coupled atmosphere-ocean mechanism together with sea surface temperature (SST) budget at the equatorial Pacific and has a significant impact on the global climate.  We consider a modeling framework that was originally developed by Majda and co-workers in (Chen et al. 2018; Thual et al. 2016), which is physically consistent and amenable to detailed analysis. The coupled model is mainly governed by the equatorial atmospheric and oceanic Kelvin and Rossby waves and it is shown that stochastic forcing gives rise to the model anomalies and unpredictable behavior. The purpose of our work is to investigate the influence of randomness on the model dynamics,  construct the appropriate model components with stochastic noise and calculate the statistical properties. We also provide analytical and numerical solutions of the model to prove the convergence of the numerical scheme developed in our work. 

We use Wiener-Chaos Expansion (WCE) to study stochastic ENSO models. The WCE method is based on reducing stochastic partial differential equations (SPDEs) into an infinite hierarchy of deterministic PDEs called propagators-Fourier modes (Lototsky and Rozovsky, 2006) and represents the stochastic solution as a spectral decomposition of deterministic components with respect to a set of random Hermite bases. We solve the WCE propagators, which are forced by a set of complete orthonormal bases,  by applying numerical integration and finite-difference methods. We compare WCE-based results with Monte Carlo simulations of SPDEs.

Our results depict that the mean and variance of the solutions obtained from the WCE method provide remarkably accurate results with a reasonable convergence rate and error range.  We first test the WCE-based method on the ocean  model with white noise and show that 10-Fourier modes are able to approach the theoretical variance values. We also show that the OU process with a specific noise strength and dissipation over a one-time period can be recovered with less than 50-Fourier modes for the ENSO model.  To illustrate the particular weight of variance, we also generate the ensembles of solutions by using different stochastic bases. We also derive the analytical formulation of propagators for the coupled model with nonlinear SST by using the properties of Wick polynomials that construct the foundation of numerical schemes. 

How to cite: Aydogdu, Y., Baxendale, P., and Namachchivaya, N. S.: Stochastic perturbations of El Nino Southern Oscillations (ENSO) : a Wiener chaos approach, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8733, https://doi.org/10.5194/egusphere-egu23-8733, 2023.

EGU23-8904 | Orals | CL2.2

Forecasting the El Niño type well before the spring predictability barrier 

Josef Ludescher, Armin Bunde, and Hans Joachim Schellnhuber

The El Niño Southern Oscillation (ENSO) is the most important driver of interannual global climate variability and can trigger extreme weather events and disasters in various parts of the globe. Depending on the region of maximal warming, El Niño events can be partitioned into 2 types, Eastern Pacific (EP) and Central Pacific (CP) events. The type of an El Niño has a major influence on its impact and can even lead to either dry or wet conditions in the same areas on the globe. Here we show that the zonal difference ΔTWP-CP between the sea surface temperature anomalies (SSTA) in the equatorial western Pacific and central Pacific is predictive of the type of an upcoming El Niño. When at the end of a calendar year, ΔTWP-CP is positive, an El Niño event developing in the following year will probably be an EP event, otherwise a CP event. Between 1950 and present, the index correctly indicates the type of 18 out of 21 El Niño events (p = 9.1⋅10-4).
For early actionable forecasts, the index has to be combined with a forecast for the actual onset of an El Niño event. The previously introduced climate network-based forecasting approach provides such forecasts for the onset of El Niño events also by the end of the calendar year before onset. Thus a combined approach can provide reliable forecasts for both the onset and the type of an event: at a lead time of about one year, 2/3 of the EP El Niño forecasts and all CP El Niño forecasts in the regarded period are correct. The combined model has considerably more predictive power than the current operational type forecasts with a mean lead time of about 1 month and should allow early mitigation measures.

How to cite: Ludescher, J., Bunde, A., and Schellnhuber, H. J.: Forecasting the El Niño type well before the spring predictability barrier, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8904, https://doi.org/10.5194/egusphere-egu23-8904, 2023.

Since the early 1990s the Pacific Walker circulation has strengthened, while SSTs in the eastern equatorial Pacific became colder, which is opposite to future model projections. Whether these trends, evident in many climate indices especially before the 2015 El Niño, reflect the coupled ocean-atmosphere response to global warming or the negative phase of the Pacific Decadal Oscillation (PDO) remains debated. Here we show that sea surface temperature (SST) trends during 1980-2020 are dominated by three signals: a spatially uniform warming trend, a negative PDO pattern, and a Northern Hemisphere/Indo-West Pacific warming pattern. The latter pattern, which closely resembles the transient ocean thermostat-like response to global warming emerging in a subset of CMIP6 models, shows cooling in the central-eastern equatorial Pacific but warming in the western Pacific and tropical Indian ocean. Together with the PDO, this pattern drives the Walker circulation strengthening. CMIP6 historical simulations appear to underestimate this pattern, contributing to the models’ inability to replicate the Walker cell strengthening. We further discuss how such changes in the Walker circulation can effect ENSO.

Reference:  Heede, U. and A.V. Fedorov, 2023: Colder eastern equatorial Pacific and stronger Walker circulation in the early 21st century: separating the forced response to global warming from natural variability. In press, GRL

How to cite: Fedorov, A. and Heede, U.: Colder eastern equatorial Pacific and stronger Walker circulation in the early 21st century: an Indo-Pacific ocean thermostat  versus natural variability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10347, https://doi.org/10.5194/egusphere-egu23-10347, 2023.

EGU23-10801 | Orals | CL2.2 | Highlight

Causes and Consequences of the Prolonged 2020-2023 La Niña 

Michael J. McPhaden, Nahid Hasan, and Yoshimitsu Chikamoto

The tropical Pacific has witnessed three successive years of unusually cold sea surface temperatures, with peak anomalies in late 2020, 2021 and 2022.  These conditions represent the first "triple dip" La Niña of the 21st century with major climatic impacts felt around the world.  Three year La Niña events are rare but not unprecedented; similar events occurred in 1998-2001 and in 1973-76.  A leading hypothesis for multi-year La Niñas is that they occur on the rebound from preceding extreme El Niños which, through recharge oscillator dynamics, drain the equatorial band of upper ocean heat content leaving a large heat deficit that takes multiple years to recover. The current multi-year La Niña does not conform to this scenario--antecedent conditions in the tropical Pacific in 2019 were characterized by a borderline El Niño that did not lead to a large upper ocean heat content discharge. What caused the this La Niña is thus a topic of considerable interest.  In this presentation we hypothesize that tropical inter-basin interactions were instrumental in initiating and prolonging the event. In particular, we suggest that the event was triggered from the Indian Ocean by a record Indian Ocean Dipole in late 2019, then boosted in 2021 by unusually warm conditions in the tropical Atlantic involving the strongest Atlantic Niño since the 1970s. Whether climate change may have played a role in these developments will be discussed.

How to cite: McPhaden, M. J., Hasan, N., and Chikamoto, Y.: Causes and Consequences of the Prolonged 2020-2023 La Niña, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10801, https://doi.org/10.5194/egusphere-egu23-10801, 2023.

EGU23-11500 | Orals | CL2.2

Representation of tropical SST trends in ECMWF seasonal hindcasts and implications for recent ENSO forecasts 

Michael Mayer, Magdalena Alonso Balmaseda, and Steffen Tietsche

Operational seasonal forecasts are routinely issued with their bias removed, which is estimated from hindcasts covering a sufficiently long period. An increased number of false alarms for the occurrence of El Nino by various dynamical forecasting systems in recent years challenges the view that forecast biases are stationary. Here we assess the ability of ECMWF’s operational seasonal prediction system SEAS5 to represent observed trends in tropical SSTs since 1993, with a focus on the Pacific.

SEAS5 hindcasts overestimate SST warming in the equatorial Pacific when compared to observations. Although present for all start dates, the trend error is most pronounced for May starts. As a result, SEAS5 forecasts in recent years tended to predict too warm ENSO states despite bias correction. The hindcasts also fail to reproduce the observed meridional dipole in SST trends in the eastern Pacific, with warming in the northern and cooling in the southern subtropics. We assess several numerical experiments to investigate the role of the evolving ocean observing system, the ocean data assimilation system, and the atmospheric model. Results show that the increase in Argo observations amplifies the spurious trends in the hindcasts, which points to biases in the ocean initial conditions when observational constraints are lacking prior to Argo. Furthermore, observed-SST experiments show that the atmospheric model is unable to reproduce the magnitude of increasingly northward winds that are observed in the eastern equatorial Pacific, which are associated with the meridional structure of observed SST trends and have been speculated to reduce ENSO variability. This suggests that shortcomings of the atmospheric model physics further contribute to the system’s inability to predict the recent triple La Nina period. The results call for more sophisticated calibration methods of seasonal forecasts and ultimately improved models and initialization to provide more reliable ENSO forecasts under varying background conditions.

How to cite: Mayer, M., Alonso Balmaseda, M., and Tietsche, S.: Representation of tropical SST trends in ECMWF seasonal hindcasts and implications for recent ENSO forecasts, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11500, https://doi.org/10.5194/egusphere-egu23-11500, 2023.

There are large interannual variations in the area integral of the Pacific-wide annual-mean net surface heat fluxes within 5o of the equator. They are shown to be very well correlated (r2 = 0.75) with the zonal-mean, annual-mean, zonal component of the surface wind stress on the equator, both in UK-HadGEM3 coupled climate simulations and in the ERA5 wind-stress and DEEPC net surface heat flux re-analyses. For the model data the corresponding correlations are small for monthly means (r2 = 0.25) but are large (r2 > 0.6) for time-mean periods between 6 months and 10 years (the latter being calculated from 700 year pre-industrial control simulations). The amplitude of these annual mean fluctuations in the DEEPC net surface heat fluxes is almost twice as large as that in the UK-HadGEM3 simulations. Comparison of the area-mean fields in the Nino3 and Nino4 regions from 4 member ensembles of N216O025 historical simulations with the ERA5 winds, DEEPC heat fluxes and EN4 ocean re-analyses shows that the model’s mean values and seasonal cycle of the zonal wind stress and net surface heat flux agree well with the re-analyses. In the Nino3 region however the model’s surface temperature is 1.5oC colder than the re-analyses and the depth of the 20oC isotherm (t20d) is between 10 and 15 m shallower than that in EN4.  Comparison of the amplitudes of El Nino and La Nina composite anomalies in the Nino3 and Nino4 regions shows that the surface temperature anomalies are well simulated but that the amplitudes of the wind stress anomalies in Nino4 and the t20d anomalies and surface heat flux anomalies in Nino3 are about half those in ERA5, EN4 and DEEPC respectively. These findings are somewhat similar to those from the (lower resolution)  Kiel Climate Model. The characteristic spatial patterns of the surface fields might be used to attribute the differences between the model and re-analysis net surface fluxes to particular component fluxes (e.g. the surface latent heat flux and the surface solar flux). It is also a plausible hypothesis that the under-estimation of these variations in the net surface heat fluxes is a significant contributor to the signal-to-noise paradox.       

 

How to cite: Bell, M.: HadGEM3  underestimates interannual variations in heat fluxes, zonal winds and thermocline displacements  in the tropical Pacific, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12824, https://doi.org/10.5194/egusphere-egu23-12824, 2023.

EGU23-13335 | ECS | Posters on site | CL2.2

Using Causal Discovery to Clarify Observed and Simulated Relationships Between ENSO and Other Ocean Basins 

Rebecca Herman and Jakob Runge

Observed sea-surface temperatures in various ocean basins are confounded by anthropogenic and natural radiative forcing and by teleconnections to modes of internal variability, especially the El Nino Southern Oscillation (ENSO). While confounding due to anthropogenic and natural forcing can be removed in coupled simulations, confounding due to ENSO is unavoidable. When not appropriately characterized and quantified, this confounding can obscure causal relationships between various ocean basins and atmospheric phenomena of huge humanitarian import, such as monsoon rainfall, with implications for attribution of past disasters and prediction of the future. These relationships have been difficult to characterize in part because observational data is limited and simulated data may not represent the observed climate system. This study uses causal discovery to examine the coupled relationships between ENSO and other ocean basins in simulations and observations. We begin by evaluating the (L)PCMCI(+) causal discovery algorithms under various conditions and assumptions on data generated by two continuous idealized models of ENSO: the classic Zebiak-Cane model and a simple stochastic dynamical model proposed by Thual, Majda, Chen, and Stechmann. We then apply the causal discovery algorithms to seasonally and spatially-averaged sea surface temperature (SST) indices for ENSO and other ocean basins in preindustrial control simulations from the Coupled Model Intercomparison Project Phase 6. We discuss the robustness of the results, and the differences between the causal relationships in different General Circulation Models. Finally, we apply the causal learning algorithm to observed SST, and discuss to what extent simulated relationships can be used to learn about the observed climate system. We additionally demonstrate the implications of this study for other scientific questions, specifically for understanding variability in Sahel Monsoon rainfall.

How to cite: Herman, R. and Runge, J.: Using Causal Discovery to Clarify Observed and Simulated Relationships Between ENSO and Other Ocean Basins, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13335, https://doi.org/10.5194/egusphere-egu23-13335, 2023.

EGU23-13812 | ECS | Orals | CL2.2

ENSO–IOD Inter-Basin Connection Is Controlled by the Atlantic Multidecadal Oscillation 

Jiaqing Xue, Jing-Jia Luo, Wenjun Zhang, and Toshio Yamagata

The interactions between El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) are known to have great implications for global climate variability and seasonal climate predictions. Observational analysis suggests that the ENSO–IOD inter-basin connection is time-varying and related to the Atlantic Multidecadal Oscillation (AMO) with weakened ENSO–IOD relationship corresponding to AMO warm phases. A suite of Atlantic pacemaker simulations successfully reproduces the decadal fluctuations in ENSO–IOD relationship and its link to the AMO. The warm sea surface temperature (SST) anomalies associated with the AMO drive a series of Indo-Pacific mean climate changes through tropical-wide teleconnections, including the La Niña-like mean SST cooling over the central Pacific and the deepening of mean thermocline depth in the eastern Indian Ocean. By modulating ocean–atmosphere feedback strength, those mean state changes decrease both ENSO amplitude and the Indian Ocean sensitivity to ENSO forcing, therefore decoupling the IOD from ENSO.

How to cite: Xue, J., Luo, J.-J., Zhang, W., and Yamagata, T.: ENSO–IOD Inter-Basin Connection Is Controlled by the Atlantic Multidecadal Oscillation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13812, https://doi.org/10.5194/egusphere-egu23-13812, 2023.

EGU23-15824 | ECS | Orals | CL2.2

Future Changes in the early winter ENSO teleconnections to the North Atlantic European region 

Muhammad Adnan Abid and Fred Kucharski

North Atlantic European (NAE) winter climate variability is strongly modulated through the stratospheric and tropospheric pathways, where El Niño-Southern Oscillation (ENSO) teleconnections play an important role. Recent studies showed intra-seasonal changes of the ENSO response in the NAE circulation anomalies from early to late winter.  One mechanism for this behavior is that the Indian Ocean (IO) dominate over the direct ENSO teleconnections in early winter favoring an in-phase North Atlantic Oscillation (NAO) response over NAE region. On the other hand, the direct ENSO response dominates in latter half of winter, where it projects onto the opposite phase of the NAO. In present study, we analyze the early to late winter ENSO-NAE teleconnections in future climate projections by adopting the sixth assessment report Coupled Model Intercomparison Project (CMIP6) model datasets. During early winter, we noted an increase in the ENSO-induced precipitation variability in the Pacific as well as over western and central Indian Ocean, while decrease is noted over the eastern IO. Moreover, a strengthening of the ENSO and Indian connections are noted in almost all models except few, where these connections are not well represented in the present climate. Interestingly, the changes in ENSO forced wave train are noted, which may lead to the negative NAO like circulation anomalies over the NAE region in future compared to the present climate. 

How to cite: Abid, M. A. and Kucharski, F.: Future Changes in the early winter ENSO teleconnections to the North Atlantic European region, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15824, https://doi.org/10.5194/egusphere-egu23-15824, 2023.

EGU23-16921 | Orals | CL2.2

The role of spatial shifting in El Niño/Southern Oscillation complexity 

Sulian Thual and Boris Dewitte

The El Niño-Southern Oscillation (ENSO) represents the most consequential fluctuation of the global climate system, with dramatic societal and environmental impacts. Here we show that the spatial shifting movements of the Walker circulation control the ENSO space-time complexity in a major way. First, we encapsulate the process in a conventional recharge-discharge oscillator for the ENSO by replacing the regionally fixed sea surface temperatures (SST) index against a warm pool edge index. By doing so, we can model essential ingredients of ENSO diversity and nonlinear behavior without increasing the complexity of the dynamical model. Second, we propose a data-driven method for estimating equatorial Pacific SST variability resulting from spatial shifting. It consists in time-averaging conditions respective to the evolving warm pool edge position, then generating back SST data with reduced dimensionality (one degree of freedom) from the movements of the resulting "shifted-mean" profile. It is shown that the shifted-mean SST generated in this fashion reasonably reconstructs observed interannual SSTs both in terms of amplitude and pattern diversity. We discuss implications of the present paradigm of spatial shifting for understanding ENSO complexity, including tropical basins interactions.

How to cite: Thual, S. and Dewitte, B.: The role of spatial shifting in El Niño/Southern Oscillation complexity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16921, https://doi.org/10.5194/egusphere-egu23-16921, 2023.

Previous studies with coarse-resolution global climate models (GCMs) have widely shown that extensive deforestation in the Amazon leads to a reduction in precipitation, with a potential irremediable loss of the rainforest past a critical threshold. However, precipitation in the Amazon region is of convective nature and thus has to be parameterized in coarse-resolution GCMs, limiting confidence in the results of such studies. To bypass this limitation, this study aims to investigate the impact of Amazon deforestation on precipitation in global climate simulations that can explicitly represent convection. The simulations are conducted with the ICON-Sapphire atmosphere-only model configuration run with a grid spacing of 5 km for two years. To understand the impacts of Amazon deforestation, we compare the results of a complete deforestation simulation with a control simulation. Results show no significant change in precipitation during the wet season and a slight decrease of precipitation during the dry season in the deforested simulation. Precipitation decreases due to decreased evapotranspiration are compensated by enhanced moisture convergence.

How to cite: Yoon, A.: The impact of Amazon deforestation on rain system using a storm-resolving global climate model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1304, https://doi.org/10.5194/egusphere-egu23-1304, 2023.

The current crisis state of the planet, commonly called the Anthropocene, emerged as the result of the Great Acceleration in human consumption and environmental impact which followed the Second World War in the middle of the 20th c. There is growing evidence suggesting that similar acceleration dynamics, characterised by exponential growth in human environmental impact, occurred locally or regionally at earlier stages in human history. It is, however, difficult to identify, quantify, and confirm such cases without high-resolution, well-dated historical or paleoenvironmental data. In this presentation, I review three cases of well-documented Anthropocene-like accelerations, from Roman Anatolia, medieval Poland, and early modern Greece. In all of these cases, it was political consolidation, even if short-lived, as well as economic integration, that created the social tipping point triggering exponential acceleration of human environmental impact. All of these acceleration phases also collapsed once the underlying social dynamics was no longer present.

How to cite: Izdebski, A.: Social tipping points of Anthropocene acceleration dynamics in European history, from Roman times to the Little Ice Age, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3151, https://doi.org/10.5194/egusphere-egu23-3151, 2023.

Many aspects of anthropogenic global change, such as land cover change, biodiversity loss, and the intensification of agricultural production, threaten the natural biosphere. Implications of these specific aspects of environmental conditions are not immediately obvious, so it is hard to obtain a bigger picture of what these changes imply and distinguish beneficial from detrimental human impacts.  Here I describe a holistic approach that provides a bigger picture and use it to understand how the terrestrial biosphere can be sustained in the presence of increased human activities.  This approach focuses on the free energy generated by photosynthesis, the energy needed to sustain both the dissipative metabolic activity of ecosystems and human activities, with the generation rate being set by the physical constraints of the environment.  One can then distinguish two kinds of human impacts on the biosphere: detrimental effects caused by enhanced human consumption of this free energy, and empowering effects that allow for more photosynthetic activity and, therefore, more dissipative activity of the biosphere.  I use examples from the terrestrial biosphere to illustrate this view and global datasets to show how this can be estimated.  I then discuss how certain aspects of modern technology can enhance the free energy generation of the terrestrial biosphere, which can then safeguard its sustenance even as human activity increasingly shapes the functioning of the Earth system.

Note: Presentation is based on this manuscript (https://arxiv.org/abs/2210.09164), accepted for publication in the INSEE journal.

How to cite: Kleidon, A.: How to sustain the terrestrial biosphere in the Anthropocene? A thermodynamic Earth system perspective, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3251, https://doi.org/10.5194/egusphere-egu23-3251, 2023.

EGU23-3443 | Orals | CL3.2.6 | Highlight

Regional Climate Expected to Continue to Change Significantly After Net-Zero CO2 Emissions Reached 

Andrew H. MacDougall, Josie Mallett, David Hohn, and Nadine Mengis

The Zero Emissions Commitment (ZEC) is the expected temperature change following the cessation of anthropogenic emissions of climate altering gases and aerosols. Recent model intercomparison work has suggested that global average ZEC for CO2 is close to zero. However there has thus far been no effort to explore how temperature is expected to change at spatial scales smaller than the global average. Here we analyze the output of nine full complexity Earth System Models which carried out standardized ZEC experiments to quantify the ZEC from CO2. The models suggest that substantial temperature change following cessation of emissions of CO2 can be expected at large and regional spatial scales. Large scale patterns of change closely follow long established patterns seen during modern climate change, while at the regional scale patterns of change are far more complex and show little consistency between different models. Analysis of model output suggest that for most models these changes far exceed pre-industrial internal variability, suggesting either higher climate variability, continuing changes to climate dynamics or both. Thus it appears likely that at the regional scale, where climate change is directly experienced, climate disruption will not end even as global temperature stabilizes. Such indefinite continued climate changes will test the resilience of local ecosystem and human societies long after economic decarbonization is complete. Overall substantial regional changes in climate are expected following cessation of CO2 emissions but the pattern, magnitude and sign of these changes remains highly uncertain.

How to cite: MacDougall, A. H., Mallett, J., Hohn, D., and Mengis, N.: Regional Climate Expected to Continue to Change Significantly After Net-Zero CO2 Emissions Reached, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3443, https://doi.org/10.5194/egusphere-egu23-3443, 2023.

EGU23-5233 | Posters on site | CL3.2.6

Association for Trans-Eurasia Exchange and Silk-Road Civilization Development 

Likun Ai, Juzhi Hou, Haichao Xie, Yanbo Yu, and Fahu Chen

Spanning more than 6,400 kilometers across Eurasia, the Silk Road played a key role in facilitating exchanges in economy, culture, politics, and religions between East and West. The ancient Silk Road was one of the most important passages for trans-Eurasia exchange and human migrations, which could be traced back to 5000-4000 years before present. To deepen understanding of the effects of environmental changes in shaping the long-term trans-Eurasia exchanges and Silk Road civilization, the Trans-Eurasia Exchange and Silk-Road Civilization Development (ATES) was launched by a group of scientists with background of climate, hydrology, environment, archaeology in 2019. There are about 118 scientists from 10 countries that with different background have joined the ATES so far. ATES now has a President, and three coordinators in the secretariat, and all the alliance members are allocated to the 5 Working Groups (WG) based on their background and research interests. The main scientific issues for the ATES are: 1) Routes and driving forces of ancient human migrations across Eurasia in the Paleolithic; 2) Relationship between the food globalization, development of agro-pastoralism in Eurasia and human migration in the Neolithic; 3) Mechanisms of establishment, shift and demise of routes and key towns along the ancient Silk Road; 4) Effects of environmental changes on the rise and fall of the Silk Road civilization as to the trans-Eurasia exchanges in terms of economy, technology and culture. What does it tell us about the future of ongoing climate change? ATES aims to set an international platform to exchange multi-discipline knowledge and the latest research achievement on the ancient Silk Road, including exchanges of culture, science, and technology along the roads, perceptions of climate change, and socio-economic development in different historical periods along the Silk Road, and effects of environmental changes on the rise and fall of the Silk Road civilization.

ATES welcomes institutes and scientists worldwide to initiate and launch relevant research programs and projects with the ATES community. By establishing several joint research and education centers with partners, ATES facilitates and supports field observations, research, and capacity building. Training of Young Scientists is one of the main tasks for ATES capacity building, which includes the training workshops and field learnings organized by ATES and its partners. In order to strengthen the interaction of the ATES community, and to enhance the exchange of new achievements and insights of the interdisciplinary study on the evolution of trans-Eurasia exchanges and Silk Road civilization, the ATES Silk Road Civilization Forum invites a world-renowned scientist to give a special lecture on the focused topic every 3 months. ATES will organize parallel sessions and side meetings in the big events such as AGU, EGU, Conference of the Parties of the UNFCCC, UNCBD, ANSO conference, et al. ATES partners and other institutes are welcome to join in organizing the above meetings.

How to cite: Ai, L., Hou, J., Xie, H., Yu, Y., and Chen, F.: Association for Trans-Eurasia Exchange and Silk-Road Civilization Development, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5233, https://doi.org/10.5194/egusphere-egu23-5233, 2023.

EGU23-5722 | ECS | Orals | CL3.2.6 | Highlight

Recurrent droughts increase risk of cascading tipping events by outpacing adaptive capacities in the Amazon rainforest 

Nico Wunderling, Arie Staal, Frederik Wolf, Boris Sakschewski, Marina Hirota, Obbe A. Tuinenburg, Jonathan F. Donges, Henrique M.J. Barbosa, and Ricarda Winkelmann

Since the foundational paper by Lenton et al. (2008, PNAS), tipping elements in the climate system have attracted great attention within the scientific community and beyond. One of the most important tipping elements is the Amazon rainforest. Under ongoing global warming, it is suspected that extreme droughts such as those in 2005 and 2010 occur significantly more often, up to nine out of ten years from the mid to late 21st century onwards (e.g. Cox et al., 2008, Nature; Cook et al., 2020, Earth’s Future).

In this work, we quantify how climates ranging from normal rainfall conditions to extreme droughts may generate cascading tipping events through the coupled forest-climate system. For that purpose, we make use of methods from nonlinear dynamical systems theory and complex networks to create a conceptual model of the Amazon rainforest, which is dependent on itself through atmospheric moisture recycling.

We reveal that, even when the rainforest is adapted to past local conditions of rainfall and evaporation, parts of the rainforest may still tip when droughts intensify. We uncover that forest-induced moisture recycling exacerbates tipping events by causing tipping cascades that make up to one-third (mean+-s.d. = 35.9+-4.9%) of all tipping events. Our results imply that if the speed of climate change might exceed the adaptation capacity of the forest, knock-on effects through moisture recycling impede further adaptation to climate change.

Further, we use a network analysis method to compare the four main terrestrial moisture recycling hubs: the Amazon Basin, the Congo Rainforest, South Asia and the Indonesian Archipelago. By evaluating so-called network motifs, i.e. local-scale network structures, we quantify the fundamentally different functioning of these regions. Our results indicate that the moisture recycling streams in the Amazon Basin are more vulnerable to disturbances than in the three other main moisture recycling hubs.

How to cite: Wunderling, N., Staal, A., Wolf, F., Sakschewski, B., Hirota, M., Tuinenburg, O. A., Donges, J. F., Barbosa, H. M. J., and Winkelmann, R.: Recurrent droughts increase risk of cascading tipping events by outpacing adaptive capacities in the Amazon rainforest, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5722, https://doi.org/10.5194/egusphere-egu23-5722, 2023.

EGU23-7871 | Posters on site | CL3.2.6 | Highlight

Is the current methane growth event comparable to a glacial/interglacial Termination event? 

Euan Nisbet, Martin Manning, David Lowry, Rebecca Fisher, and James France

Atmospheric methane shows very sharp growth since 2006. Growing evidence for methane's main sink, atmospheric OH, being relatively stable implies a major increase in methane emissions is occurring. Methane's synchronous isotopic shift to more negative d13C(CH4) values means the increase is primarily driven by rapid growth in emissions from biogenic sources, such as natural wetlands and agriculture. Recent acceleration in the increase is also strong evidence that it is too large to be caused primarily by anthropogenic sources. Instead, much of the growth may come from large-scale climate-change feedbacks affecting the productivity and balance between methanogenic and methanotrophic processes in tropical and boreal wetlands. Emissions from tropical wetlands in particular may be larger and more influenced by climate shifts than hitherto realised. If so, even despite the Global Methane Pledge, achieving the goals of the UN Paris Agreement may be much harder than previously anticipated.

Modelling indicates that, for scale and speed, the biogenic feedback component of methane's growth and isotopic shift in the 16 years from 2006-2022 is comparable to (or greater than) phases of abrupt growth and isotopic shift during glacial/interglacial terminations, from Termination V (about 430 ka BP) to Termination I that initiated the Holocene. These were rapid global-scale climate shifts when the Earth system reorganised from cold glacial to warmer interglacial conditions.  Methane's recent 2006-2022 growth in biogenic sources may be within Holocene variability, but it is also a possibility that methane may be providing the first indication that a very large-scale end-of-Holocene reorganisation of the climate system is already under way: Termination Zero.

How to cite: Nisbet, E., Manning, M., Lowry, D., Fisher, R., and France, J.: Is the current methane growth event comparable to a glacial/interglacial Termination event?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7871, https://doi.org/10.5194/egusphere-egu23-7871, 2023.

EGU23-9387 | ECS | Posters on site | CL3.2.6

Robustness of critical slowing down indicators to power-law extremes in an Amazon rainforest model 

Vitus Benson, Jonathan F. Donges, Jürgen Vollmer, and Nico Wunderling

Critical slowing down has recently been detected as an indicator of reduced resilience in remotely sensed data of the Amazon rainforest [1]. Tropical rainforests are frequently hit by disturbances such as fire, windthrow, deforestation or drought, which are known to follow a heavy-tailed amplitude distribution. Early warning signals based on critical slowing down are theoretically grounded for systems under the influence of weak, Gaussian noise. Hence, it is not imminent that they are applicable also for systems like the Amazon rainforest, which are influenced by heavy-tailed noise. Here, we extended a conceptual model of the Amazon rainforest [2] to study the robustness of critical slowing down indicators to power-law extremes. These indicators are expected to increase before a critical transition. 

We find the way by which such an increase is detected is decisive for the recall of the early warning indicator (i.e. the proportion of critical transitions detected by the indicator). If a linear slope is taken, the recall of the early warning signal is reduced under power-law extremes. Instead, the Kendall-Tau rank correlation coefficient should be used because the recall remains high in this case. Other approaches to increase robustness, like a high-pass filter or the interquartile range, are less effective. In [1], reduced resilience of the Amazon rainforest was determined through an increase in the lag-1 autocorrelation measured by the Kendall-tau rank correlation. Hence, if there was a resilience loss, they can correctly detect it even in the presence of relatively strong power-law disturbances. However, we also quantify the false positive rate, that is, how often a resilience loss is measured if the model represents a stable rainforest. At a significance level of 5% (1%, 10%) for the early warning signal detection, the false positive rate is approximately 10% (5%, 15%). For strong heavy-tailed noise, this false positive rate can deteriorate to as high as 25% (15%, 35%). This indicates, that increasing critical slowing down may not always be caused by an approaching critical transition, a false positive detection is possible.

 

[1] Boulton, C.,  Lenton, T.  and Boers, N.: “Pronounced Loss of Amazon Rainforest Resilience since the Early 2000s”. Nature Climate Change 12-3 (2022).

[2] Van Nes, E., Hirota, M., Holmgren, M. and Scheffer, M.: “Tipping Points in Tropical Tree Cover”. Global Change Biology 20-3 (2014).

How to cite: Benson, V., Donges, J. F., Vollmer, J., and Wunderling, N.: Robustness of critical slowing down indicators to power-law extremes in an Amazon rainforest model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9387, https://doi.org/10.5194/egusphere-egu23-9387, 2023.

EGU23-9954 | ECS | Posters on site | CL3.2.6

Climate tipping risks under policy-relevant overshoot temperature pathways 

Tessa Möller, Ernest Annika Högner, Samuel Bien, Carl-Friedrich Schleussner, Johan Rockström, Jonathan F. Donges, and Nico Wunderling

The risk of triggering multiple climate tipping points if global warming levels were to exceed 1.5°C has been heavily discussed in recent literature. Current climate policies are projected to result in 2.7°C warming above pre-industrial levels by the end of this century and will thereby at least temporarily overshoot the Paris Agreement temperature goal.

Here, we assess the risk of triggering climate tipping points under overshoot pathways derived from emission pathways and their uncertainties from the PROVIDE ensemble using PyCascades, a stylised network model of four interacting tipping elements including the Greenland Ice Sheet, the West Antarctic Ice Sheet, the Atlantic Meridional Overturning Circulation, and the Amazon Rainforest.

We show that up until 2300, when overshoots are limited to 2°C, the upper range of the Paris Agreement goal, the median risk of triggering at least one element would be less than 5%, although some critical thresholds may have been crossed temporarily. However, the risk of triggering at least one tipping element increases significantly for scenarios that peak above the Paris Agreement temperature range. For instance, we find a median tipping risk in 2300 of 46% for an emission scenario following current policies. Even if temperatures would stabilize at 1.5°C after having peaked at temperatures projected under current policies, the long-term median tipping risks would approach three-quarters.

To limit tipping risks beyond centennial scales, we find that it is crucial to constrain any temperature overshoot to 2°C of global warming and to stabilize global temperatures at 1.0°C or below in the long-term.

How to cite: Möller, T., Högner, E. A., Bien, S., Schleussner, C.-F., Rockström, J., Donges, J. F., and Wunderling, N.: Climate tipping risks under policy-relevant overshoot temperature pathways, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9954, https://doi.org/10.5194/egusphere-egu23-9954, 2023.

EGU23-10044 | ECS | Orals | CL3.2.6 | Highlight

The Impact of Solar Radiation Modification on Earth System Tipping Points and Threshold Free Feedbacks 

Gideon Futerman and Claudia Wieners

The modification of the climate by Solar Radiation Modification (SRM) could be a potentially important human-Earth System interaction in the Anthropocene, having potentially beneficial and adverse impacts across climatic and human indices. SRM would likely interact with Earth system resilience in many ways, with our paper exploring SRM’s interaction with Earth System tipping point which has been extremely underexplored in the literature thus far.

SRM would likely be able to reduce global mean surface temperature quickly, although its broader climate imprint, especially on precipitation and local climatic conditions, is not the same as reversing greenhouse gas emissions. Its cooling effect suggests that SRM can help stop us from hitting those tipping elements that are most temperature-dependent, while the situation is more complex for tipping elements which strongly depend on other factors such as precipitation or regional climate changes. This more complex picture could have important implications for the role (or lack of) that SRM could and ought to play in improving Earth system resilience in the Anthropocene.

We review the available literature about the influence of SRM on the tipping elements and threshold free-feedbacks identified by McKay et al. (2022), as well as reviewing the impact of SRM on relevant climatic conditions that could contribute to tipping of each element, to give an assessment of the potential beneficial or adverse impact of SRM and identify key uncertainties and knowledge gaps. We will also briefly assess how these impacts may differ with different methods of deployment and with the termination of SRM.

How to cite: Futerman, G. and Wieners, C.: The Impact of Solar Radiation Modification on Earth System Tipping Points and Threshold Free Feedbacks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10044, https://doi.org/10.5194/egusphere-egu23-10044, 2023.

EGU23-10864 | Posters on site | CL3.2.6

Towards the Anthropocene peatlands and forests – old-growth forest loss in Western Poland initiated peat growth and peatland state shifts 

Mariusz Lamentowicz, Sambor Czerwiński, Monika Karpińska-Kołaczek, Piotr Kołaczek, Mariusz Gałka, Piotr Guzowski, and Katarzyna Marcisz

During European states’ development, various past societies utilized natural resources, but their impact was not uniformly spatially and temporally distributed. Considerable changes resulted in landscape fragmentation, especially during the Middle Ages. Changes in state advances that affected the local economy significantly drove the trajectories of ecosystems’ development. The legacy of significant changes from pristine forests to farming is visible in natural archives as novel ecosystems. Here, we present two high‑resolution, densely dated multi‑proxy studies covering the last 1000 years from peatlands in CE Europe. In that case, the economic activity of medieval societies was related to the emerging Polish state and new rulers, the Piasts (in Greater Poland) and the Joannites (the Order of St. John of Jerusalem, Knights Hospitaller). Our research revealed rapid deforestation and subsequent critical land-use transition in the high and late Middle Ages and its consequences on the peatland ecosystem development. The shift from the old-growth forests correlates well with raising the local economy, deforestation and enhanced peat initiation. Along with the emerging landscape openness, the wetlands switched from wet fen with open water to terrestrial habitats. Both sites possess a different timing of the shift, but they also show that the catchment deforestation caused accelerated terrestrialization. Our data show how closely the ecological state of wetlands relates to forest microclimate. We identified a significant impact of economic development and the onset of intensive agriculture processes near the study sites. Our results revealed a surprisingly fast rate at which the feudal economy eliminated pristine nature from the studied area and led to intensive nature exploitation in the Anthropocene. In consequence, its activities led to the creation of novel peatlands types.

How to cite: Lamentowicz, M., Czerwiński, S., Karpińska-Kołaczek, M., Kołaczek, P., Gałka, M., Guzowski, P., and Marcisz, K.: Towards the Anthropocene peatlands and forests – old-growth forest loss in Western Poland initiated peat growth and peatland state shifts, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10864, https://doi.org/10.5194/egusphere-egu23-10864, 2023.

EGU23-13587 | ECS | Posters virtual | CL3.2.6

Model hierarchies and bifurcations in QE monsoon models 

Krishna Kumar S and Ashwin K Seshadri

The convective quasi-equilibrium (CQE) framework has been successfully employed in the past to build intermediate complexity models accounting for the interaction of convection and large-scale dynamics (Neelin and Zeng, 1999, JAS). As a consequence, these models find use in the study of monsoon circulations, which also experience abrupt onset among several other intriguing features. While some low-order simplifications of CQE based Quasi-equilibrium tropical circulation model (QTCM) yields insights into the mechanisms of monsoon dynamics, they are restricted in the range of processes accounted for. A hierarchy of models, on the other hand, would serve well to study monsoon dynamics and various influences. While the existence of bifurcations or 'tipping-points' in monsoon dynamics has been studied for certain simple models, a thorough investigation of this possibility across a hierarchy of models is absent. Such a hierarchy of models would provide an understanding of effects of different simplifying assumptions on dominant balances in the momentum and thermodynamic equations and resulting nonlinear dynamics, including the choice of precipitation parameterizations. This study explores a hierarchy of such models of varying complexity, based on the QTCM equations. The potential occurrence of bifurcation phenomena are considered, along with their sensitivity to various parameter changes, in the context of the role of different nonlinearities present in these models. The study builds on recent results interpreting the suppression of bifurcation phenomena in these models, as a result of shifts in equilibrium branches and consequently their physical relevance. The hierarchy of models approach, in this context, reconciles apparent contradictions between bifurcations being observed in the simplest models and the evidence from more complex models as well as observations, while identifying robust phenomena.

How to cite: Kumar S, K. and Seshadri, A. K.: Model hierarchies and bifurcations in QE monsoon models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13587, https://doi.org/10.5194/egusphere-egu23-13587, 2023.

EGU23-13620 | Orals | CL3.2.6

The Western Amazon social-ecological system at risk of tipping: A transdisciplinary modelling approach 

Benjamin Stuch, Rüdiger Schaldach, Regine Schönenberg, Katharina Meurer, Merel Jansen, Claudia Pinzon Cuellar, Shabeh Ul Hasson, Christopher Jung, Ellen Kynast, Jürgen Böhner, and Hermann Jungkunst

The Amazon rainforest is a tipping element of the global climate system due to its high carbon storage potential and its flying rivers providing rain for South America. Studies suggest that land use and land cover change (LUCC) in the Amazon, i.e. deforestation, strongly disturb regional convectional rain pattern, which could lead to an increase of drought frequencies and intensities. Under increasing drought stress, the evergreen tropical rainforest may transform into a seasonal forest or even a savannah ecosystem. Such a transformation would likely activate the Amazon tipping element and may affect global climate change by triggering other critical tipping elements of the global climate system.  

Here we present our transdisciplinary research approach in the Western Amazon rainforest developed in context of the PRODIGY research project. We apply a social-ecological system approach to account for the dynamic interactions and feedbacks between people and nature, which could either stabilize or self-enforce regional tipping cascades. For example, regional land users may suffer declining yield and net primary production from decreasing precipitation. Land users may compensate the drop in production/income e.g. by cultivating more land or seeking for other income sources. As a response, deforestation could increase which may drive a self-enforcing feedback loop that further decrease precipitation.

In a participatory process, together with regional stakeholders we develop land use related explorative scenarios. Preliminary results from the scenario exercise show that future agricultural production increases in all scenarios (crops between 20% and 200% and livestock between 0% and 300%). In the first modelling step, these  changes drive the regionally adjusted spatial land system model LandSHIFT. Simulation results indicate that deforestation increases in all scenarios depending on the production technology and the reflexivity of institutions establishing appropriate management options.

In an integrated modelling step, the calculated LUCC maps serve as input to a regional climate model (WRF), which simulates respective changes in regional temperature and precipitation. Then, temperature and precipitation changes are applied to the biogeochemical model CANDY to simulate the impact (of regional deforestation) on crop yields, Net Primary Production (NPP) and changes in soil C and N cycling. In an iterative process, the yield and NPP responses are fed back to the land-use change model to simulate the required land use adaptations, accordingly. By closing the feedback loop between deforestation, climate, yield and NPP as well as respective land use adaptation, we are able to simulate a cascade of endogenous key process in the regions social ecological system. The integrated modelling results will support the stakeholders in identifying key measures/options/policies that could increase resilience of the regional social-ecological system to prevent crossing destructive regional tipping points.

How to cite: Stuch, B., Schaldach, R., Schönenberg, R., Meurer, K., Jansen, M., Pinzon Cuellar, C., Ul Hasson, S., Jung, C., Kynast, E., Böhner, J., and Jungkunst, H.: The Western Amazon social-ecological system at risk of tipping: A transdisciplinary modelling approach, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13620, https://doi.org/10.5194/egusphere-egu23-13620, 2023.

Microbial communities in freshwater lake sediments play a crucial role in regulating geochemical cycles and controlling greenhouse gas emissions. Many of them exhibit a highly ordered structure along depth profile. Besides redox effect, sediment stratification could also reflect historical transition. Dam construction dramatically increased in the mid-20th century and is considered one of the most far-reaching anthropogenic modifications of aquatic ecosystems. Here we attempted to identify the effect of historical dam construction on sediment microbial zonation in Lake Chaohu, one of the major freshwater lakes in China. The damming event in AD 1962 was coincidentally labeled by the 137Cs peak. Physiochemical and sequencing analyses (16S amplicon and shotgun metagenomics) jointly showed a sharp transition occurred at the damming-labeled horizon which overlapped with the nitrate-methane transition zone (NMTZ) and controlled the depth of methane sequestration. At the transition zone, we observed significant taxonomic differentiation. Random forest algorithm identified Bathyarchaeota, Spirochaetes, and Patescibacteria as the damming-sensitive phyla, and Dehalococcoidia, Bathyarchaeia, Marine Benthic Group A, Spirochaetia, and Holophagae as the damming-sensitive classes. Phylogenetic null model analysis also revealed a pronounced shift in microbial community assembly process, from a selection-oriented deterministic community assembly down to a more stochastic, dispersal-limited one. These findings delineate a picture in which dam-induced changes to the lake trophic level and sedimentation rate generate great changes in sediment microbial community structure, energy metabolism, and assembly process.

How to cite: Zhou, X. and Ruan, A.: Dam construction as an important anthropogenic modification triggers abrupt shifts in microbial community assembly in freshwater lake sediments, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14360, https://doi.org/10.5194/egusphere-egu23-14360, 2023.

EGU23-14772 | Posters on site | CL3.2.6

Sustainable Pathways under Climate Variability 

Kira Rehfeld and the SPACY research group members

External forcings and feedback processes of the Earth system lead to timescale and state-dependent climate variability, causing substantial surface climate fluctuations in the past. Particularly relevant for future livelihoods, changing variability patterns could also modify the occurrence of extreme events. However, spatiotemporal mechanisms of climate variability are poorly understood. Likewise, the societal implications are weakly constrained, particularly variability’s potential to drive sustainable transformation. The SPACY project investigates climate variability from past cold and warm periods to future scenarios. One research focus is how forcing mediates climate fluctuations. Bridging the gap between Earth system models and palaeoclimate proxies, we study vegetation and water isotope changes. A second focus is exploring sustainable pathways under climate variability, addressing potential interactions between artificial carbon dioxide removal and surface climate, among others.

