OS – Ocean Sciences

OS1.1 – Improved Understanding of Ocean Variability and Climate

EGU22-1377 | Presentations | OS1.1

New estimates of observed poleward freshwater transport since 1970

Taimoor Sohail, Jan Zika, Damien Irving, and John Church

Global water cycle changes induced by anthropogenic climate change pose a growing threat to existing ecosystems and human infrastructure. However, scarce direct observations of precipitation and evaporation means historical water cycle changes remain uncertain. In this work, we apply a novel watermass-based diagnostic framework to the latest observations of ocean salinity to quantify poleward freshwater transport in the earth system since 1970. This observational estimate is not replicated in any model in the current generation of CMIP6 climate models - likely due to the inaccurate representation of surface freshwater flux intensification in such models. These results provide a first-of-its-kind baseline of observed warm-to-cold freshwater transport since 1970, and also underscore the need to further explore surface freshwater fluxes in existing climate models.

How to cite: Sohail, T., Zika, J., Irving, D., and Church, J.: New estimates of observed poleward freshwater transport since 1970, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1377, https://doi.org/10.5194/egusphere-egu22-1377, 2022.

EGU22-1309 | Presentations | OS1.1

 A regional (land – ocean) comparison of the seasonal to decadal variability of the Northern Hemisphere jet stream 1871-2011

Samantha Hallam, Simon Josey, Gerard McCarthy, and Joel Hirschi

Seasonal to decadal variations in Northern Hemisphere jet stream latitude and speed over land (Eurasia, North America) and oceanic (North Atlantic, North Pacific) regions are presented for the period 1871 – 2011 from the Twentieth Century Reanalysis dataset

Significant regional differences are seen on seasonal to decadal timescales. Seasonally, the  jet latitude range is lower over the oceans compared to land, reduced from 20° over Eurasia to 10° over the North Atlantic where the ocean meridional heat transport is greatest. The mean jet latitude range is at a minimum in winter (DJF), particularly along the western boundary of the North Pacific and North Atlantic, where the land-sea contrast and SST gradients are strongest.

The 141-year trends in jet latitude and speed show differences on a regional basis. The largest increasing trends in jet latitude and jet speed are observed in the North Atlantic, with increases in winter of 3° and 4.5ms-1, respectively. There are no trends in jet latitude or speed over the North Pacific.

Long term trends are overlaid by multi decadal variability. In the North Pacific, 20-year variability in jet latitude and jet speed are seen, associated with the Pacific Decadal Oscillation which explains 50% of the winter variance in jet latitude since 1940.

In addition, current work on a lead/lag analysis of western boundary currents/ocean variability in the North Atlantic and North Pacific and links to the northern hemisphere jet stream will be presented.

Hallam et al., A regional (land-ocean) comparison of the seasonal to decadal variability of the northern hemisphere jet stream. Climate Dynamics (2022 in revision).https://doi.org/10.21203/rs.3.rs-607067/v1

 

How to cite: Hallam, S., Josey, S., McCarthy, G., and Hirschi, J.:  A regional (land – ocean) comparison of the seasonal to decadal variability of the Northern Hemisphere jet stream 1871-2011, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1309, https://doi.org/10.5194/egusphere-egu22-1309, 2022.

EGU22-3526 | Presentations | OS1.1

A global stratification product of the thermocline based on Argo observations

Marisa Roch, Peter Brandt, and Sunke Schmidtko

Enhanced ocean stratification is projected as a result of a warming climate. Changes of upper-ocean stratification can have a potential impact on physical as well as biogeochemical and ecological processes, such as ocean circulation and redistribution of heat and salt, ocean ventilation and air-sea interactions and in addition, nutrient fluxes, primary productivity and fisheries. However, in what terms these processes might be affected still remains uncertain. This investigation particularly addresses variations of the vertical stratification maximum which is found at the depth of the thermocline/pycnocline. The analysis separates between summer and winter stratification. Trends of the vertical stratification maximum are computed for both seasons, respectively. Our intention is to show regional differences in the trends as well as to identify whether the corresponding seasonal cycle is changing. The aim of this study is further to produce a world-wide product of the stratification maximum based on Argo observations from 2006-2021. The goal is to create an algorithm that takes the uneven vertical resolution of Argo profiles into account. In order to verify our product, we compare the results of the Argo data to other CTD measurements as obtained from research vessels and buoys. With this we receive a quality-controlled global product which allows us to make a statement about the global variability of the stratification in the thermocline. Understanding the changes of the vertical stratification maximum will help to identify their impact on ocean ventilation and nutrient supply to the euphotic zone.

How to cite: Roch, M., Brandt, P., and Schmidtko, S.: A global stratification product of the thermocline based on Argo observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3526, https://doi.org/10.5194/egusphere-egu22-3526, 2022.

EGU22-4148 | Presentations | OS1.1

The global and regional structure of simulated historical ocean heat content change in CMIP6 models

Till Kuhlbrodt, Aurore Voldoire, Rachel Killick, and Matthew Palmer

Ocean heat content is arguably the most relevant metric for tracking the current global heating. Because of its enormous heat capacity, the global ocean stores about 89 percent of the excess heat in the Earth System. Time series of global ocean heat content (OHC) closely track Earth’s energy imbalance, observed as the net radiative imbalance at the top of the atmosphere. Therefore, simulated OHC time series are a cornerstone for assessing the scientific performance of Earth System models (ESM) and global climate models. Here we present a detailed global and regional analysis of the OHC change in CMIP6 simulations of the historical climate (20th century up to 2014) performed with four state-of-the art ESMs and climate models: UKESM1, HadGEM3-GC3.1-LL, CNRM-ESM2-1 and CNRM-CM6-1. All four share the same ocean component, NEMO3.6 in the shaconemo eORCA1 configuration. Analysing only a small number of models allows us to extend our analysis from a global perspective, to also consider individual ocean basins.

For the global ocean, the two CNRM models reproduce the observed OHC change since the 1960s closely, especially in the top 700 m of the ocean. The two UK models (UKESM1 and HadGEM3-GC3.1-LL) do not simulate the observed global ocean warming in the 1970s and 1980s in the top 700 m, and they warm too fast after 1991. We analyse how this varied performance across the models relates to the simulated radiative forcing of the atmosphere and its components. All four models show a larger transient climate response (TCR) than the CMIP5 ensemble mean.

For the UK models, resolving the ocean warming in depth and time shows virtually zero historical warming at intermediate depths (700 m – 2000 m) whereas the global full-depth OHC change is reasonably simulated. After 1991, regional ocean heat uptake in the North Atlantic plays a substantial role in compensating small warming rates elsewhere.

A different picture emerges from the CNRM models. Globally the simulated OHC change is closer to observations, especially for CNRM-ESM2-1. Regionally the simulated OHC change is close to observations in the Pacific and Indian basins, while tending to be too small in the Atlantic, indicating a markedly different role for the Atlantic meridional overturning circulation (AMOC) and for cross-equatorial heat transport between the CNRM models and the UK models. While the UK models simulate larger than observed historical warming below 2000 m in the Atlantic and South Pacific, the CNRM models take up heat at a larger than observed rate at intermediate depths in the South Atlantic and the South Pacific, with a much smaller role for the North Atlantic in global ocean heat uptake.  

How to cite: Kuhlbrodt, T., Voldoire, A., Killick, R., and Palmer, M.: The global and regional structure of simulated historical ocean heat content change in CMIP6 models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4148, https://doi.org/10.5194/egusphere-egu22-4148, 2022.

EGU22-12567 | Presentations | OS1.1

Scaling properties of sea surface temperature for various global warming levels in CMIP6 models

Josipa Milovac, Maialen Iturbide, Joaquin Bedia, Jesus Fernandez, and Jose Manuel Gutierrez

Mean sea surface temperature (SST) increased during the 20th century and continues to rise on average at a rate of 0.14 ºC per decade. In the last decade, mean SST showed an increase of 0.88 ºC compared to the pre-industrial era and, according to the latest IPCC report (Masson-Delmotte et al., 2021), 83% of the ocean surface will very likely continue to warm up until the end of this century in all Shared Socioeconomic Pathways (SSP). Global mean surface air temperature (GSAT) has increased by 1.09 ºC since the pre-industrial times, and it is projected to continue to rise by 1.0 - 5.7 ºC (depending on the SSP scenario) until the end of the 21st century. GSAT incorporates land surface air temperature (LSAT) and sea surface air temperature (SSAT) in the models. 

In this study we analyze the CMIP6 ensemble of global climate models to identify projected scaling properties between SST, SSAT, and GSAT under various SSP scenarios. Preliminary analysis indicates that the temperatures are linearly correlated, with the scaling factor of ~0.8 for SSAT and GSAT, ~0.7 for SST and GSAT, and ~0.87 for SST and SSAT at the global warming level of 2 ºC. Such scaling is regionally dependent, and does not apply to the polar oceanic regions. Furthermore, we explore the dependence of the scaling properties on the global warming levels, and how sensitive the results are for the coastal regions.

References:

IPCC, 2021: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press. In Press.

Acknowledgements:

We acknowledge the support from the Spanish Agencia Estatal de Investigación through the Unidad de Excelencia María de Maeztu with reference MDM-2017-0765., and the support from the projects CORDyS (PID2020-116595RB-I00) and ATLAS (PID2019-111481RB-I00), both funded by MCIN/AEI/10.13039/501100011033.

How to cite: Milovac, J., Iturbide, M., Bedia, J., Fernandez, J., and Gutierrez, J. M.: Scaling properties of sea surface temperature for various global warming levels in CMIP6 models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12567, https://doi.org/10.5194/egusphere-egu22-12567, 2022.

EGU22-6683 | Presentations | OS1.1

Deep ocean steady-state transport and decadal variability inferred from 1980-2020 CFCs and SF6 observations

Laura Cimoli, Sarah Purkey, Jake Gebbie, and William Smethie

What are the time-mean pathways and the decadal variability of the deep ocean circulation? To answer this question, we conduct a global tracer analysis with a newly developed approach, the “Time-Correction” method. This novel method leverages the information of four decades of anthropogenic transient tracer observations (1980-2020) to reconstruct their propagation in the global ocean. The Time-Correction method solves a modified least-squares problem that accounts for the uncertainty in the observations, propagates this uncertainty in our solution, and uses prior information about the system in the final solution. The method takes into account the statistical information used in the Maximum Entropy Method but is designed to be more computationally efficient.

We apply the Time-Correction method to chlorofluorocarbons (CFC-11 and CFC-12) and sulfur hexafluoride (SF6) observations to reconstruct the time evolution of their concentrations in the deep ocean. Their propagation is reconstructed at annual resolution and permits CFC snapshots from multiple decades to be put into a common context. The reconstructed tracer concentrations capture the pathways of AABW and NADW, highlighting (i) the southward flow of the different NADW components (upper, middle and lower NADW) and their equatorial recirculation in the Atlantic Ocean, and (ii) the spreading of CFC-rich AABW in the North Pacific Ocean through the Samoan Passage, its bottom-driven northward circulation in the East Indian Ocean, and its northward flow in the West Atlantic Ocean and recirculation around the Equator. These reconstructed tracer concentrations reflect the tracer distribution for time-mean ocean transport and can be used to investigate the non-steady ocean circulation decadal variability. In locations with multiple occupations of tracer data where no steady-state solution can be found, we conclude that the circulation has changed and show regional patterns of increased and decreased ventilation over the last four decades. Additional research is underway to investigate NADW formation rate variability over the 1980-2020 period at decadal and inter-annual resolution depending on the number of available occupations.

How to cite: Cimoli, L., Purkey, S., Gebbie, J., and Smethie, W.: Deep ocean steady-state transport and decadal variability inferred from 1980-2020 CFCs and SF6 observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6683, https://doi.org/10.5194/egusphere-egu22-6683, 2022.

EGU22-4259 | Presentations | OS1.1

Detection timescale of anthropogenic climate change signals in the global ocean

Jerry Tjiputra and Jean Negrel

Robust detection of anthropogenic climate change is a necessary prerequisite in developing reliable climate change mitigation and adaptation plans. Here, we use simulation data from a suite of latest Earth system model projections to establish the detection timescale of anthropogenic signals in the global ocean under a strong future climate change scenario. We focus on projections of temperature, salinity, oxygen, and pH changes from surface to 2000 m depths. Despite lack of direct interaction with anthropogenic forcing, climate changes in the interior ocean are projected to be detectable earlier than on the surface. This general feature is primarily due to the low background natural variability in the subsurface depths. Acidification signals will occur earliest, followed by warming and oxygen changes. Consistent with the global overturning circulation pathway, the interior of the Atlantic basin will experience earlier detectable signals than the Pacific and Indian basins. The model ensemble projects the subsurface tropical Pacific as the domain least susceptible to exposure of anthropogenic climate change signals over the 21st century. Our study suggests earliest detectable anthropogenic exposure can be expected in the Southern Ocean and the North Atlantic. Sustained deployment of monitoring systems, such as ARGO floats equipped with biogeochemical sensors, in these domains would be highly pertinent to timely detect the early emergence of anthropogenic climate change signals.

How to cite: Tjiputra, J. and Negrel, J.: Detection timescale of anthropogenic climate change signals in the global ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4259, https://doi.org/10.5194/egusphere-egu22-4259, 2022.

EGU22-12052 | Presentations | OS1.1

Neural-network parametric modeling of ocean surface brightness temperature polarimetric observations for Sentinel Copernicus Imaging Microwave Radiometer

Emanuele Gugliandolo, Mario Papa, Nazzareno Pierdicca, and Frank Marzano

The Copernicus Space Component Expansion program includes new missions that have been identified by the European Commission as priorities for implementation in the coming years by providing additional capabilities in support of current emerging user needs. The passive microwave imaging mission, such as the Copernicus Imaging Microwave Radiometer (CIMR) is uniquely able to observe a wide range of parameters, in particular sea ice concentration, and serve operational systems at almost all-weather conditions, day, and night. This mission shall provide improved continuity of sea ice concentration monitoring missions, in terms of spatial resolution (about 15 km), temporal resolution (sub-daily) and accuracy (in particular, near the ice edges). Additional measurement of sea surface temperature in the polar regions may also be included.

CIMR mission, to be launched in 2025, is designed to host spectral channels at 1.413 (L band), 6.925 (C band), 10.65 GHz (X band), 18.70 (K band), and 36.5 GHz (Ka band) with a radiometric sensitivity less than 0.4 K (except at Ka band where 0.7 K is goal) and a spatial resolution less than 60, 15, 15, 5, and 4 km, respectively. Such resolutions are obtained with a large deployable reflector mesh antenna of about 7-m diameter. CIMR shall be capable of measuring the full brightness temperature (BT) Stokes vector for all bands in the same way WindSat and SMAP (Soil Moisture Active Passive) spaceborne radiometers accomplished (even though not for all bands and not necessarily fully polarimetric). Most of the sea-surface retrieval techniques, developed so far, have been based on maximum likelihood approaches exploiting the sea-surface geophysical model function (SSGMF). Even though previous missions span over most CIMR channels, there is not a systematic development and synthesis of CIMR SSGMF with a polarimetric capability.

In this work we aim at modeling CIMR sea emissivity GMF Stoke vector parameters, coupled with a microwave atmosphere radiative transfer (MART) model in clear/cloudy conditions and ECMWF ReAnalyses (ERA5) input data, to simulate CIMR brightness temperatures (BT) in different sea climatic regions, i.e., Northern and Southern Atlantic Ocean and Mediterranean Sea. MART simulations are statistically validated with AMSR2 (Advanced Microwave Scanning Radiometer 2) at C, X, K and Ka band and SMAP data at L band. A feed-forward neural network (NN) is developed to simulate polarimetric CIMR BT Stokes vector directly from ERA5 inputs as well as an inverse NN to retrieve the surface wind velocity and direction, sea surface salinity and temperature from CIMR polarimetric BT data. The designed NN is built with 1 hidden layer and sigmoidal functions, 151 inputs (from ERA5 profiles) and 20 outputs (BT Stokes vector for 5 frequency channels) trained and tested on the 3 selected areas of interest. The results show a correlation coefficient between the predicted and actual values larger than 0.9, meaning that the forward and inverse NNs successfully capture the relationship between the ERA5 inputs and the simulated CIMR BT Stokes vector. Results will be illustrated and discussed, pointing out potential developments and critical issues.

How to cite: Gugliandolo, E., Papa, M., Pierdicca, N., and Marzano, F.: Neural-network parametric modeling of ocean surface brightness temperature polarimetric observations for Sentinel Copernicus Imaging Microwave Radiometer, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12052, https://doi.org/10.5194/egusphere-egu22-12052, 2022.

EGU22-2085 | Presentations | OS1.1

Adiabatic and diabatic signatures of ocean temperature variability

Ryan Holmes, Taimoor Sohail, and Jan Zika

EGU22-8895 | Presentations | OS1.1

Reconstructing upper ocean carbon variability using ARGO profiles and CMIP6 models

Katherine Turner, Richard G. Williams, Anna Katavouta, and Doug M. Smith

Historically, ocean carbon content has been poorly sampled due to the logistical difficulties inherent in carbonate chemistry measurements.  As a result, global products of ocean carbon content observations have been restricted to calculate climatologies or long-term trends. Recent innovations with machine learning have provided for observational reconstructions of multidecadal and interannual carbon variability. In this work, we create a complementary method for reconstructing historical carbon variability by drawing upon the Ensemble Optimal Interpolation method used for reconstructing historical ocean heat and salinity [1-3]. Ensemble Optimal Interpolation draws upon first-order relationships between variables and use covariances from model ensembles to propagate information from data-rich to data-sparse regions.

We test our method by conducting synthetic reconstructions of upper ocean carbon content using ARGO-style sampling distributions with CMIP6 ensemble covariance fields. Sensitivity tests of local carbon reconstructions suggest that around 50% of ocean carbon variability can be reconstructed using temperature and salinity measurements. Expanding the synthetic reconstructions to include irregular sampling consistent with ARGO profile locations results in a similar capacity to reconstruct ocean carbon variability, as the increased information provided from multiple sampling locations compensates for the propagation of errors within the CMIP6 covariance fields.  Our initial results indicate that the first-order relationships between temperature, salinity, and carbon can be used to describe a substantial proportion of historical carbon variability. In addition to showing promise for a new historical reconstruction complementary to current products, our work emphasises the important links between hydrographic and carbon variability for much of the global ocean.

 

References

[1] D. M. Smith and J. M. Murphy, 2007. "An objective ocean temperature and salinity analysis using covariances from a global climate model," JGR Oceans.

[2] L. Cheng, K. E. Trenberth, J. T. Fasullo, T. Boyer, J. T. Abraham and J. Zhu, 2017. "Improved estimates of ocean heat content from 1960 to 2015," Science Advances.

[3] L. Cheng, K. E. Trenberth, N. Gruber, J. P. Abraham, J. T. Fasullo, G. Li, M. E. Mann, X. Zhao and J. Zhu, 2020. "Improved Estimates of Changes in Upper Ocean Salinity and the Hydrological Cycle," Journal of Climate.

How to cite: Turner, K., Williams, R. G., Katavouta, A., and Smith, D. M.: Reconstructing upper ocean carbon variability using ARGO profiles and CMIP6 models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8895, https://doi.org/10.5194/egusphere-egu22-8895, 2022.

EGU22-12758 | Presentations | OS1.1

Decomposing oceanic temperature and salinity change using ocean carbon change

Charles Turner, Pete Brown, Kevin Oliver, and Elaine Mcdonagh

As the planet warms due to anthropogenic CO2 emissions, the interaction of surface ocean carbonate chemistry and the radiative forcing of atmospheric CO2 leads to the global ocean sequestering heat and carbon, in a ratio that is near constant in time: this enables patterns of ocean heat and carbon uptake to be derived. Patterns of ocean salinity also change as the earth system warms due to hydrological cycle intensification and perturbations to air-sea freshwater fluxes.
Local temperature and salinity change in the ocean may result from perturbed air-sea fluxes of heat and freshwater (excess temperature, salinity), or from reorganisation of the preindustrial temperature and salinity fields (redistributed temperature, salinity).
Here, we present a novel method in which the redistribution of preindustrial carbon is diagnosed, and the redistribution of temperature and salinity estimated using only local spatial information.
We demonstrate this technique in the NEMO OGCM coupled to the MEDUSA-2 Biogeochemistry model under a RCP8.5 scenario over 1860-2099. 
The excess changes are thus calculated.
We demonstrate that a global ratio between excess heat and temperature is largely appropriately regionally with key regional differences consistent with reduced efficiency in the transport of carbon through the mixed layer base at high latitudes.
On centennial timescales, excess heat increases everywhere, with 25+/-2 of annual global heat uptake in the North Atlantic over the 21st century.
Excess salinity meanwhile increases in the Atlantic but is generally negative in other basins, consistent with increasing atmospheric transport of freshwater out of the Atlantic.
In the North Atlantic, changes in the inventory of excess salinity are detectable in the late 19th century, whereas increases in the inventory of excess heat does not become significant until the early 21st century. This is consistent with previous studies which find salinification of the Subtropical North Atlantic to be an early fingerprint of anthropogenic climate change.

Over the full simulation, we also find the imprint of AMOC slowdown through significant redistribution of heat away from the North Atlantic, and of salinity to the South Atlantic.
Globally, temperature change at 2000m is accounted for both by redistributed and excess heat, but for salinity the excess component accounts for the majority of changes at the surface and at depth. 
This indicates that the circulation variability contributes significantly less to changes in ocean salinity than to heat content.

By the end of the simulation excess heat is the largest contribution to density change and steric sea level rise, while excess salinity greatly reduces spatial variability in steric sea level rise through density compensation of excess temperature patterns, particularly in the Atlantic.
In the Atlantic, redistribution of the preindustrial heat and salinity fields also produce generally compensating changes in sea level, though this compensation is less clear elsewhere.

The regional strength of excess heat and salinity signal grows through the model run in response to the evolving forcing.
In addition, the regional strength of the redistributed temperature and salinity signals also grow, indicating increasing circulation variability or systematic circulation change on timescales of at least the model run.

