EGU General Assembly 2023  

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 175^oE  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.

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.

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.

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.

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 Letters, 47(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 Change, 9(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 Change, 11(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 Letters, 49(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.

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.

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 28^oC, 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 90^oE, 15^oN 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 26^oC isotherm) – D12 (depth of 
26^oC 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.

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.

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.

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.

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.

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.

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.

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 (J_q(z)) to describe how WPWP mixing is regulated by seasonal, intraseasonal, and synoptic-scale atmospheric disturbances. It is argued that observed variations in J_q(z) impact seasonal-to-synoptic trends in SST and mixed-layer depth. J_q(z) is weakest during Spring (dry season), when heat fluxes into the ocean (Q_net > 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 J_q(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 (Q_net < 0) further contributed to mixing. A similar seasonality is observed at the thermocline level, where J_q(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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 coefficients 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.

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.

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.

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.

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.

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.

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.

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 : λ_s which describes the relative contribution of the buoyancy flux and the wind in the surface layer, and the Richardson number R_h 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 d_t 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.

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.

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.

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.

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, 6^oN-15^oN, 72^oE-77^oE) 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 28.5^oC) 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 2^o 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 (²³⁶U_RP) 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 ²³⁶U_RP 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.

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.

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.

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.

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.

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.

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.

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.

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.

At present, it is well-known that the fast increase in atmospheric carbon dioxide (CO₂) concentrations resulting from human activities (C_ant), 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, C_ant-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 C_ant. Despite having such an important role, the magnitude of the C_ant 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 C_ant 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 C_ant 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 C_ant concentrations and inventories obtained with the two pairs of transient tracers to one-another as well as to previous estimates of C_ant 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 C_ant 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 20^th and 21^st 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.

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 F_ov index. For example when AMOC transports net freshwater out of the Atlantic sector at 34°S (F_ov < 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 F_ov 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 F_ov index, implying that the AMOC is too stable in these climate model simulations. Here we analyse F_ov-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 F_ov index drifts from negative values to positive values over a 300-year simulation period. The F_ov 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 F_ov are responsible the difficulty in simulating realistic AMOC behaviour in climate model simulations. The implication is that climate models with an inconsistent F_ov 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.

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.

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.

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 (C_ant). The North Atlantic Ocean encompasses the highest ocean storage capacity of C_ant per unit area. In particular, the Labrador and Irminger Seas are two basins storing a high amount of C_ant due to the deep convection activity taking place there. The temporal evolution of C_ant concentration in these two basins and their C_ant inventories in the 0-1800 depth layer are estimated over the period 2011-2021. The C_ant values are estimated from Argo floats equipped with oxygen sensors, predictive neural networks (ESPER_NN and CONTENT) and a carbon-based back-calculation method (φC^O_T method). On average, C_ant 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.

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.

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.

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.

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.

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.

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.

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.

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.

The Atlantic Meridional Overturning Circulation (AMOC) is predicted to weaken in the 21^st 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 (U_10m) and SST and positive correlations between near-surface specific humidity (q_2m) and SST. However, when smaller scales (1 – 150km, i.e., encompassing the ocean mesoscale and a portion of the submesoscale) are considered, the U_10m-SST and q_2m-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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 (δ¹³C_POC) and nitrogen (δ¹⁵N_PN) 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 δ¹³C_POC 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 δ¹⁵N_PN 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 δ¹⁵N_PN 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.

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 32^oS 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.

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.

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.