 

In particular, we validate the ability of climate models to represent potential climate variability changes. Here, we focus on isotope-enabled simulations with dynamic vegetation. We find that models exhibit less local temperature and water isotope variability than paleoclimate proxies on decadal and longer timescales. Simulations with natural forcing agree much better with proxy records than unforced ones. The mean local temperature variability decreases with warming. Furthermore, we analyze potentials and limitations of terrestrial hydroclimate proxies. This includes water isotopes in speleothems and ice cores and vegetation indicators derived from pollen assemblages.

Transferring our understanding to the future, we contribute to mitigation and sustainable transitions. Weather and climate extremes determine losses and damages, but their impact on socioeconomic development is poorly examined. We scrutinize damage parametrization of economic models regarding the ability to consider variability. While large-scale sequestration of atmospheric carbon dioxide is paramount to mitigation targets, its representation in climate models is insufficient. Accounting for feedbacks of carbon dioxide removal (CDR) requires model experiments with modified land surfaces. We develop CDR representations of “artificial photosynthesis” in Earth system models. Pollen records benchmark the simulated climate–carbon dioxide–vegetation interactions. This supports modeling endogenous societal land use decisions in the future.

Our work continues to improve the understanding of long-term climate predictability. The combined knowledge from past climate studies and comprehensive modeling for future scenarios underlines the relevance of changing boundary conditions for a future within planetary boundaries.

 

 

How to cite: Rehfeld, K. and the SPACY research group members: Sustainable Pathways under Climate Variability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14772, https://doi.org/10.5194/egusphere-egu23-14772, 2023.

EGU23-16944 | ECS | Orals | CL3.2.6

Socio-Political Feedback on the Path to Net Zero 

Saverio Perri, Simon Levin, Lars Hedin, Nico Wunderling, and Amilcare Porporato

Anthropogenic emissions of CO2 must soon approach net zero to stabilize the global mean temperature. Although several international agreements have advocated for coordinated climate actions, their implementation has remained below expectations. One of the main challenges of international cooperation is the different degrees of socio-political acceptance of decarbonization.

In this contribution, we interrogate a minimalistic model of the coupled human-natural system representing the impact of such socio-political acceptance on investments in clean energy and the path to net-zero emissions. Despite its simplicity, the model can reproduce complex interactions between human and natural systems, and it can disentangle the effects of climate policies from those of socio-political acceptance on the path to net zero. Although perfect coordination remains unlikely, as clean energy investments are limited by myopic economic strategies and a policy system that promotes free-riding, more realistic decentralized cooperation with partial efforts from each actor could still lead to significant emissions cuts.

Since the socio-political feedback on the path to net zero could influence the trajectories of the Earth System for decades to centuries and beyond, climate models need to incorporate better the dynamical bi-directional interactions between socio-political groups and the environment. Our model represents a first step for incorporating this feedback in describing complex coupled human and natural systems.

How to cite: Perri, S., Levin, S., Hedin, L., Wunderling, N., and Porporato, A.: Socio-Political Feedback on the Path to Net Zero, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16944, https://doi.org/10.5194/egusphere-egu23-16944, 2023.

EGU23-17342 | ECS | Orals | CL3.2.6

Systematic assessment of climate tipping points 

Sina Loriani, Boris Sakschewski, Jonathan Donges, and Ricarda Winkelmann

Tipping elements constitute one high-risk aspect of anthropogenic climate change - after their critical thresholds are passed, self-amplifying feedbacks can drive parts of the Earth system into a different state, potentially abruptly and/or irreversibly. A variety of models of different complexity shows these dynamics in many systems, ranging from vegetation over ocean circulations to ice sheets. This growing body of evidence supports our understanding of  potential climate tipping points, their interactions and impacts.

However, a systematic assessment of Earth system tipping points and their uncertainties in a dedicated model intercomparison project is of yet missing. Here we illustrate the steps towards automatically detecting abrupt shifts and tipping points in model simulations, as well as a standardised evaluation scheme for the Tipping Point Model Intercomparison Project (TIPMIP). To this end, the model outputs of taylored numerical experiments are screened for potential tipping dynamics and spatially clustered in a bottom-up approach. The methodology is guided by the anticipated setup of the intercomparison project, and in turn contributes to the design of the TIPMIP protocol.

How to cite: Loriani, S., Sakschewski, B., Donges, J., and Winkelmann, R.: Systematic assessment of climate tipping points, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17342, https://doi.org/10.5194/egusphere-egu23-17342, 2023.

EGU23-17397 | ECS | Posters virtual | CL3.2.6

Is Arctic Permafrost a Climate Tipping Element? – Potentials for Rapid Permafrost Loss Across Spatial Scales 

Jan Nitzbon, Thomas Schneider von Deimling, Sarah Chadburn, Guido Grosse, Sebastian Laboor, Hanna Lee, Norman Julius Steinert, Simone Maria Stuenzi, Sebastian Westermann, and Moritz Langer

Arctic permafrost is yet the largest non-seasonal component of Earth's cryosphere and has been proposed as a climate tipping element. Already today, permafrost thaw and ground ice loss have detrimental consequences for Arctic communities and are affecting the global climate via carbon-cycle–feedbacks. However, it is an open question whether climatic changes drive permafrost loss in a way that gives rise to a tipping point, crossing of which would imply abrupt acceleration of thaw and disproportional unfolding of its impacts.

Here, we address this question by geospatial analyses and a comprehensive literature review of the mechanisms and feedbacks driving permafrost thaw across spatial scales. We find that neither observation-constrained nor model-based projections of permafrost loss provide evidence for the existence of a global-scale tipping point, and instead suggest a quasi-linear response to global warming. We identify a range of processes that drive rapid permafrost thaw and irreversible ground ice loss on a local scale, but these do not accumulate to a non-linear response beyond regional scales.

We emphasize that it is precisely because of this overall linear response, that there is no „safe space“ for Arctic permafrost where its loss could be acceptable. Every additional amount of global warming will proportionally subject additional land areas underlain by permafrost to thaw, implying further local impacts and carbon emissions.

How to cite: Nitzbon, J., Schneider von Deimling, T., Chadburn, S., Grosse, G., Laboor, S., Lee, H., Steinert, N. J., Stuenzi, S. M., Westermann, S., and Langer, M.: Is Arctic Permafrost a Climate Tipping Element? – Potentials for Rapid Permafrost Loss Across Spatial Scales, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17397, https://doi.org/10.5194/egusphere-egu23-17397, 2023.

EGU23-17457 | ECS | Orals | CL3.2.6 | Highlight

Indicators of changing resilience and potential tipping points in the automotive industry 

Joshua E Buxton, Chris A Boulton, Jean-Francois Mercure, Aileen Lam, and Timothy M Lenton

Through innovation and wider socio-economic processes, large sections of the economy have been known to rapidly (and often irreversibly) transition to alternative states. One such sector currently undergoing a transition is the automotive industry, which is moving from a state dominated by internal combustion engines to one characterised by low-emission vehicles. While much research has focused on early warning signals of climate and ecological tipping points, there is much to be done on assessing the applicability of these methods to social systems. Here we focus on the potential for tipping points to occur in the sale of electrical vehicles in various markets, including Norway and the UK. Early indicators that this new state is being approached are considered through the use of novel data sources such as car sales, infrastructure announcements and online advert engagement. We then map out the socio-technical feedback loops which may drive these tipping points. Consideration is also given to the resilience of the wider automotive industry to previous economic shocks. 

How to cite: Buxton, J. E., Boulton, C. A., Mercure, J.-F., Lam, A., and Lenton, T. M.: Indicators of changing resilience and potential tipping points in the automotive industry, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17457, https://doi.org/10.5194/egusphere-egu23-17457, 2023.

EGU23-38 | Orals | CL4.4

6.5 ka BP cold spell in the Nordic Seas: a potential trigger for a global cooling event? 

Maciej M. Telesiński and Marek Zajączkowski

The present interglacial is a relatively warm and stable interval, especially compared to the preceding glacial period. However, several prominent cooling events have been identified within the Holocene epoch. Most of them occurred in its early or late part, while the middle Holocene was generally considered the warmest and most stable phase. Some of the cooling events (e.g., the well-known 8.2 ka BP event) have been proven to be of overregional importance. Here we focus on an event centred around 6.5 ka BP observed in marine records from the Norwegian Sea and the Fram Strait that has not been described previously. Planktic foraminiferal records from cores along the North Atlantic Drift reveal a subsurface water cooling that in the Fram Strait was more prominent than the well-known 8.2 ka BP event. The increase in the abundance of cold water foraminiferal species is preceded by a stepwise expansion of sea ice in the eastern Fram Strait and is accompanied by a decrease in the abundance of planktic foraminiferal species, an increase in shell fragmentation and IRD deposition. At the same time, alkenone-derived surface water temperatures in the north-eastern Norwegian Sea remain high, suggesting that the cooling was related to a drop in Atlantic Water advection rather than an external forcing. We discuss the possible causes of this event and its potential consequences, including the triggering of a global climatic deterioration that occurred shortly thereafter. Understanding the mechanisms behind such cold spells occurring within a generally warm interval is invaluable for future climate predictions. This study was supported by grant no. 2020/39/B/ST10/01698 funded by the National Science Centre, Poland.

How to cite: Telesiński, M. M. and Zajączkowski, M.: 6.5 ka BP cold spell in the Nordic Seas: a potential trigger for a global cooling event?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-38, https://doi.org/10.5194/egusphere-egu23-38, 2023.

EGU23-974 | ECS | Posters on site | CL4.4

A quantitative analysis of the source of inter-model spread in Arctic surface warming response to increased CO2 concentration 

Xiaoming Hu, Yangchi Liu, Yunqi Kong, and Qinghua Yang

This study exams the main sources of inter-model spread in Arctic amplification of surface warming simulated in the abrupt-4×CO2 experiments of 18 CMIP6 models. It is found that the same seasonal energy transfer mechanism, namely that the part of extra solar energy absorbed by Arctic Ocean in summer due to sea-ice melting is temporally stored in ocean in summer and is released in cold months, is responsible for the Arctic amplification in each of the 18 simulations. The models with more (less) ice melting and heat storing in the ocean in summer have the stronger (weaker) ocean heat release in cold season. Associated with more (less) heat release in cold months are more (less) clouds, stronger (weaker) poleward heat transport, and stronger (weaker) upward surface sensible and latent heat fluxes. This explains why the Arctic surface warming is strongest in the cold months and so is its inter-model spread.

How to cite: Hu, X., Liu, Y., Kong, Y., and Yang, Q.: A quantitative analysis of the source of inter-model spread in Arctic surface warming response to increased CO2 concentration, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-974, https://doi.org/10.5194/egusphere-egu23-974, 2023.

In this study, we derived the environmental lapse rate (ELR) with the new European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data ERA5 that could cover the central Arctic area and an extended period from 1980 to this day. We focus on the Greenland region, where the melting of the Greenland ice sheet plays a vital role in global sea level rise. The temporal and spatial variability of ELR distribution over the Greenland Ice sheet is fully explored in our research and the ELR values distribution over the other central Arctic land area including the Canadian archipelago, high latitude area of North America, and Eurasian are also studied. Our results indicate that ELR values differ dramatically in different seasons and areas, and the commonly used constant ELR −6.5 K/km is not suitable for the Arctic region. The monthly averaged ELR in Greenland shows an annual seasonal cycle with the lowest value is −2.5 K/km in winter. Near-zero ELR occurs in the northeastern marginal part of Greenland for the entire year except summer months. We talked about factors that might cause the near-zero ELR values that occurred over the research area in different seasons and hence research the inversion phenomenon in detail. 

The freshwater forcing that is equivalent to ice loss from Greenland in the real world is too small to affect the AMOC in climate model experiments. The freshwater flux (FWF) is comprised of runoff(liquid) and discharge(solid). To get a real and complete FWF as a freshwater forcing to activate the hosing experiment, the first step is to downscale near-surface temperature to get a higher-resolution runoff. ELR displays how the temperature near the surface varies with altitude and has been used for downscaling the near-surface temperature which will be further used for obtaining runoff. 

Our results could not only provide a reference for future near-surface temperature research and studies about inversion phenomena in different regions, but also depict the temperature vertical changes over the Arctic land area with ELR distribution. This research could provide a useful perspective on the changes in the Arctic cryosphere in recent years and should be helpful for a better understanding of mechanisms and feedback that drive the Arctic and subarctic climate changes. 

How to cite: Zhang, Z., Bamber, J., and Igneczi, A.: Temporal and spatial variability of Environmental Lapse Rate distribution over Greenland and the central Arctic from 1980 to 2020, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1762, https://doi.org/10.5194/egusphere-egu23-1762, 2023.

After the last glaciation numerous temperature sensitive climate proxies from around the Arctic – ice cores, terrestrial and marine archives alike – show a tight connection to northern insolation with highest temperatures noted in the early Holocene. However, until the mid-Holocene (5-6ka; start of neoglaciation) all environmental change and reorganization occurred under circumstances still caused by deglaciation and global sea-level rise. Thus, the situation observed since then is interpreted to be mainly driven by a kind of ocean-atmospheric system that has little in common with the time before. In the Arctic the flooding of the vast shelves ended thereby massively expanding the area of winter sea-ice. And in the Nordic Seas water fronts were established which caused intensification of the gyre systems leading to the modern-like circulation pattern during the past 4kyrs. In several records these past 4 millennia were relatively cool. In the largest Arctic delta (Lena) peat-based island accumulation started at 4ka and another major change in growth occurred after 2.5ka in both, accumulation and species composition.

Neoglacial cooling in the colder Nordic Seas is witnessed by a persistent sedimentation of ice-rafted debris (IRD) after 6 ka, a trend which continued until recent time. Although within the eastern, Atlantic-influenced sector warm conditions persisted until about 1 ka, as seen in both planktic and benthic O-isotopes, variability among foraminiferal species would indicate major surface changes, as the abundance of the polar species increased to 70 % since then (in the Little Ice Age). That drastic increase was associated with highly variable O-isotope values throughout the entire water column. Thus, for the Little Ice Age the particular situation caused a rerouting of polar water masses and sea-ice far into the eastern Nordic seas. The major force behind such centennial-long climatic events must be sought in a complex atmosphere-surface ocean interaction rather than in the often-mentioned meridional ocean overturning circulation. Thus, spatial expansion of sea-ice impacts both the polar vortex and the temperature gradient between the high and low latitudes thereby exerting climate pressure on regions well beyond the Arctic realm.

How to cite: Bauch, H.: Effects of atmosphere-ocean interactions on late Holocene climate in the Arctic-Subarctic region, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2775, https://doi.org/10.5194/egusphere-egu23-2775, 2023.

EGU23-3186 | ECS | Posters on site | CL4.4

Spatial-temporal variations of maximum surface water temperature in Arctic Fennoscandian lakes 

Mingzhen Zhang, Jan Weckstrom, Maija Heikkila, and Kaarina Weckstrom

The remote Arctic region is covered with numerous small lakes affected my current climate warming. There are little data on their thermal features, however, which hinders our understanding of the possible ecosystem impacts of warming climate and climate feedbacks at large spatial scales. We investigated spatial - temporal variations of summer lake surface temperatures (LSTs’) in 12 Arctic lakes and explored the predominant drivers by continuous year round observations of surface water temperatures. Our results suggest the general annual cycle pattern of summer water temperature: 1) the warming - up season lasted from May to July (or August) until the water temperature reached its maximum, and then the water temperature decreased until freezing in fall; and 2) the large regional heterogeneity existed in changes of summer LSTs. Futhermore, our results illustrate that July air temperature, maximum lake depth and longitude explained most of the variance in summer LSTs (>75%), and the remaining variance was related to geographic location (e.g. altitude and latitude), lake morphometric features, such as lake area and catchment area, and geochemical characteristics, i.e. turbidity and dissolved organic carbon (DOC) content. Our results provide new insights into thermal responses of small Arctic lakes with different environmental settings to climate change.

How to cite: Zhang, M., Weckstrom, J., Heikkila, M., and Weckstrom, K.: Spatial-temporal variations of maximum surface water temperature in Arctic Fennoscandian lakes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3186, https://doi.org/10.5194/egusphere-egu23-3186, 2023.

EGU23-3252 | Posters on site | CL4.4

The Batagay megaslump in east Siberia as an archive of climate–permafrost interactions during the Middle and Late Pleistocene 

Thomas Opel, Sebastian Wetterich, Hanno Meyer, and Julian Murton

The Batagay megaslump (67.58 °N, 134.77 °E) is the largest known retrogressive thaw slump on Earth, and located in the Yana River Uplands near the town of Batagay in east Siberia. The slump headwall is about 55 m high and exposes ancient permafrost deposits that provide a discontinuous record of the Middle and Late Pleistocene that dates back to at least 650 ka.

In this contribution, we compile cryostratigraphic observations and dating results for the permafrost exposed in the Batagay megaslump. Both provide evidence for several periods of permafrost formation and degradation. Permafrost formation and stability during Marine Isotope Stage (MIS) 16 or earlier (lower ice complex), MIS 7–6 or earlier (lower sand unit), MIS 4–2 (upper ice complex), and MIS 3–2 (upper sand unit) are reflected by the presence of deposits hosting syngenetic ice wedges and composite (i.e., ice–sand) wedges. In contrast, permafrost thaw and erosion are indicated by sharp, erosional discordances above reddish and organic-rich layers and by the accumulation of woody (forest) remains in erosional downcuts below and above the lower sand unit, and above the upper ice complex. Permafrost thaw and erosion likely took place during one or several periods between MIS 16 and MIS 7–6 as well as during MIS 5 and the late Pleistocene–Holocene transition.

To gain seasonal-scale climate signals, we analyzed the stable isotope composition of ground ice (ice and composite wedges and pore ice) from all four main stratigraphic units reflecting permafrost aggradation exposed in the Batagay megaslump. Ice and composite wedges contain winter climate signals. Their distinctly depleted δ18O values reflect the extreme continentality of the region with large seasonal temperature differences. Pore ice is mostly characterized by less depleted δ18O values and rather reflects summer to annual climate signals subject to post-depositional isotopic fractionation.

To draw large-scale conclusions on climate–permafrost interactions we compare our data to independent climate and permafrost reconstructions from terrestrial (cave deposits, lake sediment cores, and permafrost deposits) and marine sediment cores across the Arctic.

How to cite: Opel, T., Wetterich, S., Meyer, H., and Murton, J.: The Batagay megaslump in east Siberia as an archive of climate–permafrost interactions during the Middle and Late Pleistocene, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3252, https://doi.org/10.5194/egusphere-egu23-3252, 2023.

EGU23-3330 | Orals | CL4.4

Tides, Internal and Near-Inertial Waves in the Yermak Pass at the Entrance of the Atlantic Water to the Arctic Ocean. 

Christine Provost, Camila Artana, Ramiro Ferrari, Clément Bricaud, Léa Poli, and Young-Hyang Park

In the crucial region of the Yermak Plateau where warm Atlantic water enters the Arctic ocean, we examined high frequency variations in the Yermak Pass Branch over a 34 months-long mooring data set. The mooring was ice covered only half of the time with ice-free periods both in summer and winter. We investigated the contribution of residual tidal currents to the low frequency flow of Atlantic Water (AW) and high frequency variations in velocity shears possibly associated with internal waves. High resolution model
simulations including tides show that diurnal tide forced an anticyclonic circulation around the Yermak Plateau. This residual circulation helps the northward penetration of the AW into the Arctic. Tides should be taken into account when examining low frequency AW inflow. High frequency variations in velocity shears are mainly concentrated in a broad band around 12 hr in the Yermak Pass. Anticyclonic eddies, observed during ice-free conditions, modulate the shear signal. Semi-diurnal internal stationary waves dominate high frequency variations in velocity shears. The stationary waves could result from the interaction of freely propagating semi-diurnal internal waves generated by diurnal barotropic tides on critical slopes around the plateau. The breaking of the stationary waves with short length scales possibly contribute to mixing of AW at the entrance to the Arctic.

How to cite: Provost, C., Artana, C., Ferrari, R., Bricaud, C., Poli, L., and Park, Y.-H.: Tides, Internal and Near-Inertial Waves in the Yermak Pass at the Entrance of the Atlantic Water to the Arctic Ocean., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3330, https://doi.org/10.5194/egusphere-egu23-3330, 2023.

EGU23-3894 | ECS | Posters on site | CL4.4

A 22,000-Year Sediment Record from Burial Lake, Alaska, Shows a Rapid Twofold Increase in Mercury Concentration in Response to Early Holocene Climate Change 

Melissa Griffore, Eitan Shelef, Matthew Finkenbinder, Joseph Stoner, and Mark Abbott

Arctic permafrost soils have recently been identified as the largest mercury (Hg) reservoir on Earth. Today, rapid warming in the high latitudes may be altering the Arctic Hg cycle by accelerating permafrost thaw, leading to changes including deepening of the active layer, increasing organic matter decay, and increasing seasonal groundwater flow. However, few studies have investigated how the Hg cycle has responded to past changes in climate, and there is a lack of Arctic records that span the late glacial to early Holocene when climate conditions changed abruptly. We propose that the geochemical and physical changes in the sediment record of Burial Lake (68.43ºN, 159.17ºW; 460 m ASL), which document climatic and environmental changes in northwestern Alaska after the Last Glacial Maximum (LGM), can be used as an analog to investigate how today’s rapid warming affects Hg mobilization from permafrost soils to surficial waters. Warming in the Northern Hemisphere between ~15.0 and 8.0 ka resulted in rapid changes in northwest Alaska, including the submergence of the Bering Land Bridge that reconnected the Pacific and Arctic Oceans (~11.0 ka), in addition to changes in the hydroclimate. Our results indicate that the Hg concentration was relatively low and stable in the Burial Lake record during the transition from the LGM to the late glacial (20.0 and 16.0 ka) with a mean concentration of 64±7 μg/kg. Mercury concentrations begin to increase after 16.0 ka. Then, coinciding with a rapid temperature increase at the beginning of the Bølling Allerød (14.7 to 12.9 ka), Hg concentrations increased by ~20% and showed higher variability as temperatures fluctuated until the end of the Younger Dryas (12.9 to 11.7 ka). At 11.0 ka, the Hg concentration increased rapidly. It peaked at 140 µg/kg, with a mean Hg concentration of 119 μg/kg between 11.0 to 8.8 ka, coinciding with evidence of a rapid increase in regional precipitation and flooding of the Bering Land Bridge. From 8.8 to 0.1 ka, the mean Hg concentration decreased to 107 μg/kg and then increased rapidly over the last 100 years to a maximum concentration of 196 μg/kg occurring during the 1990s. Throughout the majority of the Burial Lake sediment record, the Hg concentration is most strongly correlated with total organic carbon content and geochemical proxies sensitive to changes in redox conditions. We interpret this finding as an indication that a large fraction of Hg is mobilized from the lake catchment along with dissolved organic matter (DOM), iron (Fe), and manganese (Mn) that are mobilized as a result of saturation and deepening of the active layer during periods of warmer, but most importantly, wetter climate. The Hg record from Burial Lake suggests that as the climate warmed after the LGM, organic-rich permafrost soils and Hg accumulated in the catchment. The sudden increase in Hg mobilization from permafrost soils was then initiated at the onset of the Holocene due to the rapid increase in precipitation that coincided with the flooding of the Bering Land Bridge.

How to cite: Griffore, M., Shelef, E., Finkenbinder, M., Stoner, J., and Abbott, M.: A 22,000-Year Sediment Record from Burial Lake, Alaska, Shows a Rapid Twofold Increase in Mercury Concentration in Response to Early Holocene Climate Change, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3894, https://doi.org/10.5194/egusphere-egu23-3894, 2023.

EGU23-4364 | ECS | Posters on site | CL4.4

Interaction between ice sheet instability and sea surface characteristics in the Labrador Sea during the last 50 ka 

Defang You, Ruediger Stein, and Kirsten Fahl

The study on the decay of ice sheets in the past provides important insights into the interaction between ice sheet behaviours and ocean characteristics, especially under a sustained warming climate. On the one hand, the ice sheet may affect the ocean environment; on the other hand, changes in sea surface conditions may affect the instability of the ice sheets. However, interactions between ice sheet dynamics and sea surface characteristics are still not fully understood. Thus, studies of carefully selected sediment cores representing both ice-sheet and ocean characteristics can help to better predict changes in ice sheets in the future. Here, we show sedimentary records from the eastern Labrador Sea, proximal to the Laurentide Ice Sheet (LIS) and the Greenland Ice Sheet (GrIS), representing the last 50 ka, i.e., the last glacial-deglacial-Holocene period. Our XRF and biomarker data document the outstanding collapse of the LIS/iceberg discharge during Heinrich Events (i.e., HE5, HE4, HE2, and HE1) and the occurrence of meltwater plumes from the LIS and GrIS during the deglaciation. Such meltwater discharge has caused surface water freshening in the Labrador Sea and, consequently, decreased sea surface temperatures and decreased primary productivity. Enhanced Irminger Current inflow might have triggered the retreat of ice sheets/meltwater discharge, as shown in our planktic foraminifera records. In contrast to dominantly relatively low primary productivity during the glacial period, both higher sea ice algae and phytoplankton production occurred during the Last Glacial Maximum (LGM), probably caused by a polynya in front of the GrIS reaching its maximum extent at that time. During the deglaciation to Holocene time interval, primary productivity shows an increasing trend probably related to decreased meltwater discharge, decreased sea ice extent, and increased insolation.

 

How to cite: You, D., Stein, R., and Fahl, K.: Interaction between ice sheet instability and sea surface characteristics in the Labrador Sea during the last 50 ka, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4364, https://doi.org/10.5194/egusphere-egu23-4364, 2023.

EGU23-5088 | ECS | Orals | CL4.4

Sedimentation rates across Baffin Bay since the last glacial period (based on radiocarbon age control) 

Emmanuel Okuma, Jürgen Titschack, Markus Kienast, and Dierk Hebbeln

Around Baffin Bay, the large continental Laurentide, Innuitian, and Greenland ice sheets retreated from their maximum extent reaching the shelf break during the Last Glacial Maximum (LGM) to their present-day close-to-minimum extent being largely confined to onshore settings. The associated changes in ice extent, erosion patterns, and material transport modes probably greatly affected spatial and temporal patterns of sediment deposition in Baffin Bay. While for many sites in Baffin Bay, local information about temporal changes in sedimentation rates exist, a spatial analysis allowing to compare sedimentation patterns is still lacking. To fill this gap, radiocarbon ages from over 50 sediment cores (with two or more dates) across Baffin Bay were compiled to assess the spatiotemporal variability in sediment input to Baffin Bay since the LGM. Preliminary results evaluating sedimentation rates (calculated from un-calibrated 14C ages) binned to 1 ka time slices reveal that during the LGM and the early deglacial, the slope beyond the shelf break and the deep basin were the only active depocenters, however, marked by very low sedimentation rates (mainly <20 cm ka-1), suggesting a largely ice-covered bay. At ~15 ka, sediment supply to these settings increased, likely reflecting the onset of ice retreat during the deglaciation. With the beginning of deposition on the mid and outer shelves after ~10 ka, deposition on the slopes and in the basin ceased almost completely. Ongoing ice retreat progressively uncovered new depocenters in the over-deepened shelf troughs off Baffin Island and Greenland, where from ~9 ka onwards, especially the inner shelf off Greenland, experienced elevated sedimentation rates (~100-500 cm ka-1), while Baffin Island fjords received less material (mainly <100 cm ka-1). Most shelf records show a continuous decrease in sedimentation rates since the early Holocene but a few records from the Greenland shelf point to rates picking up over the last two millennia, probably reflecting the Neoglaciation. Sedimentation rates peak after ~6 ka in the wider northern Baffin Bay. These data generally reflect the transition from low glacial to enhanced deglacial sedimentation beyond the shelves, followed by a progressive landward displacement of the main depocenters towards the over-deepened inner shelf troughs. There, sediment input decreased when the ice sheets attained their minimum extent in the mid-Holocene. Only in northernmost Baffin Bay is this trend turned around, with the highest sediment input in the Late Holocene.

How to cite: Okuma, E., Titschack, J., Kienast, M., and Hebbeln, D.: Sedimentation rates across Baffin Bay since the last glacial period (based on radiocarbon age control), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5088, https://doi.org/10.5194/egusphere-egu23-5088, 2023.

EGU23-5643 | Posters on site | CL4.4

Late Quaternary history of glaciations in the northern Kara Sea and Arctic Ocean iceberg drift in marine isotope stage 6 

Robert F. Spielhagen, Blumenberg Martin, Kus Jolanta, Ovsepyan Yaroslav, Taldenkova Ekaterina, Wangner David, and Zehnich Marc

We present new data from two long sediment cores obtained off the St. Anna and Voronin troughs on the northern continental margin of the Kara Sea (eastern Arctic Ocean). According to preliminary age models based on microfossil findings and grain size data, the cores cover the last ca. 150 kyr. Coarse-grained layers with common to abundant iceberg-rafted lithic grains (IRD) were deposited when ice sheets on the Kara Sea shelf had advanced close to the shelf break and ice streams developed in the deep troughs opening towards the eastern Arctic Ocean. Terrestrial data suggest that large ice sheets in the area developed in marine isotope (sub)stages (MIS) 6, 5b, and 4, while glaciation was restricted to the westernmost Kara Sea in the last glacial maximum (MIS 2) (Svendsen et al., 2004, Quat. Sci. Rev.). Our new data reveal details of the ice extent during individual glacial phases. They suggest that only in MIS 6 both troughs were filled with ice streams and that in the younger glacial phases regional differences of ice extent developed along the continental margin.

In several layers, coal clasts up to 4 cm in size were found. We have obtained coal petrological and organic geochemical data of these particles and of coal grains found in other sediment cores from the deep-sea eastern Arctic Ocean and the Fram Strait area. The results reveal a certain variability of data (random vitrinite reflectance (VRr %), Rock-Eval hydrogen and oxygen indices, hydrocarbon biomarkers) even among samples from the same core, suggesting that the coal grains do not stem from one restricted area. Data clusters and comparison with published information on coals from circum-Arctic continents, however, allow a tentative discrimination of our samples. The coals from the northern Kara Sea area and the central Fram Strait show relatively high oxygen indices, in opposite to coals from the NE Greenland margin. The latter resemble coals from the Cretaceous/Tertiary basins on Svalbard and NE Greenland. Available stratigraphic data from the cores suggests that the layers with high coal particle abundances in deep-sea cores from the northern Kara Sea area, the central Fram Strait, and the NE Greenland margin were deposited in MIS 6. We conclude that during MIS 6 coal-bearing layers in the NE Greenland Wandel Sea Basin were eroded by an expanded North Greenland Ice Sheet and transported by icebergs southward along the adjacent continental margin. At the same time, icebergs breaking off from the large northern Eurasian Ice Sheet drifted from northern Siberia across the Eurasian Basin towards the central Fram Strait. Our results generally support the hypothesis of a cross-Arctic iceberg transport in MIS 6 but show that caution must be applied when conclusions are made on the sources of individual coal particles.

How to cite: Spielhagen, R. F., Martin, B., Jolanta, K., Yaroslav, O., Ekaterina, T., David, W., and Marc, Z.: Late Quaternary history of glaciations in the northern Kara Sea and Arctic Ocean iceberg drift in marine isotope stage 6, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5643, https://doi.org/10.5194/egusphere-egu23-5643, 2023.

EGU23-8351 | ECS | Orals | CL4.4

A high-resolution, operational pan-Arctic meltwater discharge database from 1950 to 2021 

Adam Igneczi and Jonathan Bamber

The Arctic has warmed about four times faster than the global average during the last four decades. One of the consequences of this intensive warming is increasing Arctic land ice loss. In particular, mass loss from the Greenland Ice Sheet has been estimated to have increased sixfold between 1980 and 2020. Glaciers and ice caps outside of Greenland, though receiving less attention, have also been reported to be losing mass at an increasing rate. This is caused by a combination of negative surface mass balance – due to decreasing snowfall and/or increasing melting and runoff – and increasing ice discharge. However, negative surface mass balance due to increasing melting and runoff has become the dominant cause of mass loss in Greenland and the Canadian Arctic during the last 10-15 years. This indicates the increasing role of meltwater discharge into fjords and coastal seas, influencing a wide-range of physical, chemical and biological processes and also the large-scale oceanic circulation. Despite recent advancements, no meltwater discharge data products are available that cover the entire Arctic at a high spatial (< 1 km) and temporal (sub-monthly) resolution. To fill this data gap, we use daily ~6km runoff data from a regional climate model, Modéle Atmosphérique Régional (MAR), for the period of 1950-2021 – covering Greenland, Arctic Canada, Iceland, Svalbard, and Arctic Russia. We employ a statistical downscaling algorithm that utilises a high resolution (250 m) DEM, land mask (Copernicus GLO-90), and ice mask (GIMP, RGI). A hydrological routing scheme is also applied to the downscaled runoff to provide meltwater runoff data at coastal outflow points. Meltwater components coming from non glacierized land, bare glacier ice, and glacierized area above the snowline are separated to aid further analyses. The software pipeline is designed to be fully operational so that it can be used to update the time series as soon as the input data are available, so providing a continuous time series for the entire Arctic within the framework of a project aimed to develop a holistic, integrated observing system for the Arctic (www.arctipassion.eu).

How to cite: Igneczi, A. and Bamber, J.: A high-resolution, operational pan-Arctic meltwater discharge database from 1950 to 2021, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8351, https://doi.org/10.5194/egusphere-egu23-8351, 2023.

EGU23-8460 | ECS | Orals | CL4.4

An updated view on water masses on the Northeast Greenland shelf and their link to the Laptev Sea and Lena River 

Esty Willcox, Jørgen Bendtsen, John Mortensen, Christian Mohn, Marcos Lemes, Thomas Juul-Pedersen, Marit-Solveig Seidenkrantz, Johnna Holding, Eva Møller, Mikael Sejr, and Søren Rysgaard

The Northeast Greenland shelf is a broad Arctic shelf located between Greenland and Fram Strait. It is the principal gateway for sea ice export and sea ice-associated freshwater from the Arctic Ocean. Sea ice thickness has decreased by 15% per decade since the early 1990s and meteoric freshwater discharge has increased. The consequence of changing sea-ice and freshwater conditions in the region on ocean dynamics and the biological system remains unknown. Determining the source(s) of freshwater is important to be able to understand how the area will react to future upstream change. Here we present a synoptic survey of the Northeast Greenland shelf and slope with observations of hydrography, the nutrients nitrate, phosphate and silicate, and conservative tracers δ18O, δ2H and total alkalinity during late summer 2017. We compare these to previously published values, including those which identify Pacific and Atlantic water, the Siberian shelf seas, and the 6 largest Arctic rivers. We show that a major source of freshwater on the Northeast Greenland shelf during late summer 2017 is the Laptev Sea and find no conclusive evidence of Pacific Water. Our observations provide a direct link between Northeast Greenland hydrology and processes occurring on Eurasian shelves.

How to cite: Willcox, E., Bendtsen, J., Mortensen, J., Mohn, C., Lemes, M., Juul-Pedersen, T., Seidenkrantz, M.-S., Holding, J., Møller, E., Sejr, M., and Rysgaard, S.: An updated view on water masses on the Northeast Greenland shelf and their link to the Laptev Sea and Lena River, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8460, https://doi.org/10.5194/egusphere-egu23-8460, 2023.

EGU23-9642 | Orals | CL4.4 | Highlight

ABRUPT Arctic Climate Change 

Bjørg Risebrobakken, Yunyi Wang, Chuncheng Guo, Dag Inge Blindheim, Trond Dokken, Kirsten Fahl, Eystein Jansen, Marlene Klockmann, Juliette Tessier, Amandine Tisserand, Rüdiger Stein, Guido Vetteretti, and Andrzej Witkowski

At unprecedented resolution we investigate the nature of Dansgaard-Oeschger events in the Fram Strait, the gateway between the Nordic Seas and the Arctic Ocean. The new reconstructions of biomarkers and sea ice variability, stable isotopes and IRD will be seen in context of sea ice conditions, ocean hydrography and climate of the Nordic Seas as seen in multi-model output from three transient glacial GCM simulations (NorESM, CESM, MPI-ESM) and high-resolution reconstructions from an eastern Nordic Seas transect (from the Faeroe-Shetland Channel, via the Norwegian Sea to the Fram Strait). The combined results show that ocean-atmosphere-sea ice processes and dynamics during the transition from H4 to GI8 are strongly coupled. 

 

Both model results and reconstructions suggest subsurface ocean warming and polynya events in the southern- and northernmost Nordic Seas during the cold stadial. For a short time during the stadial to interstadial transition, a corridor of open water and hence sea ice-free conditions existed from the southern Nordic Seas all the way to the Fram Strait. The breakup of the sea ice cover is likely caused by the overshoot of AMOC during the transition and the associated enhanced ocean heat transport into the Nordic Seas. After the transition, winter sea ice grows back in the Fram Strait during the interstadial state, but the Southern Nordic Seas remain ice-free.

How to cite: Risebrobakken, B., Wang, Y., Guo, C., Blindheim, D. I., Dokken, T., Fahl, K., Jansen, E., Klockmann, M., Tessier, J., Tisserand, A., Stein, R., Vetteretti, G., and Witkowski, A.: ABRUPT Arctic Climate Change, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9642, https://doi.org/10.5194/egusphere-egu23-9642, 2023.

EGU23-10585 | ECS | Orals | CL4.4

Performance evaluation of 20CRv3 downscaling using WRF over southern Alaska with focus on temperature and precipitation in glaciated areas 

Sandra Koenigseder, Timothy Barrows, Jenny Fisher, Jason Evans, and Chesley MacColl

Global warming has raised mean surface temperatures by 0.99 ± 0.15 °C from 1850-1900 to 2011-2020. The temperature rise has been greatest in the high latitudes. Alaska has one of the largest temperate and subarctic glaciated areas in the world, which is highly sensitive to climate change. Currently, the mass loss from these glaciers contributes to about a third of the global sea-level rise. For example, the tidewater glacier Columbia Glacier located within Prince William Sound is the largest single contributor to sea level rise through its rapid retreat, which started in the early 1980s. Although internal controls strongly influence the tidewater glacier cycle, the ubiquitous retreat of Alaskan tidewater glaciers indicates climatic forcing is involved. However, it is unlikely climate controls the rate of retreat. There are insufficient meteorological observations from this region to assess the role of climate across a whole tidewater cycle. This project reconstructs the regional climate of southern Alaska from 1836–2015 using dynamical downscaling of the NOAA-CIRES-DOE 20th Century Reanalysis (20CRv3). To do this, the Weather Research and Forecasting model (WRF) has been used to spatially downscale the reanalysis data to produce high-resolution 4 km (convection permitting) output for southcentral/southeastern Alaska. Five different physics parametrisations have been tested for the year 2010. The model output of these five configurations were evaluated using observational records from the Global Surface Summary of the Day (GSOD). The physics scheme that performed most realistically was identified using root mean square error, R squared and normalized mean error for temperature and precipitation. The study shows that 20CRv3 can successfully be downscaled for the study region. As a result, the leading parametrisation was used for a long-term simulation (179 years) to reconstruct local climate and weather over southern Alaska over a significant part of a tidewater glacier cycle. The results will be used to evaluate the influence of climate on these glaciers for the downscaling period from 1836 to 2015.

How to cite: Koenigseder, S., Barrows, T., Fisher, J., Evans, J., and MacColl, C.: Performance evaluation of 20CRv3 downscaling using WRF over southern Alaska with focus on temperature and precipitation in glaciated areas, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10585, https://doi.org/10.5194/egusphere-egu23-10585, 2023.

EGU23-11323 | Posters on site | CL4.4

Freshwater input and water mass interactions in the Uummannaq fjord system 

Leandro Ponsoni, Anouk Ollevier, Roeland Develter, and Wieter Boone

The climate is rapidly changing in the Arctic, where global warming is reported to be about up to four times the global average in the last two decades. Aligned with this Arctic Amplification, other climate-related phenomena are also changing, or are bound to change, on a regional scale. For instance, the accelerated glaciers’ melting is forcing a transition of some glaciers from marine- to land-terminating systems and, therefore, impacting the balance of freshwater input into the oceans. As consequence, other ocean climate-related processes (e.g., water masses (trans)formation, baroclinicity of geostrophic currents) are expected to be impacted.

Within this context, and as part of the “Innovative study on regional high-resolution imaging of glacier induced plankton dynamics in West-Greenland fjords (IOPD)” project, we visited the fjord system in the Uummannaq area, off Western Greenland, aboard the R/V Sanna, from 28/Jun to 10/Jul/2022. In this region, fjords are marked by both land- and marine-terminating glaciers. During the cruise, we performed 47 hydrographic stations of the entire water column into 5 different fjords - from their mouth to the innermost accessible location. These stations are complemented by an offshore transect from the fjord mouth to the shelf edge.

Based on the in-situ measurements described above, complemented by other historical oceanographic measurements and state-of-the-art datasets for solid and liquid freshwater input provided by the Geological Survey of Denmark and Greenland (GEUS), we aim at characterizing the fjord system in the Uummannaq area in perspective of the ongoing climate changes. More specifically, this work addresses the following questions (i) What is the long-term and recent freshwater input to the region? And, is this input undergoing changes in the latest years? (ii) How are the water masses quantitatively distributed within the fjords and adjacent continental shelf? Are there differences between fjords? And, how do the connections with the adjacent continental shelf take place? (iii) Are there differences between marine- and land-terminating systems in terms of (solid and liquid) freshwater input and water mass distribution in the region? If so, what are these differences?

How to cite: Ponsoni, L., Ollevier, A., Develter, R., and Boone, W.: Freshwater input and water mass interactions in the Uummannaq fjord system, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11323, https://doi.org/10.5194/egusphere-egu23-11323, 2023.

Radiogenic Sr, Nd, and Pb isotope compositions in marine sediments are widely used as provenance tracers delivering valuable information about past environmental conditions. Over the last ten years, several studies performing radiogenic isotope analysis on marine sediment records from Baffin Bay and Labrador Sea highlighted the strength of this method in shedding light upon past glacier dynamics and related environmental changes in Greenland and the Canadian Arctic. The main outcomes of our studies include precise information on the opening of Arctic gateways and the setting of oceanic connection from the Arctic Ocean to the Atlantic through Baffin Bay. At a more regional scale, these tracers document the late glacial to Holocene dynamics of Baffin Island glaciers, helping to understand how climate and oceanic conditions impacted glacier margin fluctuations. As importantly, our study also highlighted limitations in the sensitivity of radiogenic isotopes from Baffin Bay marine sediments as tracers. Most important for interpreting radiogenic isotope compositions is the availability of a sufficiently dense cover of their properties in bedrock and reference isotope signatures from such remote areas to better resolve potential sediment sources. Another challenge for sediment records obtained from core sites at near-proximity to ice margins is the effect of glacier dynamics on the sediment composition. Intense meltwater discharge can lead to grain size and mineral sorting, which could bias the radiogenic isotope composition of the sediment. Nonetheless, radiogenic isotopes present a significant advantage over lesser availability tracers, such as biological proxies, which can be restricted due to the harsh climate conditions. In several cases, radiogenic isotope analysis also reveals more information about sediment provenance than mineralogical assemblages. All in all, in combination with sedimentological and mineralogical features, the radiogenic Sr, Nd, and Pb isotope compositions of Arctic marine sequences can be used as reliable tracers for changes in sediment provenance.

How to cite: Hingst, J., Lucassen, F., Hillaire-Marcel, C., and Kasemann, S.: Strengths and limitations of using radiogenic isotope signatures of marine sediments from Baffin Bay for the reconstruction of ice dynamics and paleoenvironments in the Canadian Arctic and Greenland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12601, https://doi.org/10.5194/egusphere-egu23-12601, 2023.

EGU23-13560 | ECS | Orals | CL4.4

How does imposing a spatially-varying map of background vertical diffusivity with rates and spatial structure informed by observations impact the modelled Arctic Ocean state? 

Benjamin O'Connor, Stephanie Waterman, Jeffrey Scott, Hayley Dosser, and Melanie Chanona

Mixing in the Arctic Ocean drives water mass transformations critical to the heat and freshwater budgets of the Arctic Ocean, impacting sea ice extent and volume, stratification, circulation, and heat and freshwater release to the subpolar N. Atlantic. Observations indicate that mixing rates in the Arctic Ocean are highly variable, however this variability is typically not well-represented in models.

This study uses a regional Arctic Ocean model to addresses the question “How does imposing a spatially-varying map of background vertical diffusivity with rates and spatial structure informed by observations impact the modelled Arctic Ocean state?” It seeks to understand impacts based on model experiments that systematically vary the diffusivity uniformly in space.