How to cite: Turner, C., Brown, P., Oliver, K., and Mcdonagh, E.: Decomposing oceanic temperature and salinity change using ocean carbon change, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12758, https://doi.org/10.5194/egusphere-egu22-12758, 2022.

EGU22-8288 | Presentations | OS1.1

Ocean sequestration of carbon dioxide and heat under global warming in a climate model with an eddy-rich ocean

Ivy Frenger, Carolina Dufour, Julia Getzlaff, Stephen Griffies, Wolfgang Koeve, and Jorge Sarmiento

To robustly estimate how much carbon dioxide (CO2) we may still emit while staying below a certain level of global warming, we need to know uncertainty in oceanic sequestration of CO2 and heat. We here address uncertainty in oceanic CO2 and heat sequestration that arises due to the representation of ocean mesoscale features. Such features are fundamental components of ocean circulation and mixing, though with spatial scales smaller than 100 km they are typically not resolved by climate models. We compare three configurations of the GFDL climate model that differ in the spatial resolution of the ocean, namely "eddy-rich" (0.10o resolution) that simulates a rich field of mesoscale features such as mesoscale eddies and fronts, "eddy-present" (0.25o) that simulates mesoscale features to a lesser extent, and "eddy-param" (1o grid spacing) that does not resolve mesoscale features but represents effects of mesoscale eddies with parameterizations. The three models are run under preindustrial conditions and then exposed to an idealized increase of atmospheric CO2 levels of one percent per year, until CO2 doubling is reached.

We find that ocean mesoscale processes act to enhance the oceanic uptake of heat under global warming, while they act to reduce the uptake of CO2 (eddy-rich relative to eddy-present). The greater heat sequestration is due to a greater reduction of the Atlantic Meridional Overturning Circulation, which redistributes heat from the Pacific to the Atlantic oceans, but also leads to an enhanced, albeit small, net global heat gain of several percent. Potential causes for the reduced sequestration of CO2 (eddy-rich takes up 7% less relative to eddy-present) include reduced surface solubility of CO2 due to the larger heating, or a different preindustrial state, e.g., of the buffer capacity. Eddy-param appears to not capture this effect; in contrast, it sequesters 13% more CO2 than eddy-rich. While eddy-param largely captures the redistribution of heat between the Pacific and Atlantic oceans, it does not capture the enhanced net global heat gain. Despite the lower oceanic heat sequestration, eddy-param features a lower global atmospheric near surface warming of 0.4oC at CO2 doubling compared to eddy-rich and eddy-present, because heat is sequestered deeper in the ocean.

Our results suggest that opting either for resolving ocean mesoscale processes in climate models or parameterizing their effects will affect the proportion of ocean heat versus carbon sequestration, with potential implications for the relationship of cumulative CO2 emissions and global warming.

How to cite: Frenger, I., Dufour, C., Getzlaff, J., Griffies, S., Koeve, W., and Sarmiento, J.: Ocean sequestration of carbon dioxide and heat under global warming in a climate model with an eddy-rich ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8288, https://doi.org/10.5194/egusphere-egu22-8288, 2022.

Using the latest Coupled Model Intercomparison Projects phase 6 (CMIP6) abrupt-4xCO2 scenario, this study investigates the sea surface salinity (SSS) and hydrological cycle changes in response to global warming in the tropical Atlantic and tropical eastern Pacific. The analysis results reveal the enhancement of the global water cycle and the effect of El Niño-like sea surface temperature (SST) warming. Under global warming, the SSS decreases in the tropical Pacific and increases in the tropical Atlantic, following the “wet-get-wetter” mechanism. The increase of specific humidity leads to the enhancement of inter-basin moisture transport. More water vapor transports from the Atlantic to the Pacific in response to the rise of the freshwater flux gradient between the two basins, resulting in an SSS decrease in the Pacific and an increase in the Atlantic. At the same time, the increase of trans-basin SST gradient leads to the enhancement and westward shift of the Walker circulation, further resulting in the precipitation increase and the salinity decrease in the tropical Pacific. Furthermore, the El Niño-like warming induces a Wind-Evaporation-SST (WES) feedback in the tropical eastern Pacific. The reduced SST meridional gradient weakens the atmospheric circulation. Correspondingly, precipitation (salinity) decreases (increases) in the northeastern Pacific and increases (decreases) in the southeastern Pacific.

How to cite: Sun, Q. and Du, Y.: Trans-basin water vapor transport and ocean salinity changes between the Atlantic and Pacific under global warming, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6678, https://doi.org/10.5194/egusphere-egu22-6678, 2022.

Large amplitude oscillations in the meridional overturning circulation (MOC) have been found near the equator in all major ocean basins in the NEMO ocean general circulation model. With periods of 3-15 days and amplitudes of ~±100 Sv in the Pacific, these oscillations have been shown to correspond to zonally integrated equatorially trapped waves forced by winds within 10° N/S of the equator, and can be well reproduced by idealized wind-driven simulations linearized about a state of rest. Observations of dynamic height from the Tropical Atmosphere Ocean (TAO) mooring array in the equatorial Pacific also exhibit spectral peaks consistent with the dispersion relation for equatorially trapped waves. Here, we revisit the TAO observations to confirm that the amplitude of the oscillations is consistent with the simulations, supporting the modelled large amplitude MOC oscillations. We also show that the zonal structure of the frequency spectrum in both observations and simulations is predicted by changes in the baroclinic wave speed with variation in stratification across the ocean basin.

How to cite: Blaker, A., Baker, L., Bell, M., and Hirschi, J.: TAO data support the existence of large amplitude wind-driven high frequency variations in the cross-equatorial overturning circulation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5123, https://doi.org/10.5194/egusphere-egu22-5123, 2022.

This study investigates the variability of eddy activities in the Kuroshio region south of Japan using both satellite sea surface height observation and high-resolution ocean reanalysis data. It is found that the eddy kinetic energy (EKE) measuring eddy activities has a significant interannual variability. On the meanwhile, the EKE variability is negatively leading correlated with the change in the Kuroshio latitudinal position over the Izu Ridge. We further find that the baroclinic instability and advection processes are responsible for the EKE interannual variability and its relationship to the Kuroshio latitudinal position over the Izu Ridge. Specifically, before the high EKE level occurs, a cyclonic eddy generates east of Kii Peninsula. The rapid development of this eddy and its eastward movement to the Kuroshio region induce the isopycnal inclinations there and the associated horizontal density gradient, which leads to the strong baroclinic instability and promotes the evolution of eddy field. The developed strong eddies move downstream to the Izu Ridge. This pushes the Kuroshio off the shore and causes the southerly Kuroshio latitudinal position. Contrarily, when the cyclonic eddies do not appear in the Kuroshio region, the isopycnals are relatively flat, which is not conducive to the generation of baroclinic instability. Consequently, the EKE level is low and only weak eddies are advected to Izu Ridge, which does not substantially shift the Kuroshio southward and thus results in the northerly Kuroshio position. This contributes to the understanding and prediction of the Kuroshio dynamics. 

How to cite: Wang, Q.: The interannual variability of eddy activities in the Kuroshio region south of Japan and its relationship to Kuroshio latitudinal position over the Izu Ridge , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6927, https://doi.org/10.5194/egusphere-egu22-6927, 2022.

EGU22-204 | Presentations | OS1.1

Recent Observations of the Bottom Mixed Layer in the Tropical Northeast Pacific Ocean

Si-Yuan Sean Chen, Carlos Muños Royo, Raphael Ouillon, Matthew Alford, and Thomas Peacock

The bottom mixed layer (BML) is a well mixed, weakly stratified bottom boundary layer adjacent to the seafloor, with a thickness of the order 10-100 m, and is considered a common feature of the deep water column in the ocean. First observed in the 1970s and documented extensively in the deep Northwest Atlantic Ocean in the 1980s, the abyssal ocean (depth > 4000 m) BMLs have not been well observed in other regions of the global oceans, particularly in the Northeast Pacific Ocean, and the dynamical processes that lead to their formation are not well understood. Turbulent diffusivity in the BML is estimated to be greater than in the interior ocean by an order of magnitude, and the presence of such layers is often associated with elevated level of turbidity and episodic events of sediment resuspension and transport, known as the benthic storms. Without a clear understanding of the variability and dynamics of these layers, assessing potential environmental impacts of proposed commercial activities in the deep sea, such as the exploitation of polymetallic nodules in the Clarion-Clipperton Fracture Zone (CCFZ) in the tropical Northeast Pacific, is challenging. 

 

In this study, we analyze observed profiles from conductivity-temperature-depth (CTD) measurements recently collected in the German licence area of the CCFZ, a region with abyssal hills west of the East Pacific Ridge. Quasi-uniform profiles of potential temperature, salinity, and potential density extending from the seafloor to a maximum of 475 m above bottom (mab) reveal the presence of a BML in the region with a thickness of O(100 m), using a mixed-layer quantification method based on potential temperature profiles. The BML thickness and structure vary both temporally and spatially, with three major characteristics: (i) a well-developed, statically stable BML with a thickness between 200 and 475 m; (ii) a less well-developed BML with a thickness of approximately 100 m; and (iii) a well developed BML with a thickness of around 400 m and multiple intrusive layering structures, each of which with a thickness of approximately 100 m, near bathymetric reliefs. These findings confirm the preliminary findings from the 1980s that benthic stratification in the region is weak and that a mixed layer may be present at the bottom. While our preliminary findings establish the presence of BML in the region, questions regarding the dynamical processes responsible for the temporal and spatial variabilities remain to be addressed. Further analyses using data from the eastern segment of the World Ocean Circulation Experiment (WOCE) tropical North Pacific (P04E) section are ongoing to understand the spatial variability of these layers in the region. 

How to cite: Chen, S.-Y. S., Muños Royo, C., Ouillon, R., Alford, M., and Peacock, T.: Recent Observations of the Bottom Mixed Layer in the Tropical Northeast Pacific Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-204, https://doi.org/10.5194/egusphere-egu22-204, 2022.

EGU22-11866 | Presentations | OS1.1

Diurnal dynamics at the sea-atmosphere interface: The Central Adriatic campaign

Ana Cvitešić Kušan, Andrea Milinković, Abra Penezić, Saranda Bakija Alempijević, Jelena Godrijan, Blaženka Gašparović, Danijela Šantić, Mariana Ribas Ribas, Oliver Wurl, Maren Striebel, Jutta Niggemann, Carmen Cohrs, Carola Lehners, Tiera-Brandy Robinson, Lisa Gassen, Ranka Godec, Valentina Gluščić, and Sanja Frka

Nowadays, various environmental compartments are under increasing pressure from anthropogenic impact, and we as a society, have a duty, to understand the extent of the changing environment and how this may affect the functioning of global earth processes. More than 70% of the Earth’s surface is covered by the ocean whose uppermost layer, the sea surface microlayer (SML), is a specific environment at the air-sea interface, that is highly susceptible to increasing human impacts and climate change. SML has short- and long-term impacts on a range of planetary processes, including global biogeochemical cycling, air-sea exchange of gases and particles, and climate regulation. The SML is highly enriched in organic matter (OM) and has biofilm-like properties, and due to direct solar radiation, provides a challenging habitat for a wide variety of auto- and heterotrophic organisms. This makes SML a site of unique photochemical reactions that result in significant abiotic production of unsaturated and functionalized volatile organic compounds acting as precursors for the formation of marine secondary organic aerosols. The cycling of OM through the microbial food web at the sea surface determines the accumulation and enrichments of OM at SML, which directly affects the gas exchange rates and chemical composition of aerosols released from the sea surface to the atmosphere. Although the SML is involved in all ocean-atmosphere exchange processes, especially for climate-relevant gases and aerosol particles, its biogeochemical functioning during diurnal cycles is poorly characterized.

Therefore, in the summer of 2020, a multidisciplinary field campaign was conducted in the central Adriatic Sea, which included three full diurnal cycles of simultaneous sampling of the SML, with a special sampler, underlying water (ULW) and atmospheric aerosols (particulate matter < 10 µm, PM10). The results of biochemical analyses of SML and ULW including dissolved (DOC) and particulate organic carbon (POC), nutrients (NO3-, NH4+, PO43-), lipids, transparent exopolymeric particles (TEP) and Coomassie stainable particles (CSP), surface active substances (SAS), phytoplankton and heterotrophic bacteria abundance as well as results of mass concentrations and total organic carbon (OC), water soluble organic carbon (WSOC), SAS and ions (Cl-, NO3-, SO42-, Na+, NH4+, K+, Mg2+, Ca2+) determined in PM10 samples were correlated and statistically analysed depending on their solar radiation exposure. The comprehensive data-set will be discussed to investigate diurnal variations in the coupling between meteorological forcing, SML physicochemical and biological properties, and air–sea exchange of aerosol particles. This interdisciplinary diurnal study represents a promising approach in contributing to the fundamental current knowledge of ocean–atmosphere feedbacks, crucial for exploring global biogeochemical cycles, as well as predicting human impact on future climate changes.

Acknowledgment: This work has been supported by DAAD project “Diurnal dynamics on the sea-atmosphere interface" and Croatian Science Foundation under the IP-2018-01-3105 BiREADI project.

How to cite: Cvitešić Kušan, A., Milinković, A., Penezić, A., Bakija Alempijević, S., Godrijan, J., Gašparović, B., Šantić, D., Ribas Ribas, M., Wurl, O., Striebel, M., Niggemann, J., Cohrs, C., Lehners, C., Robinson, T.-B., Gassen, L., Godec, R., Gluščić, V., and Frka, S.: Diurnal dynamics at the sea-atmosphere interface: The Central Adriatic campaign, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11866, https://doi.org/10.5194/egusphere-egu22-11866, 2022.

EGU22-2615 | Presentations | OS1.1

Towards better understanding of carbon and oxygen biogeochemical rates in the Red Sea

Salma Elageed, Abdirahman Omar, Emil Jeansson, Elsheikh Ali, Ingunn Skjelvan, Knut Barthel, Truls Johannessen, and Ping Zhai

Towards better understanding of carbon and oxygen biogeochemical rates in the Red Sea    

Salma Elageed1,3 , A M. Omar2, Emil Jeansson2, Elsheikh B. Ali1 , Ingunn Skjelvan2 , Knut Barthel3 , Truls Johannessen3, P.Zhai4 

1Institute of Marine Research, Red Sea University, Port Sudan, Sudan 

2 NORCE, Norwegian Research Centre, Bjerknes Centre for Climate Research, Bergen, Norway 

3 Geophysical Institute, University of Bergen, Bergen, Norway 

4 Geoscience Dept., Princeton University, USA 

 

Abstract 

The Red Sea is one of the warmest and saltiest seas in the world, with surface water temperatures of 26–30°C and salinities of 36–41. The sea gains heat in the south and loses heat in the north and this gives a large-scale thermohaline circulation pattern with a northward surface flow and a southward flow at sill depth. At smaller spatial scales, along-coastal currents and upwelling occur.  

Here we summarise the main results from two studies that are parts of a PhD-study. We demonstrate how multi-spatial scale circulation and biological processes influence rates of: air-sea flux of carbon dioxide (CO2), oxygen utilization (OU), and removal of total alkalinity by calcification and sedimentation, i.e., alkalinity utilization (AU).  

In the first study, based on cruise data collected in the Red Sea in 2011 and 1982 (Aegaeo and MEROU cruises, respectively), we combine depth profiles of tracer-based water mass ages, AU, and OU to derive the first-ever basin-wide, long time integrated utilization rates of alkalinity (AUR) and oxygen (OUR). Results reveal that the large-scale circulation impacts the water masse ages and OU while remineralization of organic matter and calcification also influences in depth variations of OU and AU. The highest rates for OUR and AUR occur in the surface water followed by a swift attenuation of the rates towards zero for AUR and ~5 µmol kg-1 for OUR at 500 m depth.  

In the second study, new carbon and hydrography data from the Sudanese coastal Red Sea were used to investigate seasonal dynamics of sea surface partial pressure of CO2 (pCO2) and air–sea CO2 exchange. The results show that seasonal pCO2 change was primarily driven by temperature changes while along-coast advection, upwelling of CO2-rich deep water, and uptake of atmospheric CO2 also contributed to changes in dissolved inorganic carbon and total alkalinity. Furthermore, based on a compilation of historical and our new data, the region seems to have transformed from being a source of CO2 to the atmosphere throughout the year to becoming a sink of CO2 during parts of the year. 

 

How to cite: Elageed, S., Omar, A., Jeansson, E., Ali, E., Skjelvan, I., Barthel, K., Johannessen, T., and Zhai, P.: Towards better understanding of carbon and oxygen biogeochemical rates in the Red Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2615, https://doi.org/10.5194/egusphere-egu22-2615, 2022.

EGU22-13403 | Presentations | OS1.1

Atmosphere-Ocean Coupled Variability in the Arabian/Persian Gulf

Fahad Al Senafi

The present study investigates the interannual variability of the advective pathways of the Red Sea Overflow Water (RSOW) in the western Arabian Sea using Lagrangian particle tracking simulations as a proxy indicator for the poorly understood RSOW spreading. The RSOW, formed in the Red Sea interior, is the primary source of salt for the Indian Ocean intermediate layer and very likely an important source of oxygen for the oxygen-depleted mid-depth water of the Arabian Sea. However, the RSOW pathways and their interannual variability in the open ocean are barely understood. Here, we focus on the western Arabian Sea. The study is based on the Eddy rich Mercator GLORYS12 ocean reanalysis (1/12ohorizontal resolution; ~8 km in the Arabian Sea), which assimilates most satellite and in-situ observations collected between 1993 and 2018 and reproduces relatively well the climatological seasonal cycle of the RSOW to the Gulf of Aden, essential characteristics of the exchange at the Strait of Bab al-Mandab, and the Gulf’s intermediate circulation. For evaluating the pathways interannual variability, tens of thousands of particles were released each year between 1993 and 2013 (every 5-days) in the westernmost part of the Gulf of Aden within the RSOW isopycnic layer (27-27.6 kg/m3; ~600-1000 m). These particles were tracked over five years using the Parcels toolbox. Transit times from the outflow area to the western Arabian Sea are around three years. Statistical analysis of trajectories reveals a strong interannual variability in the RSOW pathways for the first time. The interannual variability of the western boundary undercurrents (Socotra and Somali) is evaluated in characterizing the pathways variability. Impacts on the intermediate-depth salinity are also investigated, although the scarcity of in-situ observations posed a significant limitation for the salinity analysis.

How to cite: Menezes, V.: Interannual Variability of Red Sea Overflow Water Pathways in the Western Arabian Sea in an Eddy Rich Ocean Reanalysis , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4438, https://doi.org/10.5194/egusphere-egu22-4438, 2022.

EGU22-4380 | Presentations | OS1.1

Unforced AMOC variations modulated by Tropical Indian Ocean SST

Brady Ferster, Leonard Borchert, and Juliette Mignot

            A potential future slowdown or acceleration of the Atlantic Meridional Overturning Circulation (AMOC) would have profound impacts on global and regional climate. Recent studies have shown that AMOC responds, among many other processes, to anthropogenic changes in tropical Indian ocean (TIO) temperature. However, internal unforced co-variations between these two basins are largely unexplored as of yet. Here, we use the ERSST5 and HadISST4 gridded observational products for the period 1870-2014, as well as dedicated simulations with coupled climate models, to illustrate how unforced changes in TIO sea surface temperature can drive teleconnections that influence internal variations of North Atlantic climate and AMOC.

            We separate the unforced observed component from the forced signal following the residuals method presented by Smith et al. (2019): the forced response is estimated from the CMIP6 multi-model ensemble mean and then subtracted from observed variability, leaving the unforced residual. In the absence of direct AMOC observation we estimate AMOC variability from a SST index first proposed by Caesar et al. (2018), the Caesar Index (CI). We find a robust observed relationship between unforced TIO and unforced CI when TIO leads by ~30 years. This time-lag is in line with a recently described mechanism of anomalous tropical Atlantic rainfall patterns that originate from TIO warming and cause anomalously saline tropical Atlantic surface water which slowly propagate northward into the subpolar North Atlantic, ultimately altering oceanic deep convection and AMOC (Ferster et al. 2021). Pre-industrial control simulations with the IPSL-CM6A-LR model confirm this relationship, indicating a time lag of ~30 years between TIO and CI variations. These simulations also confirm that the CI is representative of unforced AMOC variations when CI leads by 10 years. This work therefore indicates that an unforced pathway between TIO temperature and AMOC exists with a ~20 year lag, which opens the potential for using TIO temperature as precursor to predict future AMOC changes.

 

Caesar, L., Rahmstorf, S., Robinson, A., Feulner, G., & Saba, V. (2018). Observed fingerprint of a weakening Atlantic Ocean overturning circulation. Nature, 556(7700), 191-196.

Ferster, B. S., Fedorov, A. V., Mignot, J., & Guilyardi, E. (2021). Sensitivity of the Atlantic meridional overturning circulation and climate to tropical Indian Ocean warming. Climate Dynamics, 1-19.

Smith, D. M., Eade, R., Scaife, A. A., Caron, L. P., Danabasoglu, G., DelSole, T. M., ... & Yang, X. (2019). Robust skill of decadal climate predictions. Npj Climate and Atmospheric Science, 2(1), 1-10.

How to cite: Ferster, B., Borchert, L., and Mignot, J.: Unforced AMOC variations modulated by Tropical Indian Ocean SST, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4380, https://doi.org/10.5194/egusphere-egu22-4380, 2022.

In this study, daily outgoing longwave radiation (OLR) product is used to detect the atmospheric intraseasonal oscillation 
(ISO) in the eastern tropical Indian Ocean (TIO). A 50–80-day ISO is identifed south of the equator, peaking in boreal winter 
and propagating eastward. The mechanisms underneath are investigated using observational data and reanalysis products. 
The results suggest that the 50–80-day atmospheric ISO is enhanced by ocean dynamic processes during December–January. 
Monsoon transition in October–November causes large wind variability along the equator. Equatorial sea surface height/
thermocline anomalies appear of Sumatra due to the accumulative efects of the wind variability, leading the atmospheric 
50–80-day ISO by ~5–6 weeks. The wind-driven ocean equatorial dynamics are refected from the Sumatra coast as downwelling oceanic Rossby waves, which deepen the thermocline and contribute to the SST warming in the southeastern TIO, 
afecting local atmospheric conditions. It ofers insights into the role of ocean dynamics in the intensifcation of 50–80-day 
atmospheric ISOs over the eastern TIO and explains the seasonal peak of the eastward-propagating ISO during boreal winter. 
These results have implications for intraseasonal predictability.