It is shown that prescribing the observationally-informed mixing map results in increased heat loss, a redistribution of freshwater storage, and increased heat and freshwater export to the N. Atlantic relative to a control case with an equal-on-average-but-spatially-uniform distribution of mixing. These effects can be understood as the result of enhancing (reducing) mixing on the shelves (basins) relative to the control case. They highlight sensitivities of the Arctic Ocean heat and freshwater budgets to shelf and basin mixing respectively.

These findings are relevant to the impacts of the changing Arctic Ocean mixing environment on Arctic Ocean functioning and subpolar ocean variability. They further suggest ways in which the prescription of Arctic Ocean mixing may be important to improving model representations of Arctic Ocean dynamics.

How to cite: O'Connor, B., Waterman, S., Scott, J., Dosser, H., and Chanona, M.: How does imposing a spatially-varying map of background vertical diffusivity with rates and spatial structure informed by observations impact the modelled Arctic Ocean state?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13560, https://doi.org/10.5194/egusphere-egu23-13560, 2023.

EGU23-14677 | ECS | Posters on site | CL4.4 | Highlight

Increasing Arctic River Discharge and Its Role for the Phytoplankton Responses in the Present-day and Future Climate Simulations 

Jung Hyun Park, Seong-Joong Kim, Hyung-Gyu Lim, Jong-seong Kug, Eun Jin Yang, and Baek-Min Kim

With the unprecedented rate of Arctic warming in recent decades, the hydrological cycle over high-latitude landmass began to accelerate, which would lead to increased river discharge into the Arctic Ocean. However, the recent climate models that participated in Coupled Model Intercomparison Project 6 (CMIP6) tend to underestimate Arctic river discharge. This study elucidates the role of overlooked Arctic river discharge for the phytoplankton responses in present-day and future climate simulations. In the present-day climate simulation, the run with additional river discharge simulates the decrease in the spring phytoplankton. Freshening of Arctic seawater leads to high freezing point that increases sea ice concentration in the spring, eventually decreasing phytoplankton due to the less light availability. On the other hand, in the summer, phytoplankton increases due to the surplus of surface nitrate and the increase in the vertical mixing induced by the reduced summer sea ice melting water. In the future climate, the role played by additional input of freshwater is similar to the present-day climate. However, the major phytoplankton responses are shifted from the Eurasian Basin to the Canadian Basin and the East-Siberian Sea. This is mainly due to the shift of the marginal sea ice zone from the Barents-Kara Sea to the East Siberian-Chukchi Sea in the future.

How to cite: Park, J. H., Kim, S.-J., Lim, H.-G., Kug, J., Yang, E. J., and Kim, B.-M.: Increasing Arctic River Discharge and Its Role for the Phytoplankton Responses in the Present-day and Future Climate Simulations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14677, https://doi.org/10.5194/egusphere-egu23-14677, 2023.

EGU23-1252 | ECS | Orals | CL4.5

Deployment of the global tide and surge model for estimating sea-level trends along the Dutch coast 

Sanne Muis, Natalia Aleksandrova, Fedor Baart, Willem Stolte, and Jelmer Veentra

The monitoring of the sea level trend is important for decision-making in the near-future. For the Dutch coast, the Sea Level Monitor periodically publishes new estimates of the sea level rise trend. This observed trend, based on a selection of Dutch tide gauge stations, is used for the planning and management of our coastal defenses in the next 10-15 years. To estimate the trend in mean sea level, the influence of land subsidence, long-term tidal cycles and storm surges levels need to be removed from the observations.

In this contribution, we focus on the contribution of storm surges, that are driven by variations of atmospheric pressure and wind. We will present an updated methodology to remove the effects of these variations on the sea-level trend, which is based on monthly mean sea levels derived with a depth-averaged hydrodynamic model instead of a linear regression. A fully automated and portable workflow was developed to deploy Global Tide and Surge Model (GTSM) on a high-performance computing cluster. Leveraging recent updates of the ERA5 climate reanalysis, we extent existing GTSM simulations back to 1950 and to present-day. Based on these new simulations, we will discuss the variability in mean sea levels due to atmospheric conditions, and present how the sea-level trend changes due to the improved correction.

How to cite: Muis, S., Aleksandrova, N., Baart, F., Stolte, W., and Veentra, J.: Deployment of the global tide and surge model for estimating sea-level trends along the Dutch coast, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1252, https://doi.org/10.5194/egusphere-egu23-1252, 2023.

EGU23-3929 | ECS | Orals | CL4.5

Investigating the sea level budget in the East China Sea 

Christina Strohmenger, Ziyu Liu, Bernd Uebbing, Jürgen Kusche, Lennart Reißner, Yunzhong Shen, Wei Feng, and Qiujie Chen

Sea level change is not uniform around the globe. We focus on regional sea level change in the East China Sea (ECS), a Western Pacific marginal sea of 770.000 km2, with a densely populated and economically important coastal area. Several challenges arise when investigating past and current sea level change and budgets in this region.

Ocean mass change is observed by GRACE(-FO). However, one needs to account for hydrological signals leaking from land into the ocean, as well as for sediment discharge from rivers. Steric contributions are usually measured by Argo floats, but from the shallow inner shelf of the ECS only few data are available. Thus, ocean reanalyses should be handled with caution. Total sea level change from altimetry can be compared to tide gauge data, but gauges are sparsely distributed in the ECS area and only few stations are co-located with GNSS to account for vertical land motion.

In this contribution, we analyze and compare different data products to better understand regional sea level change and its contributors. Time series of ECS- averaged levels (total from altimetry, mass from GRACE and GRACE-FO and steric from ORAS5 reanalysis) are computed and compared in terms of trend, seasonal amplitudes and correlations. Additionally, spatial patterns are investigated, revealing that the shallow coastal regions, vast continental shelf areas and deep sea areas show distinct characteristic behaviors of sea level change. Altimetry and tide gauge data show a correlation of higher than 70% for 11 of 13 available records. Finally, we compare the individual data sets to results of a joint sea level inversion framework (Uebbing, 2022).

How to cite: Strohmenger, C., Liu, Z., Uebbing, B., Kusche, J., Reißner, L., Shen, Y., Feng, W., and Chen, Q.: Investigating the sea level budget in the East China Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3929, https://doi.org/10.5194/egusphere-egu23-3929, 2023.

EGU23-4017 | ECS | Posters virtual | CL4.5

Global Sea Level Trend, Acceleration and Its Components over 1993-2016 

Fengwei Wang, Yunzhong Shen, Qiujie Chen, and Jianhua Geng

A 24-year global mean barystatic sea level change from January 1993 to December 2016 is derived by the joint use of Tongji-LEO2021 and Tongji-Grace2018 monthly gravity field solutions, with which the global sea level budget is investigated together with altimetry, steric and four mass elements (glaciers, Greenland, Antarctica and land water storage). The derived global mean sea level changes from altimetry, steric and two Tongji solutions generally agree well with each other with three correlation coefficients all higher than 0.90. The results show that the linear trend of global mean sterodynamic sea level change is 2.85±0.30 mm/year from altimetry, close to 2.82±0.19 mm/year of barystatic (1.55±0.15 mm/year) plus steric (1.27±0.12 mm/year) and 2.94±0.13 mm/year of the sum mass contributions (1.67±0.06 mm/year) plus steric, whose misclosure ranges -0.09 to 0.03 mm/year. The acceleration of global mean barystatic sea level change is 0.139±0.019 mm/year2, which is mainly caused by four factors, 0.051±0.002 mm/year2 (~36.7%) by Greenland ice melting, 0.027±0.002 mm/year2 (~19.4%) by Antarctica ice melting, 0.027±0.001 mm/year2 (~19.4%) for other glaciers melting and 0.032±0.010 mm/year2 (~23.0%) for land water storage, respectively. The findings in this study suggested that the global sea level budget was closed from 1993 to 2016 based on altimetry, steric, Tongji solutions and mass elements data.

How to cite: Wang, F., Shen, Y., Chen, Q., and Geng, J.: Global Sea Level Trend, Acceleration and Its Components over 1993-2016, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4017, https://doi.org/10.5194/egusphere-egu23-4017, 2023.

EGU23-4176 | ECS | Posters virtual | CL4.5

Sea level variability across the Northwest Atlantic shelf 

Anrijs Abele, Sam Royston, and Jonathan Bamber

Ocean dynamics plays a prominent role in the change of sea level variability on approach to the coast. While some studies have focused on decadal changes at tide gauges, a gap remains in understanding higher frequency variability, which provides a significant proportion of total variability in the coastal region. The Northwest Atlantic, an area including the U.S. East coast and Atlantic Canada, is a known hotspot of sea level rise and shows spatial differences in lower frequency variability along the shelf. However, the higher frequency variability is rarely explored, despite being at least partly captured by the observation systems.

In this study, we evaluated the sea level variability across the sub-annual timescales on the shelf of the Northwest Atlantic and linked it to the local and far-field ocean dynamics. The drivers of sea level variability include both wind-driven and buoyancy-driven circulation. We used high-frequency tide gauge records, eddy-resolving high-resolution (1/12°) ocean reanalysis, and high-precision synthetic aperture radar (SAR) altimeter along-track data to obtain sea level anomalies for the analysis. We evaluated the coherence of sea level signal for all sources and with the drivers of ocean circulation.

How to cite: Abele, A., Royston, S., and Bamber, J.: Sea level variability across the Northwest Atlantic shelf, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4176, https://doi.org/10.5194/egusphere-egu23-4176, 2023.

EGU23-4834 | ECS | Orals | CL4.5 | Highlight

The Timing of Decreasing Coastal Flood Protection Due to Sea-Level Rise 

Tim Hermans, Victor Malagón-Santos, Caroline Katsman, Robert Jane, Dj Rasmussen, Marjolijn Haasnoot, Gregory Garner, Robert Kopp, Michael Oppenheimer, and Aimée Slangen

Sea-level rise (SLR) amplifies the frequency of extreme sea levels as it raises their baseline height. Projections of the frequency amplification of extremes are often computed for arbitrary future years and relative to the historical centennial event, which is not necessarily meaningful locally. Consequently, such projections may not provide salient information to adaptation planners, as they do not indicate when certain flood risk thresholds will be crossed given the current degree of local coastal flood protection.

To better support adaptation planning, we introduce a framework that extends the emerging timing perspective on sea-level rise to the frequency amplification of extreme sea levels. Moreover, by relating amplification factors to local flood protection standards estimated with the FLOPROS modelling approach, we project the timing of decreases in the local degree of protection. The sea-level rise required for such decreases is derived from extreme sea-level distributions inferred from GESLA3 observations and combined with the relative sea-level projections of the Sixth Assessment Report of the IPCC until 2150 to compute the timing of these decreases at tide gauges globally.

Our central estimates indicate that the estimated degrees of protection will be exceeded 10 times as frequently within the next 30 years (the lead time that large adaptation measures may take) at 26 & 32% of the tide gauges considered, and annually at 4 & 8%, for respectively a low & high emissions scenario (SSP1-2.6 & SSP3-7.0). Even though our results are based on estimated degrees of protection, they highlight that at several locations substantial decreases in the degree of protection may occur before large adaptation measures can be completed. Furthermore, we find that under SSP3-7.0, the same decreases in the degree of coastal protection will occur substantially faster in the future as sea-level rise accelerates. Our projection framework adds a new perspective on the frequency amplifications of extremes that may help adaptation planners to assess the available lead time and useful lifetime of protective infrastructure, given unacceptable decreases in the degree of coastal protection.

How to cite: Hermans, T., Malagón-Santos, V., Katsman, C., Jane, R., Rasmussen, D., Haasnoot, M., Garner, G., Kopp, R., Oppenheimer, M., and Slangen, A.: The Timing of Decreasing Coastal Flood Protection Due to Sea-Level Rise, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4834, https://doi.org/10.5194/egusphere-egu23-4834, 2023.

EGU23-5189 | Orals | CL4.5

The drivers of decadal fluctuation in the global mean sea level rise 

Hyeonsoo Cha, Jae-Hong Moon, Taekyun Kim, and Y. Tony Song

Recent advances in satellite and in-situ measurements have enabled the monitoring of GMSL budget components and provided insights into ocean effects on the Earth’s energy imbalance and hydrology. The global mean sea level rise slowed over the 2000s, which coincides with a global warming hiatus period, but has accelerated again since 2011. This decadal fluctuation in GMSL rise can be attributed to climate-related fluctuation in ocean heat and mass change. Sea level and Earth’s energy budget results demonstrate that the decadal climate variability has resulted in ocean mass loss and decreased ocean heat uptake, slowing the GMSL rise rate during the 2000s. After ~2011, the climate-driven fluctuations of ocean mass, heat, and GMSL rise rate were reversed. This result highlights the importance of natural variability in understanding the ongoing sea-level rise.

How to cite: Cha, H., Moon, J.-H., Kim, T., and Song, Y. T.: The drivers of decadal fluctuation in the global mean sea level rise, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5189, https://doi.org/10.5194/egusphere-egu23-5189, 2023.

EGU23-5341 | ECS | Posters on site | CL4.5

Coherent modes of coastal sea level variability from altimetry and tide gauge observations 

Julius Oelsmann, Francisco M. Calafat, Marcello Passaro, Chris Piecuch, Kristin Richter, Anthony Wise, Felix Landerer, Caroline Katsman, Chris Hughes, and Svetlana Jevrejeva

Sea level dynamics in the coastal zone can differ significantly from that in the open ocean. The presence of the continental slope, shallow waters and the coastlines give rise to a variety of processes that mediate the response of coastal sea level to open-ocean changes and produce distinct spatiotemporal sea level patterns. Yet how exactly this interplay occurs and, more importantly, the extent to what coastal sea level variations differ from open-ocean variability remain poorly understood. In this work, we use coastal altimetry observations in combination with tide gauge data to determine patterns of coherent coastal sea level variations and the degree of decoupling between such variations and open-ocean changes.

In a first step, we apply Bayesian mixture models to identify clusters of correlated tide gauge observations that explain a significant fraction of the coastal sea level variability. Using altimetry data, we find high regional coherency of along-shore coastal sea level variations, indicating common underlying mechanisms that cause these correlations.

In light of previous research, we confirm that the correlation structures of these coherent patterns are often confined to the continental slopes, particularly in extratropical regions. In regions like the northeastern US continental shelf, correlations decrease with increasing water depth, indicating a decoupling of shelf sea and open-ocean variability. We investigate how these differences between coastal and open ocean sea level variations change as a function of time scale, i.e., from monthly or interannual variations to long-term trends, and validate these results against tide gauge observations. We derive across-shore correlation length scales that provide insights into the space scales of coastal sea level dynamics and are useful to understand how well gridded products can resolve such processes.

We discuss possible causes of the coherent sea level fluctuations, such as wind forcing, coastally trapped waves, and large scale climate modes. The results motivate further research to better understand the driving mechanisms behind these coherent sea level variations, as well as the pathways linking remote forcing to coastal changes.

How to cite: Oelsmann, J., Calafat, F. M., Passaro, M., Piecuch, C., Richter, K., Wise, A., Landerer, F., Katsman, C., Hughes, C., and Jevrejeva, S.: Coherent modes of coastal sea level variability from altimetry and tide gauge observations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5341, https://doi.org/10.5194/egusphere-egu23-5341, 2023.

EGU23-6990 | ECS | Orals | CL4.5

Removing Internal Variability as a Means of Improving Regional Emulation of Ocean Dynamic Sea-Level Change 

Víctor Malagón-Santos, Aimée B.A. Slangen, Tim H.J. Hermans, Sönke Dangendorf, Marta Marcos, and Nicola Maher

Regional emulation tools based on statistical relationships, such as pattern scaling, provide a computationally inexpensive way of projecting ocean dynamic sea-level change for a broad range of climate change scenarios. Such approaches usually require a careful selection of one or more predictor variables of climate change so that the statistical model is properly optimized. Even when appropriate predictors have been selected, spatiotemporal oscillations driven by internal climate variability can be a large source of model disagreement. Using pattern recognition techniques that exploit spatial covariance information can effectively reduce internal variability in simulations of ocean dynamic sea level, significantly reducing random errors in regional emulation tools. Here, we test two pattern recognition methods based on Empirical Orthogonal Functions (EOF), namely signal-to-noise maximising EOF pattern filtering and low-frequency component analysis, for their ability to reduce errors in pattern scaling of ocean dynamic sea-level change. These two methods are applied to an initial-condition large ensemble (MPI-GE), so that its externally forced signal is optimally characterized. We show that pattern filtering provides an efficient way of reducing errors compared to other conventional approaches such as a simple ensemble average. For instance, filtering only two realizations by characterising their common response to external forcing reduces the random error by almost 60%, a reduction level that is only achieved by averaging at least 12 realizations. We further investigate the applicability of both methods to single realization modelling experiments, including four CMIP5 simulations for comparison with previous regional emulation analyses. Pattern scaling leads to a varying degree of error reduction depending on the model and scenario, ranging from more than 20% to about 70% reduction in global-mean mean-squared error compared with unfiltered simulations. Our results highlight the relevance of pattern recognition methods as a means of reducing errors in regional emulation tools of ocean dynamic sea-level change, especially when one or a few realizations are available.

How to cite: Malagón-Santos, V., Slangen, A. B. A., Hermans, T. H. J., Dangendorf, S., Marcos, M., and Maher, N.: Removing Internal Variability as a Means of Improving Regional Emulation of Ocean Dynamic Sea-Level Change, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6990, https://doi.org/10.5194/egusphere-egu23-6990, 2023.

EGU23-7227 | Orals | CL4.5 | Highlight

Contribution of subsidence on relative sea level in Europe 

Rémi Thiéblemont, Gonéri Le Cozannet, Daniel Raucoules, Jérémy Rohmer, Guy Wöppelmann, Floris Calkoen, and Robert J. Nicholls

While the understanding and modelling of relative sea level rise (SLR) due to ocean density and mass changes have greatly improved over the past few decades, SLR contributions due to vertical ground motions (VGMs) remain a major source of uncertainty. Here, VGMs relate to ground motions that have imprints of a few kilometers, as opposed to broad scale land motion such as Glacial Isostatic Adjustment (GIA). VGMs are caused by processes such as natural resource extraction or the load of anthropogenic infrastructure on recent sediment deposits or natural processes (e.g. sismotectonics, volcanism, landslide), all of which vary in space and time, and can strongly inflate SLR locally.

Here, we present a pan-European analysis of relative sea-level changes in Europe considering VGMs based on trends retrieved from the European Ground Motion Service (EGMS). EGMS allows identifying hot spots of robust subsidence along the European coastline such as the north Adriatic coast in Italy, areas such as Palavas (France), Groningen (Netherlands) and many coastal infrastructures such as dikes in La Rochelle (France) where subsidence was not documented earlier. Hence the service delineates where subsidence can have a significant impact to relative sea-level changes in coastal areas. This satisfies a major need from coastal adaptation stakeholders concerned with SLR. EGMS results are complemented and compared with VGMs estimates from permanent Global Navigation Satellite System (GNSS) network stations. The precision of the measurements is discussed: VGMs from GNSS stations derived from 4 different solutions (ULR, NGL, JPL and GFZ) allow accounting for uncertainty in trends estimation techniques. We estimate VGMs residual trends after removing the effect of the GIA from geophysical modelling, but also the effect of contemporary mass redistribution on solid Earth deformation. The results from both GNSS and EGMS suggest that the precision of ground motion velocities can be in the order of a millimetre per year.

Overall, these estimates and their uncertainty can be used to produce a new coastal pan-European relative sea-level set of projections that respond to one major user need, namely the identification of areas where sea level rise is amplified by subsidence. However two other user needs remain unachieved: the local attribution of observed sea-level changes to components with a submillimetric per year accuracy and a quantified projection of subsidence, which would at least require subsidence models.    

How to cite: Thiéblemont, R., Le Cozannet, G., Raucoules, D., Rohmer, J., Wöppelmann, G., Calkoen, F., and Nicholls, R. J.: Contribution of subsidence on relative sea level in Europe, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7227, https://doi.org/10.5194/egusphere-egu23-7227, 2023.

EGU23-7585 | Posters on site | CL4.5

Assessing sea-level change of the last 300 years using tide gauge and proxy records 

Fiona D. Hibbert, Marta Marcos, Andrew Valentine, Ed Garrett, and W. Roland Gehrels

Detailed sea-level budgets are now available for the 20th and 21st centuries, but separating the differing contributions of sea-level rise prior to 1900 remains difficult, in part due to additional temporal and vertical uncertainties associated with proxy records, and the spatially variable nature of driving processes.

We present tide gauge and proxy reconstructions of sea level since 1700, and analyse their structure using Gaussian process modelling which allows for continuous reconstructions with fully quantified uncertainties. This enables the timing of accelerations, magnitude and rates of change to be determined, and in turn enables site-specific sea-level budgets to be derived. The contribution of different driving mechanisms (e.g., glacio-isostatic adjustment and sterodynamic changes) for each site is assessed, and the evolution of the barystatic contribution for the last 300 years is evaluated.

How to cite: Hibbert, F. D., Marcos, M., Valentine, A., Garrett, E., and Gehrels, W. R.: Assessing sea-level change of the last 300 years using tide gauge and proxy records, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7585, https://doi.org/10.5194/egusphere-egu23-7585, 2023.

EGU23-8047 | ECS | Posters on site | CL4.5

Monthly sea level fingerprints from 1992-2017, utilising ESA CCI Essential Climate Variables in an ensemble modelling framework 

Stephen Chuter, Andrew Zammit-Mangion, Jonathan Bamber, and Jérôme Benveniste

Sea level rise is one of the greatest socio-economic impacts of climate change in the 21st Century. Whilst global mean sea level is an essential climate variable (ECV) for assessing the integrated response of the Earth system to climate change, regional sea level variability is of primary concern for policy-making decisions and the development of adaptation strategies in coastal localities. Redistribution of terrestrial mass, in the form of hydrological and land ice mass fluxes, partly drives this regional sea level variability due to its impact on the Earth’s gravity, rotation and deformation (GRD), termed ‘Sea Level Fingerprints’ or Barystatic-GRD fingerprints. With increasing mass losses projected from ice sheets and glaciers over the coming centuries, the magnitude and relative contribution of these Barystatic-GRD fingerprints to regional sea level change are expected to increase. As a result, accurately quantifying this phenomenon and its uncertainty is critical when assessing contemporary and future regional sea level variability.

Current contemporary Barystatic-GRD fingerprints are typically either calculated using a single mass loading observation source or provide discontinuous coverage since 1992 (the satellite altimetry era). Here, we present a continuous monthly Barystatic-GRD fingerprint product from 1992-2017, computed from an ensemble of mass loadings derived from differing observation techniques. To achieve this, we use the Ice Sheet and Sea Level Model (ISSM) sea level equation solver, which uses a finite element approach to solving the sea level equation at high spatial-temporal resolution, whilst maintaining computational efficiency. This enables us to use an ensemble modelling framework, ensuring the computed Barystatic-GRD fingerprint encompasses the variability between differing observation techniques. Additionally, it allows us to propagate the observation uncertainties into the fingerprint uncertainty in a robust manner. As well as the total Barystatic-GRD fingerprint, we assess the contribution of individual terrestrial components (Antarctica, Greenland, Glaciers, and hydrological mass change). This work is part of the Fingerprinting Approach to Close Regional Sea Level Budgets using ESA-CCI (FACTORS), a European Space Agency Climate Change Initiative Research Fellowship.

How to cite: Chuter, S., Zammit-Mangion, A., Bamber, J., and Benveniste, J.: Monthly sea level fingerprints from 1992-2017, utilising ESA CCI Essential Climate Variables in an ensemble modelling framework, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8047, https://doi.org/10.5194/egusphere-egu23-8047, 2023.

EGU23-8186 | Orals | CL4.5

Changes in extreme sea levels along the North Atlantic coasts, over the last century 

Lucia Pineau-Guillou, Pascal Lazure, Guy Wöppelmann, Jean-Baptiste Roustan, and Markus Reinert

Extreme sea levels are the joint contribution of mean sea level, tide and storm surges. The ClimEx project investigates changes in tide and storm surges over the last century, along the North Atlantic coasts. Concerning the tide, we investigated the long-term changes of the principal tidal component M2, from 1846 to 2018 (Pineau-Guillou et al., 2021). The M2 variations are consistent at all the stations in the North-East Atlantic. The changes started long before the 20th century and are not linear. Regarding the possible causes of the observed changes, the similarity between the North Atlantic Oscillation and M2 variations in the North-East Atlantic suggests a possible influence of the large-scale atmospheric circulation on the tide. A possible underlying mechanism is discussed. Concerning the storm surges, we found a clear shift in the storm surge season at Brest (France), between 1950 and 2000 (Reinert et al., 2021). Extreme storm surge events occurred three weeks earlier (mid-December instead of beginning of January) in the winter 2000 than in the 1950s. Analysis of additional stations in Europe reveals a large-scale process (Roustan et al., 2022). Temporal shifts are positive (later events) in northern Europe, and negative (earlier events) in southern Europe. Such a tendency is similar to the one already reported for European river floods between 1960 and 2010 (Blöschl et al., 2017).

 

References

[1] Pineau-Guillou L., Lazure P. and Wöppelmann G. (2021). Large-scale changes of the semidiurnal tide along North Atlantic coasts from 1846 to 2018. Ocean Sci., 17, 17–34. https://doi.org/10.5194/os-17-17-2021

[2] Reinert M., Pineau-Guillou L., Raillard N., Chapron B. (2021). Seasonal shift in storm surges at Brest revealed by extreme value analysis. J. Geophys. Res. Oceans, 126, e2021JC017794. https://doi.org/10.1029/2021JC017794

[3] Roustan J.-B., Pineau-Guillou L., Chapron B., Raillard N., Reinert M. (2022). Shift of the storm surge season in Europe due to climate variability. Sci. Rep., 12, 8210. https://doi.org/10.1038/s41598-022-12356-5

[4] Blöschl G., Hall J., Parajka J., Perdigão R. A. P., Merz B., Arheimer B. et al. (2017). Changing climate shifts timing of European floods. Science, 357(6351), 588–590. https://doi.org/10.1126/science.aan2506

How to cite: Pineau-Guillou, L., Lazure, P., Wöppelmann, G., Roustan, J.-B., and Reinert, M.: Changes in extreme sea levels along the North Atlantic coasts, over the last century, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8186, https://doi.org/10.5194/egusphere-egu23-8186, 2023.

EGU23-9023 | ECS | Posters on site | CL4.5

Enhancing projections of sea-level rise with changing seasonality 

Daisy Lee-Browne, Luke Jackson, Pippa Whitehouse, and Sophie Williams

There is evidence to show that anthropogenically-driven climate change will alter large-scale atmospheric circulation in the future. However, limited research has been conducted to explore how these atmospheric changes will impact seasonal sea-level change. The majority of global to local sea-level projections are made on multi-annual timescales, meaning important sub-annual changes in sea level driven by climatic oscillations are not being accounted for. Sea level on the Northwestern European Shelf (NWES) has been shown to vary in response to fluctuations in the North Atlantic Oscillation (NAO). We examine how seasonal sea level may change on the NWES in response to changes in the NAO in the near future (2023-2053). The work uses a statistical approach that incorporates the inverse barometer effect to produce projections of seasonal sea-level change. The main objectives include quantifying the sensitivity of sea level to the NAO over the 20th century using tide gauge and satellite altimetry data in combination with historical records of the NAO index. Projections of mean sea-level change are then updated to account for seasonal variability that may occur on the NWES using CMIP5 and CMIP6 model outputs of sea-level change and the NAO for the period 2023-2053. The research aims to improve understanding of short-term drivers of future sea-level change and explore the ability of a statistical method to accurately detect and project seasonal patterns.

How to cite: Lee-Browne, D., Jackson, L., Whitehouse, P., and Williams, S.: Enhancing projections of sea-level rise with changing seasonality, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9023, https://doi.org/10.5194/egusphere-egu23-9023, 2023.

EGU23-9181 | ECS | Posters on site | CL4.5

Understanding Regional Sea Level Rise Acceleration Along the North American Eastern Seaboard 

Victoria Schoenwald and Ben Kirtman

The East Coast of North America has experienced rates of sea level rise (SLR) five times larger than the global average. This steep increase in SLR contributed to a higher frequency of coastal flooding events along the southeastern seaboard and the worst nuisance flooding event in Miami, FL during the last 20 years. Using tide gauge data from several stations, empirical mode decomposition (EMD) was used to understand sea level variability along the East Coast of the U.S., and its connectivity to atmospheric and oceanic circulation and thermosteric effects. This is a unique approach in identifying the “in phase” sea level variability and how it relates to the atmosphere and the ocean on varying timescales. The EMD modes were also used to understand the “out of phase” components of sea level variability such as the “hot spot” of SLR between Cape Hatteras, NC and Key West, FL where sea levels increased at rates of 25.5mm/year compared to a global average of 4.5 mm/year. Similar techniques were then applied to climate model simulations using sea surface height at coastal locations as proxies for the tide gauge data. The EMD approach was applied at both ocean eddy parameterized and ocean eddy resolving scales. The goal was to determine if the natural variability in the models have similar characteristics to the observational estimates. And, to assess whether the modes associated with the trend in observations have appropriate analogues to the model simulations. By comparing pre-industrial simulations with historical simulations, we will be assessing whether a changing climate affects the natural variability.

How to cite: Schoenwald, V. and Kirtman, B.: Understanding Regional Sea Level Rise Acceleration Along the North American Eastern Seaboard, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9181, https://doi.org/10.5194/egusphere-egu23-9181, 2023.

EGU23-9831 | Orals | CL4.5 | Highlight

Unraveling Regional Patterns of Sea Level Change over the Altimeter Era 

R. Steven Nerem, Kristopher Karnauskas, John Fasullo, and Benjamin Hamlington

Satellite altimeters have measured the global mean and regional patterns of sea level change since 1993 with impressive detail and precision. While the global mean rate of sea level rise has been studied extensively and is readily linked to global water budgets, the regional patterns (or deviations from the global mean) are subject to diverse physical mechanisms that span the gauntlet of internal climate dynamics, and models suggest a nuanced relationship to radiative forcing (greenhouse gases, aerosols, etc.). To date, little attempt has been made to synthesize the regional patterns of sea level change across the global ocean with a common diagnostic framework. Here we combine oceanic and atmospheric observations and leverage ensembles of a state-of-the-art global climate model to unravel the mechanisms governing the basin-scale patterns of sea level change around the world ocean. By applying some bedrock principles of physical oceanography and coupled dynamics, we find a leading role for wind forcing—Ekman and Sverdrup dynamics together yield faithful reproductions of the large-scale structure of sea level change from the tropics to the midlatitudes. We argue that the global pattern of sea level rise since 1993 is set, to leading order, by changes in the wind-driven ocean circulation and their influence on sea surface height via ocean heat divergence. Importantly, wind-driven needn’t be synonymous with internal variability—indeed, much of the observed global pattern is recovered by global climate models subject to historical anthropogenic forcings, and single-forcing experiments enable further insight into which forcings are responsible for which regional phenomena. As we move forward into the uncertain future, a better understanding of the causes of regional rates of sea level rise, including distinguishing which features are driven by human activities versus modes of natural variability—or both, is critical for the successful adaptation of humanity and its infrastructure to a rapidly changing climate.

How to cite: Nerem, R. S., Karnauskas, K., Fasullo, J., and Hamlington, B.: Unraveling Regional Patterns of Sea Level Change over the Altimeter Era, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9831, https://doi.org/10.5194/egusphere-egu23-9831, 2023.

EGU23-10492 | ECS | Posters on site | CL4.5

Projection of local sea-level rise under CMIP6 scenarios (SSP1-2.6, SSP5-8.5) in the Northwestern Pacific marginal seas using dynamical downscaling.  

Yu-Kyeong Kang, Yang-Ki Cho, Yong-Yub Kim, Bong-Kwan Kim, Gwang-Ho Seo, Seok-Jae Kwon, and Hyun-Ju Oh

The global mean sea level has been rising with an acceleration since the twentieth century. Sea level rise is not spatially uniform but shows large regional variation. Local sea level can change due to various physical processes like changes in ocean circulation, atmospheric pressure, and mass redistribution. Projections of global sea level changes are available from the Coupled Model Intercomparison Project Phase 6 (CMIP6) database. However, Global climate models (GCMs) are limited in simulating spatially non-uniform sea level rise in marginal seas due to their coarse resolution and the absence of rivers and tides. High-resolution regional ocean climate models (RCMs) that consider tides and rivers were used to address these limitations in the Northwestern Pacific (NWP) marginal seas through dynamical downscaling. Four GCMs were selected for dynamical downscaling based on a performance evaluation of SST and the SSH along the RCM boundaries. A regional model with high resolution (1/20°) was simulated to project spatially non-uniform changes in the sea level under two CMIP6 scenarios (SSP1-2.6 and SSP5-8.5) from 2015 to 2100. Sea level rise in the NWP marginal seas was ~82 cm under SSP5-8.5 scenario and ~47 cm under SSP1-2.6 scenario, respectively. Under both scenarios, the predicted local sea-level rise was higher in the East/Japan Sea (EJS), where the currents and eddy motions are active, than in the Yellow and East China Seas.

 

How to cite: Kang, Y.-K., Cho, Y.-K., Kim, Y.-Y., Kim, B.-K., Seo, G.-H., Kwon, S.-J., and Oh, H.-J.: Projection of local sea-level rise under CMIP6 scenarios (SSP1-2.6, SSP5-8.5) in the Northwestern Pacific marginal seas using dynamical downscaling. , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10492, https://doi.org/10.5194/egusphere-egu23-10492, 2023.

EGU23-10639 | ECS | Orals | CL4.5

Updating sea-level reconstruction since 1900 

Jinping Wang, John Church Church, Xuebin Zhang, and Xianyao Chen

Sea-level rise integrates the responses of several components (ocean thermal expansion, mass loss from glaciers and ice sheets, terrestrial water storage). Before the satellite era, global sea-level reconstructions depend on tide-gauge records and ocean observations. However, the available global mean sea level (GMSL) reconstructions using different methods indicate a spread in sea-level trend over 1900-2008 (1.3~2.0 mm yr-1). With the improved understanding of the causes of sea-level change, here we update the original Church and White (2011) reconstruction by using the latest observations, taking the time-evolving sea-level fingerprint, sterodynamic sea level (SDSL) climate change pattern and local vertical land motion (VLM) into account. The updated trend of GMSL of 1.6 ± 0.2 mm yr-1 (90% confidence level) over 1900-2019 is consistent with the sum of contributions of 1.5 ± 0.2 mm yr-1, slightly lower than 1.8 ± 0.2 mm yr-1 from original reconstruction. The lower trend from the updated reconstruction is mainly due to including residual VLM correction. The trends at tide gauge locations from updated reconstruction agree better with the tide gauge observations, with comparable mean trend of 1.7 mm yr-1 (standard deviation; STD of 2.0 mm yr-1) from observation and 1.7 mm yr-1 (STD of 1.2 mm yr-1) from the updated reconstruction. The inclusion of sea-level fingerprint and SDSL climate change pattern are the dominant contributors for improved reconstruction skill on regional scales at tide gauge locations. This update leads to GMSL solution that are consistent with other reconstructions in terms of long-term trend and 30-year running rate.

How to cite: Wang, J., Church, J. C., Zhang, X., and Chen, X.: Updating sea-level reconstruction since 1900, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10639, https://doi.org/10.5194/egusphere-egu23-10639, 2023.

EGU23-10695 | Orals | CL4.5

Causal Mechanisms of Sea Level Variations along the U.S. West Coast 

Ian Fenty, Ou Wang, and Ichiro Fukumori

Tide-gauge records along the U.S. West Coast since the mid-1920’s show large ENSO-correlated sea-level variability and a below-average linear trend relative to the global mean over the past three decades. On weekly and longer timescales, sea-level variations in the region are primarily steric, reflecting variations in coastal ocean temperatures rather than that of mass. Previous research into sea-level variability in the region identified coastally-trapped waves forced by nonlocal winds as the main source of long-lasting sea-level variability. Here we offer a rigorous quantification of the contributions of wind-stress and buoyancy forcing anomalies across the entire Pacific Basin on the U.S. West Coast Sea level using a global data-constrained ocean and sea-ice model of the Estimating the Circulation and Climate of the Ocean (ECCO) consortium. Causal relationships are quantified using the model’s adjoint and mechanisms are elucidated via perturbation experiments.

By convolving the adjoint sensitivities with atmosphere forcing anomalies we find that long-term (>1 week) sea level variations along the U.S. West Coast are almost entirely due to wind-stress anomalies while buoyancy anomalies, in contrast, contribute virtually nothing. Interestingly, the wind stress anomalies that contribute to sea level variations in the region come from two sectors: i) a coastally-confined region from 0-45N and ii) and the open-ocean Pacific equatorial waveguide (roughly -/+ 10 degrees latitude). Wind stress anomalies in the coastally-confined sector induce coastally-trapped waves which propagate poleward, depress the thermocline, reduce upwelling/air-sea heat loss and, thereby, lead to positive ocean temperature / steric height anomalies. Zonal wind stress anomalies in the equatorial waveguide induce eastward-propagating equatorial Kelvin waves, some energy of which is converted to coastally-trapped waves upon reaching continent, which lead to positive steric height anomalies following the same causal chain.

This study highlights the benefits of applying the complimentary tools of adjoint-based convolution and perturbation experiments to explain the origin of regional sea-level anomalies.

How to cite: Fenty, I., Wang, O., and Fukumori, I.: Causal Mechanisms of Sea Level Variations along the U.S. West Coast, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10695, https://doi.org/10.5194/egusphere-egu23-10695, 2023.

EGU23-10796 | Posters on site | CL4.5 | Highlight

A worst case extreme sea levels along the global coastline by 2100 

Svetlana Jevrejeva, Joanne Williams, Michalis Vousdoukas, and Luke Jackson

We calculate the magnitude of a worst case scenario for extreme sea levels along the global coastline by 2100. Our worst case scenario for extreme sea levels is a combination of sea surface height associated with storm surge and wave (100-year return period, the 95th percentile), high tide (the 95th percentile) and a low probability sea level rise scenario (the 95th percentile). We show that by 2100 extreme sea levels have a 5% change of exceeding 4.2 m (global coastal average), compared to 2.6 m during the baseline period (1980-2014). Up to 90% of increases in magnitude of extreme sea levels are driven by future sea level rise, compare to 10% associated with changes in storm surges and waves. By 2030-2040 the present-day 100-year return period for extreme sea levels would be experienced at least once a year in tropical areas. This 100-fold increase in frequency will take place on all global coastlines by 2100. Future changes in magnitude and frequency of extreme sea levels undermine the resilience of coastal communities and ecosystems, considering that sea level rise will increase the magnitude, frequency of extreme sea levels and will reduce the time for post-event recovery.

 

How to cite: Jevrejeva, S., Williams, J., Vousdoukas, M., and Jackson, L.: A worst case extreme sea levels along the global coastline by 2100, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10796, https://doi.org/10.5194/egusphere-egu23-10796, 2023.

EGU23-11788 | Orals | CL4.5

Constraining ocean dynamic sea level projections along the coast of the Netherlands 

Dewi Le Bars, Iris Keizer, Franka Jesse, and Sybren Drijfhout

Ocean dynamic sea level (ODSL) is the local height of the sea surface above the geoid. It is computed by atmosphere-ocean coupled general circulation models from the coupled model intercomparison projects (CMIP). In many places it is one of the most important components of sea level projections for the coming century. However, because it depends on climate dynamics, there is a low agreement between climate models. Moreover, the difficulty to estimate ODSL from observations has resulted in IPCC AR5 and AR6 sea level projections using CMIP5 and CMIP6 outputs without model selection nor bias correction.

 

We use multiple lines of evidence to constrain ODSL along the coast of the Netherlands: ocean reanalyzes, sea-level budget closure using tide gauges and satellite altimetry observations, and direct integration of steric sea level change from observed temperature and salinity together with an estimation of wind influence on sea level.

 

We find that CMIP6 overestimates ODSL change along the Dutch coast and that this overestimation is not only related to the overestimation of global mean temperature increase. Based on the emergent constraint framework, we provide improved ODSL projections with reduced uncertainty and an increased level of confidence.

How to cite: Le Bars, D., Keizer, I., Jesse, F., and Drijfhout, S.: Constraining ocean dynamic sea level projections along the coast of the Netherlands, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11788, https://doi.org/10.5194/egusphere-egu23-11788, 2023.

EGU23-11825 | ECS | Posters on site | CL4.5

Characterization of changes in extreme storm surges along the North Atlantic coasts, since 1850 

Julie Cheynel, Lucia Pineau-Guillou, and Pascal Lazure

Severe storms that hit the North Atlantic coasts over the last decades, such as Xynthia storm in Europe, showed the vulnerability of coastal populations to extreme sea levels. There is a need to quantify the changes in extreme sea levels, to enable the implementation of appropriate coastal adaptation measures. Extreme sea levels are the joint contribution of mean sea level, tide and storm surges. Several authors investigated changes in storm surges. Storm surges display strong interannual and multidecadal variability, but no clear long-term trends at most sites globally (Mawdsley and Haigh, 2016; Marcos and Woodworth, 2017). The objective of the present study is to characterize changes in extreme storm surges along the North Atlantic coasts, since 1850. We selected long-term tide gauges with at least 100 years of data, from GESLA-3 dataset (Haigh et al., 2022). This conducted to consider around 30 tide gauges along the U.S. and European coasts. Extreme storm surges were evaluated yearly, using different approaches: (1) the maximum value over a period (e.g. annual maximum), the n-th percentile (e.g. 99th percentile) and (3) the return level associated to a return period (e.g. 1 year return level); this last value is obtained by fitting a Generalized Extreme Value distribution on data. At each station, we characterized changes in extreme storm surges over the last century. We compared the different approaches. We estimated long-term trends and analyzed storm surge variability in link with large-scale atmospheric forcing (e.g. North Atlantic Oscillation index). Regions of similar variations were also identified. These results are a first step towards the understanding of the physical causes behind the observed changes of extreme storm surges in the North Atlantic.

 

References

[1] Marcos, M. & Woodworth, P. L (2017). Spatiotemporal changes in extreme sea levels along the coast of the North Atlantic and the Gulf of Mexico. J. Geophys. Res. Oceans 122, 7031–7048. https://doi.org/10.1002/2017JC013065

[2] Mawdsley R. J. and Haigh I. D. (2016). Spatial and Temporal Variability and Long-Term Trends in Skew Surges Globally. Front. Mar. Sci. 3:29. https://doi.org/10.3389/fmars.2016.00029

[3] Haigh I. D., Marcos M., Talke S. A., Woodworth P. L., Hunter J. R., Hague B. S., et al. (2022). GESLA Version 3: A major update to the global higher-frequency sea-level dataset. Geosci. Data J., 00, 1–22. https://doi.org/10.1002/gdj3.174

 

How to cite: Cheynel, J., Pineau-Guillou, L., and Lazure, P.: Characterization of changes in extreme storm surges along the North Atlantic coasts, since 1850, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11825, https://doi.org/10.5194/egusphere-egu23-11825, 2023.

EGU23-12782 | Orals | CL4.5

Sources and sinks of interannual steric sea level variability 

Antoine Hochet, William Llovel, Florian Sévellec, and Thierry Huck


It is now well established that sea level rise is not uniform and presents large deviations from its global mean trend. 
Indeed, some regions such as the western Pacific ocean or the Indian ocean experience a linear rise 3 times larger than the global mean sea level trend since 1993 (Cazenave and Llovel, 2010; Llovel and Lee, 2015).
Superimposed to the long-term trend, the interannual variability may enhance or reduce sea level change over a shorter time period (few months). It is well known that these variations are linked to the interannual variability of the steric sea level driven by natural modes of climate variability such as El Nino Southern Oscillation (in the tropical Pacific ocean) and the Indian Ocean Dipole (in the north Indian ocean, Llovel et al., 2010). Therefore, investigating the mechanisms of interannual variability of steric sea level appears to be highly relevant for understanding processes at play in regional sea level variability. 

In this work, we investigate the local sources and sinks of interannual steric sea level variability using the ECCOv4 (Estimating the Circulation and Climate of the Ocean, Forget et al., 2015) state estimate over 1993-2014. We find that the variability is, in almost all regions, sustained by interannual fluctuating winds via Ekman transport and damped by both interannual variations of the net heat flux from the atmosphere and by the rectification effect of subannual oceanic circulation. 

This method allows not only the identification of the physical process at play in the interannual steric sea level variability, but also if the latter is a source or a sink of the interannual steric sea level variability. This method presents evident advantages especially to assess the reliability of coupled climate models used to predict future sea level changes.

How to cite: Hochet, A., Llovel, W., Sévellec, F., and Huck, T.: Sources and sinks of interannual steric sea level variability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12782, https://doi.org/10.5194/egusphere-egu23-12782, 2023.

EGU23-14558 | Posters on site | CL4.5

Observation-Consistent Nonlinear Ice Melt Contribution to Sea Level Rise and its Implications for Sea-Level Projections 

Sandy Avrutin, Philip Goodwin, Ivan D Haigh, and Robert Nicholls

Sea level rise is a major result of climate change that threatens coastal communities and has the potential to incur significant economic damage. Projecting sea level rise as temperatures rise is therefore crucial for policy and decision-making.

The two modelling methods currently used to project future sea level change are process-based and semi-empirical. Process-based models rely on combining outputs from coupled atmosphere/ocean models for each component of sea level rise. Semi-empirical models calculate sea level as an integrated response to either warming or radiative forcing, using parameters constrained from past observations.

Historically, there is disagreement in sea-level projections between different modelling methods. One source of the discrepancies is uncertainty in land ice response to warming; although nonlinearities exist within processes affecting this response, most existing semi-empirical models treat the relationship between warming and ice-melt as linear.

Non-linear ice melt processes may have not yet affected the observational record (such as tipping points as future warming crosses some threshold) or may have already occurred (such as non-linear effects that apply across all levels of warming, or for which the threshold has already been passed). Here, we examine the effect on semi-empirical projections of sea level rise of nonlinearities in ice melt that have already affected the observed sea level record, by adding a nonlinear term to the relationship between warming and the rate of sea level rise within a large ensemble of historically constrained efficient earth systems model simulations.

Projections reach a median sea level rise of 1.3m by 2300 following SSP245, and 2.6m by 2300 following SSP585. Results suggest that nonlinear interactions can be sub-linear, super-linear or 0, with a mainly symmetrical distribution. This includes high-impact, low-probability super-linear interactions that lead to significantly larger high-end sea level rise projections than when nonlinear interactions are not included. It is key to note that nonlinear interactions that have not yet occurred but that may occur in the future, are not considered – these will lead to an increased projection of sea level rise.

How to cite: Avrutin, S., Goodwin, P., Haigh, I. D., and Nicholls, R.: Observation-Consistent Nonlinear Ice Melt Contribution to Sea Level Rise and its Implications for Sea-Level Projections, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14558, https://doi.org/10.5194/egusphere-egu23-14558, 2023.

EGU23-15613 | ECS | Orals | CL4.5

Sensitivity of the Antarctic Ice Sheet evolution to different Earth structures using a coupled 3D GIA - ice-sheet model under different future climate scenarios 

Caroline van Calcar, Jorge Bernales, Tijn Berends, Wouter van der Wal, and Roderik van de Wal

The projected decay of the Antarctic Ice Sheet (AIS) over the coming centuries will lead to uplift of the Earth's surface due to Glacial Isostatic Adjustment (GIA). GIA slows down grounding line migration and therefore has a stabilizing effect on the ice sheet evolution. GIA acts on timescales of decades to centennial depending on the magnitude of the mantle viscosity. The mantle viscosity is several orders of magnitude higher in East Antarctica than in West Antarctica and varies with one order of magnitude within West Antarctica. Studies of the AIS evolution over the last glacial cycle have shown that including lateral variations of the Earth's mantle viscosity can lead to 1.5-kilometer thicker ice in West Antarctica at present day. However, current projections do not include GIA, or they use a laterally homogeneous GIA model. One study applied a uniform high mantle viscosity under East Antarctica and a uniform low mantle viscosity under West Antarctica and showed that, on longer timescales of hundreds of years, mass loss projections of Antarctica may be underestimated because spatially uniform GIA models overestimate the stabilizing effect of GIA across East Antarctica. We developed a coupled GIA - ice-sheet model using the ice-sheet model IMAU-ICE, and a 3D GIA finite element model that includes lateral mantle viscosity variations, and a seismic model to determine the patterns of the viscosity. The results of projections for two IPCC scenarios show that including lateral variations in the Earth's mantle viscosity leads to local ice thickness differences of up to 600 meters in West Antarctica  2300. The results underline and quantify the importance of including this local feedback effect in ice-sheet models when projecting the long-term sea level contribution from Antarctica.

How to cite: van Calcar, C., Bernales, J., Berends, T., van der Wal, W., and van de Wal, R.: Sensitivity of the Antarctic Ice Sheet evolution to different Earth structures using a coupled 3D GIA - ice-sheet model under different future climate scenarios, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15613, https://doi.org/10.5194/egusphere-egu23-15613, 2023.

EGU23-15625 | ECS | Orals | CL4.5

Wind-driven currents and sea-level variability of the northwest European shelf 

Samuel T. Diabaté, Neil J. Fraser, and Gerard D. McCarthy

The shelf northwest of Europe is home to subinertial fluctuations in sea level, whose peak-to-peak amplitude reach several tens of centimetres. These weekly-to-monthly shelf-wide sea-level variations feature at the coast, and therefore understanding their drivers is of prime importance for coastal adaptation. These sea-level changes have been previously hypothesized to reflect the strength of the European slope current (Chafik et al., 2017), a wind and density driven quasi-barotropic circulation lying in the region of the 500 to 1000 m isobaths (Huthnance & Gould, 1989). This interpretation has however not yet been validated by in-situ observations.

 

Using data from single-point current-meters and acoustic Doppler current profilers moored west of France, Ireland and Scotland, we show that the common mode of northwest European sea-level changes covaries with along-isobath currents on the shelf and on the upper part of the slope (< 400 m of water depth). However, the pattern of variability is different in the slope current and further off-shelf, , with the correlations between shelf sea levels and in-situ currents decreasing moving down-slope (> 400m of water depth).  We discuss whether or not the relationship between European sea levels and shelf and slope currents emerges from momentum balance associated with the slope current existence (joint effect of winds, baroclinicity and bathymetry). We also discuss the relevance for coastal sea levels and associated coastal vulnerability.

 

Chafik, L., Nilsen, J. E. Ø., & Dangendorf, S. (2017). Impact of North Atlantic teleconnection patterns on Northern European sea level. Journal of Marine Science and Engineering, 5(3), 43.

Huthnance, J. M., & Gould, W. J. (1989). On the northeast Atlantic slope current. In Poleward flows along eastern ocean boundaries (pp. 76-81). Springer, New York, NY.

How to cite: Diabaté, S. T., Fraser, N. J., and McCarthy, G. D.: Wind-driven currents and sea-level variability of the northwest European shelf, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15625, https://doi.org/10.5194/egusphere-egu23-15625, 2023.

EGU23-15655 | Orals | CL4.5

Towards Physically Consistent Sea Level Rise Storylines for the United Kingdom 

Benjamin Harrison, Matthew Palmer, Lesley Allison, Jonathan Gregory, Tom Howard, Anne Pardaens, and Jonathan Tinker

There is increasing awareness of the need for comprehensive information on potential future sea-level rise to inform adaptation planning and coastal decision-making. The IPCC Sixth Assessment Report (AR6) states that global mean sea level rise approaching 5 m by 2150, and more than 15 m by 2300, cannot be ruled out under high greenhouse gas emissions due to uncertainty in ice sheet processes. Moreover, local sea level rise may be further exacerbated through systematic changes in the climate system, such as a rapid weakening of the Atlantic Meridional Overturning Circulation (AMOC).

We combine the latest United Kingdom national sea-level projections (UKCP18) with recently published projections of Antarctic ice mass loss to develop a small set of physically consistent storylines of local sea-level change that extend to 2300. The storylines span the range of uncertainty assessed by AR6 and deliver continuous sea level rise information around the UK coastline. While we focus on the UK, the methods are generic and can be readily applied to other geographic locations. Further, we consider potential changes in coastal flood hazard associated with a weakening of the AMOC using dynamical downscaling and storm surge modelling of climate model projections.

How to cite: Harrison, B., Palmer, M., Allison, L., Gregory, J., Howard, T., Pardaens, A., and Tinker, J.: Towards Physically Consistent Sea Level Rise Storylines for the United Kingdom, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15655, https://doi.org/10.5194/egusphere-egu23-15655, 2023.

For sea-level projections along the coast of the Netherlands, ocean dynamic sea level (ODSL) is one of the most important contributors to sea-level rise in the 21st century. The ODSL output from the latest coupled model intercomparison projects (CMIP5 and CMIP6) is used for these projections. These CMIP models overwhelmingly use ocean models with a spatial resolution of 1° and a vertical z-level coordinate. Using these CMIP models for projections does not provide a seamless connection between observations and projections, this study aims to improve on that. To do so, we use a configuration of the Regional Ocean Modelling System (ROMS) for the North Sea with a resolution of 0.25° to downscale the spatial resolution of CMIP6 models and interpolate the vertical coordinate to topography-following sigma levels to improve the projections for the Netherlands.

First, we use ROMS to reconstruct the ODSL along the coast of the Netherlands for the observational period. The regional model is forced using an atmospheric dataset constructed from ERA-interim and ERA-5 surface data and different ocean reanalysis datasets. It is not straightforward to compare the ODSL from different ocean reanalyses, as some datasets assimilate satellite altimetry data, whereas others do not. The ODSL from the reanalysis datasets that assimilate altimetry data are corrected for land ice and terrestrial water storage contributions to correct these differences.

Then, we use ROMS to obtain new projections of ODSL for the coast of the Netherlands that seamlessly connect to the estimate of ODSL from ocean reanalysis data. We extend the forcing datasets for the regional ocean model of the observational period using the anomalies of CMIP6 variables. Using this new method, we obtain improved projections along the coast of the Netherlands.

How to cite: Keizer, I., Le Bars, D., and Drijfhout, S.: Estimating ocean dynamic sea level along the coast of the Netherlands using the regional ocean modelling system (ROMS) to seamlessly connect the observational period to projections for the 21st century., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16275, https://doi.org/10.5194/egusphere-egu23-16275, 2023.

EGU23-16517 | ECS | Posters on site | CL4.5

Forcing Mechanisms of the Interannual Sea Level Variability in the Midlatitude South Pacific during 2004-2020 

Cyril Germineaud, Denis Volkov, Sophie Cravatte, and William Llovel

Over the past few decades, the global mean sea level rise and superimposed regional fluctuations of sea level have exerted considerable stress on coastal communities, especially in low-elevation regions such as the Pacific Islands in the western South Pacific Ocean. This made it necessary to have the most comprehensive understanding of the forcing mechanisms that are responsible for the increasing rates of extreme sea level events. In this study, we explore the causes of the observed sea level variability in the midlatitude South Pacific on interannual time scales using observations and atmospheric reanalyses combined with a 1.5 layer reduced-gravity model. We focus on the 2004–2020 period, during which the Argo’s global array allowed us to assess year-to-year changes in steric sea level caused by thermohaline changes in different depth ranges (from the surface down to 2000 m). We find that during the 2015–2016 El Niño and the following 2017–2018 La Niña, large variations in thermosteric sea level occurred due to temperature changes within the 100–500 dbar layer in the midlatitude southwest Pacific. In the western boundary region (from 30°S to 40°S), the variations in halosteric sea level between 100 and 500 dbar were significant and could have partially balanced the corresponding changes in thermosteric sea level. We show that around 35°S, baroclinic Rossby waves forced by the open-ocean wind-stress forcing account for 40 to 75% of the interannual sea level variance between 100°W and 180°, while the influence of remote sea level signals generated near the Chilean coast is limited to the region east of 100°W. The contribution of surface heat fluxes on interannual time scales is also considered and shown to be negligible.

How to cite: Germineaud, C., Volkov, D., Cravatte, S., and Llovel, W.: Forcing Mechanisms of the Interannual Sea Level Variability in the Midlatitude South Pacific during 2004-2020, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16517, https://doi.org/10.5194/egusphere-egu23-16517, 2023.

EGU23-17395 | ECS | Posters virtual | CL4.5

Impact of mean sea level rise in the Rias Baixas hydrodynamics (NW Iberian Peninsula) 

Clara Ribeiro, Magda Catarina Sousa, Carina Lurdes Lopes, Inés Álvarez, and João Miguel Dias

Mean sea level rise is currently a growing and prominent consequence of climate change. The increase in the mean sea level poses a significant threat to low-lying coastal areas that often present high economic and biological value. Recent studies also show that tidal propagation in estuarine systems will be altered due to climate change, intensifying the threat it poses to these systems. The Rias Baixas located in the NW of the Iberian Peninsula, as well as the rest of the Galician coast, are areas of high primary production susceptible to alterations in their hydrodynamics induced by climate change,  negatively impacting the system.

In this context, this study aims to validate a hydrodynamic model of the Rias Baixas and to analyse the effect of mean sea level rise in the local hydrodynamics. The methodology followed comprises the application of a three-dimensional numerical model (Delft3D), with realistic bathymetry and coastline of the NW Iberian Peninsula including the Rias Baixas. The model considers the main physical processes and the main features of circulation. Ambient shelf conditions include TOPEX global tidal solution.

Firstly, the model validation was done through a qualitative and quantitative analysis. The qualitative analysis was done through a visual comparison between model results and observed time series of the water level in several sampling stations, showing good agreement. The quantitative analysis aims to assess the model performance, through the determination of the root mean square error between model results and observations and of the harmonic constituents from both types of data series. After the model validation, the main semidiurnal and diurnal constituents as well as the tidal current magnitude were determined for Ria Baixas for three mean sea level scenarios: present mean sea level and two future scenarios from CMIP6, a more optimistic one (SSP1 - 2.6) and a more pessimistic one (SSP5 - 8.5).

The model results show that the amplitude of the main semidiurnal and diurnal constituents will decreases for future scenarios, whereas the respective phase increases towards the head of the Rias. The results also highlight that tidal current magnitude generally increases with mean sea level rise for future scenarios, although a slight decrease was found at the upstream areas of the Ria Baixas.

Funding: We acknowledge financial support to CESAM by FCT/MCTES (UIDP/50017/2020+UIDB/50017/2020+ LA/P/0094/2020) through national funds.

How to cite: Ribeiro, C., Sousa, M. C., Lopes, C. L., Álvarez, I., and Dias, J. M.: Impact of mean sea level rise in the Rias Baixas hydrodynamics (NW Iberian Peninsula), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17395, https://doi.org/10.5194/egusphere-egu23-17395, 2023.

EGU23-103 | ECS | Orals | CL4.7

Energetic Constraints on Baroclinic Eddy Heat Transport in a Rotating Annulus 

Cheng Qian, Peter Read, and David Marshall

We measure baroclinic eddy heat transport in a differentially heated rotating annulus laboratory experiment to test mesoscale ocean eddy parameterization frameworks. The differentially heated rotating annulus comprises a fluid placed between two upright coaxial cylinders which are maintained at different temperatures, usually with a cooled inner cylinder and a heated outer.  The annular tank is placed on a rotating table which provides conditions for baroclinic eddies to develop and equilibrate in different flow regimes, depending upon the imposed conditions. As the rotation speed is increased, the equilibrated flow changes from a steady or periodically varying low wavenumber pattern to a more complex, time-varying flow dominated by higher wavenumbers. With a topographic beta effect produced by conically sloping upper boundary, more complex flow regimes are observed combining zonal jets and eddies forming one or more parallel storm tracks. With this possibility to explore varied flow regimes, our experimental approach combines laboratory calorimetry and visualization measurements along with numerical simulations to derive the eddy heat transport properties. In the following, we focus on the visualisation measurement to test related assumptions and parametric dependencies for eddy transport. We first test the assumptions of a down-gradient temperature flux-gradient relationship, determining coefficients of the eddy transport tensor, and exploring scaling relations for the eddy coefficients. A clear statistical scaling is found between eddy heat fluxes and physical variables such as eddy energy, the beta effect, and the temperature contrast.

How to cite: Qian, C., Read, P., and Marshall, D.: Energetic Constraints on Baroclinic Eddy Heat Transport in a Rotating Annulus, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-103, https://doi.org/10.5194/egusphere-egu23-103, 2023.

EGU23-273 | ECS | Posters virtual | CL4.7

Understanding the variability and trend of the regional Hadley Cell over Asia-Pacific 

Pratiksha Priyam Baruah and Neena Joseph Mani

The zonal mean Hadley Cell (HC) has been reported to be expanding poleward in the last few decades. However, there has been no consensus on whether the zonal mean HC is strengthening or weakening. The features of longitudinally averaged HC are collectively modulated by various regional HCs, controlled by the regional differences in land-ocean distribution and topography. However, there have not been many studies exploring the variability and trend of regional HCs in a detailed manner. In this study, we examine the variability and long-term trend of the regional HC over the Asia-Pacific and explore the different factors contributing to the regional HC variability. Moist convection can regulate regional HCs on synoptic time scales through equatorial wave dynamics. The ocean–atmosphere coupled variability associated with the El Niño-Southern Oscillation (ENSO), and the modulation of tropical convection and equatorial waves are considered to exert a dominant control on the regional HC variability in the interannual timescale. In addition to the tropical forcing, the regional HC variability is also affected by fluxes transported by the midlatitude eddies from the subtropics to the tropics. In this study, we will be quantifying the relative role of these tropical and extratropical forcings in modulating the variability of regional HC over Asia-Pacific.

 

How to cite: Baruah, P. P. and Joseph Mani, N.: Understanding the variability and trend of the regional Hadley Cell over Asia-Pacific, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-273, https://doi.org/10.5194/egusphere-egu23-273, 2023.

EGU23-972 | ECS | Posters on site | CL4.7

Vanishing the El Niño-induced delay effect on the ice mass loss of West Antarctica under global warming 

Hyunju Lee, Emilia Kyung Jin, Byeong-Hoon Kim, and Won Sang Lee

West Antarctica has been losing their ice mass due to global warming, and the El Niño has delayed the ice mass loss by inducing weakening of the Amundsen Sea Low (ASL), encouraging of poleward moisture flux and consequent extreme precipitation. However, it is not yet revealed whether the delay effect will continue in the future. We analyzed future scenarios from the CMIP6 Earth system models (ESMs) to identify future change and identified that the El Niño-driven mass increase by precipitation will vanish in the high-emission future scenarios. Precipitation anomaly in response to El Niño starts to be negative from the 2050s in the SSP5-8.5 and from the 2060s in the SSP3-7.0, which means that the El Niño-driven delay effect disappears. It is because the moisture transport into West Antarctica is prevented due to east-equatorward migration of El Niño-induced ASL anomaly as global warming intensifies. The strengthened polar jet associated with positive Southern Annular Mode (SAM) trend moves the ASL anomaly east- and equatorward under global warming.

How to cite: Lee, H., Jin, E. K., Kim, B.-H., and Lee, W. S.: Vanishing the El Niño-induced delay effect on the ice mass loss of West Antarctica under global warming, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-972, https://doi.org/10.5194/egusphere-egu23-972, 2023.

 We show that the most prominent of the work theorems, the Jarzynski equality and the Crooks relation, can be applied to the momentum transfer at the air-sea interface using a hierarchy of local models. In the more idealized models, with and without a Coriolis force, the variability is provided from a Gaussian white-noise which modifies the shear between the atmosphere and the ocean. The dynamics is Gaussian and the Jarzynski equality and Crooks relation can be obtained analytically solving stochastic differential equations. The more involved model consists of interacting atmospheric and oceanic boundary-layers, where only the dependence on the vertical direction is resolved, the turbulence is modeled through standard turbulent models and the stochasticity comes from a randomized drag coefficient. It is integrated  numerically and can give rise to a non-Gaussian dynamics. Also in this case the Jarzynski equality allows for calculating a dynamic-beta ßD of the turbulent fluctuations (the equivalent of the thermodynamic-beta  ß=(kB T)-1 in thermal fluctuations). The Crooks relation gives the ßD as a function of the magnitude of the work fluctuations. It is well defined (constant) in the Gaussian models and can show a slight variation in the  more involved models. This demonstrates that recent concepts of stochastic thermodynamics used to study micro-systems subject to thermal fluctuations can further the understanding of geophysical fluid dynamics with turbulent fluctuations.

How to cite: Wirth, A.: Jarzynski equality and Crooks relation for local models of air-sea interaction, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1066, https://doi.org/10.5194/egusphere-egu23-1066, 2023.

The Arctic is warming at a rate faster than any other oceans, a phenomenon known as Arctic amplification that has widespread impact on the global climate. In contrast, the Southern Ocean (SO) and Antarctica have been cooling over the past decades. The projection of these regions under global warming has a non-negligible model spread. Here we show that under a strong warming scenario from 1950 to 2100, comparing a cutting-edge high-resolution climate model to a low-resolution model version, the increase of Arctic amplification is 3 °C more and the SO and Antarctica warming is 2°C less. Previously ice-covered Arctic Ocean will exhibit greater SST variability under future global warming. This is due to an increased SST increase in summer due to sea ice retreat. Extreme warming events in the Arctic and SO, known as marine heat waves (MHW) that influence the ecology, are largely unknown. We find that the MHWs in the Arctic and SO are twice as strong in the high-resolution model version, where the increasing intensity of MHWs in the Arctic corresponds to strong decline (<-6% per decade) of sea ice. In both the high-resolution and low-resolution models, the duration of MHWs in the Arctic and SO shows a declining trend under global warming. The much stronger MHWs in the high-resolution model could be caused by two orders of magnitude more ocean turbulent energy. For example, the spatial patterns of SO MHW intensity correspond to the pattern of SO EKE. We conclude that the Arctic amplification and MHWs at high latitudes might be underestimated by the current generation of climate models with low resolution, and the SO and Antarctica warming might be overestimated. Our eddy- and storm-resolving model is expected to open new frontiers on how the system responds to human activities in a high CO2 world by evaluating the impact on past and future climate and environmental extremes.

How to cite: Gou, R., Lohmann, G., and Wu, L.: Increase in Arctic amplification and high-latitude marine extremes in the 21st century as obtained from high-resolution modelling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1649, https://doi.org/10.5194/egusphere-egu23-1649, 2023.

EGU23-1773 | ECS | Posters on site | CL4.7

Tropical Instability Waves in a High-Resolution Oceanic and Coupled GCM 

Li Tianyan and Yu Yongqiang

Tropical instability waves (TIWs) are the dominant mesoscale variability in the eastern equatorial Pacific Ocean. TIWs have direct impacts on the local hydrology, biochemistry and atmospheric boundary layer, and feedback on ocean circulations and climate variability. In this study, the basic characteristics of Pacific Ocean TIWs simulated by an eddy-resolving ocean model and a coupled general circulation model are evaluated. The simulated TIW biases mainly result from the mean climatology state, as TIWs extract eddy energy from the mean potential and kinetic energy. Both the oceanic and coupled models reproduce the observed westward propagating large-scale Rossby waves between approximately 2-8N, but the simulated TIWs have shorter wavelengths than the observed waves due to the shallower thermocline. Meanwhile, the weak meridional shears of background zonal currents and the less-tilted pycnocline in these two models compared to the observations causes weak barotropic and baroclinic instability, which decreases the intensity of the simulated TIWs. We then contrast the TIWs from these two models and identify the roles of atmospheric feedback in modulating TIWs. The latent heat flux feedback is similar to observation in the coupled model but absent in the ocean model, contributing to the stronger standard deviation (STD) of the TIW SST in the ocean model. The ocean model is not able to capture realistic air-sea interaction processes when forced with prescribed atmospheric forcing. However, the misrepresented atmospheric feedback in the ocean model tends to decrease the sea surface height (SSH) variability, and the current feedback damping effect is stronger in the ocean model than in the coupled model. Combined with weaker barotropic conversion rate and baroclinic conversion rate in the ocean model than in the coupled model, the STD of the TIW SSH in the ocean model is weaker.

How to cite: Tianyan, L. and Yongqiang, Y.: Tropical Instability Waves in a High-Resolution Oceanic and Coupled GCM, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1773, https://doi.org/10.5194/egusphere-egu23-1773, 2023.

EGU23-1985 | ECS | Posters on site | CL4.7

Sensitivity of ocean heat content to atmospheric forcing in the period of global warming hiatus 

Chavely Albert Fernández, Armin Köhl, and Detlef Stammer

Between 1998 and 2012, there was a smaller rate of global warming, known as the "global warming hiatus". One of the suggested causes is that during this period additional heat sequestration occurs into the deep ocean layers such that deep layers warm at a greater rate than the upper layers. This research is focused on the origins of changes in ocean heat content during the hiatus period, defined in this case as the last 10 years of adjoint model run, where the cost function is defined. Adjoint sensitivities are used to determine the influence of atmospheric forcing (heat and freshwater fluxes and wind stress) on the ocean heat content.

The MIT General Circulation Model with a resolution of 2° x 2° is used over the period 1978-2008 to determine adjoint sensitivities of the globally and temporally (over the last 10 years, defined as hiatus period) integrated vertical heat fluxes across various depth levels. The contributions of different forcing components to the vertical heat flux anomalies are obtained from the scalar product between sensitivities and the anomalies of the atmospheric forcing.  For this, the atmospheric forcing anomalies are computed with respect to the climatology calculated over the period 1948-1968 when there was almost no change in the ocean heat content.

A more pronounced increase in ocean heat uptake during the hiatus period has been evidenced by the forward run of the model. Wind anomalies represent more than half of the contribution to the increase in heat flux across 300m, suggesting that the excess of heat stored by the ocean is transferred adiabatically to the deeper layers and that the zonal wind is one of the major drivers of ocean heat uptake. In the Southern Ocean, the sensitivities to the wind stress change from positive to negative when the hiatus starts. This indicates that, during the hiatus, the rate of change of ocean heat content is opposite to the one of the wind stress. The Southern Ocean presents smaller values of the computed amplitude weighted mean time, meaning that this region has the fastest response to changes in surface atmospheric forcing.

How to cite: Albert Fernández, C., Köhl, A., and Stammer, D.: Sensitivity of ocean heat content to atmospheric forcing in the period of global warming hiatus, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1985, https://doi.org/10.5194/egusphere-egu23-1985, 2023.

 Widespread observed and projected increases in warm extremes, along with decreases in cold extremes, have been confirmed as being consistent with global and regional warming. However, based on observational datasets and state-of-the-art CMIP6 model simulations, we disclose that the decadal variation in the frequency of the surface air temperature (SAT) extremes over mid- to high latitudes over Eurasia (MHEA) in winter is primarily dominated by the thermodynamical effect of the surface heat fluxes release over the midlatitude North Atlantic induced by Atlantic multidecadal oscillation (AMO), which even masks the dynamical large-scale Rossby wave propagation. Besides, the stronger Atlantic meridional overturning circulation (AMOC) gives rise to both warm and cold extremes through increasing the variance of winter SAT over MHEA due to thermodynamical heat release and enhanced dynamical Rossby wave propagation.

How to cite: Wang, H.: Frequency of winter temperature extremes over Eurasia dominated by variabilities over the Atlantic Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3106, https://doi.org/10.5194/egusphere-egu23-3106, 2023.

EGU23-3282 | ECS | Posters on site | CL4.7

Physics of the Eddy Memory Kernel of a Baroclinic Midlatitude Atmosphere 

Elian Vanderborght, Jonathan Demeayer, Henk Dijkstra, Georgy Manucharyan, and Woosok Moon

In recent theory trying to explain the origin of low-frequency atmospheric variability, the concept of eddy-memory has been suggested. In this view, the effect of synoptic scale heat fluxes on the mean flow depends on the history of the mean meridional temperature gradient. Mathematically, this involves a convolution of an integral kernel with the mean meridional temperature gradient over past times. In atmospheric studies, it has been proposed that the shape of this integral kernel is linked to the baroclinic wave life cycle. However, this hypothesis has yet to be supported by numerical and observational evidence. In this study we use a low-order two layer quasi-geostrophic atmospheric model (Demaeyer et al., 2020). By perturbing the model with a known forcing, linear response theory can be used to estimate the shape of the integral kernel. Using this methodology, we find an integral kernel that resembles the shape of an exponentially decaying oscillation, different from the simple exponentially decaying integral kernel assumed in most previous studies. By computing the energies and performing a sensitivity analysis, we link the shape of the integral kernel to atmospheric dynamical processes.

References:

J. Demaeyer, L. De Cruz, and S. Vannitsem. qgs: A flexible python framework of reduced-order multiscale climate
models. Journal of Open Source Software, 5(56):2597, 2020.

How to cite: Vanderborght, E., Demeayer, J., Dijkstra, H., Manucharyan, G., and Moon, W.: Physics of the Eddy Memory Kernel of a Baroclinic Midlatitude Atmosphere, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3282, https://doi.org/10.5194/egusphere-egu23-3282, 2023.

EGU23-3492 | Orals | CL4.7

Variability of summer-time Arctic sea ice: the drivers and the contribution to the sea ice trend and extremes 

Mehdi Pasha Karami, Torben Koenigk, and Bruno Tremblay

Understanding the variability of summer-time Arctic sea ice at interannual to multidecadal time scales in the midst of anthropogenically forced sea ice decline is crucial for better predictions of sea ice conditions in the future climate and rapid changes in sea ice. Here, we apply time-frequency analysis to study the modes of variability, extreme events and the trend in the September Arctic sea ice in 100–150 year datasets. We extract the non-linear trend for the sea ice area and provide an estimate for the anthropogenic-driven sea ice loss. For the used dataset, the anthropogenic-related sea ice loss is found to have a rate of ~-0.25 million km2 per decade in the 1980’s and accelerating to ~-0.47 million km2 per decade in 2010’s. By assuming the same rate of sea ice loss in the future, and without the contribution of the internal variability and feedbacks, we can approximate the occurrence of summer sea-ice free Arctic to be around 2060. Regarding the dominant modes of variability for the September sea ice, we find that they have periods of around 3, 6, 17, 28 and 55 years, and show what drives these modes and how they contribute to sea ice extreme events. The main atmospheric and oceanic drivers of sea ice modes include the Arctic oscillation and Arctic dipole anomaly for the 3-year mode, variability of sea surface temperature (SST) in Gulf Stream region for the 6-year mode, decadal SST variability in the northern North Atlantic Ocean for the 17-year mode, Pacific decadal oscillation (PDO) for the 28-year mode, and Atlantic multidecadal Oscillation (AMO) for the 55-year mode. Results show that changes in the sea ice due to internal variability can be as large as forced changes thus can slow down or accelerate the background anthropogenic-driven sea ice loss. By applying the same method, we also present modes of variability and trend of sea ice in the large ensemble global model simulations of EC-Earth model (SMHI-LENS) for the future climate projections and different climate scenarios.

How to cite: Karami, M. P., Koenigk, T., and Tremblay, B.: Variability of summer-time Arctic sea ice: the drivers and the contribution to the sea ice trend and extremes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3492, https://doi.org/10.5194/egusphere-egu23-3492, 2023.

Global circulation patterns are analysed using the mean meridional circulation (MMC) from ERA-Interim for the period of 1979 – 2017. The global isentropic MMC consists of a single overturning cell in each hemisphere with net heat transport from the equator to the pole. Six clusters are identified from daily data that are associated with one of four seasons. Two solstitial MMC clusters represent either stronger or weaker circulation in the winter hemisphere. We show that long-term trends do not reflect a gradual change in the atmospheric circulation, but rather a change in the frequency of preferred short-term circulation regimes. Before the late 1990s the clusters showing a stronger (weaker) winter circulation are becoming less (more) frequent; from around year 2000 the trends have paused. These trends are in close agreement with the change in the low-stratospheric Antarctic ozone trends reported by earlier studies. Our findings also reveal a strong coupling between Southern and Northern Hemispheres during boreal winter. Following Hartmann et al. (2022), we hypothesize that anomalous polar vortex over Antarctica leads to anomalies in the sea surface temperatures (SST) in the tropical Pacific that impact the circulation in both hemispheres. Furthermore, we show that consecutive solstice season demonstrates coherent anomalies in the frequency of circulation regimes. We discuss possible reasons for such relationship.

References:
Hartmann, D. L., Kang, S., Polvani, L. & Xie, S.-P. The Antarctic ozone hole and the pattern effect on climate sensitivity. (2022) doi:10.1073/pnas.

How to cite: Rudeva, I., Boschat, G., and Lucas, C.: How can atmospheric trends be explained by changes in frequency of short-term circulation regimes and what is the role of the Antarctic ozone?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4557, https://doi.org/10.5194/egusphere-egu23-4557, 2023.

EGU23-5305 | ECS | Orals | CL4.7

Linking ITCZ migrations to the AMOC and North Atlantic/Pacific SST decadal variability 

Eduardo Moreno-Chamarro, John Marshall, and Tom L. Delworth

This contribution discusses the link between migrations in the intertropical convergence zone (ITCZ) and changes in the Atlantic meridional overturning circulation (AMOC), Atlantic multidecadal variability (AMV), and Pacific decadal oscillation (PDO). We use a coupled climate model that allows us to integrate over climate noise and assess underlying mechanisms. We use an ensemble of ten 300-yr-long simulations forced by a 50-yr oscillatory North Atlantic Oscillation (NAO)-derived surface heat flux anomaly in the North Atlantic, and a 4000-yr-long preindustrial control simulation performed with GFDL CM2.1. In both setups, an AMV phase change induced by a change in the AMOC’s cross-equatorial heat transport forces an atmospheric interhemispheric energy imbalance that is compensated by a change in the cross-equatorial atmospheric heat transport due to a meridional ITCZ shift. Such linkages occur on decadal time scales in the ensemble driven by the imposed forcing, and internally on multicentennial time scales in the control. Regional precipitation anomalies differ between the ensemble and the control for a zonally averaged ITCZ shift of similar magnitude, which suggests a dependence on timescale. Our study supports observational evidence of an AMV–ITCZ link in the twentieth century and further links it to the AMOC, whose long-time-scale variability can influence the phasing of ITCZ migrations. In contrast to the AMV, our calculations suggest that the PDO does not drive ITCZ migrations, because the PDO does not modulate the interhemispheric energy balance.

How to cite: Moreno-Chamarro, E., Marshall, J., and Delworth, T. L.: Linking ITCZ migrations to the AMOC and North Atlantic/Pacific SST decadal variability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5305, https://doi.org/10.5194/egusphere-egu23-5305, 2023.

EGU23-5962 | Orals | CL4.7

Tropical precipitation biases in nextGEMS storm-resolving Earth System Models 

Simona Bordoni, Roberta D'Agostino, and Adrian M. Tompkins

Global Earth System Models at storm-resolving resolutions (SR-ESM, with horizontal resolutions of ~4km) are being developed as part of the nextGEMS collaborative European EU’s Horizon 2020 programme. Through breakthroughs in simulation realism, these models will eventually allow us to understand and reliably quantify how the climate will change on a global and regional scale, and how the weather, including its extreme events, will look like in the future.

As part of the Storms & Ocean theme, we are exploring how resolving convective storms, ocean mesoscale eddies, and air-sea interaction on these scales influences the development of tropical SST anomalies and their influence on the mean climate (ITCZ and circulation biases) and its variability. Existing biases in the SR-ESM simulations in the first two development cycles are interpreted using the vertically integrated atmospheric energy budget to disentangle local and remote influences on tropical precipitation. More specifically, these biases are decomposed in hemispherically symmetric and antisymmetric components, which are linked, respectively, to biases in the atmospheric net energy input near the equator (tropical SST biases, low level clouds, etc) and to the cross-equatorial atmospheric energy flux (driven by inter-hemispheric contrast in net energy input, for instance biases in clouds in the southern ocean). We also explore the role that transient eddies, both of extratropical and tropical origin that are usually neglected in this framework, play in the global energetics and tropical precipitation patterns.

How to cite: Bordoni, S., D'Agostino, R., and Tompkins, A. M.: Tropical precipitation biases in nextGEMS storm-resolving Earth System Models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5962, https://doi.org/10.5194/egusphere-egu23-5962, 2023.

    Poleward atmospheric energy transport is determined by the overall equator-to-pole radiative imbalance. As this imbalance is projected to remain fairly constant in end-of-century greenhouse gas forcing scenarios, an increase in poleward latent heat transport must be accompanied by a reduction in dry static energy flux. Since midlatitude energy transport is dominated by the eddy component, changes in the energy budget go hand in hand with changes in cyclone characteristics. From a dynamical perspective, the enhanced condensation due to climate change promotes intensification, prolongs lifetime of cyclones, and can increase stationarity of anticyclones. However, it also tends to increase static stability and thereby reduce baroclinicity, which is another important driver of cyclone development. Additionally, baroclinicity is projected to increase at upper levels due to tropical amplification, to decrease at low levels as a result of Arctic amplification, and to be affected by land-sea temperature contrast changes. As these processes are at play simultaneously, isolating the role of moisture is rather complicated. Therefore, in addition to coupled climate model simulations we use idealized aquaplanet simulations to single out the effects of individual physical mechanisms and address the question: if the overall poleward energy transport remains largely unaffected by global warming, how do cyclone characteristics change in the presence of increased moisture in the atmosphere?

    For bridging the gap between the global energy flux and synoptic-scale features, we analyse the role of increasing moisture for shaping midlatitude storm tracks in present and future climates from both an Eulerian and a Lagrangian perspective. We apply the moist static energy (MSE) framework that allows partitioning atmospheric energy fluxes into eddy and mean, dry and moist components. Here, eddies are related to cyclones and anticyclones, while the mean energy flux is associated with planetary waves and the mean meridional overturning circulation. The goal is to relate the eddy MSE fluxes to feature-based results including extratropical cyclone number, lifetime, intensity, location, and tilt. By combining results from both global-scale eddy energy fluxes and synoptic-scale feature quantities, we aim to improve the understanding of the role of latent heating in shaping the mean properties of extratropical storm tracks. Therefore, a central question of this project is whether and how changes in cyclone quantities can be linked to changes in latent heat transport and release. Building on what we learn from bringing the two perspectives together, we will proceed to investigating the impact of increased latent heating on midlatitude storm tracks. 

How to cite: Zibell, J., Schemm, S., and Hermoso Verger, A.: Combining global-scale atmospheric heat transport and synoptic-scale extratropical cyclone characteristics to understand the role of latent heating for midlatitude storm tracks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7216, https://doi.org/10.5194/egusphere-egu23-7216, 2023.

EGU23-7348 | ECS | Orals | CL4.7

An Energy Transport View of ENSO Responses to Volcanic Forcing 

Shih-Wei Fang and Claudia Timmreck

El Niño-Southern Oscillation (ENSO) is one of the major climate phenomena impacting the globe. When a volcanic eruption happens, how ENSO will respond has still no consensus in proxy data though climate models tend to have an El Niño tendency. In this study, using 100 members of diverse (in locations and magnitude) idealized volcanic forcing ensembles, we found that the ENSO responses to north and south extra-tropical eruptions are related to the energy transport to the cooling hemisphere and involve direct and indirect responses through atmospheric and oceanic transport. The north extratropical forcing leads to more El Niño up to three years after eruptions, which is related to the direct atmospheric responses of the southward movement of ITCZ for transporting more energy to the north. The indirect oceanic transport then takes over afterward, leading to more La Niña due to more upwelling in the equatorial eastern Pacific. The south extra-tropical eruptions have less El Niño tendency due to the northward replacement of ITCZ. As the indirect oceanic transport also results in equatorial mean state changes, which may lead to distinct ENSO responses. The long-term ENSO responses from extra-tropical cooling will also be investigated through the simulations from the Extratropical-Tropical Interaction Model Intercomparison Project (ETIN-MIP) experiment.

How to cite: Fang, S.-W. and Timmreck, C.: An Energy Transport View of ENSO Responses to Volcanic Forcing, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7348, https://doi.org/10.5194/egusphere-egu23-7348, 2023.

EGU23-9914 | Posters on site | CL4.7

Changes in river temperature, discharge and heat flux based on new observational data for Yenisei basin and modeling 

Alexander Shiklomanov, Richard Lammers, Alexander Prusevich, Irina Panyushkina, and David Meko

River temperature plays an important role in numerous biological and ecological processes within the Yenisei River basin and it is very sensitive to changes in climatic characteristics and anthropogenic disturbances. Water temperature and river discharge characterize heat or energy flux, which is important in northern latitudes for river freeze-up and ice break-up processes and thermal riverbank erosion. The changes in heat flux in river estuary can also significantly impact various biophysical processes in coastal ocean waters.

We use new water temperature data and river discharge records for 12 observational gauges in the Yenisei River basin to analyze changes in water temperature and heat flux from upstream to downstream over 1950-2018. Preliminary results show significant increases for most gauges in maximum annual water temperature as well as in 10-days mean water temperature during May-June and September-October. There were no significant changes in river temperature during July-August unless the gauges were impacted by reservoir regulations. The river heat flux has significantly increased in central and northern parts of the Yenisei basin and decreased in the south, mainly due to discharge variability.

The gridded hydrological Water Balance Model (WBM) developed at the University of New Hampshire, that takes into account various anthropogenic activities, was used to simulate river discharge and water temperature for entire Yenisei basin with a 5 minute spatial resolution river network using several climate reanalysis products (MERRA2, ERA5 and NCEP-NCAR).  The modeled results were verified with observational data and simulations using the MERRA2 climate drivers demonstrated the best match with observations (Nash-Sutcliffe model efficiencies coefficients were greater than 0.5 for both river temperature and discharge). Maps of modeled changes in runoff, river temperature and heat flux show the opposite changes in the southern and northern parts of Yenisei basin. The model simulations correspond well with observational data even for heavily disturbed river reaches. For example, they show unfrozen water with positive temperatures during the winter below large dams and reservoirs.       

The WBM was also applied to project changes in water temperature, discharge and heat flux up to 2100 for several SSPs and GCMs from CMIP6. In spite of heterogenous projected changes in these parameters across Yenisei basin, significant increases in discharge and heat flux to the Arctic Ocean are expected.

How to cite: Shiklomanov, A., Lammers, R., Prusevich, A., Panyushkina, I., and Meko, D.: Changes in river temperature, discharge and heat flux based on new observational data for Yenisei basin and modeling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9914, https://doi.org/10.5194/egusphere-egu23-9914, 2023.

EGU23-11159 | ECS | Orals | CL4.7

Causal model evaluation of Arctic-midlatitude process during the boreal cold season in CMIP6 

Evgenia Galytska, Katja Weigel, Dörthe Handorf, Ralf Jaiser, Raphael Köhler, Jakob Runge, and Veronika Eyring

Linked to increased sea ice loss, the Arctic region has warmed at least four times faster than the global average over the past 40 years. Mutual links between amplified Arctic warming with changes and variability in midlatitude weather have been discussed in several studies. Nevertheless, the lack of consistent conclusions between observations and model simulations obfuscates the interpretation behind the mechanisms of Arctic-midlatitude teleconnections. To contribute to the understanding of Arctic-midlatitude connections that occur in conditions of amplified Arctic warming, we applied causal discovery to analyse causal and contemporaneous links. Initially, we calculated causal dependencies for monthly mean ERA5 reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF) and Hadley Centre Sea Ice and Sea Surface Temperature among local and remote processes. Then, by comparing causal graphs detected in reanalyses data with a number of climate model historical simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6), we assessed the performance of climate models and evaluated the robustness of the observed Arctic-midlatitude connections in the current climate. By comparing causal graphs from the CMIP6 historical and Scenario Model Intercomparison Project (ScenarioMIP) we estimated future changes in Arctic-midlatitude teleconnections towards the end of the century. In this study, we focus on the differences in the mechanism of Arctic-midlatitude teleconnections that occur during the boreal cold season, i.e. early winter (October-November-December), winter (December-January-February), and late winter (January-February-March). In this study, we will present the major findings of Galytska et al., 2022 discussing how causal model evaluation helps to summarize major differences between causal interdependencies detected in observations and simulated by a number of climate models. Understanding these differences can be the basis for further improvement of the representation of Arctic-midlatitude teleconnections in climate models.

References. 

Evgenia Galytska, Katja Weigel, Dörthe Handorf, Ralf Jaiser, Raphael Köhler, Jakob Runge, and Veronika Eyring. Causal model evaluation of Arctic-midlatitude teleconnections in CMIP6. Authorea. October 06, 2022. DOI: 10.1002/essoar.10512569.1

 

How to cite: Galytska, E., Weigel, K., Handorf, D., Jaiser, R., Köhler, R., Runge, J., and Eyring, V.: Causal model evaluation of Arctic-midlatitude process during the boreal cold season in CMIP6, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11159, https://doi.org/10.5194/egusphere-egu23-11159, 2023.

EGU23-12377 | ECS | Posters on site | CL4.7

Thermodynamic assessment of simulations of the last deglaciation with an Earth system model of intermediate complexity 

Muriel Racky, Irene Trombini, Klaus Pfeilsticker, Nils Weitzel, and Kira Rehfeld

As we observe and expect severe changes in the Earth’ climate, the analyses of past climate state transitions is of major value for improving our Earth system understanding. Under this objective, the last deglaciation (~ 21 ka to 9 ka before present), the transition from the Last Glacial Maximum (LGM) to the Holocene, is an ideal case study. During this transition, the orbital configuration gradually changed and greenhouse gases have risen, which caused a sharp decline in northern hemisphere ice sheets and an increase in the global mean surface temperature.