How to cite: Liang, Y. and Du, Y.: Oceanic impacts on 50–80‑day intraseasonal oscillation in the eastern tropical Indian Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6725, https://doi.org/10.5194/egusphere-egu22-6725, 2022.

EGU22-12287 | Presentations | OS1.1

Decadal Oscillations in Southern Ocean Air-Sea Exchange Arises from Zonal Asymmetries in the Atmospheric Circulation

F. Alexander Haumann, Ivana Cerovečki, Graeme A. MacGilchrist, and Jorge L. Sarmiento

Air-sea exchange of heat, freshwater, and carbon dioxide in the Southern Ocean exhibits large anomalies on decadal time scales. In particular, anomalies in the exchange of carbon-dioxide between the atmosphere and the ocean are dominated by decadal fluctuations. Since known modes of Southern Ocean climate variability, like the Southern Annular Mode, cannot explain these fluctuations, previous studies have suggested a strong link to decadal variability in the tropics. Here, we show that these fluctuations mainly arise from zonal sea-level pressure gradients between 35°S and 63°S that only correlates with tropical climate variability on regional scales. An atmospheric state of increased zonal pressure gradients leads to a stronger meridional exchange of heat and moisture. Such an enhanced meridional exchange favors air-sea fluxes either through a direct modification of the air-sea temperature and humidity gradients, or through resulting changes in ocean mixing and water-mass transformation. The latter changes have profound influences on the surface partial pressure of carbon dioxide in the surface ocean, which controls the surface carbon-dioxide flux. In order to capture this decadal mode of variability in the atmospheric circulation, we define a Southern Decadal Oscillation (SDO) index that is based on the zonal sea-level pressure gradients. This index explains more than two thirds of the variance in the total Southern Ocean carbon-dioxide flux and also dominates the variance in the surface heat and freshwater fluxes on time scales longer than five years. Our results provide an important step in understanding variations in the Southern Ocean surface climate on decadal time scales and imply that the surface ocean buoyancy forcing may control decadal variations in the water masses formed in this region.

How to cite: Haumann, F. A., Cerovečki, I., MacGilchrist, G. A., and Sarmiento, J. L.: Decadal Oscillations in Southern Ocean Air-Sea Exchange Arises from Zonal Asymmetries in the Atmospheric Circulation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12287, https://doi.org/10.5194/egusphere-egu22-12287, 2022.

EGU22-13416 | Presentations | OS1.1

Upper ocean mixing from shear microstructure and density inversions nearthe Walvis Ridge

Letizia Roscelli, Christian Mertens, and Maren Walter

The M180 cruise is part of the observational program of the TRR 181 'Energy Transfers in
Atmosphere and Ocean', and will focus on observe numerous energy compartments in
order to construct a regional oceanic energy budget for the southeast Atlantic. The study
area will be nearby the Walvis Ridge, a region of strong eddy activity and internal tides, in
the Eastern South Atlantic (0 -10 E, 30 -35 S). There, energy is converted from barotropic
to baroclinic tides at the seafloor. Additionally, in this region, the Agulhas leakage regularly
sheds eddies from the Agulhas current in the form of Agulhas rings that propagate slowly
northwestward. The location is, therefore, ideal for the study of interaction and links
between different energy compartments in the ocean and at the ocean-atmosphere
boundary.
The work will focus on energy dissipation and diapycnal mixing which, on the smallest
scales, drive the circulation in the ocean and is thus of highly significant for the global
meridional overturning circulation in the ocean and its deep ventilation. Time series
microstructure stations will be used to assess locally the temporal variability of mixing
and dissipation. From temperature, density and shear profiles obtained with a Vertical
Microstructure Profiler (VMP-250-IR), it will be possible to calculate the energy dissipation
rate of turbulent kinetic energy by assuming a statistically valid linear relationship
between the Thorpe Scale and the Ozmidov Scale. A direct comparison between the
inferred estimation of the dissipation rate and the directly calculated dissipation rate will
be presented. Moreover, in case a possible influence from Agulhas rings on dissipation is
detected, it will be investigated.

How to cite: Roscelli, L., Mertens, C., and Walter, M.: Upper ocean mixing from shear microstructure and density inversions nearthe Walvis Ridge, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13416, https://doi.org/10.5194/egusphere-egu22-13416, 2022.

The Atlantic Meridional Overturning Circulation (AMOC) influences our climate by transporting heat northwards in the Atlantic ocean. The subpolar North Atlantic plays an important role in this circulation, with transformation of water to higher densities, deep convection and formation of deep water. Recent OSNAP observations and observations of surface flux driven water mass transformation have shown that the overturning is stronger to the east of Greenland than the west.

Firstly we analyse a CMIP6 climate model at two resolutions (HadGEM3 GC3.1 LL and MM) and show both compare well with the OSNAP observations. We explore the source of low frequency variability of the AMOC and how it is related to the surface water mass transformation in different regions. We then use a set of CMIP6 climate models and show that most climate models agree with the observations that overturning in the west is small, and show biases in the overturning in the west are related to biases in temperature and salinity. We also investigate low frequency variability and find a range of behaviour.

How to cite: Jackson, L.: Overturning and Water Mass Transformation in the Subpolar North Atlantic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2651, https://doi.org/10.5194/egusphere-egu22-2651, 2022.

EGU22-8779 | Presentations | OS1.1

Monitoring the local heat content change over the Atlantic Ocean with the space geodetic approach: the 4DATLANTIC-OHC Project

Robin Fraudeau, Michael Ablain, Gilles Larnicol, Florence Marti, Victor Rousseau, Alejandro Blazquez, Benoit Meyssignac, Giuseppe Foti, Francisco Calafat, Damien Desbruyères, William Llovel, Pablo Ortega, Vladimir Lapin, Mar Rodriguez, Rachel Killick, Nick Rayner, Marie Drevillon, Karina von Schuckmann, Marco Restano, and Jérôme Benveniste

Given the major role of the Atlantic Ocean in the climate system, it is essential to characterize the temporal and spatial variations of its heat content. The 4DATLANTIC-OHC Project (https://eo4society.esa.int/projects/4datlantic-ohc/) aims at developing and testing space geodetic methods to estimate the local ocean heat content (OHC) changes over the Atlantic Ocean from satellite altimetry and gravimetry. The strategy developed in the frame of the ESA MOHeaCAN Project (https://eo4society.esa.int/projects/moheacan/) is pursued and refined at local scales both for the data generation and the uncertainty estimate. At two test sites, OHC derived from in situ data (RAPID and OVIDE-AR7W) are used to evaluate the accuracy and reliability of the new space geodetic based OHC change. The Atlantic OHC product will be used to better understand the complexity of the Earth’s climate system. In particular, the project aims at better understanding the role played by the Atlantic Meridional Overturning Circulation (AMOC) in regional and global climate change, and the variability of the Meridional Heat transport in the North Atlantic. In addition, improving our knowledge on the Atlantic OHC change will help to better assess the global ocean heat uptake and thus estimate the Earth’s energy imbalance more accurately as the oceans absorb about 90% of the excess energy stored by the Earth system.

The objectives of the 4DATLANTIC-OHC Project will be presented. The scientific requirements and data used to generate the OHC change products over the Atlantic Ocean and the first results in terms of development will be detailed. At a later stage, early adopters are expected to assess the OHC products strengths and limitations for the implementation of new solutions for Society. The project started in June 2021 for a 2-year duration.

Visit https://www.4datlantic-ohc.org to follow the main steps of the project.

How to cite: Fraudeau, R., Ablain, M., Larnicol, G., Marti, F., Rousseau, V., Blazquez, A., Meyssignac, B., Foti, G., Calafat, F., Desbruyères, D., Llovel, W., Ortega, P., Lapin, V., Rodriguez, M., Killick, R., Rayner, N., Drevillon, M., von Schuckmann, K., Restano, M., and Benveniste, J.: Monitoring the local heat content change over the Atlantic Ocean with the space geodetic approach: the 4DATLANTIC-OHC Project, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8779, https://doi.org/10.5194/egusphere-egu22-8779, 2022.

EGU22-1152 | Presentations | OS1.1

Repeat hydrography and Deep-Argo reveal a warming-to-cooling reversal of overflow-derived water masses in the Irminger Sea during 2002-2021.

Damien Desbruyères, Eva Prieto Bravo, Virginie Thierry, Herlé Mercier, and Pascale Lherminier

Sustained shipboard hydrography surveys along the A25-Ovide section (2002 – 2018) are combined with data from a regional pilot array of Deep Argo floats (2016 – 2021) to estimate the decadal variability and linear trends in the temperature of overflow-derived waters in the Irminger Sea. Removing local or remote dynamical influences (heave) enables to identify a new statistically-significant trend reversal in Iceland Scotland Overflow Water (ISOW) and Denmark Strait Overflow Water (DSOW) core temperatures (spice). The latter took place in 2014 and interrupted a long-term warming of those water masses that was prevailing since the late 1990’s. Deep-Argo floats further reveal an overall acceleration of this cooling since 2014, with a mean rate of change estimated at -18 m°C yr-1 during 2016 – 2021, as well as a boundary-intensified pattern of change. This, along with the absence of apparent reversal in the Nordic Seas and with DSOW warming and cooling twice as fast as ISOW, points out the entrainment of subpolar intermediate signals within the overflow plumes near the Greenland-Iceland-Scotland sills as a most likely driver.

How to cite: Desbruyères, D., Prieto Bravo, E., Thierry, V., Mercier, H., and Lherminier, P.: Repeat hydrography and Deep-Argo reveal a warming-to-cooling reversal of overflow-derived water masses in the Irminger Sea during 2002-2021., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1152, https://doi.org/10.5194/egusphere-egu22-1152, 2022.

EGU22-4836 | Presentations | OS1.1

Towards ocean hindcasts in coupled climate models: AMOC variability in a partially coupled model at eddying resolution.

Tobias Schulzki, Jan Harlaß, Franziska Schwarzkopf, and Arne Biastoch

While forced ocean hindcast simulations are useful for a wide range of applications, a key limitation is their inability to explicitly simulate ocean-atmosphere feedbacks. As a consequence, they need to rely on artificial sea surface salinity restoring and budget corrections. Fully coupled models overcome these limitations, but lack the correct timing of variability due to much weaker observational constraints. This leads to a mismatch between forced and coupled models on interannual to decadal timescales and requires ensemble integrations.

A possibility to combine the advantages of both modelling strategies is to apply a partial coupling, i.e. nudging surface winds in the ocean component of a coupled climate model to reanalysed wind. Using an all-Atlantic nested configuration at eddying resolution, we show that partial coupling is able to simulate the correct timing of AMOC variability at all latitudes and timescales up to 5-years. Further, partial coupling excludes model drift caused by the artificial choices for restoring and simulates reasonable long term trends directly related to the applied momentum forcing. Owing to a higher impact of buoyancy fluxes, the timing of decadal variability differs between forced and partially coupled model runs.

How to cite: Schulzki, T., Harlaß, J., Schwarzkopf, F., and Biastoch, A.: Towards ocean hindcasts in coupled climate models: AMOC variability in a partially coupled model at eddying resolution., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4836, https://doi.org/10.5194/egusphere-egu22-4836, 2022.

EGU22-7340 | Presentations | OS1.1

Future increase in Nordic Seas overturning as a response to enhanced horizontal circulation

Marius Årthun, Helene Asbjørnsen, Léon Chafik, Helen L. Johnson, and Kjetil Våge

The Atlantic meridional overturning circulation (AMOC) carries warm and saline water toward the Arctic. The North Atlantic is separated from the Arctic by the Nordic Seas. Here, the warm Atlantic inflow across the Greenland-Scotland ridge is gradually transformed by atmospheric heat loss and freshwater input as it travels along the rim of the Nordic seas and Arctic Ocean, leading to the formation of dense overflow waters that feed the lower limb of the AMOC. Recent studies have demonstrated an important role of ocean circulation and water mass transformation in the Nordic Seas for the large-scale North Atlantic circulation. Understanding future change in the Nordic Seas is therefore essential, but the impact of anthropogenic climate change on Nordic Seas circulation and overturning remains little explored.

Here we show, using large ensemble simulations and CMIP6 models, that in contrast to the overturning circulation in the North Atlantic, the Nordic Seas overturning circulation in density space shows no persistent decline in the future and is rather characterized by an increase between 2040 and 2100. This increase in Nordic Seas overturning can be explained by enhanced horizontal circulation within the interior of the Nordic Seas. The strengthened Nordic Seas overturning is furthermore found to influence overturning changes in the subpolar North Atlantic. This study thus provides evidence that the overturning circulation in the Nordic Seas could be a stabilizing factor in a weakening North Atlantic Ocean. These regionally dependent circulation changes in response to future climate change furthermore imply that current changes in the North Atlantic overturning should not be extrapolated to the Nordic Seas and Arctic Ocean.

How to cite: Årthun, M., Asbjørnsen, H., Chafik, L., Johnson, H. L., and Våge, K.: Future increase in Nordic Seas overturning as a response to enhanced horizontal circulation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7340, https://doi.org/10.5194/egusphere-egu22-7340, 2022.

EGU22-6594 | Presentations | OS1.1

Meridional connectivity between the Labrador Sea and the subtropical AMOC

Yavor Kostov, Marie-José Messias, Herlé Mercier, Helen Johnson, and David Marshall

We attempt to reconcile two seemingly conflicting paradigms regarding the north-south connectivity in the Atlantic overturning: 1) Labrador Sea buoyancy anomalies impact the subtropical Atlantic Meridional Overturning Circulation (AMOC); and 2) water mass transformation in the eastern subpolar gyre plays an overwhelmingly dominant role in AMOC variability in the subpolar regions. We thus analyze mechanisms that link the Labrador Sea with meridionally coherent adjustment in the transport along the lower limb of the AMOC throughout the North Atlantic, from the south-eastern coast of Greenland to the subtropics. The first connectivity mechanism that we identify involves a passive advection of surface buoyancy anomalies from the Labrador Sea towards the eastern subpolar gyre by the background North Atlantic Current (NAC). The second connectivity mechanism that we analyze plays a dominant role and involves a dynamical response of the NAC to surface density anomalies originating in the Labrador Sea. The adjustment of the NAC modifies its northward transport of salt and heat and affects water mass transformation in the eastern subpolar gyre. This exerts a strong positive feedback amplifying the upper ocean buoyancy anomalies that spin the subpolar gyre up or down on a timescale of several years and drive a redistribution of Lower North Atlantic Deep Water (LNADW). During the course of this subpolar adjustment, boundary-trapped waves rapidly communicate the signal to the subtropics and facilitate a meridionally coherent response in the transport of LNADW. We find evidence in the ECCO ocean state estimate that these connectivity mechanisms have affected recent historical AMOC variability.

How to cite: Kostov, Y., Messias, M.-J., Mercier, H., Johnson, H., and Marshall, D.: Meridional connectivity between the Labrador Sea and the subtropical AMOC, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6594, https://doi.org/10.5194/egusphere-egu22-6594, 2022.

EGU22-2308 | Presentations | OS1.1

Ocean observations indicate a key role for ocean dynamics in Atlantic Multidecadal Variability

Ben Moat, Bablu Sinha, Neil Fraser, Leon Hermanson, Simon Josey, Brian King, Claire Macintosh, David Berry, Simon Williams, and Marilena Oltmanns

Multidecadal changes in North Atlantic Ocean heat storage directly affect the climate of the surrounding continents, and it is important to understand how and why the changes are taking place. Here we synthesize a wide range of observational datasets to construct an upper ocean heat budget for the period 1950 to 2020. Lead-lag correlation analysis of time series of ocean heat content, horizontal heat transport, sea surface temperature and air sea fluxes are used to infer the drivers North Atlantic heat content changes. We find systematic and interconnected migration of heat content anomalies around both subtropical and subpolar gyres and between the near surface and deep ocean on multidecadal timescales. We find a significant driving/active role for ocean circulation in these migrations throughout the North Atlantic. In contrast, air sea interaction mainly plays an active/driving role in the western subpolar Atlantic. Our use of multiple independent observational estimates of the variables allows us to provide robust error/uncertainty estimates for the evolution of the North Atlantic heat budget terms.

How to cite: Moat, B., Sinha, B., Fraser, N., Hermanson, L., Josey, S., King, B., Macintosh, C., Berry, D., Williams, S., and Oltmanns, M.: Ocean observations indicate a key role for ocean dynamics in Atlantic Multidecadal Variability, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2308, https://doi.org/10.5194/egusphere-egu22-2308, 2022.

EGU22-2753 | Presentations | OS1.1

Scalar transport induced by mesoscale eddies in the North Atlantic detected by a three-dimensional algorithm

Davide Cavaliere and Chunxue Yang

EGU22-3720 | Presentations | OS1.1

The impact of stochastic mesoscale weather systems on the Atlantic Ocean

Ian Renfrew, Shenjie Zhou, and Xiaoming Zhai

The ocean is forced by the atmosphere on a range of spatial and temporal scales. In numerical models the atmospheric resolution sets a limit on these scales and for typical climate models mesoscale (<500 km) atmospheric forcing is absent or misrepresented. Here we use a novel stochastic parameterization – based on a cellular automaton algorithm – to represent spatially coherent weather systems realistically over a range of scales, including down to the ocean grid-scale. We show that the addition of mesoscale atmospheric forcing leads to coherent and robust patterns of change: a cooler sea surface in the tropical and subtropical Atlantic, deeper mixed layers in the subpolar North Atlantic, and enhanced volume transport of the North Atlantic Subpolar Gyre and the Atlantic Meridional Overturning Circulation. Convection-permitting atmospheric models predict changes in mesoscale weather systems due to climate change, so representing them in climate models would bring higher fidelity to climate projections.

How to cite: Renfrew, I., Zhou, S., and Zhai, X.: The impact of stochastic mesoscale weather systems on the Atlantic Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3720, https://doi.org/10.5194/egusphere-egu22-3720, 2022.

EGU22-11399 | Presentations | OS1.1

Past changes in Atlantic Ocean circulation at intermediate water depths from micropaleontological and geochemical proxies since the last glacial maximum

Solène Pourtout, Sophie Sépulcre, Laetitia Licari, Christophe Colin, Elisabeth Michel, and Giuseppe Siani

Ocean circulation plays a central role on climate regulation. The paleoceanographic studies of the last decades have allowed to better document the variations in the production of the North Atlantic Deep Water (NADW). However, the role of intermediate water (IW) masses through time remains to be documented and is highly controversial. Indeed, some studies have highlighted the increased contribution of the Antarctic Intermediate Water (AAIW) in all ocean basins during the cold events recorded in the North Atlantic [1] while others suggest their absence [2]. Moreover, during the last deglaciation, the Southern Ocean played a fundamental role in the Carbon transfer from the deep ocean to the atmosphere via the increased upwelling associated to the AAIW production. In order to reconstruct the dynamics of IW masses, to better understand the relationships between variations in ocean circulation in the Atlantic and in the Southern Ocean, and the impact of these changes on the global carbon cycle during Termination I, we use two marine sediment cores from the Porcupine basin MD01-2461 (1153m) and the Iberian margin SU92-28 (997m). We combine the study of benthic foraminifera assemblages sensitive to variations in their environment (nutrient content, oxygen), and different geochemical proxies such as elemental ratios (Mg/Ca, Sr/Ca, Cd/Ca, Ba/Ca, B/Ca, Li/Ca and U/Ca), stable isotopes (δ18O and δ13C) and Neodymium isotopes records (eNd). On core SU92-28, past changes in the benthic foraminiferal content exhibit strong differences in the paleo-environments, with different ecological conditions from the LGM to the Holocene, as well as during the YD and H1 events. These differences are also observed in the δ13C, oxygen concentrations and elemental ratios records obtained from Uvigerina peregrina (or U.mediterranea), Cibicidoides mundulus and Melonis affinis. Changes in the Nd record allow to distinguish changes in the IW mass sources, reflecting the balance between Northern and Southern contributions. Future analysis (e.g., 14C reservoir ages) and the comparison with core MD01-2461 records will help to better constrain the North-South connections in the Atlantic Ocean at IW depths, and their impact on global climate changes.

[1] Ma et al. (2019) Geochemistry, Geophysics, Geosystems, 20(3), 1592-1608

[2] Gu, S., et al. (2017). Paleoceanography, 32, 1036-1053.

How to cite: Pourtout, S., Sépulcre, S., Licari, L., Colin, C., Michel, E., and Siani, G.: Past changes in Atlantic Ocean circulation at intermediate water depths from micropaleontological and geochemical proxies since the last glacial maximum, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11399, https://doi.org/10.5194/egusphere-egu22-11399, 2022.

EGU22-13047 | Presentations | OS1.1

Does interactive ocean dynamics effect North Atlantic SST variability?

Olivia Gozdz, Tim DelSole, and Martha Buckley

There is currently disagreement regarding the role of active ocean dynamics in Atlantic sea surface temperature (SST) variations. We investigate this by comparing sea surface temperature variations in a fully coupled atmosphere-ocean-ice model to those in a coupled model in which the atmosphere is coupled to a motionless slab (henceforth slab ocean model). Differences in variability between the two models are diagnosed by an optimization technique that finds components whose variance differs as much as possible between the two models. This technique reveals that SST variability differs significantly between the two models. Thus, the slab and fully coupled model are statistically distinguishable. The two leading components with larger SST variance in the slab model are associated with the tripole SST pattern and the Atlantic Multidecadal Variability (AMV) pattern. This result supports previous claims that ocean dynamics are not necessary for the AMV and, in fact, may be damping it. The leading component with larger variance in the coupled model resembles the Atlantic Nino pattern, consistent with the fact that ocean dynamics are required for Atlantic Nino. The second leading component with larger variance in the coupled model is a mode of subpolar SST variability that is associated with sea surface height variations along the path of the North Atlantic current, suggesting a role for wind-driven ocean dynamics.

How to cite: Gozdz, O., DelSole, T., and Buckley, M.: Does interactive ocean dynamics effect North Atlantic SST variability?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13047, https://doi.org/10.5194/egusphere-egu22-13047, 2022.

EGU22-10451 | Presentations | OS1.1

Attributing Recent Variability in the AMOC to Surface Buoyancy-Forcing

Charlotte Marris and Robert Marsh

Variability in the Atlantic Meridional Overturning Circulation (AMOC) on interannual to multidecadal timescales can primarily be linked to the strength of deep-water formation in the subpolar North Atlantic, where surface buoyancy-forcing transforms light surface waters to the dense waters of the southward-flowing lower-limb of the AMOC. The role of surface buoyancy-forcing in driving AMOC variability is of consequence for the regional transport and distribution of heat, carbon, and nutrients, and thus its quantification is essential for predicting how the AMOC will respond to and influence future global climate change. In a water mass transformation (WMT) framework, fields of surface density flux and surface density from the GODAS ocean reanalysis are used to reconstruct the surface-forced overturning circulation (SFOC) streamfunction for the subpolar North Atlantic (45-65 °N) over 1980-2020. The SFOC reconstruction is longitudinally partitioned into an East component, comprising the Irminger/Iceland basin, and a West component, comprising the Labrador Sea. Interannual changes in the dominant location of deep-water formation in the subpolar North Atlantic are thus elucidated. The reconstructed overturning is also partitioned in density, to separate contributions from two major North Atlantic water masses – Labrador Sea Water (LSW) and Subpolar Mode Water (SPMW) – which are inherently linked to variability associated with the North Atlantic Oscillation (NAO), influencing WMT across the subpolar North Atlantic. The analysis provides transport estimates complementary to those obtained with observations from the OSNAP array since 2014, revealing that recent (post-2014) domination of overturning by SPMW formation in the eastern subpolar gyre may be transient.

How to cite: Marris, C. and Marsh, R.: Attributing Recent Variability in the AMOC to Surface Buoyancy-Forcing, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10451, https://doi.org/10.5194/egusphere-egu22-10451, 2022.

EGU22-9433 | Presentations | OS1.1

AMOC response to Perturbations in Wind and Buoyancy Forcing in the Subpolar North Atlantic

Margarita Markina, Helen Johnson, and David Marshall

Atlantic Meridional Overturning Circulation (AMOC) is an important component of climate system and understanding what governs its variability is essential for improving climate predictability. Recent observational studies show large variability of overturning circulation in the subpolar latitudes with the dominant role of the eastern subpolar gyre, while the role of the wind and buoyancy forcing over the different regions remains underpinned. In this work, we use high-resolution (1/12°) targeted sensitivity experiments with the regional configuration of MITgcm for the North Atlantic. We show that our control experiment with repeated year forcing represents the major oceanic circulation patterns reasonably well and demonstrates similar strength of overturning with observational data from the OSNAP program. We investigate the oceanic response to changes in atmospheric forcing by setting the perturbations in surface momentum and buoyancy fluxes corresponding to the strong positive and negative phases of North Atlantic Oscillation.

How to cite: Markina, M., Johnson, H., and Marshall, D.: AMOC response to Perturbations in Wind and Buoyancy Forcing in the Subpolar North Atlantic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9433, https://doi.org/10.5194/egusphere-egu22-9433, 2022.

The recent IPCC AR6 report highlighted that, in contrast to ocean variables such as sea level and ocean heat content, where predicted and simulated rises due to anthropogenic climate change are being borne out by observations, the Atlantic Meridional Overturning Circulation (AMOC) has not conclusively shown a predicted decline and that, in fact, contradictions remain between observations and simulations through the 20th century.

The AMOC is at its weakest in 1000 years based on a compilation of paleo and instrumental proxies (Caesar et al. 2021). However, a reconstruction based on in-situ hydrographic profiles and informed by AMOC variability derived from the RAPID array shows no decline in the past 30 years (Worthington et al. 2021). Here, we show that there is no contradiction between these two results: when taken with the appropriate lag, the in-situ reconstruction matches with sea surface temperature (SST) reconstructions and the pattern of paleo proxies.

Convergence is evident in observations and reconstructions of the AMOC since the 1990s but what of prior to this? Instrumental reconstructions based on SSTs show a decline in the AMOC in the mid-20th century. The impact of the AMOC on SSTs is significant, especially on long timescales, but is not the only factor impacting SSTs. Alternative explanations for the mid 20th century cooling of Atlantic SSTs are that the cooling is linked with sulphate aerosol emission (Menary et al. 2020). This surface cooling may have led to a strengthening AMOC—the converse relationship to SST-based AMOC proxies.

We conclude by considering the challenges of instrumental-based reconstructions of the AMOC and potential avenues for reconciliation of outstanding contradictions to settle a baseline from which to observe the future AMOC slowdown that is near-universally predicted by climate models.

Caesar, L., G. D. McCarthy, D. J. R. R. Thornalley, N. Cahill, and S. Rahmstorf, 2021: Current Atlantic Meridional Overturning Circulation weakest in last millennium. Nat. Geosci., 14, 118–120, doi:10.1038/s41561-021-00699-z. https://doi.org/10.1038/s41561-021-00699-z (Accessed May 14, 2021).

Menary, M. B., and Coauthors, 2020: Aerosol-Forced AMOC Changes in CMIP6 Historical Simulations. Geophys. Res. Lett., 47, e2020GL088166, doi:10.1029/2020GL088166. https://doi. (Accessed May 14, 2021).

Worthington, E. L., B. I. Moat, D. A. Smeed, J. V. Mecking, R. Marsh, and G. D. McCarthy, 2021: A 30-year reconstruction of the Atlantic meridional overturning circulation shows no decline. Ocean Sci., 17, 285–299, doi:10.5194/os-17-285-2021.

How to cite: McCarthy, G., Caesar, L., and Worthington, E.: The challenge of reconciling in-situ observations, instrumental and paleo reconstructions, and climate model simulations of the AMOC in the 20th century , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7698, https://doi.org/10.5194/egusphere-egu22-7698, 2022.

EGU22-8798 | Presentations | OS1.1

Observation-based estimates of volume, heat and freshwater exchanges between the subpolar North Atlantic interior and its boundary currents

Sam Jones, Stuart Cunningham, Neil Fraser, Mark Inall, and Alan Fox

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 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, dynamical drivers of exchange between the gyre interior and the boundary remains unclear. 

We present a novel climatology of the Subpolar Gyre boundary using quality controlled CTD and Argo hydrography tracking the 1000 m isobath north of 47° N. The net geostrophic transport into the SPG perpendicular to this boundary section is only around 2.3 Sv.  Surface Ekman flow drives net transport out of the Subpolar Gyre in all seasons and shows pronounced seasonality, varying between 2.45 Sv in the summer and 7.70 Sv in the winter. Bottom Ekman transport associated with the boundary currents flows into the Subpolar gyre and is between 2.8 and 4 Sv.  

We estimate heat and freshwater fluxes into and out of the Subpolar gyre interior and compute the magnitude of water mass transformation (overturning) within the gyre. Heat advected into the Subpolar Gyre is between 0.10 PW and 0.19 PW. Freshwater exported from the gyre is between 0.06 Sv and 0.13 Sv. These estimates approximately balance the surface heat and freshwater fluxes into the region. Overturning varies between 6.20 Sv in the autumn and 10.17 Sv in the spring, meaning that approximately 40 % of the observed overturning in the subtropics can be attributed to water mass transformation in the interior of the SPG.

How to cite: Jones, S., Cunningham, S., Fraser, N., Inall, M., and Fox, A.: Observation-based estimates of volume, heat and freshwater exchanges between the subpolar North Atlantic interior and its boundary currents, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8798, https://doi.org/10.5194/egusphere-egu22-8798, 2022.

EGU22-9947 | Presentations | OS1.1

The ocean response to freshwater forcing from Greenland as simulated by the climate model EC-Earth3

Marion Devilliers, Steffen Olsen, Shuting Yang, Annika Drews, and Torben Schmith

Climate models usually can not afford to include an interactive ice sheet component for Greenland, which leads to a wrong representation of the variability of the freshwater fluxes released from the Greenland ice melt into the North Atlantic. We propose here to force externally a climate model (EC-Earth3) over several decades (1920-2014) with an observational dataset of runoff and solid ice discharge values for Greenland and surrounding glaciers and ice caps. It has been shown in a similar study with the IPSL-CM6-LR model that an enhancement of freshwater can modify the circulation and the convection in this region. The simulated mixed layer depths in the Nordic seas and the strength of the Atlantic Meridional Overturning Circulation  will be investigated to assess the impact of these increasing freshwater fluxes on the oceanic circulation over the period. The response in salinity and stratification in the Arctic will also be analysed as well as the ability for the system to capture abrupt changes like the 1995 warming in the subpolar gyre. 

How to cite: Devilliers, M., Olsen, S., Yang, S., Drews, A., and Schmith, T.: The ocean response to freshwater forcing from Greenland as simulated by the climate model EC-Earth3, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9947, https://doi.org/10.5194/egusphere-egu22-9947, 2022.

Ocean vertical velocities are several orders of magnitude smaller than the horizontal velocity field when looking at patterns larger than the sub-mesoscales, and for this reason, direct measurement in the ocean has not yet been possible. One method for estimating in-situ vertical velocities (w) in the real ocean is through a theoretical approach using observation-based fields. In this work, the Geostrophic Linear Vorticity Balance (GLVB: βvg = f∂w/∂z) is tested in an eddy-permitting OGCM to find out to what extent it explains the large-scale circulation in the North Atlantic and can be used to reconstruct an observation-based climatological w field. In the first part, we present a thorough baroclinic analysis of the climatological GLVB. The authors find that it holds to first order within the thermocline, below the mixing layer in the interior tropical and subtropical gyres and near the African coast. Within western boundary currents, the equatorial band, and the subpolar gyre significant departures occur due to the importance of other terms in the vorticity budget such as nonlinearities or friction. These results allow us to reconstruct w from climatological ARMOR3D geostrophic meridional velocities and satellite wind field within the thermocline of the North Atlantic tropical and subtropical gyres. In the second part, we discuss discrepancies between our observation-based reconstruction and two other existing estimates of w (one Omega equation derived product and an ocean reanalysis). At last, we revisit the classical Sverdrup explanation of gyre dynamics by adding a baroclinic analysis of some major thermocline currents.

How to cite: Cortés Morales, D. and Lazar, A.: North Atlantic thermocline vertical velocity reconstruction from ARMOR3D geostrophic meridional velocity field , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10223, https://doi.org/10.5194/egusphere-egu22-10223, 2022.

North Brazil Undercurrent is a western boundary current in the tropical South Atlantic Ocean. It is generally located between 11S and 5S, and it forms as the South Equatorial Current encounters the coast of northern Brazil. It carries a large volume of water and heat and plays an important role in the Atlantic Meridional Overturning Circulation and the South Atlantic Subtropical cycle. We have used three high-resolution and one low-resolution model outputs to explore the linear trend of NBUC transport and its variability on annual and interannual time scales. We find that the linear trend and interannual variability of the geostrophic NBUC transport show large discrepancies among the datasets. Thus, the linear trend and variability of the geostrophic NBUC are associated with the model configuration. We also find that the relative contributions of salinity and temperature gradients to the geostrophic shear of the NBUC are not model-dependent. Salinity-based and temperature-based geostrophic NBUC transports tend to be opposite-signed on all time scales. Despite the limited salinity and temperature profiles, the model results are consistent with the in-situ observations on the annual cycle and interannual time scales. We have highlighted the equally important roles of temperature and salinity in driving the variability of NBUC transport.

How to cite: Liu, H.: Role of salinity and temperature on the North Brazil Undercurrent, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4132, https://doi.org/10.5194/egusphere-egu22-4132, 2022.

EGU22-7908 | Presentations | OS1.1

The cold and warm contributions to the eastern South Atlantic subtropical gyre

Anna Olivé Abelló, Josep L. Pelegrí, Camila Artana, Lea Poli, and Christine Provost

The returning limb of the Atlantic Meridional Overturning Circulation is sustained partly by the Southern waters entering from the Pacific Ocean through the Drake Passage, what is commonly referred to as the cold-fresh water route, and by the Indian waters entering through the Agulhas Current system (ACS), what is known as the warm-salty route. Here we carry out numerical simulations of Lagrangian trajectories to identify the multiple direct and indirect cold and warm intermediate-water pathways reaching the eastern South Atlantic subtropical gyre: predominant trajectories, transit times, water transformations, changes in thermohaline properties and spatiotemporal variability. These different inflows have been characterized with thousands of particles released backward in the eastern subtropical gyre along 34°S (from 10°W to 18°E, hereafter the reference section) in 2019 and tracked during 50 years, using daily velocity fields from the GLORYS12v1 reanalysis product with a 5-day resolution.

The total cold-route contribution of intermediate waters to the reference section represents 7.1 ± 0.6 %, slightly less than the 9.0 ± 1.2 % fraction reaching this section via the warm-route ACS; both contributions decrease substantially in summer: 5.9 ± 0.7 % for the cold route and 6.2 ± 3.0 % for the warm route. The cold route consists of three main pathways: direct incorporation with over 90% of particles and water particles that recirculate either in the western subtropical Atlantic or enter the Indian Ocean before flowing back to the reference section, respectively, with about 7% and 2%. Different routes can also be identified for the warm route into the reference section, largely dominated by the direct route through the ACS but also with alternative pathways characterized by recirculations within the Atlantic and Indian Oceans. We also discuss some of the water transformations, in particular the largest changes in thermohaline properties that occur in the confluence zones of Malvinas-Brazil Current and the Agulhas-South Atlantic Current. For instance, during austral summer and along their direct path from the Drake Passage, the cold-water parcels gain a mean of 0.86 ± 0.11 ºC, 0.26 ± 0.01 in salt, increasing their mean density in 0.08 kg/m3.

How to cite: Olivé Abelló, A., Pelegrí, J. L., Artana, C., Poli, L., and Provost, C.: The cold and warm contributions to the eastern South Atlantic subtropical gyre, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7908, https://doi.org/10.5194/egusphere-egu22-7908, 2022.

EGU22-414 | Presentations | OS1.1

Time-lapse Volumetric Seismic Imaging of Water Masses at a Major Oceanic Front

Xiaoqing Chen, Nicky White, Andy Woods, and Kathryn Gunn

Oceanic fronts play a key role in modulating water mass transfer. Nevertheless, detailed information about frontal structure on appropriate temporal and spatial scales is difficult to obtain. Here, we investigate the structure of a dynamic frontal system associated with intense mesoscale eddy activity at the Brazil-Falkland Confluence of the South Atlantic Ocean using a time-lapse volumetric seismic reflection (i.e. acoustic) survey. This survey was processed by adapting standard signal processing techniques. A sequence of eleven calibrated time-lapse vertical sections from this survey reveals the detailed evolution of a major front. It is manifest as a discrete planar surface that dips at less than two degrees and it is traceable to a depth of almost 2 km. The shape and surface outcrop of this front are consistent with sloping isopycnal surfaces of the calculated potential density field and with coeval sea surface temperature measurements, respectively. Within the upper 1 km, where cold fresh water subducts beneath warm salty water, a number of tilted lenses are banked up against the sharply imaged front. The biggest lens has a maximum diameter of about 35 km and a maximum height of about 800 m. It is cored by cold fresh water which is associated with an acoustic velocity anomaly. Time-lapse imagery suggests that it grew and decayed within eleven days. On the southwestern side of the advecting front, large numbers of deforming lenses and filaments with length scales of 50 to 100 km are swept toward the advecting front. Spatial patterns of diapycnal mixing rate estimated from vertical displacements of tracked reflective horizons show that the front and associated structures condition turbulent mixing in significant ways. Finally, cross-correlation techniques are used to track the dynamic movement of frontal structures on timescales of minutes to days. This unprecedented imagery has profound implications for a fluid dynamical understanding of water mass modification at frontal systems.

How to cite: Chen, X., White, N., Woods, A., and Gunn, K.: Time-lapse Volumetric Seismic Imaging of Water Masses at a Major Oceanic Front, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-414, https://doi.org/10.5194/egusphere-egu22-414, 2022.

EGU22-5416 | Presentations | OS1.1

Overturning Variations in the South Atlantic in an Ocean Reanalyses Ensemble

Jon Baker, Richard Renshaw, Laura Jackson, Clotilde Dubois, Doroteaciro Iovino, Hao Zuo, Renellys Perez, Shenfu Dong, and Marion Kersalé

The ocean's Atlantic Meridional Overturning Circulation (AMOC) has a significant influence on global climate through its meridional transport of heat and carbon. The Southern Ocean is the conduit connecting the South Atlantic Ocean to the Pacific and Indian Oceans. Thus, overturning in the South Atlantic plays a crucial role in determining the pathways of the global overturning circulation and the transports into and out of the Atlantic Ocean. Understanding the nature and causes of its multiannual to multidecadal variation in this region is critical to improve our understanding of the MOC and more accurately predict its future changes and impacts. We analyse the South Atlantic overturning at 34.5°S in an ensemble of eddy permitting ¼ degree global ocean reanalyses, constrained by observations and historical forcings, over the period 1993-2021. This overturning transport and the meridional heat transport are validated against the continuous measurements obtained along the South Atlantic Meridional Overturning Circulation – Basin-wide Array (SAMBA). The ability of each reanalysis to capture the observed changes in the overturning will be determined, providing confidence in their ability to simulate changes prior to the availability of SAMBA, and exposing their limitations. We analyse the vertical variation of the transports and their temporal variability on various timescales. This research complements previous studies investigating changes in the subtropical and subpolar North Atlantic overturning using the same reanalyses ensemble, which was shown to provide a good representation of observations.

How to cite: Baker, J., Renshaw, R., Jackson, L., Dubois, C., Iovino, D., Zuo, H., Perez, R., Dong, S., and Kersalé, M.: Overturning Variations in the South Atlantic in an Ocean Reanalyses Ensemble, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5416, https://doi.org/10.5194/egusphere-egu22-5416, 2022.

OS1.3 – The ocean surface mixed layer: multi-scale dynamics and ecosystems in a changing climate

Mesoscale eddies play an increasingly recognized role on modulating turbulence levels and associated diapycnal fluxes in the ocean, in particular with increased dissipation rates found in anticyclones. In September 2017, the last cruise of the ProVoLo project in the Nordic Seas (https://www.uib.no/en/rg/fysos/97330/provolo) intensively surveyed an energetic mesoscale anticyclone (the permanent Lofoten Basin Eddy) to characterize turbulence of the upper layer and eventually quantify the resulting vertical fluxes nutrients caused by turbulence.

The sampling strategy combined ship-borne measurements and autonomous platforms. The vessel carried out a radial transect with stations spaced by 5 km near the center and 10-20 km outside the eddy with measurements of temperature and salinity (CTD), currents (lowered ADCP) and turbulence (Vertical Microstructure Profiler, VMP2000). Water samples were analyzed to estimate the concentration of the main nutrients (nitrate, phosphate and silicate). In addition, two autonomous oceanic gliders were used. A first glider profiling 0-1000 m deep was completing a 6-month mission. A second glider was specifically deployed during the cruise (5 days). This glider was equipped with a dissolved oxygen Aanderaa optode, a WET Labs FLNTU fluorescence and turbidity sensor and a Rockland Scientific Microrider sampling turbulence. It sampled the surface layer (0-300 m) at high temporal (~30 min) and spatial (~500 m) resolution from about 60 km to 5 km of the eddy center.

By combining those measurements, we characterized the turbulence dissipation rates, vertical diffusion and its associated fluxes across the different nutriclines from the center to the outside region area of the eddy, revealing significant contrasts. Below the thermocline, turbulent patches were observed within the core with dissipation rates elevated by one order of magnitude relative to the values outside. The higher levels of dissipation rates supported 10-fold stronger vertical diffusion coefficients, substantially increasing vertical turbulent fluxes through the nutriclines. The transition between the eddy tangential velocity maximum and the zero vorticity was characterized by a frontal region exhibiting important oscillations of the thermocline, manifesting important vertical exchanges.

This study is not only relevant in a local context, but also has global implications for the ocean energy budget and highlights the need for more high-resolution observations resolving scales from the mesoscale to the dissipation.

How to cite: Bosse, A. and Fer, I.: Contrasts in turbulent vertical fluxes of nutrients across the permanent Lofoten Basin Eddy in the Nordic Seas, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5808, https://doi.org/10.5194/egusphere-egu22-5808, 2022.

In this study, we assess the ability of the ocean-sea ice general circulation models that participated in the CMIP6 Ocean Model Intercomparison Project (OMIP) to simulate the seasonal cycle of the ocean mixed layer depth in the area of the Arctic Ocean covered by multiyear sea ice. During summertime, all models understimate the mixed layer depth by about 20 m compared to the MIMOC (Monthly Isopycnal/Mixed layer Ocean Climatology) observational data. The origin of this systematic bias is unclear. 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. Since the mixed layer deepening in ice-covered regions during these seasons is largely controlled by the brine rejection associated with ice growth, the discrepancies between models might be related to differences in the modelled sea ice mass balance. However, a detailed model comparison reveals that this is not the case, all models simulating more or less the same sea ice mass balance and thus salt flux into the ocean during sea ice freezing. By applying to model outputs the analytical model developed by Martinson (1990), that allows in particular to determine the main processes responsable for maintaining stablility in polar oceans, it is finally found that most of the disagreement between models can be explained by the accuracy with which the Arctic halocline is reproduced by those models. This feature is simulated generally poorly and quite differently from one model to another, and models with less stratified halocline generally lead to deeper mixed layers. It now remains to identify the model deficiencies responsible for this situation.

How to cite: Allende, S., Fichefet, T., and Goosse, H.: On the ability of CMIP6 OMIP models to simulate the seasonalcycle of the ocean mixed layer depth in the central Arctic Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2231, https://doi.org/10.5194/egusphere-egu22-2231, 2022.