We create an ensemble of deglaciation simulations with a modified version of the Planet Simulator, an Earth system model of intermediate complexity (EMIC). We produce single and combined forcing simulations for further investigation from a thermodynamic perspective. The response to the transiently changing radiative forcing is investigated in terms of energy and entropy budgets of the atmosphere. Here, we focus on the deglacial evolution of the material entropy production (MEP). Its contributions represent the strength of major climate features such as the kinetic energy generation rate, vertical and horizontal heat transport and the hydrological cycle. Preliminary results show an increase of the global mean MEP from the LGM to the Holocene because of a strengthening of the hydrological contribution. In contrast, the relative importance of kinetic energy dissipation and turbulent heat diffusion in the boundary layer decrease. Our work can provide the basis for investigating the MEP as a diagnostic quantity with other models and for other climate state transitions.

How to cite: Racky, M., Trombini, I., Pfeilsticker, K., Weitzel, N., and Rehfeld, K.: Thermodynamic assessment of simulations of the last deglaciation with an Earth system model of intermediate complexity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12377, https://doi.org/10.5194/egusphere-egu23-12377, 2023.

EGU23-12939 | Orals | CL4.7

The contribution of Arctic marine heatwaves to the minimum sea ice extent as compound events 

Armineh Barkhordarian, David Nielsen, and Johanna Baehr

On the global scale, the frequency of marine heatwaves (MHWs) is projected to increase further in the twenty-first Century. In our earlier study we demonstrate that the high-impact major marine heatwaves over the northeast Pacific are co-located with a systematically-forced long-term warming pool, which we attribute to forcing by elevated greenhouse gases levels (GHG), and the recent industrial aerosol-load decrease (Barkhordarian et al., 2022).  In current study we show that the magnitude of Arctic MHWs has significantly increased since 2006, and has exceeded the pre-industrial climate bounds since then. We here perform statistical attribution methodologies, and provide a quantitative assessment of whether GHG forcing was necessary for the Arctic MHWs to occur, and whether it is a sufficient cause for such events to continue to repeatedly occur in the future.

The probability of necessary causation of Arctic MHWs intensity, increases with increasing the severity of MHWs, and saturate to 1.0 by the time MHWs intensity exceeds the 2°C, indicating that any MHWs over the Arctic with an intensity higher than 2°C is entirely attributable to the inclusion of GHG forcing. These amplified extreme MHWs in the Arctic have each been accompanied by a record decline in Arctic Sea ice, in particular in the years 2007, 2012, 2016 and 2020. Over the last decade, MHWs occur in the Arctic where sea ice melt in June is 4 %/year faster, the ice-free season is ~3 months longer, the ocean heat-uptake is 50 W/m2 higher, and the sea surface temperature is ~2°C warmer, in comparison with the previous decade. In autumn surface evaporation rate is increasing, the increased low clouds favor more sea ice melt via emitting stronger longwave radiation. In summary, prolonged Arctic marine heatwaves, triggered by faster early summer sea ice melt, will accelerate Arctic warming, and cause Arctic Sea ice extent to shrink even faster in the near future.

Barkhordarian, A., Nielsen, D.M. & Baehr, J. Recent marine heatwaves in the North Pacific warming pool can be attributed to rising atmospheric levels of greenhouse gases. Nature Communications Earth & Environment, 3, 131 (2022). https://doi.org/10.1038/s43247-022-00461-2

 

How to cite: Barkhordarian, A., Nielsen, D., and Baehr, J.: The contribution of Arctic marine heatwaves to the minimum sea ice extent as compound events, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12939, https://doi.org/10.5194/egusphere-egu23-12939, 2023.

EGU23-13091 | Posters on site | CL4.7

Emergent constraints on the future Arctic lapse-rate feedback 

Olivia Linke, Nicole Feldl, and Johannes Quaas

Arctic amplification (AA) is largely attributed to the effect of sea ice decline leading to greater surface solar absorption and further ice melt, and the vertical structure of the warming. The latter aspect evokes the positive lapse-rate feedback (LRF), which is commonly understood as an effect of stable stratification: The warming in the Arctic is particularly strong close to the surface, but muted aloft. This limits the outgoing long-wave radiative flux at the top-of-the-atmosphere (TOA) relative to vertically uniform warming.

We estimate the future Arctic LRF in 43 global climate models (GCMs) from the highest emission pathway SSP5 of CMIP6. The GCMs simulate a large spread of future AA (2-8 K above global warming) and Arctic LRF (1-4 K warming contribution) at the end of the century 2070-2099. Our work aims to identify emerging relationships between this spread and observable aspects of the current climate to ultimately narrow down the range of future Arctic climate predictions.

Previous studies have identified an emerging relationship for the surface-albedo feedback based on the observed seasonal cycle of Arctic sea ice. We similarly derive a positive relationship (r=0.70) between future and seasonal LRF, but due to its nature, no direct observation of the LRF exists. However, we find relationships between the future LRF and observable sea ice metrics, namely sea ice concentration, seasonality, extent and area. From these relationships, the sea ice concentration provides the strongest correlation (r=-0.76) for the area-averaged Arctic sea ice cover. This relationship implies a contribution of the LRF to future Arctic warming of approx. 2 K, which further relates to an AA of 4 K above global average at the end of the century.

We further emphasise the physical meaning behind our constraint: The negative emerging relationship implies that models with a lower Pan-Arctic sea ice concentration produce a larger LRF in the future. However, when dividing the entire sea ice area into regions of sea ice retreat (SIR) and persisting sea ice (PSI) in the future prediction, the relationship becomes positive over these two area-averaged regions. Thereby, the negative overall relationship is merely a result of the area-size distribution of SIR vs. PSI across the spread of model simulations. We conclude that while the Pan-Arctic perspective enables the emergent constraint, the physical meaning is hidden: A higher initial sea ice concentration produces a stronger positive Arctic LRF by setting the stage for greater sea ice retreat.

How to cite: Linke, O., Feldl, N., and Quaas, J.: Emergent constraints on the future Arctic lapse-rate feedback, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13091, https://doi.org/10.5194/egusphere-egu23-13091, 2023.

EGU23-14141 | Orals | CL4.7

Underestimation of Arctic warming trends in sub-seasonal forecasts 

Steffen Tietsche, Frederic Vitart, Michael Mayer, Antje Weisheimer, and Magdalena Balmaseda

The Arctic has warmed substantially over the last decades and will continue to do so owing to global warming in conjunction with polar amplification. The changing mean state poses many challenges to the construction, evaluation and calibration of subseasonal-to-seasonal forecasting systems, because it puts into question the representativeness of the system's retrospective forecasts (reforecasts). Furthermore, any inconsistencies with observed trends degrade the forecast skill and point to deficiencies in either the physical modelling or the initialization methods. Here, we assess the consistency of boreal winter trends of surface air temperature (SAT) in the Eurasian Arctic between the ERA5 reanalysis and ECMWF sub-seasonal reforecasts initialised from ERA5, for the 35-year period 1986-2021. We present methods to quantify robustness and importance of the observed trends, and to quantify the consistency of reforecast trends with these observed trends. We find that, in large parts of the marine Arctic, the reforecasts clearly underestimate the reanalsyis warming trend of about 1 K per decade at lead times beyond two weeks. For longer lead times, the reforecast trend is less than half of the reanalysis trend, with very high statistical significance. We present a series of numerical experiments to investigate potential reasons for the trend underestimation. These concern the sea-ice thermodynamic coupling to the atmosphere, impact of sea surface temperatures, and possible remote atmospheric influences from the North Atlantic and the Tropics. The outcome of these experiments provides guidance for future improvements in the physical forecast model and data assimilation methods needed to faithfully represent and predict Arctic climate variability and change.

How to cite: Tietsche, S., Vitart, F., Mayer, M., Weisheimer, A., and Balmaseda, M.: Underestimation of Arctic warming trends in sub-seasonal forecasts, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14141, https://doi.org/10.5194/egusphere-egu23-14141, 2023.

EGU23-14470 | Posters on site | CL4.7

A first look at the new PolarRES ensemble of polar regional climate model storylines to 2100 

Ruth Mottram, Priscilla Mooney, and Jose Abraham Torres and the PolarRES Consortium

The Horizon 2020 project PolarRES is coordinating a large international consortium of regional climate modelling groups in building a new ensemble of regional climate projections out to 2100. The ensemble is built at very high resolution (~12km) and using common domains, and set-ups to give directly comparable model outputs. At the same time, all regional climate models have been upgraded to a next-generation set-up, producing an ensemble of unprecedented sophistication.

We use a storyline approach, focused on Arctic amplification and cyclones in the northern hemisphere and Southern Annular Mode variability in Antarctica, to select global climate models for forcing on the boundaries. Each regional climate modelling group will downscale ERA5 and multiple global climate models. The data produced from these simulations will be used to improve process understanding under present and future conditions as well as to identify impacts of climate change in the polar regions.

Here, we present the experimental protocol developed in PolarRES and give details of the different regional climate models used, their setup, processes and domains as well as an overview of the outputs and planned applications. We show preliminary analysis of hindcast outputs to assess the performance of the ensemble. We invite other regional climate modelling groups outside the PolarRES consortium to consider using the same CORDEX -compatible model set-up and we are happy to receive suggestions of further spin-off studies or requests for collaboration.

 

How to cite: Mottram, R., Mooney, P., and Torres, J. A. and the PolarRES Consortium: A first look at the new PolarRES ensemble of polar regional climate model storylines to 2100, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14470, https://doi.org/10.5194/egusphere-egu23-14470, 2023.

EGU23-15746 | Orals | CL4.7

Simulating oceanic mesoscale eddy dynamics: A comparison of novel parameterizations and energy diagnostics and their impact on the global ocean circulation 

Stephan Juricke, Sergey Danilov, Marcel Oliver, Anton Kutsenko, and Kai Bellinghausen

In this study, we present a variety of parameterizations for simulating ocean eddy dynamics including novel viscous and kinetic energy backscatter closures. Their effect is analyzed using new diagnostics that allow for application on unstructured meshes.

Ocean mesoscale eddy dynamics play a crucial role for large-scale ocean currents as well as for the variability in the ocean and climate. The interactions between eddies and the mean flow affect strength, position and variations of ocean currents. Mesoscale eddies have a substantial impact on oceanic heat transport and the coupling between the atmosphere and ocean. However, at so-called eddy-permitting model resolutions around ¼°, eddy kinetic energy and variability is often substantially underestimated due to excessive dissipation of energy. Despite ever-increasing model resolutions, eddy-permitting simulations will still be used in uncoupled and coupled climate and Earth system simulations for years to come.

To improve the presentation of eddy dynamics in such resolution regimes, we present and systematically compare a set of viscous and kinetic energy backscatter parameterization with different complexity. These schemes are implemented in the unstructured grid, finite volume ocean model FESOM2 and tested in both idealized channel and global ocean simulations. We show that kinetic energy backscatter and adjusted viscosity parameterizations can alleviate some of the substantial eddy related biases, for example biases in sea surface height variability, mean currents and in water mass properties. We then further analyze the effect of these schemes on energy and dissipation spectra using new diagnostics that can be extended to the unstructured grid used by FESOM2. The rigorous intercomparison allows to make informed decisions on which schemes are the most suitable for a given application, considering the complexity of the schemes, their computational costs, their adaptability to various model resolutions and any simulation improvements related to a specific scheme. We will show that novel viscous and kinetic energy backscatter schemes outperform previously used, classical viscous closures. Furthermore, when compared to higher resolution simulations, they are computationally less expensive but achieve similar results.

How to cite: Juricke, S., Danilov, S., Oliver, M., Kutsenko, A., and Bellinghausen, K.: Simulating oceanic mesoscale eddy dynamics: A comparison of novel parameterizations and energy diagnostics and their impact on the global ocean circulation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15746, https://doi.org/10.5194/egusphere-egu23-15746, 2023.

The future evolution of the West Antarctic Ice Sheet (WAIS) will strongly influence the global sea-level rise in the coming decades. Ice shelf melting in that sector is partly controlled by the low-pressure system located off the West Antarctic coast, namely the Amundsen Sea Low (ASL). When the ASL is deep, an overall increase in ice shelf melting is noticed. Because of the sparse observational network and the strong internal variability, our understanding of the long-term climate changes in the atmospheric circulation is limited, and therefore its impact on ice melting as well. Among all the processes involved in the West Antarctic climate variability, an increasing number of studies have pointed out the strong impact of the climate in the tropical Pacific. However, most of those studies focus on the past decades, which prevents the analysis of the role of the multi-decadal tropical variability on the West Antarctic climate. Here, we combine annually-resolved paleoclimate records, in particular ice core and coral records, and the physics of climate models through paleoclimate data assimilation to provide a complete spatial multi-field reconstruction of climate variability in the tropics and Antarctic. This allows for studying both the year-to-year and multi-decadal variability of the tropical-Antarctic teleconnections. As data assimilation provides a climate reconstruction that is dynamically constrained, the contribution of the tropical variability on the West Antarctic climate changes can be directly assessed. Our results indicate that climate variability in the tropical Pacific is the main driver of ASL variability at the multi-decadal time scale, with a strong link to the Interdecadal Pacific Oscillation (IPO). However, the deepening of the Amundsen Sea Low over the 20th century cannot be explained by tropical climate variability. By using large ensembles of climate model simulations, our analysis suggests anthropogenic forcing as the primary driver of this 20th century ASL deepening. In summary, the 20th century ASL deepening is explained by the forcing, but the multi-decadal variability related to the  IPO is superimposed on this long-term trend.

How to cite: Dalaiden, Q., Abram, N., and Goosse, H.: Tropical Pacific variability and anthropogenic forcing are the key drivers of the West Antarctic atmospheric circulation variability over the 20th century, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-683, https://doi.org/10.5194/egusphere-egu23-683, 2023.

EGU23-991 | Orals | CR3.2

Future irreversible loss of Thwaites Glacier relative to global warming 

Emilia Kyung Jin, In-Woo Park, Hyun Joo Lee, and Won Sang Lee

The speed of West Antarctic melting is a very important factor in determining the degree of future global sea level rise. Loss of the Thwaites glacier due to global warming will have various regime changes in line with changes in the Earth system. The basal melting as a result of ocean warming can cause loss at an inhomogeneous rate across the underlying topography and overlying ice volume, while the change in precipitation from snow to rain as atmospheric warming can accelerate surface melting and trigger the irreversible loss.  

In this study, the ISSM model was driven with the ocean and atmospheric forcings obtained from the CMIP6 earth system model results, and future prediction experiments were performed until 2300. As a result, the accelerated period of melting of the Thwaites glacier related with forcings and the period of irreversible loss according to the structural characteristics and degree of warming are investigated. The mechanisms and timing that cause rapid ice loss are analyzed and the tipping point at which irreversible losses are triggered has been proposed as a function of warming.

How to cite: Jin, E. K., Park, I.-W., Lee, H. J., and Lee, W. S.: Future irreversible loss of Thwaites Glacier relative to global warming, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-991, https://doi.org/10.5194/egusphere-egu23-991, 2023.

EGU23-1329 | ECS | Orals | CR3.2

Characterizing the influence of idealized atmospheric forcings on firn using the SNOWPACK firn model 

Megan Thompson-Munson, Jennifer Kay, and Bradley Markle

The porous layer of snow and firn that blankets ice sheets can store meltwater and buffer an ice sheet’s contribution to sea level rise. A warming climate threatens this buffering capacity and will likely lead to depletion of the air-filled pore space, known as the firn air content. The timing and nature of the firn’s response to climate change is uncertain. Thus, understanding how the firn may evolve in different climate scenarios remains important. Here we use a one-dimensional, physics-based firn model (SNOWPACK) to simulate firn properties over time. To force the model, we generate idealized, synthetic atmospheric datasets that represent distinct climatologies on the Antarctic and Greenland Ice Sheets. The forcing datasets include temperature, precipitation, humidity, wind speed and direction, shortwave radiation, and longwave radiation, which SNOWPACK uses as input to simulate a firn column through time. We perturb the input variables to determine how firn properties respond to the perturbation, and how long it takes for those properties to reach a new equilibrium. We explore how different combinations of perturbations impact the firn to assess the effects of, for example, a warmer and wetter climate versus a warmer and drier climate. The firn properties of greatest interest are the firn air content, liquid water content, firn temperature, density, and ice slab content since these quantities help define the meltwater storage capacity of the firn layer. In our preliminary analysis, we find that with a relatively warm and wet base climatology representative of a location in southern Greenland, increasing the air temperature by 1 K yields a 48% decrease in firn air content and a 3% increase in the deep firn temperature 100 years after the perturbation. SNOWPACK also simulates near-surface, low-permeability ice slabs that inhibit potential meltwater storage in deeper firn. Conversely, decreasing the air temperature by 1 K yields a 7% increase in firn air content and a <1% decrease in the deep firn temperature in the same amount of time. In this scenario, the effects of warming are more extreme and have more adverse impacts on the firn’s meltwater storage capacity when compared to cooling. This work highlights the sensitivity of the firn to changing atmospheric variables and provides a framework for estimating the timescales and magnitude of firn responses to a changing climate.

How to cite: Thompson-Munson, M., Kay, J., and Markle, B.: Characterizing the influence of idealized atmospheric forcings on firn using the SNOWPACK firn model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1329, https://doi.org/10.5194/egusphere-egu23-1329, 2023.

EGU23-3405 | ECS | Orals | CR3.2

Disentangling the drivers of future Antarctic ice loss with a historically-calibrated ice-sheet model 

Violaine Coulon, Ann Kristin Klose, Christoph Kittel, Ricarda Winkelmann, and Frank Pattyn

Recent observations show that the Antarctic ice sheet is currently losing mass at an accelerating rate in areas subject to high sub-shelf melt rates. The resulting thinning of the floating ice shelves reduces their ability to restrain the ice flowing from the grounded ice sheet towards the ocean, hence raising sea level by increased ice discharge. Despite a relatively good understanding of the drivers of current Antarctic mass changes, projections of the Antarctic ice sheet are associated with large uncertainties, especially under high‐emission scenarios. This uncertainty may notably be explained by unknowns in the long-term impacts of basal melting and changes in surface mass balance. Here, we use an observationally-calibrated ice-sheet model to investigate the future trajectory of the Antarctic ice sheet until the end of the millennium related to uncertainties in the future balance between sub-shelf melting and ice discharge on the one hand, and the changing surface mass balance on the other. Our large ensemble of simulations, forced by a panel of CMIP6 climate models, suggests that the ocean will be the main driver of short-term Antarctic mass loss, triggering ice loss in the West Antarctic ice sheet (WAIS) already during this century. Under high-emission pathways, ice-ocean interactions will result in a complete WAIS collapse, likely completed before the year 2500 CE, as well as significant grounding-line retreat in the East Antarctic ice sheet (EAIS). Under a more sustainable socio-economic scenario, both the EAIS and WAIS may be preserved, though the retreat of Thwaites glacier appears to be already committed under present-day conditions. We show that with a regional near-surface warming higher than +7.5°C, which may occur by the end of this century under unabated emission scenarios, major ice loss is expected as the increase in surface runoff outweighs the increase in snow accumulation, leading to a decrease in the mitigating role of the ice sheet surface mass balance.

How to cite: Coulon, V., Klose, A. K., Kittel, C., Winkelmann, R., and Pattyn, F.: Disentangling the drivers of future Antarctic ice loss with a historically-calibrated ice-sheet model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3405, https://doi.org/10.5194/egusphere-egu23-3405, 2023.

EGU23-4042 | Posters on site | CR3.2

Experimental design for the the 2nd marine ice sheet and ocean model intercomparison project (MISOMIP2) 

Nicolas Jourdain, Jan De Rydt, Yoshihiro Nakayama, Ralph Timmermann, and Mathias Van Caspel

The 2nd Marine Ice Sheet and Ocean Model Intercomparison Project (MISOMIP2) is a natural progression of previous and ongoing model intercomparison exercises that have focused on the simulation of ice-sheet--ocean processes in Antarctica. The previous exercises motivate the move towards more realistic configurations and more diverse model parameters and resolutions. The first objective of MISOMIP2 is to investigate the robustness of ocean and ocean--ice-sheet models in a range of Antarctic environments, through comparisons to interannual observational data. We will assess the status of ocean--ice-sheet modelling as a community and identify common characteristics of models that are best able to capture observed features. As models are highly tuned based on present-day data, we will also compare their sensitivity to abrupt atmospheric perturbations leading to either very warm or slightly warmer ocean conditions than present-day. The approach of MISOMIP2 is to welcome contributions of models as they are, but we request standardised variables and common grids for the outputs. There will be two target regions, the Amundsen Sea and the Weddell Sea, chosen because they describe two extremely different ocean environments and have been relatively well observed compared to other parts of Antarctica. An observational "MIPkit" is provided to evaluate ocean and ice sheet models in these two regions.

How to cite: Jourdain, N., De Rydt, J., Nakayama, Y., Timmermann, R., and Van Caspel, M.: Experimental design for the the 2nd marine ice sheet and ocean model intercomparison project (MISOMIP2), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4042, https://doi.org/10.5194/egusphere-egu23-4042, 2023.

EGU23-6642 | ECS | Orals | CR3.2

Snow evolution through the Last Interglacial with a multi-layer snow model 

Thi Khanh Dieu Hoang, Aurélien Quiquet, Christophe Dumas, and Didier M. Roche

The Last Interglacial period (LIG), which occurred approximately between 130 and 116 kyr BP, is characterized by similar/warmer temperatures and higher sea levels compared to the present-day conditions due to the orbital variation of the Earth. Hence, the period provides insights into the behavior of the Earth's system components under stable and prolonged warm climates and their subsequent evolution into a glacial state. 

To better understand the ice sheet's surface mass balance that ultimately drives the advance and retreat of ice-sheets, we study the snow cover changes in the Northern Hemisphere during the LIG. In order to do so, we used BESSI (BErgen Snow Simulator), a physical energy balance model with 15 vertical snow layers and high computational efficiency, to simulate the snowpack evolution. First, BESSI was validated using the regional climate model MAR (Modèle Atmosphérique Régional) as forcing and benchmark for snow cover over the Greenland and Antarctica Ice Sheets under present-day climate. Using two distinct ice sheet climates helps constrain the different processes in place (e.g., albedo and surface melt for Greenland and sublimation for Antarctica). 

For the LIG simulations, the latest version of an Earth system model of intermediate complexity iLOVECLIM was used to force BESSI in different time slices to fully capture the snow evolution in the Northern Hemisphere throughout this period. Impacts of the downscaling component of iLOVECLIM, which provides higher resolution data and accounts for the influences of the topography, on BESSI performance are also discussed.  

The results show that BESSI performs well compared to MAR for the present-day climate, even with a less complex model set-up. Through the LIG, with the ability to model the snow compaction, the change of snow density and snow depth, BESSI simulates the snow cover evolution in the studied area better than the simple snow model (bucket model) included in iLOVECLIM. 

The findings suggest that BESSI can provide a more physical surface mass balance scheme to ice sheet models such as GRISLI of iLOVECLIM to improve simulations of the ice sheet - climate interactions.  

How to cite: Hoang, T. K. D., Quiquet, A., Dumas, C., and Roche, D. M.: Snow evolution through the Last Interglacial with a multi-layer snow model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6642, https://doi.org/10.5194/egusphere-egu23-6642, 2023.

EGU23-7020 | ECS | Orals | CR3.2

Uncertainties in Greenland ice sheet evolution and related sea-level projections until 2100 

Charlotte Rahlves, Heiko Goelzer, Petra Langebroek, and Andreas Born

The Greenland ice sheet is currently one of the main contributors to sea-level rise and mass loss from the ice sheet is expected to continue under increasing Arctic warming. Since sea-level rise is threatening coastal communities worldwide, reducing uncertainties in projections of future sea-level contribution from the Greenland ice sheet is of high importance. In this study we address the response of the ice sheet to future climate change. We determine rates of sea-level contribution that can be expected from the ice sheet until 2100 by performing an ensemble of standalone ice sheet simulations with the Community Ice Sheet Model (CISM). The ice sheet is initialized to resemble the presently observed geometry by inverting for basal friction. We examine a range of uncertainties, associated to stand alone ice sheet modeling by prescribing forcing from various global circulations models (GCMs) for different future forcing scenarios (shared socioeconomic pathways, SSPs). Atmospheric forcing is downscaled with the regional climate model MAR. The response of marine terminating outlet glaciers to ocean forcing is represented by a retreat parameterization and sampled by considering different sensitivities. Furthermore, we investigate how the initialization of the ice sheet with forcing from different global circulation models affects the projected rates of sea-level contribution. In addition, sensitivity of the results to the grid spacing of the ice sheet model is assessed. The observed historical mass loss is generally well reproduced by the ensemble. The projections yield a sea-level contribution in the range of 70 to 230 mm under the SSP5-8.5 scenario until 2100. Climate forcing constitutes the largest source of uncertainty for projected sea-level contribution, while differences due to the initial state of the ice sheet and grid resolution are minor.

 

 

How to cite: Rahlves, C., Goelzer, H., Langebroek, P., and Born, A.: Uncertainties in Greenland ice sheet evolution and related sea-level projections until 2100, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7020, https://doi.org/10.5194/egusphere-egu23-7020, 2023.

The precession of the equinoxes has a strong influence on the intensity of summer insolation according to most metrics and we would therefore expect the 23-Kyr and 19-Kyr precession cycles to be strongly reflected in our records of global ice volume, if summer insolation is indeed important for pacing glacial-interglacial cycles as proposed by Milutin Milankovitch. Instead, the precession signal is reduced in amplitude compared with the obliquity cycle in the Late Pleistocene, and in the Early Pleistocene (EP) precession appears completely absent in the δ18O stack. For this reason, the ‘40-Kyr world’ of the EP has been referred to as Milankovitch's other unsolved mystery. Indeed, numerous models of the Northern Hemisphere (NH) ice sheets simulated across the Plio-Pleistocene predict both a strong precessional and obliquity variability during the EP, at odds with the δ18O record. This points to the possibility of a dynamic Antarctic Ice Sheet in the EP that varied out-of-phase with the NH ice sheets at the precession period. In the original theory proposed by Raymo et al., (2006), from 3 to 1 Ma the East Antarctic Ice Sheet may have been land-terminating between 70S to 65S and sensitive to local summer insolation forcing. As precession is out-of-phase between the hemispheres, these variations could be cancelled out in globally integrated proxies of sea-level, concealing the true precession variability of both hemispheres in the marine sediment record. While studies have demonstrated  that precession-driven variations of the Antarctic Ice Sheet could cancel out NH variations in the deep-ocean record, no studies have investigated the actual feasibility of strong precession variability of the Antarctic Ice Sheet in the EP driven by local summer insolation, and whether it would have the magnitudes necessary to offset larger variations of the NH ice sheets. The question remains under what CO2 concentrations and orbital configuration can the East Antarctic Ice Sheet realistically be sensitive to local summer insolation forcing and possibly deglaciated from 70S to 65S, as postulated by Raymo et al. (2006). Can this produce the 10-30 m of sea-level necessary to offset NH variations in ice volume? To investigate the feasibility for anti-phased precession variability between the NH ice sheets and Antarctica in the EP, we use a zonally-averaged energy balance model coupled to a 1-D ice sheet model of a northern and southern hemisphere ice sheet, forced by atmospheric CO2 concentrations and daily insolation fields. The model will simulate glacial cycles across the Quaternary for different CO2 scenarios and determine whether anti-phased precessional cycles in ice volume between the hemispheres is a viable mechanism to explain the 40-Kyr world found in the δ18O record.

How to cite: Gunning, D.: Investigating precession cancellation across the MPT using a zonally averaged energy balance model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7385, https://doi.org/10.5194/egusphere-egu23-7385, 2023.

EGU23-7422 | ECS | Orals | CR3.2 | Highlight

(Ir)reversibility of future Antarctic mass loss on multi-millennial timescales 

Ann Kristin Klose, Violaine Coulon, Frank Pattyn, and Ricarda Winkelmann

Given the potentially high magnitudes and rates of future warming, the long-term evolution of the Antarctic Ice Sheet is highly uncertain. While recent projections under Representative Concentration Pathway 8.5 estimate the Antarctic sea-level contribution by the end of this century between -7.8 cm and 30.0 cm sea-level equivalent (Seroussi et al., 2020), sea-level might continue to rise for millennia to come due to ice sheet inertia, resulting in a substantially higher long-term committed sea-level change. In addition, potentially irreversible ice loss due to several self-amplifying feedback mechanisms may be triggered within the coming centuries, but evolves thereafter over longer timescales depending on the warming trajectory. It is therefore necessary to account for the timescale difference between forcing and ice sheet response in long-term sea-level projections by (i) determining the resulting gap between transient and committed sea-level contribution with respect to changing boundary conditions, (ii) testing the reversibility of large-scale ice sheet changes, as well as (iii) exploring the potential for safe overshoots of critical thresholds when reversing climate conditions from enhanced warming to present-day.

Here, we assess the sea-level contribution from mass balance changes of the Antarctic Ice Sheet on multi-millennial timescales, as well as ice loss reversibility. The Antarctic sea-level commitment is quantified using the Parallel Ice Sheet Model (PISM) and the fast Elementary Thermomechanical Ice Sheet (f.ETISh) model by fixing forcing conditions of warming trajectories from state-of-the-art climate models available from the sixth phase of the Coupled Model Intercomparison Project (CMIP6) at regular intervals in time. The ice sheet then evolves for several millennia under constant climate conditions. Finally, the climate forcing is reversed to present-day starting from different stages of ice sheet decline to test for the reversibility of ice loss.

Our results suggest that the Antarctic Ice Sheet may be committed to a strong grounding-line retreat or even a collapse of the West Antarctic Ice Sheet when keeping climate conditions constant at warming levels reached during this century. Fixing climate conditions later in time may additionally trigger a substantial decline of the East Antarctic Ice Sheet. We show that the reversibility of Antarctic ice loss as well as the potential for safe overshoots strongly depend on the timing of the reversal of the forcing.

How to cite: Klose, A. K., Coulon, V., Pattyn, F., and Winkelmann, R.: (Ir)reversibility of future Antarctic mass loss on multi-millennial timescales, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7422, https://doi.org/10.5194/egusphere-egu23-7422, 2023.

EGU23-7507 | ECS | Posters on site | CR3.2

The influence of temperature variability on the Greenland ice sheet 

Mikkel Lauritzen, Guðfinna Aðalgeirsdóttir, Nicholas Rathmann, Aslak Grinsted, Brice Noël, and Christine Hvidberg

The projected contribution of the Greenland ice sheet to sea-level rise in response to future warming relies upon the state of the present-day ice sheet, and one of the main contributors to uncertainties in projections is due to uncertainties in the initial state of the simulated ice sheet. A previous study showed that including the inter-annual climate variability in an idealized ice sheet model leads to an increased mass loss rate, but the effect on the Greenland ice sheet is not known. Here we present a study using the PISM model to quantify the influence of inter-annual variability in climate forcing on the Greenland ice sheet. 
We construct an ensemble of climate-forcing fields that account for inter-annual variability in temperature using reanalysis data products from RACMO and NOAA-CIRES, and we investigate the steady state and the sensitivity of the simulated Greenland ice sheet under these different scenarios.
We find that the steady state volume decreases by 0.24-0.38% when forced with a variable temperature forcing compared to a constant temperature forcing, corresponding to 21.7±5.0 mm of sea level rise, and the response to abrupt warming is 0.03-0.21 mm SLE a-1 higher depending on climate scenario. The northern basins are particularly sensitive with a change in volume of 1.2-0.9%. Our results emphasize the importance of including climate variability in projections of future mass loss.

How to cite: Lauritzen, M., Aðalgeirsdóttir, G., Rathmann, N., Grinsted, A., Noël, B., and Hvidberg, C.: The influence of temperature variability on the Greenland ice sheet, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7507, https://doi.org/10.5194/egusphere-egu23-7507, 2023.

EGU23-7553 | ECS | Orals | CR3.2

Examining Possible Retreat Scenarios for the Greenland Ice Sheet during the MIS-11c Interglacial 

Brian Crow, Lev Tarasov, Matthias Prange, and Michael Schulz

The interglacial period spanning ca. 423 to 398 ka and known as Marine Isotope Stage (MIS) 11c has been the subject of much study, due largely to the unique evolution of global temperatures, greenhouse gas levels, and sea levels relative to other interglacials of the late Pleistocene. Particularly concerning is some geological evidence and prior modeling studies which have suggested that a large majority of the Greenland ice sheet (GrIS) disappeared during this period, despite global mean air temperatures only modestly higher than those of the preindustrial period. However, uncertainty is high as to the extent and spatiotemporal evolution of this melt due to a dearth of direct geological constraints. Our study therefore endeavors to better constrain these large uncertainties by using spatiotemporally interpolated climate forcing from CESM v1.2 time slice simulations and an ensemble of ice sheet model parameter vectors derived from a GrIS history matching over the most recent glacial cycle from the Glacial Systems Model (GSM). The use of different ice sheet initialization states from simulations of the previous glacial-interglacial transition helps to capture the large initial condition uncertainty. Two different regional present-day climate modeling datasets are utilized for anomaly correction of CESM precipitation and temperature fields. 

Preliminary analysis indicates that the most robust retreat across most ensemble members happens in the northern, western, and central portions of the ice sheet, while the higher terrain of the south and east retain substantial amounts of ice. This is broadly consistent with indications that ice may have survived the MIS-11c interglacial at the Summit ice core location, but not at DYE-3. Simulations indicate a maximum MIS-11c sea level contribution from the GrIS centered between 408 and 403 ka, with minimum GrIS volumes reaching between 25% and 70% of modern-day values. In part due to the prior constraint of ice-sheet model ensemble parameters from history matching, ensemble parameters controlling downscaling and climate forcing bias correction are the largest parametric sources of output variance in our simulations.  Though CESM uncertainties are unassessed in this study, it is likely they dominate given that the choice of present-day reference temperature climatology for anomaly correction of the climate model output has the largest effect on the GrIS melt response in our simulations.

How to cite: Crow, B., Tarasov, L., Prange, M., and Schulz, M.: Examining Possible Retreat Scenarios for the Greenland Ice Sheet during the MIS-11c Interglacial, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7553, https://doi.org/10.5194/egusphere-egu23-7553, 2023.

EGU23-7920 | ECS | Orals | CR3.2

The Divergent Futures of Greenland Surface Mass Balance Estimates from Different Regional Climate Models 

Quentin Glaude, Brice Noel, Martin Olesen, Fredrik Boberg, Michiel van den Broeke, Ruth Mottram, and Xavier Fettweis

Arctic amplification is causing global warming to have a more intense impact on arctic regions, with consequences on the surface mass balance and glacier coverage of Greenland. The glaciers of Greenland are also shrinking, contributing to sea level rise as well. Projecting the future evolution of these changes is crucial for understanding the likely impacts of climate change on sea level rise.

In this study, we compared three state-of-the-art Regional Climate Models (RCMs) (MAR, RACMO, and HIRHAM) using a common grid and forcing data from Earth System Models to assess their ability to project future changes in Greenland's surface mass balance up to 2100. We also considered the impact of different Earth System Models and Shared Socioeconomic Pathways.

The results of this comparison showed significant differences in the projections produced by these different models, with a factor-2 difference in mass loss between MAR and RACMO on cumulative Surface Mass Balance anomalies. These differences are important as RCMs are often used as inputs for ice sheet models, which are used to make predictions about sea level rise. Furthermore, we aim to investigate the causes of these differences, as understanding them will be key to improving the accuracy of sea level rise projections.

The uncertainty of the RCMs projections are translated into uncertainties in Sea-Level-Rise projections. The results presented here open the door for deeper investigations in the climate modeling community and the physical reasons linked to these divergences. Our study highlighted the importance of continued research and development of RCMs to better understand the physics implemented in these models and ultimately improve the accuracy of future sea level rise projections.

How to cite: Glaude, Q., Noel, B., Olesen, M., Boberg, F., van den Broeke, M., Mottram, R., and Fettweis, X.: The Divergent Futures of Greenland Surface Mass Balance Estimates from Different Regional Climate Models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7920, https://doi.org/10.5194/egusphere-egu23-7920, 2023.

EGU23-8341 | Orals | CR3.2 | Highlight

Antarctic Ice Sheet tipping points in the last 800,000 years 

David Chandler, Petra Langebroek, Ronja Reese, Torsten Albrecht, and Ricarda Winkelmann

Stability of the Antarctic Ice Sheet in the present-day climate, and in future warming scenarios, is of growing concern as increasing evidence points towards the prospect of irreversible ice loss from the West Antarctica Ice Sheet (WAIS) with little or no warming above present. Here, in transient ice sheet simulations for the last 800,000 years (9 glacial-interglacial cycles), we find evidence for strong hysteresis between ice volume and ocean temperature forcing through each glacial cycle, driven by rapid WAIS collapse and slow recovery. Additional equilibrium simulations at several climate states show this hysteresis does not arise solely from the long ice sheet response time, instead pointing to consistent tipping-point behaviour in the WAIS. Importantly, WAIS collapse is triggered when continental shelf bottom water is maintained above a threshold of 0 to 0.25°C above present, and there are no stable states for the WAIS in conditions warmer than present. Short excursions to warmer temperatures (marine isotope stage 7) may not initiate collapse (‘borrowed time’), while the more sustained interglacials (stages 11, 9, 5e) demonstrate an eventual WAIS collapse. Cooling of ca. 2°C below present-day is then required to initiate recovery. Despite the differing climatic characteristics of each glacial cycle, consistency between both the transient and equilibrium behaviour of the ice sheet through several cycles shows there is some intrinsic predictability at millennial time scales, supporting the use of Pleistocene ice sheet simulations and geological evidence as constraints on likely future behaviour.

How to cite: Chandler, D., Langebroek, P., Reese, R., Albrecht, T., and Winkelmann, R.: Antarctic Ice Sheet tipping points in the last 800,000 years, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8341, https://doi.org/10.5194/egusphere-egu23-8341, 2023.

EGU23-8690 | ECS | Orals | CR3.2

Antarctic sensitivity to oceanic melting parameterizations 

Antonio Juárez-Martínez, Javier Blasco, Marisa Montoya, Jorge Alvarez-Solas, and Alexander Robinson

Ice in Antarctica has been experiencing dramatic changes in the last decades. These variations have consequences in terms of sea level, which could have an impact on human societies and life on the planet in the future. The Antarctic Ice Sheet (AIS) could become the main contributor to sea-level rise in the coming centuries, but there is a great uncertainty associated with its contribution, which is due in part to the complexity of the coupled ice-ocean processes. In this study we investigate the contribution of the AIS to sea-level rise in the coming centuries in the context of the Ice Sheet Model Intercomparison Project (ISMIP6), but covering a range beyond 2100, using the higher-order ice-sheet model Yelmo. We test the sensitivity of the model  to basal melting parameters using several forcings and scenarios for the atmosphere and ocean, obtained from different GCM models. The results show a strong  dependency on variations of the parameter values of the basal melting laws and also on the forcing that is chosen. Higher values of the heat exchange velocity between ice and ocean lead to higher sea-level rise, varying the contribution depending on the forcing. Ice-ocean interactions therefore can be expected to contribute significantly to the uncertainty associated with the future evolution of the AIS.

 

How to cite: Juárez-Martínez, A., Blasco, J., Montoya, M., Alvarez-Solas, J., and Robinson, A.: Antarctic sensitivity to oceanic melting parameterizations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8690, https://doi.org/10.5194/egusphere-egu23-8690, 2023.

EGU23-8853 | ECS | Orals | CR3.2

Sensitivity of Heinrich events to boundary forcing perturbations in a coupled ice sheet-solid Earth model 

Clemens Schannwell, Uwe Mikolajewicz, Marie Kapsch, and Florian Ziemen

Heinrich events are one of the prominent signals of glacial climate variability. They are characterised as abrupt, quasi-periodic episodes of ice-sheet instabilities during which large numbers of icebergs are released from the Laurentide ice sheet. These events affect the evolution of the global climate by modifying the ocean circulation through the addition of freshwater and the atmospheric circulation through changes in ice-sheet height. However, the mechanisms controlling the timing and occurrence of Heinrich events remain enigmatic to this day. Here, we present simulations with a coupled ice-sheet solid Earth model that aim to quantify the importance of different boundary forcings for the timing of Heinrich events. We focus the analysis on two prominent ice streams of the Laurentide ice sheet with the land-terminating Mackenzie ice stream and the marine-terminating Hudson ice stream. Our simulations identify different surge characteristics for the Mackenzie ice stream and the Hudson ice stream. Despite their different glaciological and climatic settings, both ice streams exhibit responses of similar magnitude to perturbations to the surface mass balance and the geothermal heat flux. However, Mackenzie ice stream is more sensitive to changes in the surface temperature. Changes to the ocean temperature and the global sea level have a negligible effect on the timing of Heinrich events in our simulations for both ice streams. We also show that Heinrich events for both ice streams only occur in a certain parameter space. Transitioning from an oscillatory Heinrich event state to a persistent streaming state can lead to an ice volume loss of up to 30%. Mackenzie ice stream is situated in a climate that is particularly close to this transition point, underlining the potential of the ice stream to have contributed to prominent abrupt climate events during glacial-interglacial transitions.

How to cite: Schannwell, C., Mikolajewicz, U., Kapsch, M., and Ziemen, F.: Sensitivity of Heinrich events to boundary forcing perturbations in a coupled ice sheet-solid Earth model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8853, https://doi.org/10.5194/egusphere-egu23-8853, 2023.

EGU23-8973 | ECS | Posters on site | CR3.2

How does the Greenland ice sheet respond on a medium-term time scale to various levels of warming? 

Alison Delhasse, Johanna Beckmann, and Christoph Kittel

The Greenland ice sheet is considered as one of the main causes of sea level rise (SLR) at the end of the 21st century. But what if it is already too late to reverse the loss of ice from the Greenland ice sheet? The mass balance (MB) resulting from the coupling between the Regional Atmospheric Model (MAR, ULiège) and the Parallel Ice Sheet Model (PISM, PIK) over Greenland following the CESM2 ssp585 climate indicates that even if we stop the CESM2 warming in 2100 and continue with a +7°C climate until 2200 with respect to the reference period (1961-1990), the GrIS continues to lose mass up to a contribution equivalent to 60 cm of SLR in 2200. From this coupling experiment, we ran several coupled simulations by stabilizing the warming at different thresholds (+ 1, 2, 3, ... °C) with respect to our reference period in order to highlight a kind of tipping point of the ice sheet with respect to atmospheric warming. Other experiments have been launched by reversing the climate imposed by CESM2 from 2100 to 2000, for example, with the aim of identifying whether the GrIS could gain ice mass again with a climate as warm as the present one.

How to cite: Delhasse, A., Beckmann, J., and Kittel, C.: How does the Greenland ice sheet respond on a medium-term time scale to various levels of warming?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8973, https://doi.org/10.5194/egusphere-egu23-8973, 2023.

EGU23-9449 | Posters on site | CR3.2

Interactive coupling of the Antarctic Ice Sheet and the global ocean 

Moritz Kreuzer, Willem Huiskamp, Torsten Albrecht, Stefan Petri, Ronja Reese, Georg Feulner, and Ricarda Winkelmann

Increased sub-shelf melting and ice discharge from the Antarctic Ice sheet has both regional and global impacts on the ocean and the overall climate system. Additional meltwater, for example, can reduce the formation of Antarctic Bottom Water, potentially affecting the global thermohaline circulation. Similarly, increased input of fresh and cold water around the Antarctic margin can lead to a stronger stratification of coastal waters, and a potential increase in sea-ice formation, trapping warmer water masses below the surface, which in turn can lead to increased basal melting of the ice shelves.

So far these processes have mainly been analysed in simple unidirectional cause-and-effect experiments, possibly neglecting important interactions and feedbacks. To study the long-term and global effects of these interactions, we have developed a bidirectional offline coupled ice-ocean model framework. It consists of the global ocean and sea-ice model MOM5/SIS and an Antarctic instance of the Parallel Ice Sheet Model PISM, with the ice-shelf cavity module PICO representing the ice-ocean boundary layer physics. With this setup we are analysing the aforementioned interactions and feedbacks between the Antarctic Ice Sheet and the global ocean system on multi-millenial time scales.

How to cite: Kreuzer, M., Huiskamp, W., Albrecht, T., Petri, S., Reese, R., Feulner, G., and Winkelmann, R.: Interactive coupling of the Antarctic Ice Sheet and the global ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9449, https://doi.org/10.5194/egusphere-egu23-9449, 2023.