EGU22-11925 | Presentations | OS1.3 | Highlight

Impact of Ocean Warming and Natural Variability on the Stratification and Mixed Layer Depth around Iceland

Angel Ruiz-Angulo, Esther Portela, Maria Dolores Perez-Hernandez, Solveig Rosa Ólafsdóttir, Andreas Macrander, Thomas Meunier, and Steingrimur Jonsson

The ocean around Iceland witnesses some of the most important transformations of water masses that drive the Global Ocean Circulation. Here, we analyze 28 years of continuous four-yearly hydrographic sections around Iceland from 1990 to 2018. The water-mass properties around Iceland show important spatial variability. From their temperature, salinity and stratification structure, we classified the Icelandic waters in three distinct regions with similar characteristics: the Southwest, the North and Northeast regions. The warm and salty Atlantic Waters that dominate the Southwest show the deepest winter mixed layer (~500m) while the North and Northeast have relatively shallow (< 100m) to moderate (~100m) winter mixed layer depth.  
Based on the decomposition of the total stratification into temperature and salt contributions, we find that the subsurface summer stratification is mainly dominated by temperature except for the North and Northwest regions where salinity dominates. 

The interannual variability of the mixed layer and its water properties is also large around Iceland. Mixed layer waters were generally colder in the 90's, then warmed until approximately 2015, and became colder again from 2015 to 2018.  Except for the southwestern region, the observed interannual variability seems unrelated with the North Atlantic Oscillation, and its main forcing remains an open question to address in future studies. Only in the northeastern region a multidecadal mixed layer warming trend clearly emerges from the interannual variability. This is associated with the Atlantification of the Arctic, which is also observed from the northward displacements of the isotherms derived from satellite SST. Elsewhere, rather than clear trends, we observe changes in the structure of the mixed layer temperature and salinity that compensate in density.  The present study provides an unprecedented and detailed regional description of the seasonal to decadal variability of the mixed layer depth and the stratification, and their link with the changing North Atlantic under global warming.

How to cite: Ruiz-Angulo, A., Portela, E., Perez-Hernandez, M. D., Ólafsdóttir, S. R., Macrander, A., Meunier, T., and Jonsson, S.: Impact of Ocean Warming and Natural Variability on the Stratification and Mixed Layer Depth around Iceland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11925, https://doi.org/10.5194/egusphere-egu22-11925, 2022.

EGU22-917 | Presentations | OS1.3

The daily-resolved Southern Ocean mixed layer: regional contrasts assessed using glider observations

Marcel du Plessis, Sebsastiaan Swart, Louise C. Biddle, Isabelle S. Giddy, Pedro M.S. Monteiro, Chris Reason, Andrew F. Thompson, and Sarah A. Nicholson

Water mass transformation in the Southern Ocean is vital for closing the large-scale overturning circulation, altering the thermohaline characteristics of upwelled Circumpolar Deep Water before returning to the ocean interior. Using profiling gliders, this study investigates how buoyancy forcing and wind-driven processes lead to intraseasonal (1-10 days) variability of the mixed layer temperature and salinity in three distinct locations associated with different Southern Ocean regions important for water mass transformation - the Subantarctic Zone (SAZ, 43°S), Polar Frontal Zone (PFZ, 54°S) and Marginal Ice Zone (MIZ, 60°S). Surface heat fluxes drive the summertime mixed layer buoyancy gain in all regions, particularly evident in the SAZ and MIZ, where shallow mixed layers and strong stratification further enhance mixed layer warming. In the SAZ and MIZ, the entrainment of denser water from below is the primary mechanism for reducing buoyancy gain. In the PFZ, turbulent mixing by mid-latitude storms result in consistently deep mixed layers and suppressed mixed layer thermohaline variability. Intraseasonal mixed layer salinity variability in the polar regions (PFZ and MIZ) is dominated by the lateral stirring of meltwater from seasonal sea ice melt. This is evident from early summer in the MIZ, while in the PFZ, meltwater fronts are proposed to be dominant during late summer, indicating the potential for seasonal sea ice freshwater to impact a region where the upwelling limb of overturning circulation reaches the surface. This study reveals a regional dependence of mixed layer thermohaline properties to small spatio-temporal processes, which suggests a similar regional dependence to surface water mass transformation in the Southern Ocean.

How to cite: du Plessis, M., Swart, S., Biddle, L. C., Giddy, I. S., Monteiro, P. M. S., Reason, C., Thompson, A. F., and Nicholson, S. A.: The daily-resolved Southern Ocean mixed layer: regional contrasts assessed using glider observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-917, https://doi.org/10.5194/egusphere-egu22-917, 2022.

EGU22-12610 | Presentations | OS1.3

Seasonal impact of optically significant water constituents on radiative heat transfer in the Western Baltic Sea

Bronwyn Cahill, Ulf Graewe, Lena Kritten, John Wilkin, and Piotr Kowalczuk

Heating rates induced by optically significant water constituents (OACs), e.g. phytoplankton and coloured dissolved organic matter (CDOM), contribute to the seasonal modulation of thermal energy fluxes across the ocean-atmosphere interface in coastal and regional shelf seas. This is investigated in the Western Baltic Sea, a region characterised by considerable inputs of nutrients, CDOM and freshwater, and complex bio-optical and hydrodynamic processes. Using a coupled bio-optical-ocean model (ROMS-BioOptic), the underwater light field is spectrally-resolved in a dynamic ocean and the inherent optical properties of different water constituents are modelled under varying environmental conditions. We estimate the relative contribution of these water constituents to the divergence of the heat flux and heating rates and find that phytoplankton dominates absorption in spring, while CDOM dominates absorption in summer and autumn. In the Pomeranian Bight, water constituent-induced heating rates in surface waters are estimated to be up to 0.1oC d-1 in spring and summer, predominantly as a result of increased absorption by phytoplankton and CDOM, respectively during these periods. Warmer surface waters are balanced by cooler subsurface waters. Surface heat fluxes (latent, sensible and net longwave) all increase in response to warmer sea surface temperatures. We find good agreement between our modelled water constituent absorption, and in situ and satellite observations. More rigorous co-located heating rate calculations using an atmosphere-ocean radiative transfer model provide further evidence of the suitability of ROMS-BioOptic model for this purpose. The study shows that seasonal and spatial changes in optically significant water constituents in the Western Baltic Sea have a small but noticeable impact on radiative heating in surface waters and consequences for the exchange of energy fluxes across the air-sea interface and the distribution of heat within the water column. The importance of the light attenuation coefficient, Kd, in shelf seas as a bio-optical driver which provides a pathway to estimating heating rates and connects biological activity with energy fluxes is highlighted.

How to cite: Cahill, B., Graewe, U., Kritten, L., Wilkin, J., and Kowalczuk, P.: Seasonal impact of optically significant water constituents on radiative heat transfer in the Western Baltic Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12610, https://doi.org/10.5194/egusphere-egu22-12610, 2022.

EGU22-1109 | Presentations | OS1.3 | Highlight

How nonlinearities of the equation of state of seawater generate the polar halocline and promote sea ice formation

Fabien Roquet, David Ferreira, Romain Caneill, and Gurvan Madec

The equation of state of seawater determines how density varies with temperature and salinity. Although it has long been known that the equation of state is nonlinear, there seems to be an overall feeling in the physical oceanography community that associated effects might be secondary in importance. This can be seen for example from the fact that most current theories of the large-scale circulation pre-assume a linear equation of state. Yet we contend here that these nonlinearities are responsible for the main transition in mixed layer properties observed in the World Ocean, the one separating so-called alpha regions (stratified by temperature) and beta regions (stratified by salinity). Beta regions are characterized by a halocline shielding surface cold waters from the influence of warmer deep waters, a condition for sea ice to form in polar region. Through numerical experiments where different equations of state are tested, we show that nonlinear effects of the equation of state: 1) strongly modulate surface buoyancy forcings, especially in mid- to high-latitudes, 2) generate the polar halocline by reducing there the influence of temperature on density, and consequently 3) enables sea ice formation in polar regions. The main nonlinear effect comes from the fact that the thermal expansion coefficient reduces to nearly zero at the freezing point, decreasing drastically the influence of surface cooling on the polar stratification. Other nonlinear effects, such as cabbeling or thermobaricity, are found of lesser importance although they have historically been the focus of intense research.

How to cite: Roquet, F., Ferreira, D., Caneill, R., and Madec, G.: How nonlinearities of the equation of state of seawater generate the polar halocline and promote sea ice formation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1109, https://doi.org/10.5194/egusphere-egu22-1109, 2022.

EGU22-11443 | Presentations | OS1.3

A seasonal climatology of the upper ocean pycnocline

Guillaume Sérazin, Anne-Marie Tréguier, and Clément de Boyer Montégut

Climatologies of the mixed layer depth have been provided using several definitions based on temperature/density thresholds or hybrid approaches. The upper ocean pycnocline (UOP) that sits below the mixed layer base, sometimes referred to as the transition layer or as the seasonal pycnocline, remains poorly characterised though it is an ubiquitous feature of the ocean surface layer. The UOP often consists in a rapid change in density with depth and enhanced vertical shear that connects the well-mixed surface layer to the stratified ocean interior. The UOP is important for the ventilation of the ocean as it represents a barrier to mixing between the upper ocean and the ocean interior.

Available hydrographic profiles (e.g., Argo, CTD on marine mammals) provide near-global coverage of the world's oceans and allow the characterisation of spatial and seasonal variations of the upper ocean vertical stratification, including the UOP. Based on these profiles, we estimate the depth, thickness and intensity of the UOP, and assess when and where the UOP can be considered as a layer with constant thickness. We provide monthly maps of the UOP complementing the available MLD climatologies and we compare the UOP characteristics with the depth and stratification of the mixed layer. We  aim at assessing the UOP intensity in winter and spring when the stratification is usually weak and submesoscale vertical motions can penetrate below the mixed layer base. During these seasons, the UOP intermittency must be taken into account because restratification may occur with intermittent events.

How to cite: Sérazin, G., Tréguier, A.-M., and de Boyer Montégut, C.: A seasonal climatology of the upper ocean pycnocline, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11443, https://doi.org/10.5194/egusphere-egu22-11443, 2022.

EGU22-3968 | Presentations | OS1.3

Submesoscale eddies and sea ice interaction 

Lily Greig and David Ferreira

The submesoscale has been defined dynamically as those processes with Rossby and Richardson numbers approaching O(1). This scale is of emerging interest within oceanography due to the role it plays in surface layer nutrient and tracer transport. Submesoscale baroclinic eddies or mixed layer eddies (MLEs), if energised in the marginal ice zone (MIZ), have the potential to impact both the rate of ice melt/formation and the magnitude of air-sea heat fluxes in the vicinity of the ice edge. 

In this study, an MITgcm idealised high resolution simulation is used to quantify the impact of MLEs in the vicinity of the ice edge, focusing on the thermodynamic component. The domain (75 km by 75 km at 250 m resolution) is a zonally re-entrant channel with ice-free/ice-covered conditions in the South/North, representing a lead or the MIZ. To measure the eddy impact on both sea ice and air-sea heat fluxes, comparisons are made between a 3D simulation with eddies and a 2D simulation with no eddies (no zonal extension, but otherwise identical to the 3D version). Typical conditions (stratification, forcing) of the Arctic/Antarctic and summer/winter seasons are considered. 

When eddies are permitted to energize and develop within these simulations, their impacts are numerous and coupled: under summer Artic conditions, meridional heat transport to the ice-covered region is tripled with eddies present, which leads to a first order impact on the sea ice melt and a doubling of the average heat storage in the ice-covered ocean. Novel analysis into the direct impact of these eddies on air-sea heat fluxes also shows that - due the partial absorption of downwelling solar radiation by sea ice cover - the solar heat flux into the ice-covered mixed layer increases by 20% when eddies are present. Computing the residual overturning stream function, responsible for driving warmer waters under the ice, reveals the ocean dynamics behind these impacts. The overturning, weakly present in the 2D model due to frontogenesis, increases threefold in the 3D case with submesoscale eddies. Tests with the Fox-Kemper parameterization within the 2D set-up are also helping evaluate to which extent this parameterization can capture the influence of MLE eddies in these polar conditions. 

How to cite: Greig, L. and Ferreira, D.: Submesoscale eddies and sea ice interaction , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3968, https://doi.org/10.5194/egusphere-egu22-3968, 2022.

EGU22-12181 | Presentations | OS1.3

Reconstructing meso- and submesoscale dynamics in ocean eddies from current observations

Tim Fischer, Johannes Karstensen, Marcus Dengler, Reiner Onken, and Martin Holzapfel

We reconstruct the 3-D meso- and submesoscale structure of selected oceanic eddies from ship-based field observations of current velocity, in the mixed layer and below, in order to explore two main questions: what information on upwelling/downwelling can be derived; and inside what eddy radius is water trapped and transported.

The selected eddies have been intensively surveyed during the collaborative project REEBUS (Role of Eddies in the Carbon Pump of Eastern Boundary Upwelling Systems) in the eastern tropical North Atlantic. Making use of vertical sections of current velocities we fit an optimum eddy-like structure to the data. The structure is assumed a slowly drifting, circular symmetric but not necessarily linear velocity field, separated in horizontal layers. The composition of the reconstructed layers provides a 3-D velocity structure which is used to calculate derived variables as vorticity and divergence. We find submesoscale divergence patterns which support vertical flux occurring in the eddies. We further use current velocities from a high-resolution regional model based on ROMS to validate the method and estimate uncertainties.

How to cite: Fischer, T., Karstensen, J., Dengler, M., Onken, R., and Holzapfel, M.: Reconstructing meso- and submesoscale dynamics in ocean eddies from current observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12181, https://doi.org/10.5194/egusphere-egu22-12181, 2022.

EGU22-833 | Presentations | OS1.3

Four-dimensional temperature, salinity and mixed layer depth in the Gulf Stream, reconstructed from remote sensing with physics-informed deep learning.

Etienne Pauthenet, Loïc Bachelot, Anne-Marie Tréguier, Kevin Balem, Guillaume Maze, Fabien Roquet, Ronan Fablet, and Pierre Tandeo

Despite the ever-growing amount of ocean's data, the interior of the ocean remains poorly sampled, especially in regions of high variability such as the Gulf Stream. The use of neural networks to interpolate properties and understand ocean processes is highly relevant. We introduce OSnet (Ocean Stratification network), a new ocean reconstruction system aimed at providing a physically consistent analysis of the upper ocean stratification. The proposed scheme is a bootstrapped multilayer perceptron trained to predict simultaneously temperature and salinity (T-S) profiles down to 1000m and the Mixed Layer Depth (MLD) from satellite data covering 1993 to 2019. The inputs are sea surface temperature and sea level anomaly, complemented with mean dynamic topography, bathymetry, longitude, latitude and the day of the year. The in-situ profiles are from the CORA database and include Argo floats and ship-based profiles. The prediction of the MLD is used to adjust a posteriori the vertical gradients of predicted T-S profiles, thus increasing the accuracy of the solution and removing vertical density inversions. The root mean square error of the predictions compared to the observed in situ profiles is of 0.66 °C for temperature, 0.11 psu for salinity and 39 m for the MLD.
The prediction is generalized on a 1/4° daily grid, producing four-dimensional fields of temperature and salinity, with their associated confidence interval issued from the bootstrap. The maximum of uncertainty is located north of the Gulf Stream, between the shelf and the current, where the variability is large. To validate our results we compare them with the observation-based Armor3D weekly product and the physics-based ocean reanalysis Glorys12. The OSnet reconstructed field is coherent even in the pre-ARGO years, demonstrating the good generalization properties of the network. It reproduces the warming trend of surface temperature, the seasonal cycle of surface salinity and presents coherent patterns of temperature, salinity and MLD. While OSnet delivers an accurate interpolation of the ocean's stratification, it is also a tool to study how the interior of the ocean's behaviour reflects on the surface data. We can compute the relative importance of each input for each T-S prediction and analyse how the network learns which surface feature influences most which property and at which depth. Our results are promising and demonstrate the power of deep learning methods to improve the predictions of ocean interior properties from observations of the ocean surface.

How to cite: Pauthenet, E., Bachelot, L., Tréguier, A.-M., Balem, K., Maze, G., Roquet, F., Fablet, R., and Tandeo, P.: Four-dimensional temperature, salinity and mixed layer depth in the Gulf Stream, reconstructed from remote sensing with physics-informed deep learning., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-833, https://doi.org/10.5194/egusphere-egu22-833, 2022.

EGU22-9776 | Presentations | OS1.3

Identifying and tracking surface-attached vortices in free-surface turbulence from above: a simple computer vision method 

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

Turbulence close beneath a free surface leaves recognisable imprints on the surface itself. The ability to identify and quantify long-lived coherent turbulent features from their surface manifestations only could open up possibilities for remote sensing of the near-surface turbulent environment, e.g., for assimilation into ocean models. Our work concerns automatic detection of one type of surface feature – “dimples” in the surface due to surface-attached “bathtub” vortices – based solely on the surface elevation as a function of time and space. 

Two-dimensional continuous wavelet transformations are used together with criteria for eccentricity and persistence in time, to identify candidate surface-attached vortices and track their motion. We develop and test the method from direct numerical simulation (DNS) data of turbulence influenced – and influencing – a fully nonlinear, deformable free surface.  

Comparison with the vertical vorticity in a plane close beneath the surface reveals that the method is able to identify long-lived vortical structures with a high degree of accuracy. Further tests of success rate included the vortex core identification method of Jeong and Hussain (1995). Different mother wavelets were tested, showing that the simplest option – the Mexican hat – outperforms more advanced options. 

Jeong, J., & Hussain, F. (1995). On the identification of a vortex. Journal of fluid mechanics, 285 69-94. 

How to cite: Babiker, O., Bjerkebæk, I., Xuan, A., Shen, L., and Ellingsen, S. Å.: Identifying and tracking surface-attached vortices in free-surface turbulence from above: a simple computer vision method , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9776, https://doi.org/10.5194/egusphere-egu22-9776, 2022.

EGU22-10579 | Presentations | OS1.3

Intense Downwelling and Diffuse Upwelling in a Nonlinear Ekman Layer

Nikki Rahnamaei and David Straub

It has long been appreciated that Ekman transport and pumping velocities are modified through interactions with underlying geostrophic currents. Nonlinearity involving interaction of the Ekman flow with itself is, however, typically neglected. This nonlinearity occurs when the Rossby number based on the Ekman velocity and horizontal length scale approaches order one values. Such values are common, for example, in the ice-ocean stress field across sharp gradients such as leads in the sea ice cover. Recent work has shown strong asymmetry in the pumping velocities, with cyclonic forcing producing diffuse upwelling and anticyclonic forcing producing sharp downwelling fronts. To better understand this dynamics, we consider the steady response to a simple specified prescription of the stress. In the (x-z) plane perpendicular to the stress, dynamics are described by the 2-D Navier-Stokes equation, with a forcing term dependent on vertical shear of velocity in the y-hat direction, specified by a pressureless momentum equation. An expansion in an Ekman-velocity based Rossby number is used to solve the system and to better understand the asymmetry. Interactions with stratification and underlying geostrophic currents are also considered, and examples of where these effects might be important are given.

How to cite: Rahnamaei, N. and Straub, D.: Intense Downwelling and Diffuse Upwelling in a Nonlinear Ekman Layer, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10579, https://doi.org/10.5194/egusphere-egu22-10579, 2022.

EGU22-11160 | Presentations | OS1.3

Local energy release by extreme vertical drafts in stratified geophysical flows

Raffaello Foldes, Silvio Sergio Cerri, Raffaele Marino, Fabio Feraco, and Enrico Camporeale

Investigating energy injection mechanisms in stratified turbulent flows is critical to understand the multi-scale dynamics of the atmosphere and the oceans. Geophysical fluids are characterized by anisotropy, supporting the propagation of gravity waves. Classical paradigms of homogeneous isotropic turbulence may therefore not apply, the energy transfer in these frameworks being determined by the interplay of waves and turbulence as well as by the presence of structures emerging intermittently in space and time. In particular, it has been observed that stably stratified fluids can develop large-scale intermittent events in the form of extreme vertical velocity drafts, in a specific range of Froude numbers ([1]). These events were found to be associated with the enhancement of small-scale intermittency ([2]) and local dissipation ([3]). Here we verify the possibility that such extreme vertical drafts may release energy to the flow, affecting its overall dynamics and energetics. The analysis presented consists in the implementation of a space-filtering technique ([4]) applied to three-dimensional direct numerical simulations of the Boussinesq equations.

The strength of this approach relies on dealing with quantities (referred to as “sub-grid terms”) which are a reliable proxies of the classical Fourier flux terms but defined locally in the physical space, allowing for a scale analysis of the energy transfer at specific location of the domain flow. By investigating the correlation between values of the sub-grid terms and the presence of the extreme values of the vertical velocity, we found an increase in the energy transfer at intermediate scales that is likely to be associated with the development of vertical drafts in the flow. In the range of the governing parameters (namely the Froude and the Reynolds numbers) in which the extreme vertical drafts are detected in stratified turbulent flows, enhancement of the coupling between kinetic and potential energy modes is also observed, feeding in turn the scale-to-scale potential energy transfer.

 

[1] Feraco et al., EPL, 2018

[2] Feraco et al., EPL, 2021

[3] Marino et al., PRF, in review

[4] Camporeale et al., PRL, 2018

How to cite: Foldes, R., Cerri, S. S., Marino, R., Feraco, F., and Camporeale, E.: Local energy release by extreme vertical drafts in stratified geophysical flows, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11160, https://doi.org/10.5194/egusphere-egu22-11160, 2022.

EGU22-8271 | Presentations | OS1.3

Effect of Langmuir circulation on mixing and carbon dynamics in a shallow lagoon

Yoana G. Voynova, Marc P. Buckley, Michael Stresser, Marius Cysewski, Jan Bödewadt, Martina Gehrung, and Jochen Horstmann

In fall 2020 and 2021, two field surveys examined the water column dynamics and surface mixing in a shallow lagoon, Szczecin (Stettin) Lagoon, located at the border between Germany and Poland. This was part of a larger experiment, looking into water column and air-sea interactions, and momentum fluxes, but this study is focused on how the presence of proposed Langmuir circulation affects the carbon and oxygen dynamics, and primary production in this shallow lagoon.

Measurements were collected from a station in Szczecin Lagoon, located near the Polish border, with water depth of about 4 meters. Measurements at and around the station were made using mobile FerryBox systems, or Pocket FerryBoxes, which measured almost continuously water temperature, salinity, dissolved oxygen, chlorophyll fluorescence, pH, turbidity, colored dissolved organic matter (CDOM) and in 2021 partial pressure of CO2 (pCO2). In addition, water column measurements of currents (ADCP) and water level were available, as well as surface drifters, and drone aerial measurements.

We found that during low wind conditions, the water column was well-mixed to a depth controlled by expected Langmuir cells, and bottom waters below this depth were quite different in most of the biogeochemical parameters measured. Therefore Langmuir circulation most likely controlled water column structure in large regions of the Szczecin Lagoon, consequently influencing the community, carbon and dissolved gas distributions in this shallow lagoon, and most likely the air-sea gas exchange rate. Only during short storm events, these conditions changed, and the water column structure and concentrations of biogeochemical parameters were altered.

How to cite: Voynova, Y. G., Buckley, M. P., Stresser, M., Cysewski, M., Bödewadt, J., Gehrung, M., and Horstmann, J.: Effect of Langmuir circulation on mixing and carbon dynamics in a shallow lagoon, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8271, https://doi.org/10.5194/egusphere-egu22-8271, 2022.

OS1.4 – Ocean ventilation and its consequences for ocean biogeochemistry and ecosystems: from small-scale mixing to basin scale

EGU22-7924 | Presentations | OS1.4

Diapycnal fluxes and overturning from a tracer release experiment in a tidal canyon

Marie-Jose Messias, Herle Mercier, James Ledwell, Alberto Naveira Garabato, Raffaele Ferrari, and Matthew Alford

The overturning of the ocean has been classically described by sinking at high latitudes and upwelling of deep water in the ocean interior. However, measurements showing bottom enhanced mixing have suggested that the ocean interior experiences downwelling, and it has been recently proposed that the upwelling of deep water should arise over sloping boundaries. The Bottom Boundary Layer Turbulence and Abyssal Recipes project was set up to test this paradigm in the Rockall Trough, a natural laboratory of the deep ocean overturning. We conducted a tracer experiment that began by the injection of 15 kg of long lived inert SF5CF3 on the deep part of a tidal canyon in July 2021. The injection was performed in the bottom boundary layer, ~7 meters above the bottom, along streaks between 1800 m and 2000 m depth, tagging water at potential temperature of 3.6°C within a temperature window of 0.1°C. Within 24 hours we started the tracer survey along the full canyon length for two weeks (totalling 81 stations) and we report here on the integrated diapycnal fluxes (upwellings and downwellings) at key locations between 900 m and 2600 m depth, at different time steps from neap to spring tides. The tracer dispersion along the canyon unprecedently documents a rapid diapycnal upwelling of the tracer ranging from 50 to 300 meters per day driven by tidal mixing implying an overturning circulation. As the tracer evolved in the canyon under tidal sloshing, its leading edge was detected reaching 8.5°C at the canyon head as we entered spring tides. We will also report  on the tracer chase outside of the canyon   to explore the contribution of sloping boundary mixing to ventilation at the scale of the Rockall Trough.
 

How to cite: Messias, M.-J., Mercier, H., Ledwell, J., Naveira Garabato, A., Ferrari, R., and Alford, M.: Diapycnal fluxes and overturning from a tracer release experiment in a tidal canyon, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7924, https://doi.org/10.5194/egusphere-egu22-7924, 2022.

EGU22-6957 | Presentations | OS1.4

Gulf Stream and Deep Western Boundary Currents are key to constrain the future North Atlantic Carbon Uptake

Nadine Goris, Klaus Johannsen, and Jerry Tjiputra

As one of the major carbon sinks in the global ocean, the North Atlantic is a key player in mediating the ongoing global warming. However, projections of the North Atlantic carbon sink in a high-CO2 future vary greatly among models, with some showing that a slowdown in carbon uptake has already begun and others predicting that this slowdown will not occur until nearly 2100.  

For an ensemble of 11 CMIP5-models, we identify two indicators of contemporary model behavior that are highly correlated with a model´s projected future carbon-uptake in the North Atlantic. The first indicator is the high latitude winter pCO2sea-anomaly, which is tightly linked to winter mixing and nutrient supply, but also to deep convection. The second indicator is the fraction of the anthropogenic carbon-inventory stored below 1000-m depth, indicating the efficiency of dissolved inorganic carbon transport into the deep ocean.  

We further use a genetic algorithm to identify sub-regions of different shapes and sizes that optimise the correlations between our indicators and the future carbon uptake in the North Atlantic. Independent of size and shape, the genetic algorithm persistently identifies the gulf stream region as optimal for the first indicator as well as the pathway of the deep western boundary current for the second indicator. When extracting the simulated contemporary AMOC-strengths for the central latitudes and depths of these optimal regions, we also find high correlations between AMOC-values and the North Atlantic future carbon uptake.  

Our regional optimisation shows that modelled discrepancies in the future North Atlantic carbon uptake originate in different transport efficiencies of dissolved inorganic carbon from the surface to the deep ocean. We find a strong and highly important link between a model’s performance for gulf stream and deep western boundary currents and a model’s ability to accurately project the future carbon uptake in the North Atlantic.  

How to cite: Goris, N., Johannsen, K., and Tjiputra, J.: Gulf Stream and Deep Western Boundary Currents are key to constrain the future North Atlantic Carbon Uptake, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6957, https://doi.org/10.5194/egusphere-egu22-6957, 2022.

EGU22-11982 | Presentations | OS1.4

Does the Natural DIC Affect the Storage of Total Inorganic Carbon in the Central Labrador Sea?

Lorenza Raimondi, Toste Tanhua, Kumiko Azetsu-Scott, and Doug Wallace

The Labrador Sea plays a central role in the oceanic storage of carbon. In particular, several studies have shown that this region has amongst the highest integrated column inventories of anthropogenic carbon (Cant) in the world’s ocean. The rate at which Cant is stored in this region appears to be connected to changes in ocean circulation and can therefore vary over time. Nevertheless, it is still unclear whether the temporal variability of the total Dissolved Inorganic Carbon (DIC) inventory is solely due to the changes in Cant concentrations or whether there is a contribution of the natural component of DIC to this signal.

The Bedford Institute of Oceanography has been maintaining the Atlantic Zone Off-Shore Monitoring Program (AZOMP) in the Labrador Sea since the early 1990s. The AZOMP involves annual occupations of the AR7W line that crosses the Labrador Sea and includes sampling of DIC, as well as multiple transient tracers such as CFC-12 and SF6.  

By using observations of DIC along the AR7W line, as well as previous estimates of Cant obtained with transient tracers (using a refined version of the Transit Time Distribution method; TTD) and new estimates of Cant based on the extended Multiple Linear Regression (eMLR) method, we provide a first insight on the role that the natural component of DIC plays in the temporal variability of inorganic carbon in the central Labrador Sea between 1993 and 2016.

We show that different methods to estimate Cant can lead to different conclusions on the role of the natural variability of DIC and that these discrepancies could be related to the assumptions implied in the Cant estimation methods. In particular an analysis of Cant estimates obtained with our refined version of the TTD method in different water masses, highlighted that further refinement of the tracers’ saturation assumption could be necessary in this region. This refinement could reconcile the Cant estimates from the two methods and therefore lead to an unambiguous role of the natural DIC in this region.

How to cite: Raimondi, L., Tanhua, T., Azetsu-Scott, K., and Wallace, D.: Does the Natural DIC Affect the Storage of Total Inorganic Carbon in the Central Labrador Sea?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11982, https://doi.org/10.5194/egusphere-egu22-11982, 2022.

EGU22-2005 | Presentations | OS1.4

Ventilation and oxygen export in the Labrador Sea

Jannes Koelling, Dariia Atamanchuk, Johannes Karstensen, Patricia Handmann, and Douglas W.R. Wallace

The Labrador Sea is one of the few regions where ventilation can replenish oxygen to the deep ocean, owing to wintertime deep convection that occurs primarily in the center of the basin. While some recent studies have aided in quantifying the amount of oxygen taken up during Labrador Sea Water (LSW) formation, less is known about how different spreading pathways of LSW contribute to the export of oxygen.

In this study, we use oxygen data from the 53N mooring array in the boundary current at the exit of the Labrador Sea, together with Argo float data, in order to investigate the connection between deep convection, spreading of LSW, and oxygen export. We find that the annual cycle of the oxygen concentration is driven largely by an increased input of newly formed LSW into the boundary current in the spring and summer. The resulting oxygen increase is a result of a fast, direct southward pathway of LSW, and we estimate that the associated oxygen export accounts for about half of the uptake in the interior. The 4-year record that is presently available also indicates that the strength of the oxygen export varies interannually, which may be related to changing convection patterns.

Overall, our results highlight the important role that the Labrador Sea plays in supplying oxygen to the deep ocean, and represent a first step towards better understanding the ventilation pathways out of this critical region.

How to cite: Koelling, J., Atamanchuk, D., Karstensen, J., Handmann, P., and Wallace, D. W. R.: Ventilation and oxygen export in the Labrador Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2005, https://doi.org/10.5194/egusphere-egu22-2005, 2022.

EGU22-3692 | Presentations | OS1.4

Subpolar gyre decadal variability explains the recent oxygenation in the Irminger Sea

Charlene Feucher, Esther Portela, Nicolas Kolodziejczyk, and Virginie Thierry

The North Atlantic is one of the hot-spot for ocean oxygen ventilation due to cold surface water and strong winter convection. This region is subjected to large interannual to multidecadal variability, which is suspected to strongly impact the regional and temporal oxygen ventilation and inventory.
Here we investigate the oxygen variability over 1991-2018 and driving mechanisms of the two main water masses of the Irminger Sea: the Labrador Sea Water (LSW) and the Island Scotland Overflow Water (ISOW). For this, we combined the most recent Argo dataset with ship-based hydrographic data in the Irminger Sea. The dissolved oxygen concentration of the LSW oscillated between 300 mu mol/kg in the early 90's and between 2016 and 2018, and 280 mu mol/kg in the period 2002-2015. The temporal changes in oxygen concentration are less pronounced in the underlying Iceland Scotland Overflow Water (ISOW).
We show that, while solubility changes partly explain the variability of the dissolved oxygen concentration within the Labrador Sea Water (LSW), the main driver of oxygen variability is the Apparent Oxygen Utilisation (AOU). 
In the early 90's and between 2015 and 2018, the deep convection was more intense and led to less stratified, thicker, colder, and more oxygenated LSW than during the period 1995-2015. This was attributed to larger ocean heat loss, stronger wind stress, and colder subpolar gyre under positive NAO conditions.   
The observed oxygen variability in the Irminger Sea between 1991 and 2018 does not show any significant linear trend. This study provides the first observational evidence of the impact of the subpolar gyre decadal variability on the oxygen ventilation in the Irminger Sea and advocates for continuing the monitoring of oxygen concentration and content in the subpolar gyre to separate any possible warming-induced long-term changes from the large decadal natural variability.

How to cite: Feucher, C., Portela, E., Kolodziejczyk, N., and Thierry, V.: Subpolar gyre decadal variability explains the recent oxygenation in the Irminger Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3692, https://doi.org/10.5194/egusphere-egu22-3692, 2022.

EGU22-101 | Presentations | OS1.4

Subtropical contribution to Subantarctic Mode Waters

Bieito Fernández Castro, Matthew Mazloff, Richard G Williams, and Alberto Naveira Garabato

Subantarctic Mode Waters (SAMW), forming in the deep winter mixed layers in the Subantarctic Zone (SAZ) to the north of the Antarctic Circumpolar Current (ACC), connect the ocean thermocline with the atmosphere, contributing to ocean carbon and heat uptake and transporting high-latitude nutrients northward, to fuel primary production at low latitudes. Many aspects of SAMW formation are poorly understood due to the data scarcity during Austral winter. Here, we use biogeochemical Argo float observations to investigate the seasonal development, origin and significance of a subsurface salinity maximum in the SAMW formation regions. This conspicuous feature develops every summer in the seasonal thermocline of the SAMW formation regions as a consequence of the advection along the ACC of warmer and saltier waters from the western boundaries of the subtropical gyres, in particular the Agulhas Return current. The salinity maximum acts as a gatekeeper for SAMW ventilation, since it controls the seasonal evolution of stratification at the base of the mixed layer, modulating its rate of deepening during autumn and winter and re-stratifying the SAMW pool when winter mixing ceases. We also show that the subtropical influx, often overlooked, is key to understand the variability of SAMW properties, since it represents a leading order term in the heat and salt budgets at the formation regions. Finally, the analysis of the nitrate seasonal cycle at the SAMW formation regions as recorded by the Argo floats, revealed that the seasonal salinity increase goes along with a decrease in the concentration of this nutrient, as a consequence of the advection of subtropical waters containing low preformed nitrate. These results suggest that nutrient concentration in SAMW is controlled not only by the rate of upwelling of high-nutrient waters south of the ACC and the degree of biological drawdown during their northward transit, as frequently assumed, but also by the influx of subtropical waters, pointing to previously overlooked feedbacks in the redistribution of nutrients between high and low latitudes.

How to cite: Fernández Castro, B., Mazloff, M., Williams, R. G., and Naveira Garabato, A.: Subtropical contribution to Subantarctic Mode Waters, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-101, https://doi.org/10.5194/egusphere-egu22-101, 2022.

EGU22-2848 | Presentations | OS1.4

Turbulent kinetic energy dissipation rate and attendant fluxes in the western tropical Atlantic estimated from ocean glider observations

Peter Sheehan, Gillian Damerell, Philip Leadbitter, Karen Heywood, and Rob Hall

Ocean gliders enable us to collect the ocean microstructure observations necessary to calculate the dissipation rate of turbulent kinetic energy, ε, on timescales of weeks to months: far longer than is normally possible using traditional ship-based platforms. Slocum gliders have previously been used to this end;  here, we report the first detailed estimates of ε calculated using observations collected by a Seaglider. Seaglider 620 was deployed in the western tropical Atlantic in early 2020 and was equipped with a FP07 fast thermistor. We use these same fast thermistor observations to calculate ε following the Thorpe scale method. We find very good agreement between estimates of ε calculated following the two methods. The Thorpe scale method yields the larger values of ε, but the average difference, less than an order of magnitude, is smaller than reported elsewhere. The spatio-temporal distribution of ε is comparable for both methods. Maximum values of ε (10-7 W kg-1) are observed in the surface mixed layer; relatively high values (10-9 W kg-1) are also observed between approximately 200 and 500 m depth. These two layers are separated by a 100 m thick layer of low ε (10-10 W kg-1), which is co-located with a high-salinity layer of Subtropical Underwater and a peak in the strength of stratification (i.e. in N2). We calculate the turbulent heat and salt fluxes associated with the observed turbulence that act to ventilate deeper layer of the ocean. Between 200 and 500 m, ε induces downward (i.e. negative) fluxes of both properties that, if typical of the annual average, would have a very small influence on the heat and salt content of the salinity-maximum layer above. We compare these turbulent fluxes with estimates of fluxes due to double diffusion, having objectively identified those regions of the water column where double diffusion is likely to occur. While the downward heat flux due to double diffusive mixing is lower than that due to mechanical mixing, the downward salt flux due to double diffusive mixing is six times greater.

How to cite: Sheehan, P., Damerell, G., Leadbitter, P., Heywood, K., and Hall, R.: Turbulent kinetic energy dissipation rate and attendant fluxes in the western tropical Atlantic estimated from ocean glider observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2848, https://doi.org/10.5194/egusphere-egu22-2848, 2022.

EGU22-4856 | Presentations | OS1.4

The Impact of Zonal Jets on the Atlantic Oxygen Minimum Zones 

Paulo H. R. Calil

Oxygen is an essential component of the ocean biogeochemistry.  Relatively small variations in its content may have a significant impact on ocean productivity, biodiversity and fisheries and thus affect ocean health and ecosystem services.  Over the last decade, several studies have shown that regions with low oxygen concentrations are expanding over the world's oceans, a phenomenon which has been termed ocean deoxygenation. These changes are driven by a combination of anthropogenic climate change and the natural variability of the ocean. As climate change warms the upper ocean it reduces oxygen solubility,  increases upper ocean stratification and thus reduces oxygen mixing as well as induces changes in respiration rates. Disentangling the natural and anthropogenically-induced oxygen variability requires the use of models as prognostic or diagnostic tools, as they can be forced with different conditions which may or may not include the effects of climate change and allow a detailed examination of specific processes. In this work,  we compare two basin-scale coupled physical-biogeochemical simulations of the Tropical Atlantic ocean at different horizontal resolutions and show that more robust zonal jets at intermediate depths in the higher resolution simulation have a major impact on the overall structure of the North and South Atlantic OMZs by limiting their westward extent and supplying oxygen to the OMZ core regions between 300 m and 500 m. 

How to cite: R. Calil, P. H.: The Impact of Zonal Jets on the Atlantic Oxygen Minimum Zones , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4856, https://doi.org/10.5194/egusphere-egu22-4856, 2022.

EGU22-4183 | Presentations | OS1.4

Old and cold contributions to the oxygen minimum zones

Xabier Davila, Geoffrey Gebbie, Elaine McDonagh, Siv Lauvset, Ailin Brakstad, and Are Olsen

Oxygen minimum zones (OMZs) are oxygen-poor layers in the water column of great importance for marine ecosystems and biogeochemical processes. The position, size and extent of the OMZs are set by the source water properties, transport timescales, as well as respiration, both upstream of and within OMZs. Here we use an adjoint ocean circulation model built upon observations of ocean tracers to explore the complex interplay between chemical, biological and physical processes. Specifically, we determine the contributions of different water masses to the volume and oxygen deficiency of the OMZs. Among the tracers used, phosphate, oxygen and radiocarbons are included. These allow to first, constrain the ocean circulation and its timescales, and second, to determine where in the ocean oxygen utilization takes place. Here we show that the OMZs are ventilated at a wide range of timescales, ranging from a few years from adjacent regions in the tropics and subtropics, to more than 3000 years from distant deep water formation areas. Preliminary results suggest that the Antarctic marginal seas are key source water regions. While the fraction of water volume that originates in the Ross and Weddell Sea is relatively low (~20-30%), the contribution to the OMZs oxygen deficit is as large as ~40%, i.e., 40% of the apparent oxygen utilization is associated with these waters. This is a consequence of the long transit times involved, about 3000 years. Our results stress the importance of the contributions of the Ross and Weddell Seas to the climate sensitivity of the OMZs.

How to cite: Davila, X., Gebbie, G., McDonagh, E., Lauvset, S., Brakstad, A., and Olsen, A.: Old and cold contributions to the oxygen minimum zones, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4183, https://doi.org/10.5194/egusphere-egu22-4183, 2022.

EGU22-926 | Presentations | OS1.4

Lagrangian Ocean Ventilation: Improved Subgrid-Scale Dispersion on Neutral Surfaces

Daan Reijnders, Eric Deleersnijder, and Erik van Sebille

Mesoscale eddies play a major role in ocean ventilation by stirring ocean tracers, such as carbon, along sloping surfaces of neutral buoyancy. To capture the effects of these turbulent eddies, coarse resolution ocean models resort to tracer diffusion parameterizations that take into account neutral surface slopes. Likewise, when studying tracer pathways in a Lagrangian framework, the effect of eddy dispersion needs to be parameterized when coarse models are used.

Dispersion in Lagrangian simulations is traditionally parameterized by random walks, equivalent to diffusion in Eulerian models. Beyond random walks, there is a hierarchy of stochastic parameterizations, where stochastic perturbations are added to Lagrangian particle velocities, accelerations, or hyper-accelerations. These parameterizations are referred to as the 1st, 2nd and 3rd order ‘Markov models’ (Markov-N) respectively. Most previous studies investigate these parameterizations in two-dimensional setups, often restricted to the ocean surface. The few studies that investigated Lagrangian dispersion parameterizations on three-dimensional neutral surfaces have focused only on random walk (Markov-0) dispersion.

Here, we present a three-dimensional isoneutral formulation of the Markov-1 model. We also implement an anisotropic, shear-dependent formulation of Lagrangian random walk dispersion, originally formulated as a Eulerian diffusion parameterization by Le Sommer et al (2011). Random walk dispersion and Markov-1 are compared using an idealized setup as well as more realistic coarse and coarsened (50 km) ocean model output. While random walk dispersion and Markov-1 produce similar particle distributions over time, Markov-1 yields more realistic Lagrangian trajectories and leads to a smaller spurious dianeutral flux.

How to cite: Reijnders, D., Deleersnijder, E., and van Sebille, E.: Lagrangian Ocean Ventilation: Improved Subgrid-Scale Dispersion on Neutral Surfaces, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-926, https://doi.org/10.5194/egusphere-egu22-926, 2022.

EGU22-1535 | Presentations | OS1.4

Using dye tracers to understand the development of the T–-S structureof the ocean thermocline

A. J. George Nurser and Alice Marzocchi

Understanding what sets the T--S relation within the thermocline, and
how long and what volume of ventilated waters in each T--S class stay in the sub-surface
thermocline is a key question for climate prediction. In particular the sparsity of
the T--S distribution has been a puzzle since the days of
Stommel. Here we use runs performed for the TICTOC project, in which water is labelled by its
year of ventilation and its source region, to understand how the
volumetric T--S relation is laid down year on year, and  evaluate the
importance of near-surface (mostly vertical) mixing in the first year of ventilation
against longer term mixing (much of which is isopycnal) in specifying the T--S distribution.

How to cite: Nurser, A. J. G. and Marzocchi, A.: Using dye tracers to understand the development of the T–-S structureof the ocean thermocline, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1535, https://doi.org/10.5194/egusphere-egu22-1535, 2022.