EGU23-9747 | Orals | CR3.2

Climate variability as a major forcing of recent Antarctic ice-mass change 

Matt King, Kewei Lyu, and Xuebin Zhang

Antarctica has been losing ice mass for decades, but its link to large-scale modes of climate forcing is not clear. Shorter-period variability has been partly associated with El Niño Southern Oscillation (ENSO), but a clear connection with the dominant climate mode, the Southern Annular Mode (SAM), is yet to be found. We show that space gravimetric estimates of ice-mass variability over 2002-2021 may be substantially explained by a simple linear relation with detrended, time-integrated SAM and ENSO indices, from the whole ice sheet down to individual drainage basins. Approximately 40% of the ice-mass trend over the GRACE period can be ascribed to increasingly persistent positive SAM forcing which, since the 1940s, is likely due to anthropogenic activity. Similar attribution over 2002-2021 could connect recent ice-sheet change to human activity.

How to cite: King, M., Lyu, K., and Zhang, X.: Climate variability as a major forcing of recent Antarctic ice-mass change, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9747, https://doi.org/10.5194/egusphere-egu23-9747, 2023.

EGU23-9842 | ECS | Orals | CR3.2

The choice of present-day climate forcing can significantly affect modelled future and past Antarctic Ice Sheet evolution 

Christian Wirths, Johannes Sutter, and Thomas Stocker

Model simulations of past and future Antarctic ice sheet (AIS) evolution depend on the applied climatic forcing. To model the present and future Antarctic ice sheet, several different forcings from regional climate models are available. It is therefore critical to understand the influence and the resulting model differences and uncertainties associated with the choice of present-day reference forcing.  

We apply present-day climatic forcings from regional models (RACMO2.3p2, MAR3.10, HIRHAM5 and COSMO-CLM2) combined with climate anomalies from a global climate model (HadGEM2-ES). With this setup, we investigate the future evolution of the AIS under the RCP2.6, RCP4.5 and RCP8.5 scenarios using the Parallel Ice Sheet Model (PISM). We find substantial differences in the future evolution of the AIS depending on the choice of the present-day reference field even under an extreme scenario such as RCP8.5. We discuss the influence of those forcing choices on the projected future AIS dynamics and sea-level contribution, considering a variety of ice sheet model parameterizations. 

With this analysis, we aim to gain a better understanding of the role of climate forcing choices and parameterization-induced uncertainties of sea-level rise projections. 

 

How to cite: Wirths, C., Sutter, J., and Stocker, T.: The choice of present-day climate forcing can significantly affect modelled future and past Antarctic Ice Sheet evolution, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9842, https://doi.org/10.5194/egusphere-egu23-9842, 2023.

EGU23-9904 | Orals | CR3.2

Response of the Greenland Ice Sheet to temperature overshoot scenarios  

Michele Petrini, Heiko Goelzer, Petra Langebroek, Charlotte Rahlves, and Jörg Schwinger

As there is no evidence for the implementation of sufficiently ambitious global CO2 emission reductions, it is very unlikely that we will be able to keep the global mean warming at the end of the century below the 1.5 C limit set in the Paris Agreement. However, the development of CO2 removal techniques could potentially allow us to reach the 1.5 C target after a period of temperature overshoot, by offsetting past and current high levels of emissions with net-negative emissions in the future. To assess the effectiveness and the risks associated to such mitigation options, we need to better understand the impact of temperature overshoot scenarios on various components of the Earth System.  

Here, we focus on the Greenland Ice Sheet. We force an ice-sheet model (CISM2) with Surface Mass Balance (SMB) from an ensemble of 400 years-long idealized overshoot simulations, carried out with the Norwegian Earth System Model NorESM2. The SMB, which is calculated in NorESM2 using an energy balance scheme at multiple elevation classes, is downscaled during runtime to the ice-sheet model grid, thus allowing to account explicitly for the SMB-height feedback. In this presentation, we will assess the sea-level contribution of the Greenland Ice Sheet for overshoot pathways, compared to reference pathways without overshoot. Moreover, we will assess the impact of individual processes, such as the SMB-height feedback and the ocean-driven mass loss at marine-terminating margins, on the sea-level contribution of the Greenland Ice Sheet.  

How to cite: Petrini, M., Goelzer, H., Langebroek, P., Rahlves, C., and Schwinger, J.: Response of the Greenland Ice Sheet to temperature overshoot scenarios , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9904, https://doi.org/10.5194/egusphere-egu23-9904, 2023.

EGU23-10165 | Orals | CR3.2

Competing climate feedbacks of ice sheet freshwater discharge in a warming world 

Dawei Li, Robert DeConto, and David Pollard

Earth's polar ice sheets are projected to undergo significant retreat in the coming centuries if anthropogenic warming were to continue unabated, injecting freshwater stored on land over millennia into oceans and raise the global mean sea level. Ice sheet freshwater flux alters the status of ocean stratification and ocean-atmosphere heat exchange, inducing oceanic surface cooling and subsurface warming, hence an impact on the global climate. How the climate effects of ice sheet freshwater would feedback to influence the retreat of ice sheets, however, remains unsettled. Here we develop a two-way coupled climate-ice sheet modeling tool to assess the interactions between retreating polar ice sheets and the climate, considering a variety of greenhouse gas emission scenarios and modeled climate sensitivities. Results from coupled ice sheet-climate modeling show that ice sheet-ocean interactions give rise to multi-centennial oscillations in ocean temperatures around Antarctica, which would make it challenging to isolate anthropogenic signals from observational data. Future projections unveil both positive and negative feedbacks associated with freshwater discharge from the Antarctic Ice Sheet, while the net effect is scenario-dependent. The West Antarctic Ice Sheet collapses in high-emission scenarios, but the process is slowed significantly by cooling induced by ice sheet freshwater flux.

How to cite: Li, D., DeConto, R., and Pollard, D.: Competing climate feedbacks of ice sheet freshwater discharge in a warming world, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10165, https://doi.org/10.5194/egusphere-egu23-10165, 2023.

EGU23-10204 | ECS | Orals | CR3.2

Parameter ensemble simulations of the North American and Greenland ice sheets and climate of the Last Glacial Maximum with Famous-BISICLES 

Sam Sherriff-Tadano, Niall Gandy, Ruza Ivanovic, Lauren Gregoire, Jonathan Owen, Charlotte Lang, Jonathan Gregory, Robin Smith, and Tamsin Edwards
Testing the ability of climate-ice sheet coupled models to simulate past ice sheets and climates can provide a way to evaluate the models. One example is the Last Glacial Maximum (LGM), when huge ice sheets covered the Northern Hemisphere, especially over the North America. Here, we perform 200 ensemble member simulations of the North American and Greenland ice sheets and climate of the LGM with an ice sheet-atmosphere-slab ocean coupled model Famous-BISICLES. 16 parameters associated with climate and ice dynamics are varied. The simulated results are evaluated against the LGM global temperature, the total ice volume and the ice extent at the southern margin of the North American ice sheet. In the ensemble simulations, the global temperature is controlled by the combination of precipitation efficiency in the large-scale condensation and entrainment rate in the cumulus convection. Under reasonable LGM global temperature, we find that the surface albedo and Weertman coefficient in the basal sliding law control the North American ice volume. In contrast, the ice volume of Greenland is found to be controlled by the Weertman coefficient. Based on the constraints, the model produces 6 good simulations with reasonable global temperature and North American ice sheet. We also find that warm summer surface temperature biases at the ice sheet interior as well as downscaling of surface mass balance based on altitude can cause strong local ice melting. This implies the need of better representing the atmospheric conditions and surface mass balance in the ice sheet interior.

How to cite: Sherriff-Tadano, S., Gandy, N., Ivanovic, R., Gregoire, L., Owen, J., Lang, C., Gregory, J., Smith, R., and Edwards, T.: Parameter ensemble simulations of the North American and Greenland ice sheets and climate of the Last Glacial Maximum with Famous-BISICLES, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10204, https://doi.org/10.5194/egusphere-egu23-10204, 2023.

EGU23-10231 | ECS | Orals | CR3.2

The effect of an evolving Greenland ice sheet in NorESM2 projections 

Konstanze Haubner, Heiko Goelzer, Petra Langebroek, and Andreas Born

The Greenland ice sheet's mass loss is increasing and so is its impact to the climate system. Yet, Earth System models mostly keep ice sheets at a constant extent or treat interactions with the ice sheets fairly simple.

Here, we present the first simulations of NorESM2 coupled to the ice sheet model CISM over Greenland. We compare NorESM2 simulations from 1850 to 2300 with and without an evolving ice sheet over Greenland based on the ssp585 scenario and its extension to 2300. Ocean and atmosphere horizontal resolution are on 1deg, while the coupled ice sheet module CISM is running on 4km. The coupling setup is based on CESM2. Ice extent and elevation are provided to the atmosphere every 5years and the land model every year. Whereas the ice sheet receives updated surface mass balance every year.
We show the evolution of the Greenland ice sheet and changes in atmosphere, ocean and sea ice.

Overall global mean surface air temperatures (SAT) change from 14°C to 24°C by 2300 with the steepest increase between 2070-2200.
Over the Southern ocean and Antarctica, SAT are increasing by 10°C, while over the Northern hemisphere we see a change of 15-28°C by 2300. 
At the end of the simulations (year 2300), SAT over Greenland are 6°C warmer when including an evolving ice sheet. In contrast, the ocean surrounding Greenland shows SAT that are 2°C colder in the coupled system, compared to the simulation with a fixed Greenland ice sheet. Sea surface temperatures show the same ~2°C difference around Greenland in coupled and uncoupled simulation. The overall change in sea surface temperatures is 12°C.
Minimum and maximum sea ice extent differs only slightly with and without the coupling, indicating that the overall warming seems to dictate speed of the sea ice retreat.

How to cite: Haubner, K., Goelzer, H., Langebroek, P., and Born, A.: The effect of an evolving Greenland ice sheet in NorESM2 projections, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10231, https://doi.org/10.5194/egusphere-egu23-10231, 2023.

The mid-Pleistocene Transition (MPT) from 41 kyr to 100 kyr glacial cycles was one of the largest changes in the Earth system over the past 2 million years. The transition happened in the absence of a relevant change in orbital forcing. As such, it presents a challenge for the Milankovitch theory of glacial cycles. A change from a low to high friction bed under the North American Ice Complex through the removal of pre-glacial regolith has been hypothesized to play a critical role in the transition. For testing, this hypothesis requires constraint on pre-glacial regolith cover and topography as well as mechanistic constraint on whether the appropriate amount of regolith can be removed from the required regions to enable MPT occurrence at the right time. To date, however, Pleistocene regolith removal has not been simulated for a realistic, 3D North American ice sheet fully resolving relevant basal processes. A further challenge is very limited constraints on pre-glacial bed elevation and sediment thickness.

Herein, we address these challenges with an appropriate computational model and ensemble-based analysis addressing parametric and initial mean sediment cover uncertainties. We use the 3D Glacial Systems Model that incorporates relevant glacial processes. Specifically, it includes: 3D thermomechanically coupled hybrid SIA/SSA ice physics, fully coupled sediment production and transport, subglacial linked-cavity and tunnel hydrology, isostatic adjustment from dynamic loading and erosion, and climate from a 2D non-linear energy balance model and glacial index. The sediment model includes quarrying and abrasion for sediment production with both englacial and subglacial transport. The coupled system is driven only by atmospheric CO2 and insolation.

We show that the ice, climate, and sediment processes encapsulated in this fully coupled glacial systems model enables capture of the evolution of the Pleistocene North American glacial system. Specifically and within observational uncertainty, our model captures: the shift from 41 to 100 kyr glacial cycles, early Pleistocene extent, LGM ice volume, deglacial ice extent, and the broad present-day sediment distribution. We also find that pre-glacial sediment thickness and topography have a strong influence on the strength and duration of early Pleistocene glaciations.

How to cite: Drew, M. and Tarasov, L.: The pre-Pleistocene North American bed from coupled ice-climate-sediment physics and its strong influence on glacial cycle evolution, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10318, https://doi.org/10.5194/egusphere-egu23-10318, 2023.

EGU23-10677 | Orals | CR3.2 | Highlight

Impacts of regional sea-level changes due to GRD effects on multi-centennial projections of Antarctic Ice Sheet under the ISMIP6-2300 experimental protocol  

Holly Han, Matt Hoffman, Xylar Asay-Davis, Trevor Hillebrand, and Mauro Perego

Evolution of ice sheets contribute to sea-level change globally by exchanging mass with the ocean, and regionally by causing the solid Earth deformation and perturbation of the Earth’s rotation and gravitational field, so-called “gravitational, rotational and deformational (GRD) effects”. In the last decade, much work has been done to establish the importance of coupling GRD effects particularly in modeling of marine-based ice sheets (e.g., West Antarctic Ice Sheet; WAIS) to capture the interactions between ice sheets, sea level and the solid Earth at the grounding lines. However, coupling of GRD effects has not yet been done widely within the ice-sheet modeling community; for example, GRD effects were not included in any of the ice sheet models that contributed to the most recent recent ice-sheet model intercomparison through 2100 (Ice Sheet Model Intercomparison Project for CMIP6: ISMIP6-2100; Serrousi et al., 2020) cited by the latest IPCC AR6 report.

In this work, we couple the US Department of Energy’s MPAS-Albany Land Ice model (which was one of the models that participated in the ISMIP6-2100 project) to a 1D sea-level model and perform coupled simulations of Antarctica under the new ISMIP6-2300 protocol in which climate forcing is extended beyond 2100 to 2300. Comparing to the standalone ice-sheet simulations with fix bed topography without GRD effects, the results from our coupled simulations show multi-decadal to centennial-scale delays in the retreat of the Thwaites glacier in the West Antarctica. Our results further suggest that the strength of the negative feedback of sea-level changes on the WAIS retreat becomes weaker as the strength of the applied forcing increases, implying the pertinence of our commitment to limiting greenhouse gas emissions. In addition, within our coupled ice sheet-sea level modeling frame, we introduce a new workflow work in which the ISMIP6 protocol-provided ocean thermal forcing is re-extrapolated based on the updated ocean bathymetry. Our preliminary results indicate that bedrock uplift due to ice mass loss can block the bottom warm ocean, providing additional negative feedback, but also can block cold water when/if the vertical ocean temperature profile gets inverted due to climate change (e.g., as represented in the UKESM model - SSP585 scenario results).

How to cite: Han, H., Hoffman, M., Asay-Davis, X., Hillebrand, T., and Perego, M.: Impacts of regional sea-level changes due to GRD effects on multi-centennial projections of Antarctic Ice Sheet under the ISMIP6-2300 experimental protocol , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10677, https://doi.org/10.5194/egusphere-egu23-10677, 2023.

EGU23-11678 | ECS | Posters on site | CR3.2

Antarctic ice sheet response to AMOC shutdowns during the penultimate deglaciation 

Maxence Menthon, Pepijn Bakker, Aurélien Quiquet, and Didier M. Roche

According to geological records, the sea level during the Last Interglacial (∼ 129–116 ka) peaked 6 to 9 m higher than during the pre-industrial with a major contribution from the Antarctic ice sheet (Dutton et al. 2015). According to Clark et al. 2020, a longer period of reduced Atlantic Meridional Overturning Circulation (AMOC) during the penultimate deglaciation compared to the last deglaciation could have led to greater subsurface warming and subsequent larger Antarctic Ice Sheet retreat.

Here we study the response of the Antarctic ice sheet to climate forcing with a forced AMOC shutdown at different timing and duration during the penultimate deglaciation (∼ 138–128 ka). The simulations are done with the Earth System Model of Intermediate Complexity iLOVECLIM (Roche et al. 2014) and the ice sheet model GRISLI (Quiquet et al. 2018), using the recently implemented sub-shelf melt module PICO (Reese et al. 2018). In the present simulations the GRISLI is forced with the iLOVECLIM simulations and is a step towards a fully coupled climate - ice sheet set up to take into account the climate - ice sheet interactions in a physical way.

We hypothesize that both the duration and timing of reduced AMOC can significantly affect the sensitivity of the Antarctic Ice Sheet. A longer period of AMOC reduction will lead to a larger subsurface warming in the Southern Ocean and subsequently a larger ice sheet retreat. On the other hand, an AMOC reduction earlier (later) in the deglaciation implies that the ice sheet that is affected by this subsurface warming is still fairly large (already small). We will discuss both the individual as well as combined effect of duration and timing on the ice sheet evolution.

How to cite: Menthon, M., Bakker, P., Quiquet, A., and Roche, D. M.: Antarctic ice sheet response to AMOC shutdowns during the penultimate deglaciation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11678, https://doi.org/10.5194/egusphere-egu23-11678, 2023.

EGU23-11845 | ECS | Orals | CR3.2

An Adimensional Ice-Sheet-Climate Model for glacial cycles 

Sergio Pérez-Montero, Jorge Alvarez-Solas, Alexander Robinson, and Marisa Montoya

Although the ultimate trigger of glacial cycles is Milankovitch insolation cycles, there are still uncertainties concerning their timing and transitions. These unknowns are believed to be due to intrinsic nonlinearities in the climate system, and there is a deep interest in their solution. However, the longer timescales involved make it infeasible to use comprehensive climate models because of the large computational cost involved. In this context, conceptual models are built to mimic complex processes in a simpler, computationally efficient way. Here we present an adimensional ice-sheet–climate model (AMOD), which aims to study these outstanding paleoclimatic topics. AMOD represents ice sheet dynamics by using common assumptions as in state-of-the-art ice-sheet models, adapted to its dimensionless nature, and it solves surface mass balance processes and the aging of snow and ice. In this way, AMOD is able to run several glacial cycles in seconds and produces results comparable to those of paleoclimatic proxies. Preliminary results indicate nonlinearities related to both ice dynamics and snow aging that determine the timing and shape of deglaciations.

How to cite: Pérez-Montero, S., Alvarez-Solas, J., Robinson, A., and Montoya, M.: An Adimensional Ice-Sheet-Climate Model for glacial cycles, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11845, https://doi.org/10.5194/egusphere-egu23-11845, 2023.

EGU23-12206 | ECS | Orals | CR3.2

The Glacier-climate Interaction over the High-Mountain Asia during the Last Glacial Maximum 

Qiang Wei, Yonggang Liu, Yongyun Hu, and Qing Yan

Glacier advances affect the local climate, and in turn, can either promote or prohibit its own growth. Such feedback has not been considered in modeling the High-Mountain Asia (HMA) glaciers during the Last Glacial Maximum (LGM; ~28-23 ka), which may contribute to the large spread in some of the published modeling work, with some notable discrepancy with existing reconstruction data. By coupling an ice sheet model (ISSM) with a climate model (CESM1.2.2), we find that the total glacial area is reduced by 10% due to the glacier-climate interaction; glacier growth is promoted along the western rim of HMA, and yet reduced in the interior. Such changes in spatial pattern improve model-data comparison. Moreover, the expansion of glaciers causes an increase in the winter surface temperature of the eastern Tibetan Plateau by more than 2 K, and a decrease of precipitation almost everywhere, especially the Tarim basin, by up to 60%. These changes are primarily due to the increase in surface elevation, which blocks the water vapor brought by westerlies and southwesterlies, reducing precipitation and increasing surface temperatures to the east and northeast of the newly grown glaciers.

How to cite: Wei, Q., Liu, Y., Hu, Y., and Yan, Q.: The Glacier-climate Interaction over the High-Mountain Asia during the Last Glacial Maximum, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12206, https://doi.org/10.5194/egusphere-egu23-12206, 2023.

The Greenland ice sheet comprises a volume of 7.4 m sea level equivalent and is losing mass rapidly as a result of global warming. It is widely thought that the ice sheet will exhibit tipping behaviour in a warmer climate. In other words, due to ice sheet – climate feedbacks (some of) its contribution to sea level rise may become irreversible once critical thresholds are crossed. This would severely affect the increasing number of people living in low-lying coastal areas worldwide. However, the current understanding of such thresholds and tipping behaviour is very limited, because most modelling studies up to date do not include (local) interactions or feedbacks between the ice sheet (topography and ice extent) and other climate system components (surface mass balance and atmosphere).

To investigate the irreversibility of Greenland’s ice mass loss and the associated processes, we coupled our high-resolution Greenland Ice Sheet Model (GISM) with a renowned high-resolution regional climate model, the Modèle Atmosphérique Régional (MAR). The two-way coupling between both models provides a (more) realistic representation of (local) ice sheet – climate interactions for future ice sheet simulations.

Like all regional climate models, MAR needs 6 hourly atmospheric forcing from a global climate model (GCM). Several coupled model runs with forcing from different GCMs are envisioned over the coming months and years. As they are computationally intensive, simulations up to the end of the century and beyond take several weeks to a few months to complete.

The poster will present the preliminary results from our first coupled model run in an envisioned series of experiments: a two-way coupled MAR-GISM run forced by the IPSL-CM6 6 hourly output, which is available up to 2300. For this timescale, our coupled models can still be run in fully interactive mode, which means the information (surface mass balance and ice sheet extent/topography) between both models can be exchanged on a yearly basis. In addition to its long duration, the IPSL forcing is of particular interest as it is on the high end of the CMIP6 model ensemble projections regarding warming over Greenland. We thus expect the experiment to provide valuable insights regarding Greenland’s potential contribution to future sea-level rise and the associated ice sheet – climate interactions or feedbacks.  

How to cite: Paice, C. M., Fettweis, X., and Huybrechts, P.: Quantifying the response of the Greenland ice sheet in a high-end scenario until 2300 from a coupled high-resolution regional climate and ice sheet model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12281, https://doi.org/10.5194/egusphere-egu23-12281, 2023.

EGU23-13350 | Orals | CR3.2

Large effects of ocean circulation change on Greenland ice sheet mass loss 

Miren Vizcaino, Julia Rudlang, Laura Muntjewerf, Sotiria Georgiou, Raymond Sellevold, and Michele Petrini

The Greenland ice sheet (GrIS) is currently losing mass at an accelerated rate, due to atmospheric and ocean warming causing respectively enhanced melt and ice discharge to the ocean. A large part of the uncertainty on future GrIS contribution to sea level rise relates to unknown atmospheric and ocean circulation change. For the later, AR6 models project a weakening of the North Atlantic Meridional Overturning Circulation (NAMOC) during the 21st century. The magnitude of this weakening depends on the greenhouse gas scenario and model, but none of the models project a complete collapse.

Projections of future GrIS evolution in the last IPCC report AR6 are mostly based on simulations with ice sheet models forced with the output of climate models (e.g., Goelzer et al. (2020)). This method permits large ensembles of simulations, however the coupling between climate and GrIS is not represented. Here, we use a coupled Earth System and Ice Sheet Model (ESM-ISM), the CESM2-CISM2 (Muntjewerf et al. 2021) to examine the multi-millennial evolution of the GrIS surface mass balance for a middle-of-the-road CO2 scenario. The model couples realistic simulation of global climate (Danabasoglu et al. 2020), surface processes (van Kampenhout et al. 2020) and ice dynamics (Lipscomb et al. 2019). We use an idealized scenario of 1% CO2 increase until stabilization at two times pre-industrial values.  compare our results with pre-industrial and 1% to 4xCO2 simulations (Muntjewerf et al. 2020).

We find small increases and even reduction of annual temperatures in the GrIS area in connection with strong NAMOC weakening in the first two centuries of simulation. Summer temperatures and surface melt increase moderately with respect to pre-industrial. From simulation year 500, the NAMOC recovers, resulting in strong increases in GrIS melt rates and contribution to sea level rise. We compare the deglaciation pattern over a period of 3,000 years with deglaciation simulations with the same model for the last interglacial (Sommers et al. 2021).

 

How to cite: Vizcaino, M., Rudlang, J., Muntjewerf, L., Georgiou, S., Sellevold, R., and Petrini, M.: Large effects of ocean circulation change on Greenland ice sheet mass loss, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13350, https://doi.org/10.5194/egusphere-egu23-13350, 2023.

EGU23-13907 | ECS | Orals | CR3.2

First results of RACMO2.4: A new model version with updated surface and atmospheric processes 

Christiaan van Dalum and Willem Jan van de Berg

In recent years, considerable progress in surface and atmospheric physics parameterizations has been made by the scientific community that could benefit regional climate modelling of polar regions. Therefore, we developed a major update to the Regional Atmospheric Climate Model, referred to as RACMO2.4, that includes several new and updated parameterizations. Most importantly, the surface and atmospheric processes from the European Center for Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS), which are embedded in RACMO, are updated to cycle 47r1. This includes, among other changes, updates in the cloud, aerosol and radiation scheme, a new lake model, and a new multilayer snow module for non-glaciated regions. Furthermore, a new spectral albedo and radiative transfer scheme in snow scheme, which has been introduced and evaluated in a previous, yet inoperative version, is now operational. Here, we shortly introduce the aforementioned changes and present the first results of RACMO2.4 for several domains, particularly of the Greenland ice sheet.

How to cite: van Dalum, C. and van de Berg, W. J.: First results of RACMO2.4: A new model version with updated surface and atmospheric processes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13907, https://doi.org/10.5194/egusphere-egu23-13907, 2023.

EGU23-14088 | Posters on site | CR3.2

Reconstructing the Greenland ice sheet in past warm climates 

Christine S. Hvidberg, Mikkel Lauritzen, Nicholas M. Rathmann, Anne M. Solgaard, and Dorthe Dahl-Jensen

The stability of the Greenland ice sheet through past glacial-interglacial cycles provides knowledge that can contribute to understanding the future mass loss and contribution to sea level from the Greenland ice sheet in a warmer climate. Paleo-climatic records from ice cores provide constraints on the past climate and ice sheet thickness in Greenland through the current interglacial, the Holocene, 11.7 kyr to present, but is limited to a few ice cores from the central areas. In the previous interglacial period, the Eemian, 130 kyr to 110 kyr before present, the ice core constraints are sparse, and beyond the Eemian, the climate evolution is known from Antarctic ice cores and marine sediments. The limited constraints on the past climate in Greenland presents a challenge for reconstructions based on ice flow modelling. Here we present initial results from an ice flow modelling study using the PISM ice flow model to simulate the evolution of the Greenland ice sheet in the Eemian and the Holocene periods. We discuss how paleo-climatic data from ice cores and marine sediments can be combined with ice flow modelling. We find that the Greenland ice sheet retreated to a minimum volume of up to ∼1.2 m sea-level equivalent smaller than present in the early or mid-Holocene, and that the ice sheet has continued to recover from this minimum up to present day. In all our runs, the ice sheet is approaching a steady state at the end of the 20th century. Our studies show that the Greenland ice sheet evolves in response to climate variations on shorter and longer timescales, and that assessment of future mass loss must take into account the history and current state.

How to cite: Hvidberg, C. S., Lauritzen, M., Rathmann, N. M., Solgaard, A. M., and Dahl-Jensen, D.: Reconstructing the Greenland ice sheet in past warm climates, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14088, https://doi.org/10.5194/egusphere-egu23-14088, 2023.

EGU23-14236 | ECS | Orals | CR3.2

Sensitivity of future projections of ice sheet retreat to initial conditions 

Tijn Berends, Jorjo Bernales, Caroline van Calcar, and Roderik van de Wal

Both the Greenland and Antarctic ice sheets are expected to experience substantial mass loss in the case of unmitigated anthropogenic climate change. The exact rate of future mass loss under high warming scenarios remains uncertain, depending strongly on physical quantities that are difficult to constrain from observations, such as basal sliding and sub-shelf melt. We apply a novel model initialisation protocol, that combines elements from existing approaches such as the equilibrium spin-up, basal inversion, and palaeo spin-up, to models of both the Greenland and Antarctic ice sheets. We show the results in term of sea-level projections including the uncertainties, under different warming scenarios, following the ISMIP6 protocol.

This abstract is a companion to “On the initialisation of ice sheet models: equilibrium assumptions, thermal memory, and present-day states” by Bernales et al. We hope that, if both abstracts are lucky enough to be accepted, the conveners can program the two talks in sequence.

How to cite: Berends, T., Bernales, J., van Calcar, C., and van de Wal, R.: Sensitivity of future projections of ice sheet retreat to initial conditions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14236, https://doi.org/10.5194/egusphere-egu23-14236, 2023.

EGU23-14412 | ECS | Orals | CR3.2

Self-adaptive Laurentide Ice Sheet evolution towards the Last Glacial Maximum 

Lu Niu, Gregor Knorr, Uta Krebs-Kanzow, Paul Gierz, and Gerrit Lohmann

Northern Hemisphere summer insolation is regarded as a main control factor of glacial-interglacial cycles. However, internal feedbacks between ice sheets and other climate components are non-negligible. Here we apply a state-of-the-art Earth system model (AWI-ESM) asynchronously coupled to the ice sheet model PISM, focusing on the period when ice sheet grows from an intermediate state (Marine isotope stage 3, around 38 k) to a maximum ice sheet state (the Last Glacial Maximum). Our results show that initial North American ice sheet differences at 38 k are erased by feedbacks between atmospheric circulation and ice sheet geometry that modulate the ice sheet development during this period. Counter-intuitively, moisture transported from the North Atlantic warm pool during summer is the main controlling factor for the ice sheet advance. A self-adaptative mechanism is proposed in the development of a fully-grown NA ice sheet which indicates how the Earth system stabilizes itself via interactions between different Earth System components.

How to cite: Niu, L., Knorr, G., Krebs-Kanzow, U., Gierz, P., and Lohmann, G.: Self-adaptive Laurentide Ice Sheet evolution towards the Last Glacial Maximum, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14412, https://doi.org/10.5194/egusphere-egu23-14412, 2023.

EGU23-14469 | ECS | Orals | CR3.2 | Highlight

Has the (West) Antarctic Ice Sheet already tipped? 

Ronja Reese, Julius Garbe, Emily A. Hill, Benoît Urruty, Kaitlin A. Naughten, Olivier Gagliardini, Gael Durand, Fabien Gillet-Chaulet, G. Hilmar Gudmundsson, David Chandler, Petra M. Langebroek, and Ricarda Winkelmann

Observations of ocean-driven grounding line retreat in the Amundsen Sea Embayment in Antarctica raise the question of an imminent collapse of the West Antarctic Ice Sheet. Here we analyse the committed evolution of Antarctic grounding lines under the present-day climate. To this aim, we run an ensemble of historical simulations with a state-of-the-art ice sheet model to create model instances of possible present-day ice sheet configurations. Then, we extend the simulations to investigate their evolution under constant present-day climate forcing and bathymetry. We test for reversibility of grounding line movement at different stages of the simulations to analyse when and where irreversible grounding line retreat, or tipping, is initiated.

How to cite: Reese, R., Garbe, J., Hill, E. A., Urruty, B., Naughten, K. A., Gagliardini, O., Durand, G., Gillet-Chaulet, F., Gudmundsson, G. H., Chandler, D., Langebroek, P. M., and Winkelmann, R.: Has the (West) Antarctic Ice Sheet already tipped?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14469, https://doi.org/10.5194/egusphere-egu23-14469, 2023.

EGU23-14648 | ECS | Orals | CR3.2

On the initialisation of ice sheet models: equilibrium assumptions, thermal memory, and present-day states 

Jorjo Bernales, Tijn Berends, Caroline van Calcar, and Roderik van de Wal

A significant portion of the spread in future projections of ice sheet volume changes is attributed to uncertainties in their present-day state, and the way this state is represented in ice-sheet models. The scientific literature already contains a variety of classic initialisation approaches used by modelling groups around the globe, each with its own advantages and limitations. We propose a generalised protocol that allows for the quantification of the impact of individual initialisation choices, such as steady-state assumptions, the inclusion of internal paleoclimatic thermal signals, sea level and glacial isostatic effects, and calibration methods. We then apply this protocol to an ensemble of multi-millennia model spin-ups of the present-day Greenland and Antarctic ice sheets and show the importance of the choices made during initialisation.

[This abstract is a companion to “Sensitivity of future projections of ice sheet retreat to initial conditions” by Berends et al. We hope that, if both abstracts are lucky enough to be accepted, the conveners can program the two talks in sequence.]

How to cite: Bernales, J., Berends, T., van Calcar, C., and van de Wal, R.: On the initialisation of ice sheet models: equilibrium assumptions, thermal memory, and present-day states, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14648, https://doi.org/10.5194/egusphere-egu23-14648, 2023.

EGU23-14666 | Orals | CR3.2

Sensitivity of of coupled climate and ice sheet of modern Greenland to atmospheric, snow and ice sheet parameters 

Charlotte Lang, Tamsin Edwards, Jonathan Owen, Sam Sherriff-Tadano, Jonathan Gregory, Ruza Ivanovic, Lauren Gregoire, and Robin S. Smith

As part of a project working to improve coupled climate-ice sheet modelling by studying the response of ice sheets to changes in climate across different periods since the Last Glacial Maximum, we present an analysis of an ensemble of coupled climate and ice sheet simulations of the modern Greenland using the FAMOUS-BISICLES model and statistical emulation.

FAMOUS-BISICLES, a variant of FAMOUS-ice (Smith et al., 2021a), is a low resolution (7.5°X5°) global climate model that is two-way coupled to a higher resolution (minimum grid spacing of 1.2 km) adaptive mesh ice sheet model, BISICLES. It uses a system of elevation classes to downscale the lower resolution atmospheric variables onto the ice sheet grid and calculates surface mass balance using a multilayer snow model. FAMOUS-ice is computationally affordable enough to simulate the millennial evolution of the coupled climate-ice sheet system as well as to run large ensembles of simulations. It has also been shown to simulate Greenland well in previous work using the Glimmer shallow ice model (Gregory et al., 2020).

The ice sheet volume and area are sensitive to a number of parametrisations related to atmospheric and snow surface processes and ice sheet dynamics. Based on that, we designed a perturbed parameters ensemble using a Latin Hypercube sampling technique and ran simulations with climate forcings appropriate for the late 20th century.

Gaussian process emulation allows us explore parameter space in a more systematic and faster way than with more complex earth system models and make predictions at input parameter values that are not evaluated in the simulations. We find that the mass balance is most correlated to three parameters:

  • n, the exponent in Glen’s flow law, and beta, the coefficient of the basal drag law, both influencing the amount of ice lost through discharge
  • rho_threshold, a parameter setting the minimum value the dense firn albedo can possibly reach

Finally, using a history matching approach, we built an implausibility metric (based on surface mass balance, ice volume loss, near-surface and sea-surface temperature) to identify the regions of the parameter space that produce plausible runs.

How to cite: Lang, C., Edwards, T., Owen, J., Sherriff-Tadano, S., Gregory, J., Ivanovic, R., Gregoire, L., and Smith, R. S.: Sensitivity of of coupled climate and ice sheet of modern Greenland to atmospheric, snow and ice sheet parameters, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14666, https://doi.org/10.5194/egusphere-egu23-14666, 2023.

EGU23-15230 | Posters on site | CR3.2

Antarctic RINGS to characterize the Antarctic Ice Sheet coastal zone and Antarctic contribution to the global sea-level rise 

Kenichi Matsuoka, Xiangbin Cui, Fausto Ferraccioli, Rene Forsberg, Tom Jordan, Felicity McCormack, Geir Moholdt, and Kirsty Tinto and the Antarctic RINGS

Regions where the Antarctic Ice Sheet reaches the coast are fundamental to our understanding of the linkages between Antarctica and the global climate system. These coastal regions contain multiple potential tipping points for the Antarctic Ice Sheet in the ongoing 2oC warming world, which must be better understood to predict future sea-level rise. The Antarctic Ice Sheet constitutes the largest uncertainty source in future sea-level projections, and this uncertainty is mainly rooted in poorly known bed topography under the ice sheet. Bed topography matters the most in the coastal regions as it controls the stability of the ice sheet. Together with an overview of the current multidisciplinary understandings of the Antarctic coastal regions, we present ensemble analysis of published datasets to present data and knowledge gaps, and their regional distribution is discussed in the context of ice-sheet evolution and instability. Finally, we identify outstanding science priorities and discuss protocols of airborne surveys to develop a comprehensive dataset uniformly all-around Antarctica.

How to cite: Matsuoka, K., Cui, X., Ferraccioli, F., Forsberg, R., Jordan, T., McCormack, F., Moholdt, G., and Tinto, K. and the Antarctic RINGS: Antarctic RINGS to characterize the Antarctic Ice Sheet coastal zone and Antarctic contribution to the global sea-level rise, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15230, https://doi.org/10.5194/egusphere-egu23-15230, 2023.

EGU23-15361 | ECS | Posters virtual | CR3.2

Sea ice extent and subsurface temperatures in the Labrador Sea across Heinrich events during MIS 3 

Henrieka Detlef, Mads Mørk Jensen, Rasmus Andreasen, Marianne Glasius, Marit-Solveig Seidenkrantz, and Christof Pearce

Heinrich events associated with millennial-scale climate oscillations during the last glacial period are prominent events of ice-sheet collapse, characterized by the dispersal of ice(berg) rafted debris and freshwater across the North Atlantic. Hudson Strait has been suggested as one of the predominant iceberg source regions. One potential mechanism triggering iceberg release invokes cryosphere-ocean interactions, where subsurface warming destabilizes the Laurentide ice sheet. Subsurface warming is facilitated by the expansion of sea ice in the Labrador Sea in combination with a slow down of the Atlantic Meridional Overturning Circulation, which prevents the release and downward mixing of heat in the water column.

Here we present high-resolution reconstructions of sea ice dynamics in the outer Labrador Sea at IODP Site U1302/03 between 30 ka and 60 ka. Sea ice reconstructions are based on a suite of sympagic and pelagic biomarkers, including highly branched isoprenoids and sterols. The results suggest a transition from reduced/seasonal to extended/perennial sea ice conditions preceding the onset of iceberg rafting associated with Heinrich event 3, 4, 5, and 5a by ~0.9 ± 0.5 ka. Ongoing work on the same core and sample material will have to confirm the timing and extent of subsurface warming compared to sea ice advances. 

How to cite: Detlef, H., Mørk Jensen, M., Andreasen, R., Glasius, M., Seidenkrantz, M.-S., and Pearce, C.: Sea ice extent and subsurface temperatures in the Labrador Sea across Heinrich events during MIS 3, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15361, https://doi.org/10.5194/egusphere-egu23-15361, 2023.

EGU23-16930 | ECS | Orals | CR3.2 | Highlight

Multistability and transient response of the Greenland ice sheet to anthropogenic CO2 emissions 

Dennis Höning, Matteo Willeit, and Andrey Ganopolski

Ongoing CO2 emissions into the atmosphere and associated temperature rise have dramatic consequences for the ice sheets on our planet. In this presentation, we focus on the Greenland ice sheet, which holds so much ice that a complete melting would cause the global sea level to rise by seven meters. However, a prediction of future mass loss of the Greenland ice sheet is challenging because it is a strongly non-linear function of temperature and occurs over very long timescales. With the fully coupled Earth system model of intermediate complexity CLIMBER-X, we study the stability of the Greenland ice sheet and its transient response to CO2 emissions over the next 20 kyr. We find two bifurcation points within a global mean surface air temperature anomaly of 1.5°C. Each of these bifurcation points corresponds to a critical ice volume. If the Greenland ice sheet volume decreases below these critical values, returning to a previous atmospheric CO2 concentration would not cause the ice sheet to grow back to its previous state. We also find increased mass loss rates and increased sensitivity of mass loss to cumulative CO2 emission in the vicinity of these critical ice volumes. Altogether, our results suggest that global warming near the lower 1.5°C limit of the Paris agreement would already cause the Greenland ice sheet to irreversibly melt, although a complete melting would take thousands of years.

How to cite: Höning, D., Willeit, M., and Ganopolski, A.: Multistability and transient response of the Greenland ice sheet to anthropogenic CO2 emissions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16930, https://doi.org/10.5194/egusphere-egu23-16930, 2023.

EGU23-119 | ECS | Orals | CR7.3 | Highlight

Antarctic Atmospheric Rivers in the Past and Future Climates 

Michelle Maclennan, Andrew Winters, Christine Shields, Jonathan Wille, Rebecca Baiman, Léonard Barthelemy, and Vincent Favier

Atmospheric rivers (ARs) are long, narrow bands of warm and moist air that travel poleward from the midlatitudes. While Antarctic atmospheric rivers (ARs) occur only 1-3% of the time over the ice sheet, they are a significant contributor to Antarctic surface mass balance: they contribute 10% on average, and more than 20% locally, of Antarctic precipitation each year. Here we use an Antarctic-specific AR-detection algorithm to identify ARs in MERRA-2 and ERA5 reanalyses and the Community Earth System Model version 2 (CESM2). We use this algorithm to quantify the frequency, location, and precipitation attributed to Antarctic ARs for the period 1980-2014 and use these statistics to identify CESM2 biases relative to MERRA-2 and ERA5. We then apply the AR-detection algorithm to CESM2 for the future period (2015-2100) to examine how the frequency and intensity of ARs, AR-attributed total precipitation, and year-to-year variability in AR precipitation changes in the future under the SSP370 emissions scenario. Our results quantify past and future impacts of ARs on Antarctic annual precipitation, interannual variability, and trends, and ultimately provide an early assessment of future AR-driven changes in Antarctic surface mass balance.

How to cite: Maclennan, M., Winters, A., Shields, C., Wille, J., Baiman, R., Barthelemy, L., and Favier, V.: Antarctic Atmospheric Rivers in the Past and Future Climates, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-119, https://doi.org/10.5194/egusphere-egu23-119, 2023.

EGU23-814 | ECS | Orals | CR7.3 | Highlight

Evaluation of Greenland extreme snow melting patterns and their synoptic drivers 

Josep Bonsoms, Marc Oliva, and Juan Ignacio López-Moreno

 

Greenland Ice Sheet (GrIS) snow melting rates have drastically increased since the 1990s, with relevant implications in the entire ecosystem. According to climate projections, extreme weather events will potentially increase in the coming decades over the GrIS. Thus, it is necessary to analyze the past temporal evolution of GrIS extreme melting patterns, as well as their climate drivers. This work analyzes the GrIS summer extreme snow melting spatiotemporal evolution and trends (1990 to 2021). Further, we determine the contribution of synoptic weather types that drive extreme snow melting events. Results evidence that the frequency, magnitude, and the relative contribution of extreme snow melting to the total summer snow melting differs depending on the GrIS sector. Maximum extreme snow melting days per season are observed in western GrIS, whereas minimums are observed in northern sectors. The average extreme snow melting during summer is non-statistically significant increasing in the entire GrIS, which is consistent with the increase of the average snow melting for the same temporal period. Extreme snow melting days as well as the contribution of extreme snow melting to the total snow melting per season show an upward trend, except in the central and northern zones. The analysis of twenty summer circulation weather types reveals that extreme snow melting episodes for most of the GrIS sectors are mainly explained by a few synoptic systems; characterized by a high-pressure system located in central, southern, and eastern GrIS. During these synoptic episodes, stable weather conditions prevail, and the energy available for snow melting is mainly controlled by positive shortwave radiation heat fluxes leading to positive 850 hPa air temperature anomalies. Results presented in this work are relevant for a better understanding of extreme weather events over GrIS within a changing climate context.

How to cite: Bonsoms, J., Oliva, M., and López-Moreno, J. I.: Evaluation of Greenland extreme snow melting patterns and their synoptic drivers, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-814, https://doi.org/10.5194/egusphere-egu23-814, 2023.

EGU23-2311 | ECS | Orals | CR7.3

The Atmospheric effects of Southern Ocean open-ocean polynyas onto coastal polynyas in EC-Earth3 

Jakob Gunnarsson, Lu Zhou, and Céline Heuzé

Polynyas are recurrent areas of open water or thin ice within the ice pack, which alter the local heat and moisture exchange and high-latitude atmosphere-ocean circulation interannual variability. They are differentiated as coastal (latent heat) or open-ocean (sensible heat) polynya according to their forming location. Especially, coastal polynyas are critical sources of dense water and the formation of Antarctic Bottom Water (AABW) following the brine enrichment of surface waters during sea-ice formation, and easily influenced by the local atmosphere conditions. However, few studies have examined the atmospheric response of open-ocean polynyas on the coastal polynyas given the fact that open-ocean polynyas have capability to re-adjust mesoscale atmosphere circulation. To better understand the surrounding impact of large open-ocean polynya events, output from CMIP6 historical experiment synoptic scale EC-Earth3 is adopted. Our results show an increasing coastal polynya frequency and extent accompanying with more active open-ocean polynya years in the Weddell Sea. The results are explained by near-surface wind speed differences in the coastal regions, which are found statistically significant between more and less active open-ocean polynya years. Furthermore, those intensifications of winds are found in days where easterly-dominated winds north-westerly to north-easterly and easterly to south-easterly cross the open-ocean polynya. Increased near-surface air temperatures as well as a deepening in sea level pressure are also observed during the years with more active open-ocean polynya events. The findings contribute to a better understanding of coastal polynya opening processes, as well as how we might expect to see the different type of polynya interact by their influence and dependence on surrounding atmospheric conditions.

How to cite: Gunnarsson, J., Zhou, L., and Heuzé, C.: The Atmospheric effects of Southern Ocean open-ocean polynyas onto coastal polynyas in EC-Earth3, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2311, https://doi.org/10.5194/egusphere-egu23-2311, 2023.