Ocean ventilation provides the primary control of how the ocean takes up  excess carbon and heat supplied to the earth system due to carbon emissions. Ventilation involves an atmospheric source supplying a tracer to the mixed layer, which is then physically transported into the thermocline and deep ocean by the ocean circulation. For this physical transfer of tracer, there are two characteristic timescales: (i) a fast adjustment controlled by the depth of the mixed layer and (ii) a slow adjustment controlled by the rate of mass transfer to the ocean interior. However, this physical transfer is modified for heat and carbon by climate feedbacks and carbonate chemistry respectively. Here, we use a conceptual 2-dimensional ocean model that is designed to address the ocean adjustment to carbon emissions on yearly to multi-centennial timescales. The model includes  a source, an ocean mixed-layer and interior adjustments, and a feedback mechanism that includes a surface temperature feedback  (such as from clouds) and the effects of carbonate chemistry; the model ignores any seasonality, biological processes and chemical weathering. Using this conceptual model, we reveal  the similarities and differences in how ventilation controls the uptake of heat and carbon involving changes in how the fast and slow adjustments are controlled.  In summary, despite the physical transfer of fluid being determined by ocean ventilation, the effects of climate feedbacks and carbonate chemistry lead to differences in the ocean thermal and carbon adjustments to an increase in atmospheric CO2.

How to cite: Katavouta, A. and Williams, R.: Ventilation controls of ocean heat and carbon uptake: similarities and differences in the response to carbon emissions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1626, https://doi.org/10.5194/egusphere-egu22-1626, 2022.

OS1.5 – Chaotic variability and modeling uncertainties in the ocean

EGU22-6754 | Presentations | OS1.5

Zonal jets in the eastern North Pacific in an ensemble of eddy-resolving ocean general circulation model runs

Ryo Furue, Masami Nonaka, and Hideharu Sasaki

It has been known for some time that the ocean basins are populated by what is known as ‘‘zonal jets’’, ‘‘deep zonal jets’’, or ‘‘striations’’. Since the oceanic flow is, at least weakly, chaotic, it is not known whether the positions of the jets are ‘‘deterministic’’, that is, entirely determined by external parameters. A number of theories have been proposed to explain them, some of them predicting zonal jets at fixed latitudes and others implying that the positions of the jets are random. To investigate how deterministic the zonal jets are in the eastern North Pacific, a ten-member ensemble of long-term integrations of a semi-global, eddy-resolving ocean general circulation model is analyzed.

The positions of the equatorial jets, even their variability, seem to obey deterministic dynamics and some of the jets in the tropics (5°–15°N) migrate poleward coherently (similarly between ensemble members). The jets in the subtropics (15°–45°N) systematically migrate equatorward but their positions are less coherent; the jets in the subpolar region (45°N–) are random and without systematic migration. Jets near the coast of North and South America tend to have shorter meridional wavelengths than interior ones and those in the northern hemisphere are fairly coherent whereas those in the southern hemisphere seem more random. There are a few quasi-barotropic jets which are anchored to steep bottom topographic features and which also appear to trap shallower counter-flows on their poleward and equatorward flanks.

How to cite: Furue, R., Nonaka, M., and Sasaki, H.: Zonal jets in the eastern North Pacific in an ensemble of eddy-resolving ocean general circulation model runs, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6754, https://doi.org/10.5194/egusphere-egu22-6754, 2022.

EGU22-2167 | Presentations | OS1.5

Intrinsic low-frequency variability of the Mediterranean Sea circulation studied using a multilayer ocean model

Angelo Rubino, Stefano Pierini, Sara Rubinetti, and Davide Zanchettin

Intrinsic chaotic variability in the oceans is an active field of research in modern oceanography, with important implications concerning the understanding and predictability of the ocean system. The focus is mainly on open ocean basins while very little attention is devoted to enclosed or semi-enclosed seas. The intrinsic variability of the Mediterranean Sea, in particular, has not yet been investigated. Here, results obtained with an eddy-resolving nonlinear multilayer ocean model are presented shedding light on relevant aspects of the intrinsic low-frequency variability of the Mediterranean Sea circulation.

An ensemble of multi-centennial ocean runs is performed to allow for a significant statistical analysis. The statistically stationary state obtained after long simulations shows a robust meridional structure consistent with the observed Mediterranean mean state. Among the various features emerging in the decadal and multidecadal temporal ranges are abrupt shifts in the water mass stratification structure. Differences and similarities with observed patterns are finally discussed. 

How to cite: Rubino, A., Pierini, S., Rubinetti, S., and Zanchettin, D.: Intrinsic low-frequency variability of the Mediterranean Sea circulation studied using a multilayer ocean model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2167, https://doi.org/10.5194/egusphere-egu22-2167, 2022.

EGU22-1973 | Presentations | OS1.5

The Characteristics and Significance of Hydrodynamical Internal Variability in Modelling Dynamics in Marginal Seas

Lin Lin, Hans von Storch, Xueen Chen, and Shengquan Tang

Internal variability, unprovoked by external forcing, emerges in the hydrodynamics of the marginal seas. Ensemble ocean simulations are used to analyze the characteristics, scales, and intensities of such variability in the Bohai, Yellow Sea, and South China Sea. With the signal defined as the covariation in the ensemble, and the noises as the independent variations, a scale dependency of the Signal-to-Noise Ratio (S/N ratio) is found in the Bohai, Yellow Sea, and South China Sea. The external forcing and related signal are dominant for large scales, while most of the internal variability is generated for small scales. The intensities of internal variability of the Bohai and Yellow sea are about half of the intensities of South China Sea, likely because eddies are less energetic in the Bohai and Yellow Sea, which likely is the main source of noise in South China Sea.

In addition, we investigate the effect of tides on internal variability in the Bohai and Yellow Sea by three ensembles of numerical experiments with tidal forcing, with half tidal forcing, and without tidal forcing. When the tides are weakened or turned off, the S/N ratios are reduced in large and medium scales, more so in the Yellow Sea than in the Bohai. The increase in the S/N ratio is largest for large scales and for depth-averaged velocity. The reduction in tidal forcing results in an approximately 30% increase in S/N ratios in the Bohai at large scales. Thus, the absence of tidal forcing favours the emergence of unprovoked variability at large and medium scales but not at small scales. We suggest that the main mechanism for the increase of covarying variability when tides are active, is the additional mixing induced by the tides.

How to cite: Lin, L., von Storch, H., Chen, X., and Tang, S.: The Characteristics and Significance of Hydrodynamical Internal Variability in Modelling Dynamics in Marginal Seas, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1973, https://doi.org/10.5194/egusphere-egu22-1973, 2022.

EGU22-2849 | Presentations | OS1.5

The Structure of North Atlantic Kinetic Energy Spectra

William K. Dewar, Takaya Uchida, Quentin Jamet, and Andrew Poje

An ensemble of North Atlantic simulations is analyzed, providing estimates of kinetic energy spectra.  A wavelet transform technique is used permitting comparisons to be made between spectra at different locations in this highly inhomogeneous environment.  We find a strong tendency towards anisotropy in the spectra, with meridional spectra typically stronger than zonal spectra.  This holds even in the gyre interior where conditions might be expected to be homogeneous.  The spectra show reasonable ranges consistent with a downscale enstrophy cascade, but also a persistent tendency to exhibit steeper slopes at smaller scales.  The only location where the presence of an upscale cascade is supported is the Gulf Stream extension.  This is amongst first attempts to quantify and compare spectra and their differences in the inhomogeneous setting of the North Atlantic.

How to cite: Dewar, W. K., Uchida, T., Jamet, Q., and Poje, A.: The Structure of North Atlantic Kinetic Energy Spectra, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2849, https://doi.org/10.5194/egusphere-egu22-2849, 2022.

EGU22-770 | Presentations | OS1.5

Diagnosing the thickness-weighted averaged eddy-mean flow interaction from an eddying North Atlantic ensemble

Takaya Uchida, Quentin Jamet, William Dewar, Julien Le Sommer, Thierry Penduff, and Dhruv Balwada

The analysis of eddy-mean flow interaction provides key insights into the structures and dynamics of inhomogeneous and anisotropic flows such as atmospheric and oceanic jets. As the divergence of Eliassen-Palm (E-P) flux formally encapsulates the interaction, the community has had a long-standing interest in accurately diagnosing this term. Here, we revisit the E-P flux divergence with an emphasis on the Gulf Stream, using a 48-member, eddy-rich (1/12°) ensemble of the North Atlantic ocean partially coupled to identical atmospheric states amongst all members via an atmospheric boundary layer model. This dataset allows for an unique decomposition where we define the mean flow as the ensemble mean, and interpret it as the oceanic response to the atmospheric state. The eddies are subsequently defined as fluctuations about the ensemble mean. Our results highlight two points: i) the implementation of the Thickness-Weighted Averaged (TWA) framework for a realistic ocean simulation in diagnosing the E-P flux divergence, and ii) validity of the ergodic assumption where one treats the temporal mean equivalent to the ensemble mean, which is questionable for a temporally varying system such as the ocean and climate.

How to cite: Uchida, T., Jamet, Q., Dewar, W., Le Sommer, J., Penduff, T., and Balwada, D.: Diagnosing the thickness-weighted averaged eddy-mean flow interaction from an eddying North Atlantic ensemble, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-770, https://doi.org/10.5194/egusphere-egu22-770, 2022.

EGU22-1428 | Presentations | OS1.5

Non-local eddy-mean kinetic energy transfers in submesoscale-permitting ensemble simulations

Quentin Jamet, Stephanie Leroux, William K. Dewar, Thierry Penduff, Julien Le Sommer, Jean-Marc Molines, and Jonathan Gula

Understanding processes associated with eddy-mean flow interactions helps our interpretation of the ocean energetic balance, and guides the development of parameterizations. Here, we focus on the non-local nature of Kinetic Energy (KE) transfers between mean (MKE) and turbulent (EKE) reservoirs. Following previous studies, we interpret these transfers as non-local when the energy extraction from the mean flow does not locally sustain energy production of the turbulent flow, or vice versa. The novelty of our approach is to use ensemble statistics, rather than time averaging or coarse-graining methods, to define the mean and the turbulent flow. Based on KE budget considerations, we first rationalize the eddy-mean separation in the ensemble framework, and discuss the interpretation of a mean flow (<u>) driven by the prescribed (surface and boundary) forcing and a turbulent flow (u') driven by non-linear dynamics sensitive to initial conditions. Our results, based on the analysis of 120-day long, 20-member ensemble simulations of the Western Mediterranean basin run at 1/60o, suggest that eddy-mean kinetic energy exchanges are largely non-local at small scales. Our main contribution is to recognize the prominent contribution of the cross energy term (<u>.u') to explain this non-locality, providing a strong constraint on the horizontal organization of eddy-mean flow KE exchanges since this term vanishes identically for perturbations (u') orthogonal to the mean flow ( Our results also highlight the prominent contribution of vertical turbulent fluxes for energy exchanges within the surface mixed layer. Analyzing the scale dependence of these non-local energy exchanges supports the local approximation usually made in the development of meso-scale, energy-aware parameterizations for non-eddying models, but points out to the necessity of accounting for these non-local effects in the meso-to-submeso scale range.

How to cite: Jamet, Q., Leroux, S., Dewar, W. K., Penduff, T., Le Sommer, J., Molines, J.-M., and Gula, J.: Non-local eddy-mean kinetic energy transfers in submesoscale-permitting ensemble simulations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1428, https://doi.org/10.5194/egusphere-egu22-1428, 2022.

EGU22-750 | Presentations | OS1.5

Adiabatic, Constrained, Stochastic Eddy Parameterisation

Chris Wilson, Chris W. Hughes, Simon D. P. Williams, and Adam T. Blaker

Mesoscale eddy-permitting ocean models will be needed as a component of climate ensemble projections most likely for the next decade or more.   However, the kinetic energy and other measures of variability are typically an order of magnitude too weak at this nominal 0.25 degree lon-lat resolution.    This is predominantly due to excessive gridscale damping of momentum, needed for computational stability, which is believed to kill a large fraction of the energy source of the kinetic energy inverse cascade.   The KE inverse cascade is associated with the generation of intrinsic chaotic variability and ensemble spread, hence the estimation of potential predictability, but also with slower, larger-scale variability associated with climate.  The familiar Gent and McWilliams (1990) eddy parameterisation is problematic when applied to eddy-permitting models, where eddies are partially resolved, and it also tends to damp variability rather than energise it.   In response to this problem, several recent studies have focussed on the KE backscatter problem, which each attempt to increase the upscale transfer of KE, either deterministically or stochastically.

Stochastic parameterisation of sub-gridscale eddies has recently become a popular approach in ocean modelling, having been used in atmospheric modelling for many years, but there is still a diverse range of approaches for constraining either the underlying physics (how the forcing is applied) or the statistics (the spatiotemporal signature of the forcing).   This study explores some basic recipes for constructing the stochastic model from statistics of either observations or higher-resolution models.  The stochastic forcing, representing the sub-gridscale effects of eddies in our eddy-permitting simulations, is also applied adiabatically – to mimic the predominant behaviour of mesoscale eddies in the ocean interior and to preserve large-scale watermasses.   A theoretical challenge, which we explore, is to connect the applied, weakly imbalanced forcing, to a response in kinetic energy and upscale transfer.  This must also be applied without generating numerical instability.  

How to cite: Wilson, C., Hughes, C. W., Williams, S. D. P., and Blaker, A. T.: Adiabatic, Constrained, Stochastic Eddy Parameterisation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-750, https://doi.org/10.5194/egusphere-egu22-750, 2022.

EGU22-5287 | Presentations | OS1.5

Quasi-geostrophic coupled model under location uncertainty

Long Li, Etienne Mémin, Bertrand Chapron, and Noé Lahaye

In this work, we aim to describe atmosphere-ocean coupling through a physically-based stochastic formulation. We adopt the framework of modelling under Location Uncertainty (LU) [Bauer2020a], which is based on a temporal-scale separation and a stochastic transport principle. One important characteristic of such random model is that it conserves the total energy of the resolved flow. This representation has been successfully tested for ocean-only models, such as the barotropic quasi-geostrophic (QG) model [Bauer2020b], the multi-layered QG model [Li2021], as well as the rotating shallow-water model [Brecht2021]. Here, we consider the ocean-atmosphere coupled QG model [Hogg2003]. The LU scheme has been tested for coarse-grid simulations, in which the spatial structure of ocean uncertainty is calibrated from eddy-resolving simulation data while the atmosphere component is parameterized from the ongoing simulation. In other words, the ocean dynamics has a data-driven stochastic component whereas the large-scale atmosphere dynamics is fully parameterized. Two major benefits of the resulting random model are provided on the coarse mesh: it enables us to reproduce the ocean eastward jet and its adjacent recirculation zones; it improves the prediction of intrinsic variability for both ocean and atmosphere components. These capabilities of the proposed stochastic coupled QG model are demonstrated through several statistical criteria and an energy transfers analysis.

References:

  • [Bauer2020a] W. Bauer, P. Chandramouli, B. Chapron, L. Li, and E. Mémin. Deciphering the role of small-scale inhomogeneity on geophysical flow structuration: a stochastic approach. Journal of Physical Oceanography, 50(4):983-1003, 2020.
  • [Bauer2020b] W. Bauer, P. Chandramouli, L. Li, and E. Mémin. Stochastic representation of mesoscale eddy effects in coarse-resolution barotropic models. Ocean Modelling, 151:101646, 2020.
  • [Li2021] Li, L., 2021. Stochastic modelling and numerical simulation of ocean dynamics. PhD Thesis. Université Rennes 1.
  • [Brecht2021] Rüdiger Brecht, Long Li, Werner Bauer and Etienne Mémin. Rotating Shallow Water Flow Under Location Uncertainty With a Structure-Preserving Discretization. Journal of Advances in Modeling Earth Systems, 13, 2021MS002492.
  • [Hogg2003] A.M. Hogg, W.K. Dewar, P.D. Killworth, J.R. Blundell. A quasi-geostrophic coupled model (Q-GCM). Monthly Weather Review, 131:2261-2278, 2003.

 

How to cite: Li, L., Mémin, E., Chapron, B., and Lahaye, N.: Quasi-geostrophic coupled model under location uncertainty, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5287, https://doi.org/10.5194/egusphere-egu22-5287, 2022.

EGU22-7907 | Presentations | OS1.5

Stochastic data-driven model of mesoscale and submesoscale eddies in gyre circulation

Francesco Tucciarone, Long Li, and Etienne Memin

Planetary flows and large scale circulation systems are characterised by an interaction between scales that range over several orders of magnitude, with contributions given by mesoscale and submesoscale dynamics. Resolving numerically  such interactions for realistic configuration is, however, far beyond reach. Any large-scale simulation must then rely on parameterizations of the effects of the small scales on  the large scales. In this work, a stochastic parameterization is proposed based on a decomposition of the flow in terms of a smooth-in-time large-scale contribution and a random fast-evolving uncorrelated small-scale part  accounting for  mesoscales and submesoscales unresolved eddies. This  approach, termed modelling under location uncertainty (LU) [4], relies on a stochastic version of Reynolds Transport Theorem to cast physically meaningful conservation principles in this scale-separated framework. Such a scheme has been successfully applied to several large-scale models of the  ocean dynamics [1, 2, 3, 5]. Here a LU version of the  hydrostatic primitive equations is  implemented within the  NEMO community code (https://www.nemo-ocean.eu) with a data-driven approach to establish the spatial correlation of the fast evolving scales. In comparison to a corresponding deterministic counterpart, this stochastic large-scale representation  is shown to improve, in terms of the eastward jet resolution and variabilities, the  flow prediction of an idealized wind forced double gyre circulation. The results are assessed through several statistical criterion as well as an energy transfer analysis [2,5].
[1] W. Bauer, P. Chandramouli, B. Chapron, L. Li, and E. Mémin. Deciphering the
role of small-scale inhomogeneity on geophysical flow structuration: a stochastic approach.
Journal of Physical Oceanography, 50(4):983-1003, 2020.
[2] W. Bauer, P. Chandramouli, L. Li, and E. Mémin. Stochastic representation of
mesoscale eddy effects in coarse-resolution barotropic models. Ocean Modelling, 151:101646,
2020.
[3] Rüdiger Brecht, Long Li, Werner Bauer and Etienne Mémin. Rotating Shallow
Water Flow Under Location Uncertainty With a Structure-Preserving Discretization. Journal of
Advances in Modeling Earth Systems, 13, 2021MS002492.
[4], E. Mémin Fluid flow dynamics under location uncertainty,(2014), Geophysical & Astrophysical Fluid Dynamics, 108, 2, 119–146.
[5] V. Resseguier, L. Li, G. Jouan, P. Dérian, E. Mémin, B. Chapron, (2021), New trends in ensemble forecast strategy: uncertainty quantification for coarse-grid computational fluid dynamics, Archives of Computational Methods in Engineering.

How to cite: Tucciarone, F., Li, L., and Memin, E.: Stochastic data-driven model of mesoscale and submesoscale eddies in gyre circulation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7907, https://doi.org/10.5194/egusphere-egu22-7907, 2022.

EGU22-3668 | Presentations | OS1.5

Uncertainty in ocean biogeochemical simulation: Application of ensemble data assimilation to a one-dimensional model

Nabir Mamnun, Christoph Völker, Mihalis Vrekoussis, and Lars Nerger

Marine biogeochemical (BGC) models are important tools in the hands of scientists and policymakers when assessing the impacts of climate change. Therefore, including an ocean BGC component in Earth System Modeling efforts is essential for climate simulation and predictions. However, current BGC models, used to simulate and thus better understand the marine ecosystem processes, are associated with large undefined uncertainties. Similar to other geoscientific models, complex biological and chemical processes are converted to simplified schemes in BGCs, a methodology known as parameterization. However, these parameter values can be poorly constrained and also involve unknown uncertainties. In turn, the uncertainty in the parameter values translates into uncertainty in the model outputs. Therefore, a systematic approach to properly quantify the uncertainties of the parameters is needed. In this study, we apply an ensemble data assimilation method to quantify the uncertainty arising from the parameterization within BGC models. We apply an ensemble Kalman filter provided by the parallel data assimilation framework (PDAF) into a one-dimensional vertical configuration of the biogeochemical model Regulated Ecosystem Model 2 (REcoM2) at two BGC time-series stations: the Bermuda Atlantic Time-series Study (BATS) and the Dynamique des Flux Atmosphériques en Méditerranée (DYFAMED). Satellite chlorophyll-a concentration data and in situ net primary production data are assimilated to estimate ten selected biogeochemical parameters and the model state. We present convergence and interdependence features of the estimated parameters in relation to the major biological processes and discuss their uncertainties. The major improvements on the parameters involved changes in phytoplankton photosynthesis rate, chlorophyll degradation, and grazing. In general, the change in the estimates of these parameters results in improvements in the model prediction and reduced prediction uncertainty. 

How to cite: Mamnun, N., Völker, C., Vrekoussis, M., and Nerger, L.: Uncertainty in ocean biogeochemical simulation: Application of ensemble data assimilation to a one-dimensional model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3668, https://doi.org/10.5194/egusphere-egu22-3668, 2022.

EGU22-3044 | Presentations | OS1.5

Linking contemporary parametric model uncertainties to projections of biogeochemical cycles

Ulrike Löptien and Heiner Dietze

Anthropogenic emissions of greenhouse gases, such as CO2 and N2O, warm the earth. This in turn modulates the atmospheric greenhouse gas concentrations. The underlying feedback mechanisms are complex and can be counterintuitive. Earth system models have recently matured to standard tools tailored to assess and understand these feedback mechanisms. Along comes the need to determine poorly-known model parameters. This is especially problematic for the ocean biogeochemical component where respective observational data, such as nutrient concentrations and phytoplankton growth, are rather sparse in time and space. In the present study, we illustrate common problems when attempting to estimate such parameters based on typical model evaluation metrics. We find very different parameter sets which are, on the one hand, equally consistent with (synthetic) historical observations while, on the other hand, they propose strikingly differing projections into a warming climate. By the example of simulated oxygen concentrations we propose a step forward by applying variance-based sensitivity analyses to map the respective parameter uncertainties onto their local manifestations - for both contemporary climate and climate projections. The mapping relates local uncertainties of projections to the uncertainty of specific model parameters. In a nutshell, we present a practical approach to the general question of where the present-day model fidelity may be indicative for reliable projections.