EGU23-3192 | ECS | Orals | CR7.3

Spatio-temporal variability of air temperature lapse rate in the glacierised catchment of the Chandra basin, western Himalaya using in-situ measurements 

Sunil N. Oulkar, Parmanand Sharma, Bhanu Pratap, Lavkush Patel, Sourav Laha, and Meloth Thamban

The air temperature lapse rate (TLR) plays an important role in estimating ice and snow melt in high mountain regions. The TLR can vary depending on several factors, including the topography of the catchments and the microclimate. TLR calculations are typically not precise in the Himalayan glacierised regions due to a lack of in-situ observation of meteorological parameters. Therefore, a dense in-situ monitoring network over a high altitudinal gradient is needed to estimate the TLR accurately. We have obtained in-situ measurements of air temperature data from five automatic weather stations (AWS) installed at the best possible locations in the Chandra basin catchment of the semi-arid zone of the western Himalaya from October 2019 to September 2022. The altitudinal range for air temperature measurement varied between ~4000 and 5000 m a.s.l. We utilise the air temperature data to estimate the TLR by regressing the temperature with the corresponding elevations.
Comparing all the estimated TLR, the mean annual value (4.9°C/km) was significantly lower than the standard environmental lapse rate (6.5 °C/km) with substantial seasonality. The maximum TLR (~6.8 °C/km) during the summer is likely due to the high-altitude range and thin air and the presence of cold air pools at higher altitudes. However, the significantly lower TLR (~1.9 °C/km) during winters is likely due to the low air temperature and high moisture content in the region due to western disturbance. Further, we observed strong diurnal variations of TLR, which was highest during the daytime and lowest at night. This study highlighted that the TLR was potentially influenced by the local topography, particularly with higher lapse rates at higher elevations. TLR vary topographically and temporally significantly in the Chandra basin, indicating that the air temperature in this region is more sensitive to climatic variations. The findings of this study will play an important role in glacio-hydrological models, where TLR is one of the essential inputs.

How to cite: Oulkar, S. N., Sharma, P., Pratap, B., Patel, L., Laha, S., and Thamban, M.: Spatio-temporal variability of air temperature lapse rate in the glacierised catchment of the Chandra basin, western Himalaya using in-situ measurements, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3192, https://doi.org/10.5194/egusphere-egu23-3192, 2023.

The Arctic climate system has been suggested to be ‘en route’ to a new state with seasonally ice-free conditions expected within two-three decades under high-emissions scenarios. Here we show the prospect of its delayed emergence stemming from a consideration of observed and modelled Arctic cryosphere sensitivity to atmospheric circulation changes. While the observed Arctic warming contains a substantial contribution from large-scale circulation, it is not reflected in the modelled forced response. Numerical model simulations with the CESM2 with an active Greenland ice sheet model (CISM2), where model winds are nudged towards the observed state, advocate for the need to have a circulation-based model sensitivity evaluation metric. Hence a recalibration is proposed by matching the warming signals free of atmospheric circulation impacts in observations and models over 1979-2020. This constraint yields a ~decade delay in the projected timing of the first seasonally sea-ice free Arctic and widespread Greenland melting. Accounting for the role of large-scale atmospheric forcing in Arctic climate change offers new perspectives of estimating Arctic sea- and land-ice sensitivity to anthropogenic forcing and understanding the recently emerging issue of some CMIP6 climate models being ‘too hot’.

How to cite: Topal, D. and Ding, Q.: Atmospheric circulation-constrained model sensitivity recalibrates Arctic climate projections, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3971, https://doi.org/10.5194/egusphere-egu23-3971, 2023.

EGU23-5852 | ECS | Posters on site | CR7.3

CARRA-driven simulation of Greenland Ice Sheet surface mass balance at 2.5 km resolution 

Mathias Larsen, Ruth H. Mottram, and Peter L. Langen

Projections of present and future ice mass loss of the Greenland Ice Sheet are important for assessing its contribution to future sea-level rise. Critical for the total mass balance is the surface mass balance (SMB) which can be estimated from models, and improving these models can help to further constrain the uncertainties in future projections.

In this project, we use the CARRA reanalysis dataset generated from the HARMONIE-AROME weather forecast system to force an SMB model. The CARRA dataset is remarkable for its 2.5 km horizontal resolution providing unprecedented spatial detail. This is particularly important at the ice-sheet margins where both accumulation and ablation processes are impacted by strong topographic gradients. For example, the greater spatial detail is expected to provide more realistic profiles of accumulation and drying of airmasses from the coast toward the interior, in turn improving the SMB simulation.

The SMB model utilizes a subsurface scheme that consists of columns with 32 layers in the vertical. Driven by the atmospheric input, the SMB model computes all the interactions between the atmosphere and subsurface layers, such as accumulation, melting, percolation, refreezing and runoff. Using this SMB model, we performed a CARRA-driven simulation over the period 1991-2020 on the 2.5 km CARRA grid.

Our initial results show the CARRA-driven SMB model yielding somewhat higher SMB values compared to other published SMB products. The ice sheet-wide totals of accumulation and melt are comparable to other products. However, the location of maximum melt contributions is shifted further towards the interior of the ice sheet in the CARRA-driven simulation. This allows for larger refreezing and contributes significantly to the high SMB seen in the CARRA-driven simulation. Here, we evaluate the SMB model output and driving fluxes against PROMICE data and satellite observations and provide a new updated assessment of Greenland ice sheet SMB.

How to cite: Larsen, M., H. Mottram, R., and L. Langen, P.: CARRA-driven simulation of Greenland Ice Sheet surface mass balance at 2.5 km resolution, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5852, https://doi.org/10.5194/egusphere-egu23-5852, 2023.

EGU23-6243 | ECS | Posters on site | CR7.3

A systematic polar-induced signature in infrasound database highlithed by machine learning models 

Sentia Goursaud Oger, Alexandre Junqueira, and Mathilde Mougeot

Polar lows are intense but short duration maritime cyclones occurring in both hemispheres. In the northern pole, they are mainly located in the Barents and Norwegian seas, with significant damages for coastal populations. So far, a fully understanding of the physical processes at play is still lacking. This is due to the suddenness of such events, as well as a scarcity of meteorological observations in these areas. Infrasounds are sound waves with frequency ranges below the audible domain. It was shown that polar lows can be a source of infrasound. Only one study looked at the infrasound signature for two particular polar lows using data obtained from two stations, in Northern Norway and on Svalbard. Here we show the potentiality of a systematic polar low-induced signature in infrasound data.

Within the frame of the Comprehensive nuclear-test-ban treaty organization, infrasound stations were set up worldwide. One was settled in northern Norway (IS37NO) in 2003 and made fully operational since 2004. Its records consist in a timeseries of sub-daily pressure data, that are processed through a Progressive Multi Channel Cross Correlation method, resulting in variables such as the mean frequencies, azimuths and amplitudes of the detections, and covering 17 complete years (2004-2021). These variables were used to train statistical models to learn the occurrence of polar lows refered in a polar low database. Our models yield very good results, specially in term of precision and recall. They provide a basis for different research opportunities, such as the prediction of polar lows and a deeper comprehension of its climate controls.

How to cite: Goursaud Oger, S., Junqueira, A., and Mougeot, M.: A systematic polar-induced signature in infrasound database highlithed by machine learning models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6243, https://doi.org/10.5194/egusphere-egu23-6243, 2023.

EGU23-8805 | ECS | Orals | CR7.3

Extreme temperature events for the past 19 years in the McMurdo Dry Valleys, Antarctica linked to mesoscale meteorological variability 

Eva Bendix Nielsen, Marwan Katurji, Peyman Zawar-Reza, and Hanna Meyer

The McMurdo Dry Valleys (MDVs) in Antarctica have a unique environment classified as a hyper-arid desert with glacier runoff being the main source of liquid water. Previous studies have identified winds as the controlling factor of the climate in this region and especially the occurrence of foehn induced warming. Episodic foehn warming during the austral summer can contribute to above freezing temperatures sustained for multiple days. Years with extreme glacial runoff leading to flooding have been correlated with a higher occurrence of foehn induced warming events. Understanding the temporal availability of meltwater caused by extreme meteorological events is highly important since it is a dependant variable to the functioning of the area’s fragile ecosystem. Synoptic scale circulations in the surrounding Ross Sea Region are a driving factor for the occurrence of foehn warming in the MDVs with the local mesoscale meteorology modulating the spatiotemporal variability of the foehn-induced near-surface warming. AntAir ICE, a newly developed daily mean near surface air temperature dataset with a spatial grid resolution of 1 km2 has proven capable of capturing these mesoscale temperature variabilities for multiple seasons within the complex topography of the MDVs.

 

A case study on the 2nd of January 2020 where the maximum temperature measured in a Lake Vanda automatic weather station was above +9 degrees Celsius with multiple valleys experiencing foehn induced warming, displayed a clear warming signal for the MDVs in AntAir ICE. The atmospheric dynamic analysis from the numerical weather prediction model the Antarctic Mesoscale Prediction System (AMPS) indicated a clear foehn signature. This event was linked to a meso-low located in the Ross Sea which was detected in the climate re-analysis ERA5 mean sea level pressure dataset. By confidently identifying these warming events within the MDVs where there is a relatively high availability of Automatic Weather Stations and AMPS predictions, has allowed for further exploration of extreme sustained warming and potentially foehn induced warming along the terrestrial coastal margin of Antarctica. Using AntAir ICE, warming events during the austral summer season from November to February for the period 2003 to 2021 with sustained daily mean temperatures above freezing for multiple days have been identified for the Ross Sea Region. This study aims at capturing the mesoscale meteorological and climatological variability for multiple seasons within the Ross Sea Region, while linking these extreme warming events to larger scale circulation patterns can allow for understanding local extreme events in context of shifting large scale circulation drivers.

How to cite: Bendix Nielsen, E., Katurji, M., Zawar-Reza, P., and Meyer, H.: Extreme temperature events for the past 19 years in the McMurdo Dry Valleys, Antarctica linked to mesoscale meteorological variability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8805, https://doi.org/10.5194/egusphere-egu23-8805, 2023.

EGU23-10042 | ECS | Orals | CR7.3 | Highlight

Global Sources of Moisture for Atmospheric Rivers over Antarctica 

Rajashree Datta, Adam Herrington, Luke Trusel, David Schneider, Jesse Nusbaumer, and Ziqi Yin

 

The quantity and characteristics of atmospheric rivers over Antarctica, which import heat and moisture towards the continent, are a major source of uncertainty in future sea level rise estimates. We employ a new variable-resolution grid over Antarctica, using CESM2 (VR-CESM2), which balances the extensibility of a GCM with the high computational costs of a high-resolution climate model. This setup uses observed sea surface temperature and sea ice concentration, implements moisture-tagging (linking precipitation to a moisture source region on the globe), and produces high spatial and temporal resolution atmosphere and ice sheet surface outputs, which can be used to detect atmospheric rivers and to estimate their impact.

As a baseline for experiments testing the relative importance of large-scale drivers, we first quantify, over an idealized 10-year period, the global sources of moisture and the portion of total precipitation that reaches the ice sheet during large-scale vs atmospheric river events (and their associated synoptic characteristics). Beyond this baseline, we will use this setup to perform initial test scenarios assessing the relative impact of reduced sea ice combined with enhanced ocean heat at lower latitudes.

How to cite: Datta, R., Herrington, A., Trusel, L., Schneider, D., Nusbaumer, J., and Yin, Z.: Global Sources of Moisture for Atmospheric Rivers over Antarctica, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10042, https://doi.org/10.5194/egusphere-egu23-10042, 2023.

The current period of Arctic amplification has been characterized by a pronounced reduction in high-latitude snow and ice cover that is reflective of rapidly changing thermodynamic environment. Given this change in the local background conditions, it is not surprising that the Greenland Ice Sheet (GrIS) has undergone drastic surface mass loss since the turn of the century; however, research has shown that the recent acceleration of runoff from the GrIS is strongly linked to a shift in the large-scale atmospheric circulation over the same period that has brought more frequent and intense bouts of summer Greenland blocking. While this atmospheric dynamical change may merely be a manifestation of internal variability, there is growing evidence that widespread changes in surface cover and near-surface thermal gradients under Arctic amplification may favor persistent extremes such as the episodes of Greenland blocking that have encouraged melt of the ice sheet.

Here, we explore whether the change in summer atmospheric circulation over Greenland may be a dynamical response to Arctic amplification and attendant snow cover loss. Our results suggests that low North American spring snow cover and a weakened meridional temperature gradient combine to encourage the high-amplitude Omega blocking patterns that we show to have driven the recent trend in summer Greenland blocking. We show that this delayed response to anomalous spring snow cover follows from the snow-hydrological effect, whereby low spring snow cover causes early depletion of soil moisture and anomalously warm surface temperature over eastern North America. The consequent stationary Rossby wave response enforces an anomalous anticyclone, centered over Baffin Bay, that resembles that of high-amplitude Omega blocks and the atmospheric conditions which have promoted melt of the northern GrIS. Together, these results provide evidence that Arctic amplification, and thus anthropogenic climate change, has contributed to recent atmospheric dynamical forcing of GrIS surface mass loss. However, regardless of how strong this link between climate change and atmospheric circulation over Greenland may be, the change in the local thermodynamic environment under Arctic amplification represents a far more robust climate change signal. We also examine the thermodynamic contribution to GrIS surface mass loss using the regional climate model, Modèle Atmosphérique Régional (MAR) associated with blocking circulation. MAR output of surface temperature, meltwater production, and runoff are used to assess the differential impact of blocking events across the ice sheet.

How to cite: Preece, J., Mote, T., and Wachowicz, L.: Examining Atmospheric Dynamical Forcing of Greenland Ice Sheet Surface Mass Loss Within the Context of Arctic Amplification, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10541, https://doi.org/10.5194/egusphere-egu23-10541, 2023.

EGU23-10672 | ECS | Posters on site | CR7.3

Comparing the response to meteorological drivers at Taylor and Commonwealth glacier, McMurdo Dry Valleys, Antarctica. 

Marte Hofsteenge, Nicolas Cullen, Jono Conway, Marwan Katurji, Carleen Reijmer, and Michiel van den Broeke

In the McMurdo Dry Valleys (MDV) of Antarctica thrives a unique ecosystem under extreme cold and dry conditions. The limited snowfall that falls on the valley floor quickly sublimates and therefore glacial melt is the most important input to the streams and ice-covered lakes that provide water for the ecosystem. Understanding what drives the variability and changes in glacial meltwater is therefore of great importance to foresee ecosystem changes in a warming world. To assess the temporal variability and meteorological drivers of glacial melt in Taylor Valley, a 22-year surface energy balance (SEB) record is constructed for Taylor and Commonwealth glacier. Automatic weather station observations from the Long-term Ecological Research (LTER) Program in the ablation zone of each glacier are gap filled and completed using locally-tuned parameterisations. The two SEB records are compared to understand the different response of two nearby glaciers (~30 km apart) to local and regional climate forcing. The more melt dominated Commonwealth glacier shows strong seasonal variability in ablation. The closer proximity of Commonwealth glacier to the ocean leads to more rapid changes in albedo as controlled by summer snowfall events. Not only does the presence of snow but also the larger variability in ice albedo compared to Taylor glacier explains much of the seasonal variability in melt. Another major driver of melt are the number of degree days above freezing for both glaciers, which is strongly linked to foehn wind events in Taylor Valley. The further inland Taylor glacier experiences drier and windier conditions and therefore sublimation dominates ablation and melt occurrence. Cloud cover and snowfall in summer switch off glacial melt in summer on both glaciers. We have also used ERA5 fields to study the moisture sources of the MDV precipitation and clouds. This will help us understand how changes in moisture and regional circulation patterns might impact the MDV glaciers and ecosystem in a warming climate.

How to cite: Hofsteenge, M., Cullen, N., Conway, J., Katurji, M., Reijmer, C., and van den Broeke, M.: Comparing the response to meteorological drivers at Taylor and Commonwealth glacier, McMurdo Dry Valleys, Antarctica., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10672, https://doi.org/10.5194/egusphere-egu23-10672, 2023.

EGU23-13291 | ECS | Orals | CR7.3

Spatial response of Greenland’s firn layer to NAO variability 

Max Brils, Peter Kuipers Munneke, and Michiel van den Broeke

Changes in the Greenland ice sheet (GrIS) firn layer may impact its ability to retain meltwater. These changes also need to be accounted for when converting measured ice sheet volume changes to mass changes. With a firn model (IMAU-FDM v1.2G) forced by a regional climate model (RACMO2.3p2), we investigate how the GrIS firn layer depth and pore space have evolved since 1958 in response to variability in the large-scale atmospheric circulation. On interannual timescales, the firn layer’s depth and pore space shows a spatially heterogeneous response to variability in the North Atlantic Oscillation (NAO). Notably, a stronger NAO following the record warm summer of 2012 led the firn layer in the south and east of the ice sheet to regain thickness and pore space after a period of thinning and reduced pore space. The main driving forces behind these changes vary between GrIS sectors: in the southwest, a decrease in melt dominates, whereas in the east an increase in snow accumulation dominates. However, these trends are not uniform across the GrIS, and over the same period, the firn in the northwest continued to lose pore space. The NAO is also stronger in winter than in summer and we observe that this impacts the seasonal cycle of the firn. In the wet southeastern GrIS, most of the snow accumulates during the winter, when firn compaction is slow, amplifying the seasonal cycle in firn depth and pore space. The opposite occurs in other regions, where snowfall peaks in summer or autumn, at the same time as densification and melt, damping the seasonal oscillations in the firn thickness and pore space.

How to cite: Brils, M., Kuipers Munneke, P., and van den Broeke, M.: Spatial response of Greenland’s firn layer to NAO variability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13291, https://doi.org/10.5194/egusphere-egu23-13291, 2023.

EGU23-13345 | Posters on site | CR7.3

Three-decades of quality controlled Greenland Climate Network (GC-Net) weather station data 

Jason Box, Baptiste Vandecrux, Andreas Ahlstrøm, Robert Fausto, William Colgan, Nanna Karlsson, Signe Andersen, Patrick Wright, Derek Houtz, Daniel McGrath, Nicolas Cullen, Nicolas Bayou, and Konrad Steffen

The Greenland Climate Network (GC-Net) is a collection of automatic weather stations (AWS)  across the Greenland Ice Sheet. The first site was initiated in 1990, and the project has operated almost continuously since 1995, under the leadership of the late Pr. Konrad Steffen. The network consists of 19 long-running weather stations, and 14 AWS sites active under five years. As part of the continuation of the GC-Net by the Geological Survey of Denmark and Greenland (GEUS), the AWS data have recently undergone a reprocessing with new attention to erroneous data filtering, correction and derivation of additional variables: continuous surface height, instrument heights, turbulent heat fluxes.  This new augmented GC-Net level 1 (L1) AWS dataset is now available at https://doi.org/10.22008/FK2/VVXGUT and will continue to be refined. The processing scripts, the latest data and a data-user forum are available at https://github.com/GEUS-Glaciology-and-Climate/GC-Net-level-1-data-processing. In addition to the AWS data, a comprehensive compilation of valuable metadata is provided: maintenance reports, yearly pictures of the stations and the moving station positions through time. This unique dataset provides more than 320 station-years of weather data of improved quality and is made available in compliance under FAIR open data and code principles.

How to cite: Box, J., Vandecrux, B., Ahlstrøm, A., Fausto, R., Colgan, W., Karlsson, N., Andersen, S., Wright, P., Houtz, D., McGrath, D., Cullen, N., Bayou, N., and Steffen, K.: Three-decades of quality controlled Greenland Climate Network (GC-Net) weather station data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13345, https://doi.org/10.5194/egusphere-egu23-13345, 2023.

EGU23-13864 | ECS | Orals | CR7.3

New non-hydrostatic polar regional climate model HCLIM-AROME: analysis of the föhn event on 27 January 2011 over the Larsen C Ice Shelf, Antarctic Peninsula 

Kristiina Verro, Willem Jan van de Berg, Andrew Orr, Oskar Landgren, and Bert van Ulft

Recently, the climate version (HCLIM) of the regional numerical weather prediction model system ALADIN–HIRLAM of the ACCORD consortium, has been set up for the Arctic and Antarctic domains. Within the PolarRES project, HCLIM will be run, along with other regional climate models such as RACMO, MetUM, and MAR, to study the interactions between the atmosphere, oceans, and sea ice in the Arctic and Antarctic. For the Antarctic Peninsula, kilometre-scale horizontal resolution and non-hydrostatic model dynamics are essential to accurately resolve the complex topography and to capture small-scale processes such as the föhn winds that occur over ice shelves on the Antarctic Peninsula. 

Here, we present an analysis of the föhn event on 27 January 2011 over the Larsen C Ice Shelf, Antarctic Peninsula. The output of the non-hydrostatic HCLIM-AROME model, run at 2.5 km resolution, is evaluated against automatic weather station and radiosonde measurements and simulations of the non-hydrostatic regional climate model MetUM. We analyse the modelled air pressure, near-surface and tropospheric temperatures, wind speed and wind direction, and other atmospheric variables, demonstrating the strengths and weaknesses of the HCLIM-AROME model for this polar application. 

How to cite: Verro, K., van de Berg, W. J., Orr, A., Landgren, O., and van Ulft, B.: New non-hydrostatic polar regional climate model HCLIM-AROME: analysis of the föhn event on 27 January 2011 over the Larsen C Ice Shelf, Antarctic Peninsula, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13864, https://doi.org/10.5194/egusphere-egu23-13864, 2023.

EGU23-14194 | ECS | Posters on site | CR7.3

The microclimate and mass balance of Qaamarujup Sermia, West Greenland 1929-2022 

Florina Schalamon, Jakob Abermann, Sebastian Scher, Andreas Trügler, and Wolfgang Schöner

Understanding the interaction of the atmosphere and cryosphere is critical for predicting the consequences of the rapidly changing climate, particularly in the Arctic. To accurately represent feedback mechanisms between ice and climate in physical models, their thorough quantification at the local scale is required. This study analyses two high-resolution datasets from the Qaamarujup Sermia outlet glacier (West Greenland) that were collected 90 years apart (1929-1931 and 2022 onward). The first is a dataset from Alfred Wegener's last expedition 1929-31, including sub-daily atmospheric observations as well as monthly to (bi-)weekly mass balance measurements. An almost identical monitoring network was installed in 2022 with the goal of observing changes in microclimate and their impact on the glacier. Both periods cover far above-normal air temperatures. The newly installed monitoring network consists of two automatic weather stations (AWS), of which one is placed near the coast and the other one on the ice sheet in approx. 940 m a.s.l.. The station network is supplemented with three temperature and humidity sensors in 50, 270 and 950 m a.s.l. . Further, there are four autonomous ablations sensors and six ablation stakes to quantify the surface mass balance of the glacier. During the field campaign in 2022, 39 vertical drone flights were performed to investigate temperature and humidity profiles of the lowest 400 m of the atmosphere. Preliminary findings show that a surface-based temperature inversion above the glacier surface is present on all days investigated during the study period (2-10.7.2022). An elevated temperature inversion above the ice-free valley part is also present at 50% of the days, with one day reaching further inland than the glacier front. Both types of inversion occur in combination on three out of the eight study days. Comparison of the historic surface mass balance with data from a regional climate model shows reasonable agreement for locations 950 m a.s.l., while the complex topography in the valley is not represented sufficiently. Our results emphasize the value of validation data on a small spatial scale as well as the potential of short-term observations almost a century apart to investigate changing feedback mechanisms of the ice/climate interaction.  

How to cite: Schalamon, F., Abermann, J., Scher, S., Trügler, A., and Schöner, W.: The microclimate and mass balance of Qaamarujup Sermia, West Greenland 1929-2022, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14194, https://doi.org/10.5194/egusphere-egu23-14194, 2023.

This study investigates the reasons for the decrease in the water level of Beysehir Lake and the shrinkage in the lake's surface area in recent years. For this purpose, the lake water level was determined from multi-mission satellite altimeter data, and the lake area was calculated using high-resolution optical satellite images. Data from Copernicus Global Land Service was used for multi-mission satellite altimeter data, and the lake level trend between 1993-2022 was calculated with the least squares method. European Space Agency's (ESA) Sentinel-2 high-resolution optical images were used to determine the change in the lake surface area between 2015 and 2020. These high-resolution optical images were processed with The Sentinel Application Platform (SNAP) software. The Normalized Difference Water Index (NDWI) and Modified Normalized Difference Water Index (MNDWI) were calculated based on processed optical images, and these indexes reflect the changes in water surface area. From the satellite altimeter data, a decreasing trend of 2.5 ± 0.5 cm/yr in the lake water level in the last ten years and shrinkage of approximately 8 km2 in the last 6 years from the satellite images were determined. The possibility of one of the most important reasons being drought was emphasized, and monthly average air temperature data and monthly average precipitation data were obtained from the Turkish General Directorate of Meteorology. With these data, 3- and 12-month Standardized Precipitation Evapotranspiration Index (SPEI) were calculated. Regarding these calculated drought indexes, moderate, extreme, and severe hydrological drought has been determined in the region. According to the analysis, drought is thought to be the most important reason for the decrease in the lake water level and shrinkage in the lake surface area.

Keywords : Geodesy for Climate, Lake Water Level, Satellite Altimetry, In-situ observation, Sentinel-2

How to cite: Erkoç, M. H.: Examination of Causes for Decrease in the Water Level of Beysehir Lake and Shrinkage in the Lake's Surface Area., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-258, https://doi.org/10.5194/egusphere-egu23-258, 2023.

Gravity Recovery and Climate Experiment (GRACE) and GRACE-FollowOn (GFO) satellites can monitor the global spatio-temporal changes in terrestrial water storage anomalies (TWSA) with monthly temporal and ~300 km spatial resolutions. Since these native resolutions may not be adequate for various studies requiring better localization of TWSA signal both in spatial and temporal domains, in recent years, considerable efforts have been devoted to downscaling TWSA to higher resolutions. However, the majority of these studies have focused on spatial downscaling; only a few studies attempted to improve the temporal resolution. Here, we utilized an in-house developed Deep Learning (DL) based model to downscale the monthly GRACE/GFO Mass Concentration (Mascon) TWSA to daily resolution across the Contiguous United States (CONUS). The simulative performance of the DL algorithm is tested by comparing the simulations to independent (non-GRACE) dataset and the land hydrology models. In addition, we assessed the potential of our daily simulations to detect long- and short-term variations in TWSA. The validation results show that our DL-aided simulations do not overestimate or underestimate GRACE/GFO TWSA and can monitor variations in the water cycle at a higher temporal resolution.

How to cite: Uz, M., Akyılmaz, O., and Shum, C.: Deep Learning-aided Temporal Downscaling of Satellite GravimetryTerrestrial Water Storage Anomalies Across the Contiguous United States (CONUS), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-632, https://doi.org/10.5194/egusphere-egu23-632, 2023.

EGU23-1929 | ECS | Posters on site | G3.1

Hydrospheric mass loading for Europe from GNSS vertical displacement and a hydrological model 

Gael Kermarrec, Anna Klos, Henryk Dobslaw, Janusz Bogusz, and Annette Eicker

The interpretation of hydrospheric changes in the context of climate change can be enhanced using Global Navigation Satellite System (GNSS) displacement time series (DTS) combined with the one of a hydrological model. Our methodology is based on a computationally filtering strategy called the Savitzky-Golay filter and applied to selected stations in Europe. We use the GNSS solutions provided by the International GNSS Service (IGS) and, for the first time, the Nevada Geodetic Laboratory (NGL). The new hybrid dataset shows a high correspondence with DTS derived from the Gravity Recovery and Climate Experiment (GRACE) gravity mission but allows the identification of local and station-specific effects. Prior to this analysis, we eliminate various effects such as non-tidal atmospheric and oceanic loadings, glacial isostatic adjustment, barystatic sea-level changes, or thermoelastic deformation from GNSS DTS.

How to cite: Kermarrec, G., Klos, A., Dobslaw, H., Bogusz, J., and Eicker, A.: Hydrospheric mass loading for Europe from GNSS vertical displacement and a hydrological model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1929, https://doi.org/10.5194/egusphere-egu23-1929, 2023.

EGU23-2734 | ECS | Orals | G3.1

Reconstructing a new terrestrial water storage deficit index to detect and quantify drought in the Yangtze River Basin 

Xuewen Wan, Nengfang Chao, Ying Hu, Jiangyuan Wang, Zheng Liu, and Kaihui Zou

With the intensification of global climate change, droughts have occurred frequently in the Yangtze River Basin (YRB), which has caused significant impacts on human production, life, and socio-economic development. To reduce the damage caused by drought in the YRB, the drought characteristics must be comprehensively detected and quantified. Here, the spatial and temporal variability of precipitation, runoff, soil moisture, terrestrial water storage, and groundwater in the YRB from the Gravity Recovery and Climate Experiment (GRACE), hydrological and in situ observations were comprehensively estimated by decomposing them into seasonal, subseasonal, trend, and interannual observations. The new weighted GRACE drought standardisation index (WGDSI) was reconstructed using the component contribution ratio and compared with the standardised soil moisture index (SSI), standardised precipitation index (SPI06), and standardisation runoff index (SRI). Additionally, the drought characteristics identified based on observations of the water storage deficit, severity, peak, duration, and recovery time were also quantified using the WGDSI over the YRB. The results indicated that changes in soil moisture, terrestrial water storage, and groundwater in the YRB increased from 2003 to 2019 and mainly based on seasonal and interannual signals. The correlation coefficients between the WGDSI and the SSI, SPI06, and SRI were 0.92, 0.62, and 0.79, respectively, which represented increases of 9%, 14%, and 21% compared to that with the unweighted GRACE drought standardisation index, respectively. The interannual variability of the hydrologic variables was more consistent with drought events in the YRB, which was beneficial for detecting drought. Two serious droughts occurred in the YRB from 2003 to 2019. In 2006, a continuous 7-month-long drought occurred, with a peak at -28.974 km3, severity of -174.767 km3∙month, average drought recovery rate of 0.83 km3/month, and recovery time of 30 months, while in 2011, a continuous 5-month-long drought occurred, with a peak at -18.384 km3, severity of -78.106 km3/month, average drought recovery rate of 0.40 km3/month and recovery time of 39 months. The above results indicate that the WGDSI can be used to monitor and quantify drought over the YRB. The index proposed in this study can be applied to generate new datasets and methods for detecting and quantifying global drought.

How to cite: Wan, X., Chao, N., Hu, Y., Wang, J., Liu, Z., and Zou, K.: Reconstructing a new terrestrial water storage deficit index to detect and quantify drought in the Yangtze River Basin, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2734, https://doi.org/10.5194/egusphere-egu23-2734, 2023.

EGU23-4048 | ECS | Posters on site | G3.1

Empirical GNSS-derived terrestrial water storage-streamflow relationship in the Sierra Nevada ranges, California 

Nicholas Lau, Ellen Knappe, and Adrian Borsa

One of the most dynamic components of Earth surface mass variability is the constant global redistribution of terrestrial water storage (TWS) across temporal scales of hours to decades. Mass loading and unloading from TWS changes induce instantaneous elastic deformation of the solid earth, producing predominantly vertical transient displacements that are observable by geodetic methods. The global expansion of Global Navigation Satellite Systems (GNSS) networks during the last decade have provided new opportunities of directly estimating changes in TWS at high spatial and temporal resolutions. While contemporary GNSS studies have demonstrated the ability to map regional-scale water storage variability, incorporating these geodetic TWS estimates with in-situ hydrologic measurements can provide further insights on the physical mechanisms underlying the terrestrial water cycle.

 

In this study, we investigate the potential of using GNSS-derived TWS estimates to infer individual watershed condition along California’s Sierra Nevada, a major water source for urban and agricultural use. Utilizing the dense GNSS network in the western United States, we invert vertical displacements for TWS change at subbasin scale spatial resolution (USGS HUC-8). Joint analysis of our TWS estimates and stream gauge data shows contrasting seasonal behaviours in the northern and southern Sierra Nevada. The snow-dominated southern section exhibits a significant time lag between maximum storage and maximum baseflow from March to May, indicating wet-season decoupling between surface storage and the subsurface reservoirs that drive baseflow. In contrast, the northern section exhibits little to no lag, indicative of persistent surface-to-subsurface coupling, consistent with the higher rain-to-snow ratio in the north. Furthermore, we demonstrate that GNSS-derived TWS estimates can be used to infer watershed antecedent storage conditions, in which interannual variability in summer storage (dry season) influences streamflow recession behaviours during early precipitation season. Continued development of GNSS-based water storage estimates and future assimilation with hydrologic models should provide additional understanding of the water budget and hillslope hydrology in the Sierra Nevada.

How to cite: Lau, N., Knappe, E., and Borsa, A.: Empirical GNSS-derived terrestrial water storage-streamflow relationship in the Sierra Nevada ranges, California, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4048, https://doi.org/10.5194/egusphere-egu23-4048, 2023.

EGU23-4288 | ECS | Posters on site | G3.1

Quality assessment of the gridded climate indices estimated from GNSS displacements for the European area 

Artur Lenczuk, Anna Klos, and Janusz Bogusz

For more than 30 years, the Global Navigation Satellite System (GNSS) has successfully detected local crust deformations. These changes in deformation are caused, among other things, by changes in Total Water Storage (TWS), which reflect regular changes in the water system, but are also coupled with changes resulting from unexpected climate change. Current water conflicts caused by climate variability, increased human activity, population growth and food demand are leading to an increased importance of monitoring the abundance of the terrestrial hydrosphere. Such monitoring is increasingly being carried out using GNSS observations, mainly due to the impressive number of permanent stations distributed on Earth. However, the distribution of GNSS stations is irregular, and the displacement time series is often incomplete. Moreover, because of systematic errors, consistency of several parameters estimated for nearby GNSS stations may be very low. To eliminate the impact of these errors, but still capture regular changes in the climate system, we estimated drought severity index (DSI) using GNSS displacement time series over Europe, and interpolated these station-based DSI values over European area in a 1 per 1 degree grid. The quality of interpolated GNSS-DSI values has been assessed using four external datasets: (1) the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) data, (2) combination of GRACE/-FO data with the Satellite Laser Ranging (SLR) data, provided by the University of Bonn, (3) combination of SLR data and high-low Satellite-To-Satellite Tracking (hlSST) data, provided by Leibniz University Hannover, and (4) the self-calibrating Palmer Drought Severity Index (scPDSI). The external datasets have low spatial resolution, when compared to station-dependent GNSS-DSI and the scPDSI index is unable to capture several real water changes. Using GNSS displacements for estimated of DSI reduces these limitations. Our results show that GNSS-based DSI is spatially coherent with indicators derived from other datasets and is able to map dry and wet periods occurring over Europe. GNSS-DSI are also able to capture extreme short events not observed by other datasets. We note that the GRACE-DSI values show the least consistency with GNSS-DSI values. We find also that the DSI values estimated from combined GRACE and SLR indices have largest root-mean-square values for Europe. Our results show that GNSS displacements can be applied to study human and/or climate impact on water changes in small spatial and temporal scales, which may be averaged out in the other datasets; this hold the true especially in regions where GNSS stations are densely distributed.

How to cite: Lenczuk, A., Klos, A., and Bogusz, J.: Quality assessment of the gridded climate indices estimated from GNSS displacements for the European area, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4288, https://doi.org/10.5194/egusphere-egu23-4288, 2023.

EGU23-4554 | ECS | Orals | G3.1 | Highlight

A Detection of the Sea Level Fingerprint of Greenland Ice Sheet Melt 

Sophie Coulson, Sonke Dangendorf, Jerry X. Mitrovica, Mark Tamisiea, Linda Pan, and David Sandwell

Rapid melting of ice sheets and glaciers drives a unique geometry, or fingerprint, of sea-level change, including a sea-level fall in the vicinity of the ice sheet that is an order of magnitude greater than the associated global mean sea-level rise of the melt event. The detection of individual fingerprints has been challenging due to sparse sea surface height measurements at high latitudes and the difficulty of disentangling ocean dynamic variability from the signal. Efforts to date have analyzed sea level records outside the zone of major sea-level fall, where the gradients and amplitudes of the fingerprint signal are significantly lower. We predict the fingerprint of Greenland Ice Sheet (GrIS) melt using new ice mass loss estimates from radar altimetry data and model reconstructions of nearby glaciers, and compare this prediction to an independent, altimetry-derived sea surface height trend corrected for ocean dynamic variability in the region adjacent to the ice sheet. The two fields show consistent gradients across the region, with the expected strong drawdown of the sea surface toward GrIS. A statistically significant correlation between the two fields (p < 0.001) provides the first unambiguous observational detection of the near-field sea level fingerprint of recent GrIS melting in our warming world. This detection provides a robust map of the impact of ice mass flux on global oceans since the early 1990s, and validates theoretical and numerical developments in the sea level modelling community.

How to cite: Coulson, S., Dangendorf, S., Mitrovica, J. X., Tamisiea, M., Pan, L., and Sandwell, D.: A Detection of the Sea Level Fingerprint of Greenland Ice Sheet Melt, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4554, https://doi.org/10.5194/egusphere-egu23-4554, 2023.

EGU23-5086 | ECS | Orals | G3.1

The global land water storage data set GLWS 2.0: assimilating GRACE and GRACE-FO into a global hydrological model 

Helena Gerdener, Jürgen Kusche, Kerstin Schulze, Petra Döll, and Anna Klos

The satellite mission Gravity Recovery And Climate Experiment (GRACE) provided and its successor GRACE-FollowOn (GRACE-FO) provides a great opportunity to derive observations of the global water cycle from space. The missions have contributed and largely increased our knowledge about various hydrological processes on Earth, for example the melting of glaciers in Greenland or groundwater depletion in India. Nonetheless, the spatial resolution of about 300 km, missing months in the time series and the multi-month gap between GRACE and GRACE-FO complicate or even impede the usage in some applications. Further, separating single storage information, e.g. groundwater, from the GRACE/-FO derived total water storage anomalies (TWSA) is still difficult.

In recent decades, data assimilation techniques were used to downscale and disaggregate the GRACE/-FO TWSA, however, to our knowledge they focus on hydrological instead of geodetic applications, only a few assimilate GRACE/-FO TWSA on a global scale and open access is rare. Therefore, we provide the new Global Land Water Storage (GLWS2.0) data set that offers total water storage anomalies on a 0.5° monthly grid covering the global land except Greenland and Antarctica for the time period 2003 to 2019 without missing months and the GRACE/GRACE-FO gap and will soon be publicly available. GLWS2.0 is derived by assimilating GRACE and GRACE-FO TWSA into the WaterGAP model using the Ensemble Kalman Filter considering uncertainties.

We contrast the GLWS2.0 data with the GRACE/-FO observations and the model simulations in the spatial domain via linear trends, annual amplitudes and non-seasonal TWSA and in the spectral domain via degree variances, c20 coefficients and other representation of spherical harmonics. Worldwide, 1030 GNSS stations are used to validate GLWS2.0 by analyzing the vertical loading at short-term, seasonal and long-term temporal bands and we find that GLWS2.0 agrees better with GNSS than GRACE/-FO. In addition, a good agreement to another global data assimilation product is found, which assimilates GRACE/-FO TWSA into the Catchment Land Surface Model by NASA’s Goddard Space Flight Center.

How to cite: Gerdener, H., Kusche, J., Schulze, K., Döll, P., and Klos, A.: The global land water storage data set GLWS 2.0: assimilating GRACE and GRACE-FO into a global hydrological model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5086, https://doi.org/10.5194/egusphere-egu23-5086, 2023.

The Gravity Recovery and Climate Experiment (GRACE) mission has monitored total water storage anomalies (TWSA) globally with unprecedented resolution and accuracy since 2002. However, many applications require a data-based, multi-decadal extended record of TWSA prior to the GRACE period as well as bridging the eleven-months gap between GRACE and its successor GRACE-FO. Statistical and machine-learning 'reconstruction' approaches have been developed to this end, mostly via identifying relations of GRACE-derived TWSA to climate variables, and some regional or global land data sets are now publicly available.

In this contribution, we  compare the two global reconstructions by HUMPHREY AND GUDMUNDSSON (2019) and LI ET AL. (2021) mutually and against output from the the WaterGAP hydrological model from 1979 onwards, against large-scale mass-change derived from geodetic satellite laser ranging from 1992 onwards, and finally against differing GRACE/-FO solutions from 2002 onwards. 

We find that the reconstructions agree surprisingly well in many regions at seasonal and sub-seasonal timescales, even in the pre-GRACE era. We find larger differences at inter-annual timescales which we speculate are in part due to the way reconstructions are trained and in part on which specific GRACE solution they are trained as well as the climatological characteristic of the region. Our comparisons against independent SLR data reveal that reconstructions (only) partially succeed in representing anomalous TWSA for regions that are influenced by large climate modes such as ENSO.

How to cite: Hacker, C.: How realistic are multi-decadal reconstructions of GRACE-like total water storage anomalies?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5486, https://doi.org/10.5194/egusphere-egu23-5486, 2023.

EGU23-5889 | ECS | Orals | G3.1

Influence of GIA uncertainty on climate applications from satellite gravimetry 

Lennart Schawohl, Annette Eicker, Meike Bagge, and Henryk Dobslaw

Global coupled climate models are important for predicting future climate conditions. Due to sometimes large and often systematic model uncertainties, it is crucial to evaluate the outcome of model experiments against independent observations. Changes in the distribution and availability of terrestrial water storage (TWS), which can be measured by the satellite gravimetry missions GRACE and GRACE-FO, represent an important part of the climate system. However, the use of satellite gravity data for the evaluation of coupled climate models has only very recently become feasible. Challenges arise, e.g., from the still rather short time series of satellite data and from signal separation issues related to GRACE/-FO observing all mass change including non-water related variations such as glacial isostatic adjustment. Apart from climate model uncertainties, these challenges might be the reason for a disagreement between the direction of linear water storage trends of models and observations in several regions of the world, one of them located in Eastern Canada.

This presentation will highlight the latest results achieved from our ongoing research on climate model evaluation based on the analysis of an ensemble of models from the Coupled Model Intercomparison Project Phase 6 (CMIP6). We will focus on long-term wetting and drying conditions in TWS. Using an ensemble of 52 GIA models that differ in the applied ice history, solid Earth rheology, and numerical code, this presentation will discuss how GIA modeling uncertainty does influence (i) the determination of water storage trends from GRACE/FO data, and (ii) the (dis-)agreement between drying/wetting trends in satellite gravimetry and CMIP6 climate models. We will show that the apparent disagreement between observations and models in highly GIA-affected regions in North America crucially depend on the particular model chosen for reducing the GIA effect from the GRACE satellite data.

How to cite: Schawohl, L., Eicker, A., Bagge, M., and Dobslaw, H.: Influence of GIA uncertainty on climate applications from satellite gravimetry, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5889, https://doi.org/10.5194/egusphere-egu23-5889, 2023.

EGU23-6919 | ECS | Orals | G3.1

Combined GNSS Reflectometry/Refractometry for Continuous In Situ Surface Mass Balance Estimation on an Antarctic Ice Shelf 

Ladina Steiner, Holger Schmitthüsen, Jens Wickert, and Olaf Eisen

We developed a methodology for deriving automated and continuous specific surface mass balance time series for fast moving parts of ice sheets and shelves (>10m/a) by an accurate and simultaneous estimation of continuous in-situ snow density, snow water equivalent (SWE), and snow deposition and erosion, averaged over an area of several square meters and independent on weather conditions. Reliable in-situ surface mass balance estimates are scarce due to limited spatial and temporal data availability. While surface accumulation can be obtained in various ways, conversion to mass requires knowledge of the snow density, which is more difficult to obtain.

A combined Global Navigation Satellite Systems reflectometry and refractometry (GNSS-RR) approach based on in-situ refracted and reflected GNSS signals is developed. The individual GNSS-RR methods have already been successfully applied on stationary grounds and seasonal snowpacks and are now combined and transferred to moving surfaces like ice sheets. We installed a combined GNSS-RR system in November 2021 on the fast moving (~150m/d), high latitude Ekström ice shelf in the vicinity of the Neumayer III station in Antarctica. Continuous snow accumulation reference data is provided by a laser distance sensor at the same test site and manual density observations. Refracted and reflected GNSS observations from site are post-processed for SWE, snow accumulation, and snow density estimation with a sub-daily temporal resolution. Preliminary results of the first year of data show a high level of agreement with reference observations, calculated from snow accumulation data collected by the laser distance sensor and linearly interpolated monthly snow density observations of the uppermost layer equivalent to the height of snow above the buried antenna.

The deployed devices are geared towards prototype applications for reliable low-cost applications, which will allow large-scale retrieval of surface mass balance for general cryospheric applications, not only on ice sheets or shelves, but also sea ice. Regional climate models, snow modelling, and extensive remote sensing data products will profit from calibration and validation based on the derived field measurements, once such sensors can be deployed on larger scales.