 

How to cite: Löptien, U. and Dietze, H.: Linking contemporary parametric model uncertainties to projections of biogeochemical cycles, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3044, https://doi.org/10.5194/egusphere-egu22-3044, 2022.

EGU22-6041 | Presentations | OS1.5

Greenhouse gas forcing a necessary, but not sufficient, causation for the northeast Pacific marine heatwaves 

Armineh Barkhordarian, David M. Nielsen, and Johanna Baehr

Over the last decade, the northeast Pacific experienced strong marine heatwaves (MHWs) that produced devastating marine ecological impacts and received major societal concerns. Here, we assess the link between the well-mixed greenhouse gas (GHG) forcing and the occurrence probabilities of the duration and intensity of the North Pacific MHWs. We investigate whether GHG forcing was necessary for the North Pacific MHWs to occur and whether it is a sufficient cause for such events to continue to repeatedly occur in the 21st Century. To begin with, we apply attribution technique on the long-term SST time series, and detect a region of systematically and externally-forced SST increase -- the long-term warming pool -- co-located with the past notably Blob-like SST anomalies. We further show that the anthropogenic signal has recently emerged from the natural variability of SST over the warming pool, which we attribute primarily to increased GHG concentrations, with anthropogenic aerosols playing a secondary role.

After we demonstrate that the GHG forcing has a dominant influence on the base climate state in the region, we pursue an event attribution analysis for MHWs on a more localized region. Extreme event attribution analysis reveals that GHG forcing is a necessary, but not sufficient, causation for the multi-year persistent MHW events in the current climate, such as that happened in 2014/2015 and 2019/2020. However, the occurrence of the 2019/2020 (2014/2015) MHW was extremely unlikely in the absence of GHG forcing. Thus, as GHG emissions continue to firmly rise, it is very likely that GHG forcings will become a sufficient cause for events of the magnitude of the 2019/2020 record event.

 

 

How to cite: Barkhordarian, A., Nielsen, D. M., and Baehr, J.: Greenhouse gas forcing a necessary, but not sufficient, causation for the northeast Pacific marine heatwaves , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6041, https://doi.org/10.5194/egusphere-egu22-6041, 2022.

EGU22-8332 | Presentations | OS1.5

Dynamical Landscape and Noise-induced Transitions in a Box Model of the Atlantic Meridional Overturning Circulation

Reyk Börner, Valerio Lucarini, and Larissa Serdukova

The multistability of the Atlantic Meridional Overturning Circulation (AMOC) challenges the predictability of long-term climate evolution. In light of an observed weakening in AMOC strength, it is crucial to study the probabilities of noise-induced transitions between the different competing flow regimes. From a dynamical systems perspective, the phase space of a multistable system can be characterised as a non-equilibrium potential landscape, with valleys corresponding to the different basins of attraction. Knowing the potential, one can infer the statistics and pathways of noise-induced transitions. Particularly, in the weak-noise limit, transition paths lead through special regions of the basin boundaries, called Melancholia states. Recent studies have applied these concepts to climate models of low and intermediate complexity. Here, we investigate the quasi-potential landscape of a three-box model of the AMOC, based on the popular model by Rooth. We analyse noise-induced transitions between the two stable circulation states and elucidate the role of the Melancholia state. Forcing the model with different noise laws, which represent fluctuations caused by different physical processes, we discuss how the properties of transitions change when considering non-Gaussian processes, specifically Lévy noise. Simulated transition rates are related to their theoretical values using the quasi-potential landscape. Our results yield a comprehensive picture of the dynamical properties of an inter-hemispheric three-box AMOC model under stochastic forcing. By relating the deterministic structure of this simple model to the statistics of critical transitions, we hope to build a basis for transferring this approach to more complex models of the AMOC.

How to cite: Börner, R., Lucarini, V., and Serdukova, L.: Dynamical Landscape and Noise-induced Transitions in a Box Model of the Atlantic Meridional Overturning Circulation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8332, https://doi.org/10.5194/egusphere-egu22-8332, 2022.

OS1.6 – Changes in the Arctic Ocean, sea ice and subarctic seas systems: Observations, Models and Perspectives

EGU22-6930 | Presentations | OS1.6

Physical manifestations and ecological implications of Arctic Atlantification

Karen M. Assmann, Randi B. Ingvaldsen, Raul Primicerio, Maria Fossheim, Igor V. Polyakov, and Andrey V. Dolgov

The Atlantic gateway to the Arctic Ocean is influenced by vigorous inflows of Atlantic Water. Particularly since 2000, the high-latitude impacts of these inflows have strengthened due to climate change driving so-called ‘Atlantification’ - a transition of Arctic waters to a state more closely resembling that of the Atlantic. In this review, we discuss the physical and ecological manifestations of Atlantification in a hotspot region of climate change reaching from the southern Barents Sea to the Eurasian Basin. Atlantification is driven by anomalous Atlantic Water inflows and modulated by local processes. These include reduced atmospheric cooling, which amplifies warming in the southern Barents Sea; reduced freshwater input and stronger influence

of ice import in the northern Barents Sea; and enhanced upper ocean mixing and air–ice–ocean coupling in the Eurasian Basin. Ecosystem responses to Atlantification encompass increased production, northward expansion of boreal species (borealization), an increased importance of the pelagic compartment populated by new species, an increasingly connected food web and a gradual reduction of the ice-associated ecosystem compartment.

How to cite: Assmann, K. M., Ingvaldsen, R. B., Primicerio, R., Fossheim, M., Polyakov, I. V., and Dolgov, A. V.: Physical manifestations and ecological implications of Arctic Atlantification, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6930, https://doi.org/10.5194/egusphere-egu22-6930, 2022.

EGU22-5807 | Presentations | OS1.6

A multidecadal model estimate of pan-Arctic coastal erosion rates and associated nutrient fluxes

Stefanie Rynders and Yevgeny Aksenov

Arctic coastal erosion is an environmental hazard expected to increase under climate change, due to decreasing sea ice protection along with increasing wave heights. In addition to the impact on land, this affects the marine environment, as coastal erosion is a source of organic matter, carbon and nutrients for the coastal waters and shelf seas in the Arctic. Following Barnhart et al., we adapted the White model for iceberg melt to calculate pan-coastal erosion rates. The approach combines ice, ocean and wave model output with permafrost model output and geological characteristics from observations. The calculated erosion rates show large spatial variability, similar to observations, as well as a large seasonal cycle. Additionally, it brings to light the increasing trend between the 1980s and 2010s, with a lengthening of the erosion season, plus inter-annual variability. Using observed nutrient ratios, the erosion rates are converted to biogeochemical sources, which can be used for marine ecosystem models. The approach could be used on-line in earth system models, providing both projections of future erosion rates as well as improved biogeochemistry projections. We acknowledge financial support from Advective Pathways of nutrients and key Ecological substances in the Arctic (APEAR) project (NE/R012865/1, NE/R012865/2, #03V01461), as part of the Changing Arctic Ocean programme, jointly funded by the UKRI Natural Environment Research Council (NERC) and the German Federal Ministry of Education and Research (BMBF), and from the European Union’s Horizon 2020 research and innovation programme under project COMFORT (grant agreement no. 820989), for which the work reflects only the authors’ view; the European Commission and their executive agency are not responsible for any use that may be made of the information the work contains.

How to cite: Rynders, S. and Aksenov, Y.: A multidecadal model estimate of pan-Arctic coastal erosion rates and associated nutrient fluxes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5807, https://doi.org/10.5194/egusphere-egu22-5807, 2022.

EGU22-6421 | Presentations | OS1.6

Evolution of the wintertime salt budget of the Arctic Ocean mixed layer observed during MOSAIC

Torsten Kanzow, Benjamin Rabe, Janin Schaffer, Ivan Kuznetsov, Mario Hoppmann, Sandra Tippenhauer, Tao Li, Volker Mohrholz, Markus Janout, Luisa von Albedyll, Timothy Stanton, Lars Kaleschke, Christian Haas, Kirstin Schulz, and Ruibo Lei

In wintertime, the Arctic Ocean mixed layer (ML) regulates the transport of oceanic heat to the sea ice, and transfers both momentum and salt between the ice and the stratified ocean below. Between October, 2019, and May, 2020, we recorded time series of wintertime ML-relevant properties at unprecedented resolution during the MOSAIC expedition. Vertical and horizontal salt and temperature gradients, vertical profiles of horizontal velocity, turbulent dissipation of kinetic energy, growth of both level and lead ice, and ice deformation were obtained from both the Central Observatory and the Distributed Network around it.  

We find that the ML deepened from 20 m at the onset of the MOSAIC drift to 120 m at the end of the winter. The ML salinity showed a decrease between early November 2019 and mid-January 2020 followed by a pronounced increase during February and March 2020 - marking the coldest period of the observations. Applying the equation of salt conservation to the ML as a guiding framework, we combine the abovementioned observations, to intercompare the temporal evolutions of the different processes affecting salinity. Overall, brine rejection associated with thermodynamic ice growth turns out to be the largest salt flux term in the ML salt budget. Thereby the observed amplitudes of upward ocean heat fluxes into the mixed layer are too small for them to have a relevant impact on limiting ice growth. Horizontal salt advection in the ML is the second-most important flux term, actually representing a net sink of salt, thus counteracting brine release. It displays considerably larger temporal variability than brine release, though, due to the variable of ocean currents and horizontal salt gradients. Vertical ocean salt fluxes across the mixed layer base represent the third-most important salt flux term, showing particularly elevated values during storm events, when small-scale turbulence in the ML is triggered by the winds. The results presented will be interpreted in the context of the changes in the regional and temporal ocean, atmosphere and sea ice properties encountered during the MOSAIC drift.

How to cite: Kanzow, T., Rabe, B., Schaffer, J., Kuznetsov, I., Hoppmann, M., Tippenhauer, S., Li, T., Mohrholz, V., Janout, M., von Albedyll, L., Stanton, T., Kaleschke, L., Haas, C., Schulz, K., and Lei, R.: Evolution of the wintertime salt budget of the Arctic Ocean mixed layer observed during MOSAIC, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6421, https://doi.org/10.5194/egusphere-egu22-6421, 2022.

EGU22-7240 | Presentations | OS1.6

Arctic Ocean Heat Content as a Driver of Regional Sea Ice Variability

Elena Bianco, Doroteaciro Iovino, Stefano Materia, Paolo Ruggieri, and Simona Masina

The Arctic Ocean is transitioning from permanently ice-covered to seasonally ice-free, with thinner and more dynamic sea ice. This strengthens the coupling with the atmosphere and the ocean, which exert a strong influence on sea ice via thermodynamic and dynamic forcing mechanisms. Short-term predictions are met with the challenge of disentangling the preconditioning processes that regulate sea ice variability, as these often trigger a response that is not uniform in time nor in space.  This study assesses the role of ocean heat content (OHC) as a driver of sea ice variability for five different regions of the Arctic Ocean. We choose to focus on a sub-seasonal time frame, with the goal of investigating whether anomalies in ocean heat content offer a source of predictability for sea ice in the following months and whether this coupling varies across different regions and seasons. To account for the different processes that regulate the Arctic Ocean heat budget, we consider ocean heat content in the mixed layer (OHCML) and in the upper 300 m (OHC300), computed from the CMCC Global Ocean Reanalysis C-GLORSv5 for the period 1979-2017. Time-lagged correlations of linearly detrended anomalies suggest a link between heat content and sea ice variability in the following months. This source of predictability is stronger during the melt season and peaks in autumn, with highest correlations in the Kara and Chukchi regions. Consistent with previous studies, a distinctive response is observed for the Barents Sea, where sea ice is more strongly coupled with the ocean during the freezing season.  Our preliminary results support a central role of OHC as a driver of sea ice thermodynamic changes at sub-seasonal scales, a mechanism that is likely to become stronger under ice-depleted conditions.   

How to cite: Bianco, E., Iovino, D., Materia, S., Ruggieri, P., and Masina, S.: Arctic Ocean Heat Content as a Driver of Regional Sea Ice Variability, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7240, https://doi.org/10.5194/egusphere-egu22-7240, 2022.

EGU22-7793 | Presentations | OS1.6

Decadal variability in the transient tracer distribution in the upper Arctic Ocean

Wiebke Körtke, Maren Walter, Oliver Huhn, and Monika Rhein

The Arctic is warming stronger and faster than other regions during the climate change. Within this development, the Arctic Ocean’s water masses and ventilation processes are changing as well. Transient anthropogenic tracers can be used to track water masses and to investigate ventilation and mixing processes. For these tracers, e.g. chlorofluorocarbons (CFCs), the atmosphere is the only source to the ocean and they are conservative in the water. In this study, we analyse CFC-12 (CCl2F2) along two transects in the Canadian basin of the central Arctic Ocean covered in different decades (T1: 1994 and 2015, T2: 2005 and 2015), with additional hydrographic data for context. We find differences in both the tracer concentration and the hydrographic properties between the years and transects. Along the first transect (located at ~180°W), the difference in saturation between 2015 and 1994 is largest in the layer of the Atlantic Water at high latitudes (> 82°N). A similar strong increase in CFC-12 saturation is observed along the second transect (located at 150°W). In contrast to the saturation increase in the Atlantic Water layer, we find a decrease close to the surface, which is correlated to oversaturations in 2005 in this region. At the same time, the surface waters were more saline in 2005 indicating a mixing event. Oversaturation is present in all years, except in 1994. Existence of oversaturation can be caused by special events, either inside the ocean (by mixing processes) or at the sea ice-ocean-atmosphere interface (by the occurrence of changes in the sea ice concentration or atmospheric forcing). We compare the tracer results with hydrographic properties, as well as with wind and ice conditions present during the time of measurements, to investigate the causes of the observed changes. Further, the time dependent atmospheric concentrations of CFCs are used to determine the age of water masses. Here, we use the simplest possible approach of age determination to identify the age of the Atlantic Water along the transects, assuming no interaction or exchange with the surrounding water masses after the Atlantic Water left the surface in Fram Strait. Due to the decreasing CFC-12 atmospheric concentration after 2003/04, it is necessary to use sulfur hexafluoride (SF6) as an additional tracer for 2015. Along the first transect, the tracer age of CFC-12 for 1994 is compared to the tracer age of SF6 in 2015. In 2015 the tracer age is much higher in the region south of 80°N compared to 1994, while the ages are quite similar at higher latitudes. The higher age in the southern part of the transect indicates a water mass, that is much older in 2015 than it was in 1994, a sign of a possible circulation change. A similar result is found along the second transect, where the new tracer SF6 is available in both years. Along this transect, the water is also older in 2015 than in 2005.

How to cite: Körtke, W., Walter, M., Huhn, O., and Rhein, M.: Decadal variability in the transient tracer distribution in the upper Arctic Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7793, https://doi.org/10.5194/egusphere-egu22-7793, 2022.

EGU22-8234 | Presentations | OS1.6

Towards Late Quaternary sea ice reconstructions in the Arctic with sedimentary ancient DNA. 

Tristan Cordier, Danielle M. Grant, Kristine Steinsland, Katja Häkli, Dag Inge Blindheim, Agnes Weiner, Aud Larsen, Jon Thomassen Hestetun, Jessica Louise Ray, and Stijn De Schepper

Sea ice has a pivotal role in the regulation of the Arctic climate system, and by extension to the global climate. Our knowledge of its historical variation and extent is limited to the satellite records that only cover the last several decades, which considerably hampers our understanding on how past climate has influenced sea ice extent in the Arctic. Latest modelling efforts indicate that the Arctic may be sea ice free in summer by 2050, making the appreciation of the effects that such major change will have on Arctic ecosystems of paramount importance. Here, we will present the first results of the AGENSI project (www.agensi.eu) aiming at reconstructing the past sea ice evolution with sedimentary ancient DNA. Based on a large collection of surface sediments collected along multiple gradients of sea ice cover in the Arctic, we show that plankton DNA sinking to the seafloor can be used to predict the variation of surface sea ice cover. Further, we will present our current efforts to utilize this dataset to reconstruct the past sea ice variation in Late Quaternary sediment cores.

How to cite: Cordier, T., Grant, D. M., Steinsland, K., Häkli, K., Blindheim, D. I., Weiner, A., Larsen, A., Hestetun, J. T., Ray, J. L., and De Schepper, S.: Towards Late Quaternary sea ice reconstructions in the Arctic with sedimentary ancient DNA. , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8234, https://doi.org/10.5194/egusphere-egu22-8234, 2022.

EGU22-9899 | Presentations | OS1.6 | Highlight

Changes in Arctic Halocline Waters along the East Siberian Slope and in the Makarov Basin from 2007 to 2020

Cécilia Bertosio, Christine Provost, Marylou Athanase, Nathalie Sennéchael, Gilles Garric, Jean-Michel Lellouche, Joo-Hong Kim, Kyoung-Ho Cho, and Taewook Park

The Makarov Basin halocline receives contributions from diverse water masses of Atlantic, Pacific, and East Siberian Sea origin. Changes in surface circulation (e.g., in the Transpolar Drift and Beaufort Gyre) have been documented since the 2000s, while the upper ocean column in the Makarov Basin has received little attention. The evolution of the upper and lower halocline in the Makarov Basin and along the East Siberian Sea slope was examined combining drifting platforms observations, shipborne hydrographic data, and modelled fields from a global operational physical model.

In 2015, the upper halocline in the Makarov Basin was warmer, fresher, and thicker compared to 2008 and 2017, likely resulting from the particularly westward extension of the Beaufort Gyre that year. From 2012-onwards, cold Atlantic-derived lower halocline waters, previously restricted to the Lomonosov Ridge area, progressed eastward along the East Siberian slope, with a sharp shift from 155 to 170°E above the 1000 m isobath in winter 2011-2012, followed by a progressive eastward motion after winter 2015-2016 and reached the western Chukchi Sea in 2017. In parallel, an active mixing between upwelled Atlantic water and shelf water along the slope, formed dense warm water which also supplied the Makarov Basin lower halocline.

The progressive weakening of the halocline, together with shallower Atlantic Waters, is emblematic of a new Arctic Ocean regime that started in the early 2000s in the Eurasian Basin. Our results suggest that this new Arctic regime now may extend toward the Amerasian Basin.



How to cite: Bertosio, C., Provost, C., Athanase, M., Sennéchael, N., Garric, G., Lellouche, J.-M., Kim, J.-H., Cho, K.-H., and Park, T.: Changes in Arctic Halocline Waters along the East Siberian Slope and in the Makarov Basin from 2007 to 2020, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9899, https://doi.org/10.5194/egusphere-egu22-9899, 2022.

EGU22-11202 | Presentations | OS1.6

Upper Arctic Ocean hydrography during the year-round MOSAiC expedition in the context of historical observations

Myriel Vredenborg, Benjamin Rabe, Sandra Tippenhauer, and Kirstin Schulz and the Team MOSAiC OCEAN

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 unique physical, biogeochemical and ecosystem processes and their recent changes, the year-round ice drift experiment Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) was conducted from autumn 2019 to autumn 2020.

In this study we analyse temperature and salinity measurements of the upper Arctic Ocean taken during MOSAiC with different devices, i.e. on an ice-tethered profiler, a microstructure profiler and water sampler rosettes operated from the ship as well as through an ice hole on the ice floe. Combining all these measurements provides us an exceptional data resolution along the MOSAiC track. Moreover, we compare these observations with a comprehensive dataset of historical hydrographic data from the region.

Along the MOSAiC track we find signatures of a convective lower halocline (Fram Strait branch), as well as advective-convective lower halocline (Barents Sea branch). We see pronounced changes in the salinity and temperature of the lower halocline in comparison to the historical data, in particular, at the beginning of the drift. Furthermore, we show polar mixed-layer and upper halocline conditions in relation to seasonality and local surface conditions. We put the warm Atlantic Water temperature in the context of historical observations and investigate indications for the presence of Pacific Water.

How to cite: Vredenborg, M., Rabe, B., Tippenhauer, S., and Schulz, K. and the Team MOSAiC OCEAN: Upper Arctic Ocean hydrography during the year-round MOSAiC expedition in the context of historical observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11202, https://doi.org/10.5194/egusphere-egu22-11202, 2022.

EGU22-11518 | Presentations | OS1.6

Differential summer melt rates of ridges, first- and second-year ice in the central Arctic Ocean during the MOSAiC expedition

Evgenii Salganik, Benjamin Lange, Christian Katlein, Ilkka Matero, Julia Regnery, Igor Sheikin, Philipp Anhaus, Knut Høyland, and Mats Granskog

During the melt season, sea ice melts from the surface and bottom. The melt rates substantially vary for sea ice ridges and undeformed first- and second-year ice. Ridges generally melt faster than undeformed ice, while the melt of ridge keels is often accompanied by further summer growth of their consolidated layer. This summer consolidation is related to refreezing of less saline meltwater, originating from snowmelt and ridge keel melt. We examine the spatial variability of ice melt for different types of ice from in situ drilling, coring, and from multibeam sonar scans of remotely operated underwater vehicle (ROV). Seven ROV scans, performed from 24 June 2020 to 28 July 2020 during the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) expedition were analyzed. The area investigated by the ROV (400 by 200 m) consisted of several ice ridges, surrounded by first- and second-year ice. Seven ice drilling transects were additionally performed to validate ROV measurements. The maximum keel depth of the ridge investigated by ice drilling was 6.5 m. We show a substantial difference in melt rates of first-year ice, second-year ice, and sea ice ridge keels. We also show how ridge keels decay depending on keel depth, width, steepness, and orientation relative to the ice drift direction. These results are important for quantifying ocean heat fluxes for different types of ice during advanced melt, and for estimation of the ridge contribution to the total ice mass and summer meltwater balances of the Arctic Ocean.

How to cite: Salganik, E., Lange, B., Katlein, C., Matero, I., Regnery, J., Sheikin, I., Anhaus, P., Høyland, K., and Granskog, M.: Differential summer melt rates of ridges, first- and second-year ice in the central Arctic Ocean during the MOSAiC expedition, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11518, https://doi.org/10.5194/egusphere-egu22-11518, 2022.