How to cite: Steiner, L., Schmitthüsen, H., Wickert, J., and Eisen, O.: Combined GNSS Reflectometry/Refractometry for Continuous In Situ Surface Mass Balance Estimation on an Antarctic Ice Shelf, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6919, https://doi.org/10.5194/egusphere-egu23-6919, 2023.

EGU23-7836 | Orals | G3.1

GNSS-derived Precipitable Water Vapor for Climate Monitoring 

Galina Dick, Florian Zus, Jens Wickert, Benjamin Männel, and Markus Bradke

Global Navigation Satellite System (GNSS) is now an established observing system for atmospheric water vapour with high spatiotemporal resolution. Water vapour is under-sampled in the current climate-observing systems and obtaining and exploiting more high-quality humidity observations is essential for climate monitoring.

The Global Climate Observing System (GCOS), supported by the World Meteorological Organization (WMO), is establishing a reference climate observation network, the GCOS Reference Upper Air Network (GRUAN). Currently, this network comprises 30 reference sites worldwide, designed to detect long-term trends of key climate variables such as temperature and humidity in the upper atmosphere. GRUAN observations are required to be of reference quality, with known biases removed and with an associated uncertainty value, based on thorough characterization of all sources of measurement. In support of these goals, GNSS precipitable water (GNSS-PW) measurement has been included as a priority one measurement of the essential climate variable water vapor. The GNSS-PW program produces a nearly continuous reference measurement of PW and is therefore a substantial part of GRUAN.

GFZ contributes to GRUAN with its expertise in processing of ground-based GNSS network data to generate precise PW products. GFZ hosts a central processing facility for the GNSS data and is responsible for the installation of GNSS hardware, data transfer, processing and archiving, as well as derivation of GNSS-PW products according to GRUAN requirements including PW uncertainty estimation. Currently half of the GRUAN sites are equipped with GNSS receivers. GNSS-PW products for GRUAN and the results of validation studies will be presented.

 

How to cite: Dick, G., Zus, F., Wickert, J., Männel, B., and Bradke, M.: GNSS-derived Precipitable Water Vapor for Climate Monitoring, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7836, https://doi.org/10.5194/egusphere-egu23-7836, 2023.

EGU23-8001 | ECS | Posters virtual | G3.1

Revisiting the global mean ocean mass budget over 2005-2020 

Anne Barnoud, Julia Pfeffer, Anny Cazenave, Robin Fraudeau, Victor Rousseau, and Michaël Ablain

We investigate the performances of GRACE and GRACE Follow-On satellite gravimetry missions in assessing the ocean mass budget at global scale over 2005-2020. For that purpose, we focus on the last years of the record (2015-2020) when GRACE and GRACE Follow-On faced instrumental problems. We compare the global mean ocean mass estimates from GRACE and GRACE Follow-On to the sum of its contributions from Greenland, Antarctica, land glaciers, terrestrial water storage and atmospheric water content estimated with independent observations. Significant residuals are observed in the global mean ocean mass budget at interannual time scales. Our analyses suggest that the terrestrial water storage variations based on global hydrological model likely contributes to a large part to the misclosure of the global mean ocean mass budget at interannual time scales. We also compare the GRACE-based global mean ocean mass with the altimetry-based global mean sea level corrected for the Argo-based thermosteric contribution (an equivalent of global mean ocean mass). After correcting for the wet troposphere drift of the radiometer on-board the Jason-3 altimeter satellite, we find that mass budget misclosure is reduced but still significant. However, replacing the Argo-based thermosteric component by the ORAS5 ocean reanlaysis or from CERES top of the atmosphere observations leads to closure of the mass budget over the 2015-2020 time span. We conclude that the two most likely sources of error in the global mean ocean mass budget are the thermosteric component based on Argo and the terrestrial water storage contribution based on global hydrological models. The GRACE and GRACE Follow-On data are unlikely to be responsible on their own for the non-closure of the global mean ocean mass budget.

How to cite: Barnoud, A., Pfeffer, J., Cazenave, A., Fraudeau, R., Rousseau, V., and Ablain, M.: Revisiting the global mean ocean mass budget over 2005-2020, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8001, https://doi.org/10.5194/egusphere-egu23-8001, 2023.

EGU23-8590 | ECS | Posters on site | G3.1

Multi-decadal Satellite Gravity Mission Simulations Comparing Resolving Capabilities of a Long-term Trend in the Global Ocean Heat Content 

Marius Schlaak, Pail Roland, Alejandro Blazquez, Benoit Meyssignac, and Jean-Michel Lemoine

Satellite gravity missions have been almost continuously observing global mass transports for more than two decades. The resulting data record already improved our understanding of large-scale processes of the water cycle and is reaching a timespan, which has significance concerning climate related mass transport signals such as changes in the essential climate variables terrestrial water storage (TWS) and sea level. The observations of the currently flown GRACE-FO mission will be continued by NASA’s Mass Change (MC) Mission and extended to the Mass change And Geosciences International Constellation (MAGIC) by ESA’s Next Generation Gravity Mission (NGGM), setting anticipation for higher spatial and temporal resolution of satellite gravity observations in the near future.

This contribution presents initial results of multi-decadal closed loop simulations of current and future satellite gravity observations, comparing their capabilities to allow a direct estimation of long-term trends in changes of TWS and ocean mass. The observed climate signal is based on components of the TWS, as well as mass change signals of oceans, ice sheets, and glaciers extracted from CMIP6 climate projection following the shared socio-economic pathway scenario. A special focus here is on the long-term trend over the oceans. By subtracting the observed ocean mass change from the overall sea level change, the global ocean heat content can be computed from the steric component of the sea-level rise. The resulting long-term trends are then compared to initial inputs to the simulation to illustrate the difference in performance between current and future satellite gravity constellations.

How to cite: Schlaak, M., Roland, P., Blazquez, A., Meyssignac, B., and Lemoine, J.-M.: Multi-decadal Satellite Gravity Mission Simulations Comparing Resolving Capabilities of a Long-term Trend in the Global Ocean Heat Content, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8590, https://doi.org/10.5194/egusphere-egu23-8590, 2023.

EGU23-9933 | Posters on site | G3.1

Water Mass Fluxes and Budgets at Catchment-Scale over Europe in the Collaborative Research Cluster 'DETECT' 

Benjamin D. Gutknecht, Anne Springer, and Jürgen Kusche

Terrestrial Water Storage (TWS) is a measure of the total amount of net-accumulated water in all continental storage compartments. The Global Climate Observing System programme (GCOS) has recently approved TWS Anomalies as an Essential Climate Variable (ECV). With GRACE and GRACE-FO we have the ability to look back on an observable that can be interpreted as monthly TWS change since the year 2002. In the continental water mass budget equation, this change balances the water fluxes from precipitation, evapotranspiration and runoff. 

Within the framework of the new Collaborative Research Cluster 1502 'DETECT', we analyse terrestrial/atmospheric and surface water fluxes and associated budget contributions from model simulations, reanalyses and remote sensing observations for all larger river basins in Europe and combine them with catchment-integrating TWS variability. While, as a first step, we are updating previous budget analyses with latest available data sets, the project's central objective is to quantify to what extent regional changes of land and water use contribute to observed budget changes.

In this presentation, we introduce our central objectives and show first results of the latest continuation of catchment-wide water mass flux time-series analysis over Europe. We discuss our budgeting strategies as well as opportunities and hurdles concerning data availability and uncertainties --- also in view of the recently launched SWOT mission and future GRACE successors.

How to cite: Gutknecht, B. D., Springer, A., and Kusche, J.: Water Mass Fluxes and Budgets at Catchment-Scale over Europe in the Collaborative Research Cluster 'DETECT', EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9933, https://doi.org/10.5194/egusphere-egu23-9933, 2023.

EGU23-10758 | Orals | G3.1

Resolving the discrepancy betweenthe seasonal oscillation of Earth's fluid envelope estimated with SLR and that assumed in GRACE 

Donald Argus, Felix Landerer, David Wiese, and Geoffrey Blewitt

For 25 years, geodesists have inferred that the displacement of the "geocenter" estimated from (SLR) satellite laser ranging represents fluctuation of Earth's fluid envelope relative to solid Earth.  However, SLR determines the displacement of the (CN) center of network of geodetic sites relative to the (CM) center of mass of Earth, consisting of solid Earth, the oceans, the atmosphere, and continental water, snow, and ice. Because solid Earth's surface is deforming in elastic response to the changing load of continental water, atmosphere and oceans, CN only roughly approximates the (CE) center of mass of solid Earth.  In this study, estimate the velocity of CM relative to the (CE) center of mass of Earth by first correcting SLR site displacements (estimated by the International Laser Ranging Service 2020) for their elastic response relative to CE produced by fluctuations of continental water, atmosphere and oceans.  We maintain that by correcting for loading displacements relative to CE, we arrive at an estimate of the displacement of CE.  We find that transforming the SLR series from CN to CE reduces the discrepancy between the seasonal oscillation of Earth's fluid envelope estimated by SLR and that assumed by GRACE (using the technique of Sun et al. 2017) by 40 per cent.  In both SLR and GRACE, a total of 0.5 x 1016 kg of mass moves between hemispheres from southern oceans in August to snow-covered areas in North America and Europe (in particular in Canada and Siberia).  The primary remaining difference between the two techniques is that mass in the northern hemisphere is maximum on February 5 in SLR, 20 days before it is maximum on Feb 25 in GRACE.  Knowing the total transfer of the mass of between hemispheres places a boundary constraint on global models of circulation of water on land and in the oceans and atmospheres (that may be applied to forecasting extreme events such as flooding and drought).

How to cite: Argus, D., Landerer, F., Wiese, D., and Blewitt, G.: Resolving the discrepancy betweenthe seasonal oscillation of Earth's fluid envelope estimated with SLR and that assumed in GRACE, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10758, https://doi.org/10.5194/egusphere-egu23-10758, 2023.

EGU23-12155 | ECS | Posters on site | G3.1 | Highlight

Mass change of Antarctica from new GRACE/GRACE-FO releases 

Barbara Jenny, Nicolaj Hansen, Tim Jensen, and René Forsberg

An important application of the NASA/GFZ GRACE and GRACE-FO satellites is the derivation of ice mass changes in the arctic regions from the gravity field changes. Looking at climate change, it is important to know how fast the ice caps are melting for global sea level rise estimation and validation of climate models. We use recently released L2 GRACE/GRACE-FO models, including the latest CSR release 6.1, which show major improvement over earlier models, especially for Antarctica, as well as the latest TU Graz models.  We also compare the GRACE results to a new surface mass balance model, and joint high-resolution inversion with ESA’s Earth Explorer CryoSat altimetry data, highlighting areas of dynamic changes and giving a higher resolution on the main mass change areas. The study is a precursor to a project for demonstrating use of Level-1 laser data for glacial change detection.

How to cite: Jenny, B., Hansen, N., Jensen, T., and Forsberg, R.: Mass change of Antarctica from new GRACE/GRACE-FO releases, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12155, https://doi.org/10.5194/egusphere-egu23-12155, 2023.

EGU23-12215 | Orals | G3.1

Study on combination approaches for hydrological angular momentum determined from climate data 

Jolanta Nastula, Tomasz Kur, Justyna Śliwińska, Małgorzata Wińska, and Aleksander Partyka

Geophysical interpretation of polar motion (PM) and finding the sources of its excitation is an important but challenging task that takes place on the boundary between geodesy and geophysics. Especially the role of hydrological signals in PM excitation is not yet fully understood, mainly because of the lack of agreement between estimates of hydrological angular momentum (HAM) computed from different data sources (e.g., land surface models, global hydrological models, satellite gravity measurements).

The recently observed climate changes affect the global distribution and transport of continental water mass, which may also influence the HAM. Projections of past and future changes in the physical and chemical properties of the atmosphere, ocean, and hydrosphere caused by climate change are delivered by climate models, which are collected and made available to the public in the frame of the sixth phase of the Coupled Model Intercomparison Project (CMIP6). Such models provide many of variables, including variations in soil moisture and snow water storage, which are necessary for HAM computation. However, CMIP6 models differ in terms of initial conditions, physical properties of atmosphere, oceans, hydrosphere, and climate forcing. Such divergences obviously contribute to the differences between various CMIP6-based HAM series.

In this study, we investigate various groups of models according to providing institute, mean of selected models and more sophisticated combinations determined using different methods like e.g., variance components estimation, three cornered hat method. The obtained series are analyzed and evaluated in several spectral bands. The goal of such study is to check whether grouping or combining the models could improve the consistency between CMIP6-based HAM and hydrological signal in geodetically observed PM excitation. To evaluate the combined CMIP6-based HAM series, we compare them with geodetic residuals (GAO) obtained from geodetic angular momentum reduced by atmospheric and oceanic signals, as well as with HAM computed from data from Gravity Recovery and Climate Experiment (GRACE) mission. Generally, the analyses confirm the results obtained from previous studies (Nastula et al. 2022). It is possible to find grouped CMIP6 models that provide HAM series as or more compliant with GAO than HAM determined from GRACE.

How to cite: Nastula, J., Kur, T., Śliwińska, J., Wińska, M., and Partyka, A.: Study on combination approaches for hydrological angular momentum determined from climate data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12215, https://doi.org/10.5194/egusphere-egu23-12215, 2023.

EGU23-12349 | Orals | G3.1 | Highlight

ICESat-2 Ice Sheet Mass balance: Going below the surface 

Nicolaj Hansen, Louise S. Sørensen, Giorgio Spada, Daniele Melini, Rene Forsberg, Ruth Mottram, and Sebastian B. Simonsen

We use the land-ice surface height data product (ATL06 release 5) from NASA’s latest satellite laser altimetry, the Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) to compute surface elevation changes (SEC) from October 2018 to September 2021 over both Antarctica and Greenland. To convert the SEC to mass change we need to remove the non-ice related SEC processes. To remove the signal from the firn compaction, we use an offline surface energy and firn model. The model is driven by outputs from the atmospheric regional climate model HIRHAM5, forced with reanalysis dataset ERA5, and it simulates the physics of the firn pack. The vertical bedrock movement also creates non-ice related signals, the glacial isostatic adjustment has been computed using the ICE-7G model and SELEN4, and the elastic rebound has been computed using a modified version of the REAR code. 

When the SEC are corrected for signals that are not associated with a change in snow or ice mass, we convert to mass change by multiplying the height change with an appropriate density.  The corrected SEC can result from a change in either melt, snow accumulation, or dynamical behavior, this means that the appropriate density depends on which physical processes are driving the observed SEC. In this study, we have made a new density parametrization to convert the volume change into mass change. The density parametrization determines if one should multiply with snow densities (250-350 kg/m³) or ice density (917 kg/m³) based on a number of criteria; the sign of SEC, ice flow velocity, and the altitude of the area.
With our new density parametrization, we get that the Greenland Ice Sheet has lost 237.5±10.3 Gt/year and the grounded Antarctic Ice Sheet has lost -137.6±27.2 Gt/year in the period. These results are in agreement with other mass balance estimates derived with different methods.

How to cite: Hansen, N., Sørensen, L. S., Spada, G., Melini, D., Forsberg, R., Mottram, R., and Simonsen, S. B.: ICESat-2 Ice Sheet Mass balance: Going below the surface, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12349, https://doi.org/10.5194/egusphere-egu23-12349, 2023.

EGU23-12485 | ECS | Orals | G3.1

Data-Driven and Scaling Factor methods of GRACE leakage correction: Can they be reconciled? 

Vasaw Tripathi, Bramha Dutt Vishwakarma, and Martin Horwath

Time variable satellite gravimetry, realized with the missions GRACE and GRACE-FO, allows for the only global observation of total water storage (TWS) changes. These observations are inherently smoothed due to the upward continuation of the gravity field at the satellite orbits. Additionally, the correlated errors seen as north-south stripes in global maps require further filtering to separate signal from noise. This causes the signal at any region to be biased by signal at neighboring regions, better known as leakage effect. Various methods have been proposed to mitigate leakage and to spatially assign TWS changes at smaller spatial scales than the satellite data is available by using auxiliary information. Unfortunately, there is a large spatio-temporally variable degree of discrepancy in the agreement or the disagreement within these methods, leaving the non-geodetic users of GRACE TWS changes with the complex question of choosing an appropriate method. The scaling factor approach and the Data-Driven Correction (DDC) approach are the most widely used methods. The scaling factor approach uses a numerical model output of TWS changes, whereas the DDC approach uses only GRACE observations to account for leakage.
Tripathi et al., 2022 (10.5194/hess-26-4515-2022) found for the Indus basin, that a newly proposed variant of the scaling factor method, called Frequency-Dependent scaling, using the WaterGAP (Water Global Assessment and Prognosis) hydrology model (WGHM v2.2d), produced results with a striking agreement against the results from the DDC approach. Therefore, this contribution extends the comparison of Frequency-Dependent scaling using WGHM v2.2d against the DDC method for 189 global hydrological basins. We achieved an agreement between the results from both methods well within the uncertainties of GRACE TWS observations for almost 85-90% of the global hydrological basins. Such an agreement can bring a much-needed consolidation in the treatment of leakage effect across the user community. The disagreement in the rest of the basins varies across time scales, such as long-term trends and periodic signals, and is being further analysed.

How to cite: Tripathi, V., Vishwakarma, B. D., and Horwath, M.: Data-Driven and Scaling Factor methods of GRACE leakage correction: Can they be reconciled?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12485, https://doi.org/10.5194/egusphere-egu23-12485, 2023.

EGU23-13048 | ECS | Orals | G3.1

Prospects of Space Geodesy to Monitor Atmospheric Moisture and Atmospheric Net-Water Fluxes 

Kyriakos Balidakis, Henryk Dobslaw, Florian Zus, Annette Eicker, Robert Dill, and Jens Wickert

Accurate representation of the time-variable atmospheric state is achieved by assimilating numerous and disparse observations into numerical weather models (NWM). The four-dimensional atmospheric density distribution, a derivative of essential meteorological variables, affect among else how electromagnetic signals propagate through Earth’s atmosphere and how satellites orbit through Earth’s gravity field. Atmospheric refraction to which microwave signals are subjected as they traverse the electrically neutral atmosphere is quantified e.g., during the GNSS data analysis, and holds valuable information about the water vapor distribution in the vicinity of the ground stations. Satellite gravimetry as realized by the GRACE and GRACE-FO missions is sensitive to mass redistribution within Earth’s fluid envelope, including but not limited to the atmosphere and the terrestrail water storage, and also to high-frequency variations stemming from the time-integrated effect of precipitation and evapotranspiration. In this contribution we employ two state-of-the-art meso-beta scale NWM (ECMWF’s latest reanalysis ERA5 and DWD’s operational model ICON-global) as well as ERA5‘s ensemble members to demonstrate that tropospheric mosture distribution and net atmospheric freshwater fluxes are quite uncertain in modern NWM in comparison to other quantities such as hydrostatic atmospheric mass and that certain space geodetic observing systems such as GNSS and GRACE-FO are appropriate tools to monitor them, thus enhancing the accuracy of weather prediction.

How to cite: Balidakis, K., Dobslaw, H., Zus, F., Eicker, A., Dill, R., and Wickert, J.: Prospects of Space Geodesy to Monitor Atmospheric Moisture and Atmospheric Net-Water Fluxes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13048, https://doi.org/10.5194/egusphere-egu23-13048, 2023.

EGU23-13524 | Posters on site | G3.1

Unravelling watershed fluxes to detect emerging changes of the water balance 

Roelof Rietbroek, Sedigheh Karimi, and Amin Shakya

In a warming climate, atmospheric water vapour will increase, intensifying the global water cycle. However, this ”wet-get-wetter” and ”dry-get-drier” paradigm does not hold on regional scales and models seem to contradict observations. Furthermore, it is unknown whether modelled atmospheric moisture fluxes, entering and leaving the watersheds, are mass consistent with river discharge and sinks and sources such as aquifers, soil layers and surface waters. Consequently, observational evidence of the changing water cycle components is crucial for scrutinizing models. It is also essential to assess climatic water cycle trends which have far reaching ecological and socio-economic consequences, through the occurrence of heat waves, flooding, forest fires and water availability.

In this contribution, we introduce a 5 year research project, which was recently funded through the Vidi talent scheme programme of the Dutch Research Council. We will explain how we plan to use satellite gravimetry, radar altimetry, in a joint inversion scheme, to estimate water fluxes in and out of the watersheds of the North Sea region, and those of the Greater Horn of Africa. Furthermore, we’ll show how regional sea level change and vertical land motion will be consistently accounted for in the proposed estimation scheme.

How to cite: Rietbroek, R., Karimi, S., and Shakya, A.: Unravelling watershed fluxes to detect emerging changes of the water balance, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13524, https://doi.org/10.5194/egusphere-egu23-13524, 2023.

EGU23-15762 | Posters on site | G3.1

Reduced order rainfall-discharge model for hydro-climatic data assimilation: a data-driven approach 

Karim Douch, Peyman Saemian, and Nico Sneeuw

Hydro-climatic variables such as precipitation (P), evapotranspiration (ET), terrestrial water storage (TWS) or river discharge define the terrestrial water cycle at local and global scales. The robust detection and quantification of steady trends in these variables require analysing sufficiently long time series of observations. Yet, historical discharge records may suffer from long data gaps or simply be too short; different reanalyses or data-driven models of P and ET often show large discrepancies and the associated uncertainty is not systematically provided. Finally, TWS has been observed only since the launch of GRACE in 2002 and also suffers from dozens of missing epochs.

Here, we present a 3-step approach to consistently reconstruct the historical time series of TWS and discharge at the catchment scale. In the first step, we use in-situ discharge observations and TWS anomaly derived from GRACE(-FO) observations to identify a reduced-order and mass-conserving rainfall-discharge model of the catchment. In the second step, the model is run with different precipitation and evapotranspiration data sets to select the pair P and ET reproducing most accurately the observed discharge and TWS. If necessary, the resulting net water flux (P-ET) is adjusted with a bias to improve the simulation accuracy. lastly, we apply a Bayesian smoother such as the Rauch–Tung–Striebel smoother to estimate TWS and discharge along with their respective uncertainty over the period covered by the P-ET time series. Critical to the proposed approach is the rainfall-discharge model identification. Here, we assume that the observed monthly-averaged discharge at the outlet is primarily driven by the TWS in the upstream catchment. As a consequence, we first estimate a storage-discharge model in the form of a continuous-time differential equation. This equation is subsequently coupled with the water mass balance equation to form the rainfall-discharge model. Remarkably, this final model is estimated independently of any P and ET models.

Finally, we apply the proposed approach to Amazonian and Siberian catchments for a period spanning from 1980 to 2020. In the first case, linear and time-invariant models capture with reasonable accuracy the observed drainage dynamics. In contrast, non-linear or linear and time-variable models are necessary to take correctly into account the temperature-dependent snow and ice accumulation and thaw in the case of Siberian catchments.

How to cite: Douch, K., Saemian, P., and Sneeuw, N.: Reduced order rainfall-discharge model for hydro-climatic data assimilation: a data-driven approach, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15762, https://doi.org/10.5194/egusphere-egu23-15762, 2023.

EGU23-16296 | ECS | Orals | G3.1 | Highlight

Sea-level change along the South American Atlantic coastline 

Carolina M.L. Camargo, Theo Gerkema, Yochi Okta Andrawina, and Aimée B.A. Slangen

In comparison with the number of tide gauges measuring in-situ sea-level change along the Northern Hermisphere coastlines, the Southern Hemisphere has a poor spatial distribution of stations. For example, along the South American Atlantic coastline, only 12 tide gauges are registered at the Permanent Service for Mean Sea-level (PSMSL), of which only two have been updated in the last three years. While satellite altimetry can be used to provide data in locations where there is no in-situ data, estimating coastal sea-level change using altimetry data is challenging due to the distortion of the satellite signal close to the land. Consequently, sea-level change along the South American Atlantic coastline is still poorly understood. Here, we fill this gap by using coastal altimetry products together with a new network of tide gauges deployed along the coast of Brazil (by the SIMCosta project). Via a sea-level budget analysis, we look at the regional drivers of sea-level change along the coast.

 

Recently, a large effort has been put towards developing algorithms that improve the accuracy of standard radar altimetry in coastal regions. Here, we compare both a coastal altimetry product (XTRACT/ALES) and a standard altimetry product (from CMEMS) to the local tide gauges. Previous studies have shown that, for some regions, coastal sea level is driven by open ocean sea-level change ( e.g., Dangendorf et al, 2021). Following this approach, we use clusters of coherent sea-level variability (Camargo et al., 2022), extracted with a network detection algorithm (delta-Maps), that extend to the open ocean, as proxies of the drivers of sea-level change along the coast.  The northern part of the study region, covering the Amazon Plateau, has a good match between the coastal altimetry-observed sea-level change and the sum of the drivers. The sum of the drivers and coastal altimetry trends also match, considering the uncertainty bars, for the most southern part, covering the Patagonian Shelf. For the other regions, we find a large difference between the coastal altimetry-observed sea-level change and the sum of the drivers. Thus, it is possible that these regions cover large-scale features, which are not strongly correlated with coastal sea level.

 

References

Camargo, C. M. L., Riva, R. E. M., Hermans, T. H. J., Schütt, E. M., Marcos, M., Hernandez-Carrasco, I., and Slangen, A. B. A.: Regionalizing the Sea-level Budget With Machine Learning Techniques, EGUsphere [preprint, accepted], https://doi.org/10.5194/egusphere-2022-876, 2022.

Dangendorf, S., Frederikse, T., Chafik, L., Klinck, J. M., Ezer, T., & Hamlington, B. D.: Data-driven reconstruction reveals large-scale ocean circulation control on coastal sea level. Nature Climate Change, 11, 514-520. https://doi.org/10.1038/s41558-021-01046-1, 2021.

How to cite: M.L. Camargo, C., Gerkema, T., Okta Andrawina, Y., and B.A. Slangen, A.: Sea-level change along the South American Atlantic coastline, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16296, https://doi.org/10.5194/egusphere-egu23-16296, 2023.

EGU23-16692 | ECS | Orals | G3.1

Geodetic sensing of mass variations due to climatic conditions 

Jagat Dwipendra Ray, Swapnali Patar, and Rebarani Mahata

The Earth Surface undergoes continuous deformation due to surface mass variations. These mass variations are primarily caused by the hydrological cycle, snowfall, ice melt and glacial isostatic adjustment (GIA). Modern geodetic sensing techniques like the Global Navigational Satellite System (GNSS) can sense these mass variations with unprecedented accuracy.  Therefore, the GNSS positioning time series provides a unique opportunity to study these mass variations and their causes.

In this study, we have used the GNSS time series from the region of Africa and Antarctica to analyse the mass variations. Conditions like draught and ice melting characterise these two regions. Therefore this current study will look at the signals of these two physical conditions. The results obtained are discussed and analysed.

How to cite: Ray, J. D., Patar, S., and Mahata, R.: Geodetic sensing of mass variations due to climatic conditions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16692, https://doi.org/10.5194/egusphere-egu23-16692, 2023.

OS2 – Coastal Oceans, Semi-enclosed and Marginal Seas

EGU23-1019 | Posters on site | OS2.1

Decomposition of estuarine circulation and residual stratification under land-fast sea ice 

Hans Burchard, Karsten Bolding, Xaver Lange, and Alexander Osadchiev

For Arctic estuaries which are characterized by land-fast sea-ice cover during the winter season, processes generating estuarine circulation and residual stratification have not yet been investigated, although some of the largest estuaries in the world belong to this class. Land-fast sea ice provides a no-slip surface boundary condition in addition to the bottom boundary, such that frictional effects are expected to be increased. For this study of estuarine circulation and residual stratification under land-fast sea ice, first a simple linear analytical model is used. To include tidally varying scenarios, a water-column model is applied with a second-moment turbulence closure to juxtapose free-surface and ice-covered estuaries. Well-mixed and strongly stratified tidally periodic scenarios are analyzed by means of a decomposition of estuarine circulation into contributions from gravitational circulation, eddy viscosity - shear covariance (ESCO), surface stress and river run-off. A new method is developed to also decompose tidal residual salinity anomaly profiles. Estuarine circulation intensity and tidally residual potential energy anomaly are studied for a parameter space spanned by the Simpson number and the Unsteadiness number. These are the major results of this study that will support future scenario studies in Arctic estuaries under conditions of accelerated warming:
(i) Residual surface drag under ice opposes estuarine circulation;
(ii) Residual differential advection under ice destabilizes the near-surface flow;
(iii) Reversal of ESCO during strong stratification does not occur under land-fast sea ice;
(iv) Tidal pumping (s-ESCO) contributes dominantly to residual stratification also with sea-ice cover.

How to cite: Burchard, H., Bolding, K., Lange, X., and Osadchiev, A.: Decomposition of estuarine circulation and residual stratification under land-fast sea ice, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1019, https://doi.org/10.5194/egusphere-egu23-1019, 2023.

EGU23-2732 | Posters on site | OS2.1

Physical/biogeochemical modelling of the global coast with ICON-coast – the impact of continental runoff 

Kai Logemann, Moritz Mathis, and Corinna Schrum

ICON-coast is the coastal version of the newly developed global ocean model ICON-O, which is itself part of the ICON (Icosahedral Non-hydrostatic) earth system modelling framework, developed by the Deutscher Wetterdienst and the Max-Planck-Institute for Meteorology. ICON-coast uses an unstructured, triangular computational mesh with a regular bisection-type mesh refinement technique to increase the horizontal resolution along the global coast. The global tides are included and crucial shelf-specific processes are added to the pre-installed biogeochemical sub-model (HAMMOC). Furthermore, an interface to the FABM 1.0 framework was implemented, which enables a coupling with the biogeochemical model ECOSMO. We present first ICON-coast/ECOSMO experiments in order to investigate the impact of the continental runoff and its related eutrophication on the global coastal ecosystem.

How to cite: Logemann, K., Mathis, M., and Schrum, C.: Physical/biogeochemical modelling of the global coast with ICON-coast – the impact of continental runoff, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2732, https://doi.org/10.5194/egusphere-egu23-2732, 2023.

The circulation within marginal seas subject to periodic winds, and their exchange with the open ocean, are explored using idealized numerical models and theory. This is motivated by the strong seasonal cycle in winds over the Nordic Seas and the exchange with the subpolar North Atlantic Ocean through the Denmark Strait and Faroe Bank Channel, although the analysis is general in nature and relevant to other marginal seas. Two distinct regimes are identified: an interior with closed 𝑓 /ℎ contours and a shallow shelf region that connects to the open ocean. The interior develops a strong oscillating along-topography circulation with weaker ageostrophic radial flows. The relative importance of the bottom Ekman layer and interior ageostrophic flows depends only on 𝜔ℎ/𝐶d , where 𝜔 is the forcing frequency, ℎ is the bottom depth, and 𝐶d is a linear bottom drag coefficient. The dynamics on the shelf are controlled by the frictional decay of coastal waves over an along-shelf scale 𝐿 = 𝑓0 𝐿s 𝐻s /𝐶d , where 𝑓0 is the Coriolis parameter, and 𝐿s and 𝐻s are the shelf width and depth. For 𝐿 much less than the perimeter of the basin, the surface Ekman transport is provided primarily by overturning within the marginal sea and there is little exchange with the open ocean. For 𝐿 on the order of the basin perimeter or larger, most of the Ekman transport is provided from outside the marginal sea. There is also an opposite exchange through the deep part of the strait, as required to conserve mass within the marginal sea. This demonstrates a direct connection between the dynamics of coastal waves on the shelf and the exchange of deep waters through the strait, some of which is derived from below sill depth.

How to cite: Spall, M. A.: Wind-forced seasonal exchange between marginal seas and the open ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2794, https://doi.org/10.5194/egusphere-egu23-2794, 2023.

EGU23-3370 | ECS | Posters on site | OS2.1 | Highlight

Improving the representation of freshwater input to shelf seas in an intermediate-complexity global climate model 

Katie Sieradzan, Andreas Schmittner, Sophie-Berenice Wilmes, Mattias Green, and Tom Rippeth

Whilst accounting for only 7% of the global ocean surface area, shelf seas are important links between multiple environments (including terrestrial, deep marine, and atmospheric) and modulate the freshwater influx from rivers before it reaches the open ocean. This freshwater acts as a buoyancy forcing, and, together with solar heating and tidal mixing, affects the seasonal stratification of shelf seas. As stratification impacts numerous processes within the shelf seas, such as heat uptake, ocean currents and biogeochemistry which may further be of global importance, it is important that freshwater fluxes are accurately simulated within models.
Despite their importance, due to coarse model resolution, shelf seas are generally poorly represented in intermediate-complexity global climate models. Here, we examine the accuracy of shelf sea representation in the intermediate-complexity UVic Earth System Climate Model, with a primary focus on the North Sea. Using observational data, we show that the river basin configuration and freshwater discharge in the control model set up has large errors. As a result, the North Sea receives almost double the expected freshwater discharge on an annual scale, impacting the flushing time, seasonal stratification and biogeochemistry of the region. Through a series of simulations rerouting freshwater through more realistic drainage basins, and removing excess freshwater, we improve simulation results, with variations in freshwater fluxes having a significant impact on shelf sea processes. Our results indicate that the over-freshening of shelf seas may not solely be restricted to the UVic model but may be an issue in other global Earth system models due to their low spatial resolution.

How to cite: Sieradzan, K., Schmittner, A., Wilmes, S.-B., Green, M., and Rippeth, T.: Improving the representation of freshwater input to shelf seas in an intermediate-complexity global climate model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3370, https://doi.org/10.5194/egusphere-egu23-3370, 2023.

EGU23-3661 | ECS | Orals | OS2.1

Mesoscale dynamics and its influence on coastal upwelling in the northern Gulf of Guinea 

Abdoul Karim Thiam, Gaël Alory, Isabelle Dadou, Yves Morel, Dante Napolitano, Camille Cardot, Micael Aguedjou, Guillaume Morvan, and Julien Jouano

Very little is known on mesoscale dynamics in the northern Gulf of Guinea, off West Africa. The purpose of our work is to quantify these mesoscale eddies dynamics in this region (0°N-7°N, 10°W-10°E) and their impact on the near-surface ocean and particularly in the coastal upwelling along the northern coast between 2°W and 2°E. We used a regional simulation of the NEMO model at 1/36° resolution of the year 2016 with daily outputs, validated with in situ and satellite data. On average, four cyclonic and four anticyclonic eddies were detected per day with a mean radius of 75 km and 72 km, respectively. Their lifetime is of the order of few days to a month with associated sea level anomaly from 0.5 cm to more than 1cm. The largest eddies with a relatively long life span are located between 2°N and 4°N, east of Cape Palmas (Ivory Coast) and Cape Three Points (Ghana). We then focused on the July-August-September upwelling period, during which we detected a cyclonic eddy east of the Cape of Three Points, from mid-July to mid-August 2016 with an average radius of 75 km. This cyclone is quasi-stationary and is located in the core of coastal upwelling.

Using a heat budget, we show that this eddy has an influence on sea surface temperature (SST) with a double effect. It expands offshore the upwelled cold and salty waters from July 14 to 24, then from July 25 until the dissipation of the cyclone, it weakens this upwelling by advection of warm offshore waters towards the coast, which mix with the upwelling cold waters and warm them.
A lagrangian study shows that the eddy waters come from the coastal upwelling, then mix with warmer offshore waters and later are transported eastward by the Guinea Current.
In conclusion, this study demonstrates the key role of eddies in SST intra-seasonal variability in the northern Gulf of Guinea.
Keywords : Gulf of Guinea, Modeling, Eddy, Coastal upwelling, Lagrangian simulation.

How to cite: Thiam, A. K., Alory, G., Dadou, I., Morel, Y., Napolitano, D., Cardot, C., Aguedjou, M., Morvan, G., and Jouano, J.: Mesoscale dynamics and its influence on coastal upwelling in the northern Gulf of Guinea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3661, https://doi.org/10.5194/egusphere-egu23-3661, 2023.

EGU23-4696 | ECS | Orals | OS2.1 | Highlight

Stochastic properties and statistics of salt intrusion in estuaries in a warming climate 

Jiyong Lee, Bouke Biemond, Huib de Swart, and Henk Dijkstra

Salt intrusion is part of natural estuary dynamics, where saline water enters inland by tides and exchange flow, and is diluted by fresh river water. These processes and feedbacks are complicated and inherently stochastic; and it is not yet well understood how they determine the statistical behavior of the salt intrusion length. More importantly, there are large uncertainties regarding future changes of the forcing of the salt intrusion in a warming climate. In this presentation, we will introduce a new stochastic model that computes temporal changes of the salt balance equation with the decompositions of river discharge and salt intrusion length into deterministic and stochastic components. The developed framework is applied to field observations in the San Francisco Bay (USA) and shown to well reproduce general statistics of salt intrusion length. Next, the model is applied to estuaries in Europe under projected river discharge distributions up to 2100 using two large ensembles of the Community Earth System Model. The key assumption in the model is that the changes in the river discharge are the main driver that induces variability and changes in salt intrusion length in the coming decades. Our results show that there will be significant increase of salt intrusion during dry periods in many European estuaries, especially those at low latitudes. The analysis stresses that, for adequate water management, great attention is needed in monitoring and predicting salt intrusion lengths in the future.

How to cite: Lee, J., Biemond, B., de Swart, H., and Dijkstra, H.: Stochastic properties and statistics of salt intrusion in estuaries in a warming climate, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4696, https://doi.org/10.5194/egusphere-egu23-4696, 2023.

EGU23-5369 | ECS | Orals | OS2.1

Observational study on mixing in a stratified scour hole due to the wind-driven lateral circulation in a semi-closed estuary 

Fateme Ebrahimi Erami, Vasileios Kitsikoudis, Bart Vermeulen, and Suzanne Hulscher

Salt intrusion occurring in estuarine environments can be aggravated by the presence of scour holes in the bed. Saltwater has a higher density than freshwater. Therefore saltwater accumulates in scour holes which may exacerbate the salinization of freshwater once it is entrained by wind or other forcings. Wind can contribute to estuarine circulation and stratification through three main mechanisms: direct wind mixing, wind straining and wind-driven lateral circulation. Previous studies (Csanady, 1982; Winant, 2004), suggest that horizontal wind circulation mostly comes into play in estuaries with laterally varying bathymetry, which is the case for our study site. The Haringvliet estuary in the Rhine-Meuse delta is a former tidal basin in the western part of the Netherlands; it varies in bathymetry and has been closed off by floodgates. The gates are only opened during ebb tide to discharge river into the sea, and also for a short period of time during flood tide for ecological purposes. The complex geomorphology of the estuary is composed of shoals and deep scour holes. An extensive field campaign was carried out for over six months in the Haringvliet, at the locations of the scour holes, in which we measured flow velocity, salinity, discharge, and wind speed and direction. Results indicate that, under an axial wind over the estuary, a horizontal circulation forms by downwind flow over shoals and upwind flow in the deep channels. Based on the collected dataset, a change from a down-estuary to an up-estuary wind direction occurred while the floodgates were closed. As a result of the wind influence, the flow direction in the stratified deep channel changed quickly, which provided sufficient shear and turbulence in the whole water column for vertical mixing. The sharp drop in the salinity concentration corresponding to the mixing and flushing in the scour hole occurred due to the wind-induced lateral circulation without having high river discharge. This research shows that, in a semi-closed estuary like the Haringvliet, lateral currents and the momentum transfer corresponded to that can exert a predominant control on estuarine circulation and stratification.

Acknowledgments: This research was funded by the Netherlands Organisation for Scientific Research (NWO), research program SALTISolutions with project number P18-32. Rijkswaterstaat, the Dutch Ministry of   Infrastructure and Water Management, is thanked for providing extensive field data for this research.

 

References:

Csanady, G.T., 1981. Circulation in the coastal ocean. In Advances in geophysics (Vol. 23, pp. 101-183). Elsevier.

Winant, C.D., 2004. Three-dimensional wind-driven flow in an elongated, rotating basin. Journal of Physical Oceanography, 34(2), pp.462-476.

How to cite: Ebrahimi Erami, F., Kitsikoudis, V., Vermeulen, B., and Hulscher, S.: Observational study on mixing in a stratified scour hole due to the wind-driven lateral circulation in a semi-closed estuary, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5369, https://doi.org/10.5194/egusphere-egu23-5369, 2023.

EGU23-6048 | Orals | OS2.1 | Highlight

An advanced towed CTD chain for high resolution physical-biological in situ measurements in the upper ocean 

Thomas Kock, Paulo Calil, Florian Wobbe, Gerd Seidel, Rolf Riethmüller, Stephan Deschner, Martina Heineke, and Burkard Baschek

Eddies, fronts, and filaments of varying scales populate the upper ocean and are particularly important in coastal regions. These features play a vital role on biogeochemical and mixing processes as well as in the energy budget. To capture their high spatial variability, it is desirable to simultaneously resolve the horizontal and vertical gradients of hydrographic properties on scales from O(10) m to O(100) km. We present an improved towed CTD chain for rapid quasi-synoptic in situ measurements of submesoscale oceanographic features to fill this observational gap. The advanced towed CTD chain is robust, lighter and scientifically more useful than previous versions. Added flexibility in terms of freely adaptable chain and sensor setup enables tailor-made surveys for a variety of research questions. The advanced towed CTD chain collects data at a very high horizontal resolution in O(1) m with a vertical resolution between 1 to 10 m, depending on CTD probe count and spacing. Individual CTD probes used within the chain are self-contained instruments equipped with temperature, conductivity, pressure and either fast response dissolved oxygen or fluorescence sensors placed at multiple depths enabling simultaneous hydrographic and biogeochemical studies at high resolution. With the flexible probe hardware it is possible to collect data either with real-time data visualisation for adaptive sampling missions or - in a much simpler and lighter setup - log data internally for offline evaluation. Together with the towed CTD chain a set of software tools and techniques for processing CTD chain data has been developed to provide an easy-to-use and complete system. Data examples collected in various areas like the Amazonas river plume and Cape Verde Island wake highlight the advanced CTD chains robustness, flexibility and scientific capabilities.

How to cite: Kock, T., Calil, P., Wobbe, F., Seidel, G., Riethmüller, R., Deschner, S., Heineke, M., and Baschek, B.: An advanced towed CTD chain for high resolution physical-biological in situ measurements in the upper ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6048, https://doi.org/10.5194/egusphere-egu23-6048, 2023.

EGU23-6255 | ECS | Posters on site | OS2.1

Effect of the sea surface heat flux on upper layer circulation of the East/Japan Sea 

Daehyuk Kim, Hong-Ryeol Shin, Cheol-Ho Kim, Naoki Hirose, and Eun-Chul Chang

The East/Japan Sea (hereafter the East Sea) is one of the semi-closed marginal seas surrounded by Korea, Russia, and Japan. The Tsushima Warm Current (TWC) that is one of the major upper layer circulations flows into the East Sea through the Korea/Tsushima Strait (KTS). This TWC mainly bifurcates into three main branches in the southern part of the East Sea: the Nearshore Branch (NB), East Korea Warm Current (EKWC), and the Offshore Branch (OB). The upper layer circulations are greatly influenced by various external forcings such as wind stress, bottom topography, volume transport flowing through the KTS, and thermal forcing. Among them, the thermal forcing impact on the upper layer circulation of the East Sea is not well known. In this study, a three-dimensional numerical ocean model (RIAMOM) of Kyushu University is employed to investigate the impact of surface heat flux on the upper layer circulation of the East Sea. Numerical results show that the branching of the TWC is simulated regardless of applying wind stress. However, the EKWC that is one of the TWC branches does not appear without surface heat flux. Most of the TWC flows along the Japanese coast in the form of the NB. Furthermore, since there is no thermal interaction with the atmosphere, the heat supplied through the KTS tends to accumulate in the upper layer, thereby forming a thick upper layer in the overall East Sea. As the upper layer thickness increases, the TWC is significantly influenced by bottom topography. Therefore, most of the TWC flows along the developed continental shelf off the Japanese coast. In contrast, without the continental shelf, the TWC flows only along the Korean coast as a western boundary current regardless of surface heat flux. This result suggests that the surface heat flux can play an important role in controlling the bottom topographic and planetary beta effects on the upper layer circulation in the branching of the TWC.

 

Keywords: Tsushima Warm Current, East Korea Warm Current, Surface heat flux, bottom topographic effect, planetary beta effect

Acknowledgment: This work was funded by the Korea Meteorological Administration Research and Development Program under Grant KMI2022-01210.

How to cite: Kim, D., Shin, H.-R., Kim, C.-H., Hirose, N., and Chang, E.-C.: Effect of the sea surface heat flux on upper layer circulation of the East/Japan Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6255, https://doi.org/10.5194/egusphere-egu23-6255, 2023.