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OS1.1

EDI
Improved Understanding of Ocean Variability and Climate

This session will focus on variability in the ocean and its role in the wider climate system using both observations and models. Areas to be considered will include both ocean heat uptake and circulation variability as well as exploring the use of sustained ocean observing efforts and models to make progress in understanding the ocean’s role in the climate system. More than 90% of the excess heat in the climate system has been stored in the ocean, which mitigates the rate of surface warming. Better understanding of ocean ventilation mechanisms, as well as the uptake, transport, and storage of oceanic heat are therefore essential for reducing the uncertainties on global warming projections. Circulation variability and connectivity, particularly from the South Atlantic to the North Atlantic and Arctic Ocean, are also of interest as well as how they are driven by local-, large- or global-scale processes or teleconnections. Sustained observations at sea are being made within a wide variety of programmes and are leading to significant advances in our ability to understand and model climate. Thus, this session will also explore ongoing and planned sustained ocean observing efforts and illuminate their roles in improving understanding of the ocean’s role in the climate system. For example, air-sea flux moorings are being maintained at select sites to assess models and air-sea flux fields. Deep temperature and salinity measurements are being made at time series moorings and will be made by deep Argo floats. Significant advances are also being made using Argo floats for biogeochemistry and carbon measurements. Such observations provide the means to develop linkages between sustained ocean observing and climate modelling. In conclusion, the session will consider key aspects of ocean variability and its climate relevance, as well as encouraging the use of observations and models to enhance understanding of these areas.

Convener: Simon Josey | Co-conveners: Levke CaesarECSECS, Léon Chafik, Yavor KostovECSECS, Iselin Medhaug
Presentations
| Fri, 27 May, 08:30–11:50 (CEST), 13:20–16:40 (CEST)
 
Room L3

Fri, 27 May, 08:30–10:00

Chairpersons: Simon Josey, Levke Caesar, Joke Lübbecke

08:30–08:35
Introduction

08:35–08:41
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EGU22-1377
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ECS
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Virtual presentation
Taimoor Sohail et al.

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.

08:41–08:47
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EGU22-1309
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ECS
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Virtual presentation
Samantha Hallam et al.

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.

08:47–08:53
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EGU22-3526
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ECS
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Virtual presentation
Marisa Roch et al.

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.

08:53–08:59
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EGU22-4148
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Virtual presentation
Till Kuhlbrodt et al.

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.

08:59–09:05
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EGU22-12567
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ECS
Josipa Milovac et al.

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.

09:05–09:11
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EGU22-6683
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ECS
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On-site presentation
Laura Cimoli et al.

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.

09:11–09:17
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EGU22-4259
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On-site presentation
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.

09:17–09:23
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EGU22-12052
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ECS
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Virtual presentation
Emanuele Gugliandolo et al.

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.

09:23–09:29
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EGU22-2085
Adiabatic and diabatic signatures of ocean temperature variability
(withdrawn)
Ryan Holmes et al.
09:29–09:35
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EGU22-8895
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ECS
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Virtual presentation
Katherine Turner et al.

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.

09:35–09:41
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EGU22-12758
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ECS
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On-site presentation
Charles Turner et al.

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.

09:41–09:47
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EGU22-8288
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ECS
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On-site presentation
Ivy Frenger et al.

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.

09:47–10:00
Discussion

Fri, 27 May, 10:20–11:50

Chairpersons: Levke Caesar, Léon Chafik, Ivy Frenger

10:20–10:23
Introduction

10:23–10:29
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EGU22-6678
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ECS
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Virtual presentation
Qiwei Sun and Yan Du

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.

10:29–10:35
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EGU22-5123
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Virtual presentation
Adam Blaker et al.

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.

10:35–10:41
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EGU22-6927
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Virtual presentation
Qiang Wang

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.

10:41–10:47
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EGU22-204
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ECS
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Si-Yuan Sean Chen et al.

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.

10:47–10:53
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EGU22-11866
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ECS
|
Virtual presentation
Ana Cvitešić Kušan et al.

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.

10:53–10:59
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EGU22-2615
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ECS
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Virtual presentation
Salma Elageed et al.

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.

10:59–11:05
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EGU22-13403
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On-site presentation
Atmosphere-Ocean Coupled Variability in the Arabian/Persian Gulf
(withdrawn)
Fahad Al Senafi
11:05–11:11
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EGU22-4438
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Virtual presentation
Viviane Menezes

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.

11:11–11:17
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EGU22-4380
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ECS
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Virtual presentation
Brady Ferster et al.

            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.

11:17–11:23
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EGU22-6725
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ECS
|
Virtual presentation
Yun Liang and Yan Du

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.

11:23–11:29
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EGU22-12287
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ECS
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Virtual presentation
F. Alexander Haumann et al.

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.

11:29–11:35
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EGU22-13416
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ECS
|
Letizia Roscelli et al.

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.

11:35–11:50
Discussion

Fri, 27 May, 13:20–14:50

Chairpersons: Léon Chafik, Yavor Kostov, Laura Cimoli

13:20–13:23
Introduction

13:23–13:29
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EGU22-2651
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Virtual presentation
Laura Jackson

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.

13:29–13:35
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EGU22-8779
|
Virtual presentation
Robin Fraudeau et al.

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.

13:35–13:41
|
EGU22-1152
|
Virtual presentation
Damien Desbruyères et al.

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.

13:41–13:47
|
EGU22-4836
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ECS
|
|
On-site presentation
Tobias Schulzki et al.

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.

13:47–13:53
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EGU22-7340
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Virtual presentation
Marius Årthun et al.

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.

13:53–13:59
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EGU22-6594
|
ECS
Yavor Kostov et al.

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.

13:59–14:05
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EGU22-2308
Ben Moat et al.

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.

14:05–14:11
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EGU22-2753
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ECS
Scalar transport induced by mesoscale eddies in the North Atlantic detected by a three-dimensional algorithm
(withdrawn)
Davide Cavaliere and Chunxue Yang
14:11–14:17
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EGU22-3720
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Virtual presentation
Ian Renfrew et al.

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.

14:17–14:23
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EGU22-11399
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ECS
Solène Pourtout et al.

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.

14:23–14:29
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EGU22-13047
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Virtual presentation
Olivia Gozdz et al.

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.

14:29–14:50
Discussion

Fri, 27 May, 15:10–16:40

Chairpersons: Yavor Kostov, Ivy Frenger, Laura Cimoli

15:10–15:13
Introduction

15:13–15:19
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EGU22-10451
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ECS
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Virtual presentation
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.

15:19–15:25
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EGU22-9433
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ECS
Margarita Markina et al.

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.

15:25–15:31
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EGU22-7698
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On-site presentation
Gerard McCarthy et al.

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.

15:31–15:37
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EGU22-8798
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ECS
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Virtual presentation
Sam Jones et al.

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.

15:37–15:43
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EGU22-9947
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ECS
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Virtual presentation
Marion Devilliers et al.

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.

15:43–15:49
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EGU22-10223
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ECS
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On-site presentation
Diego Cortés Morales and Alban Lazar

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.

15:49–15:55
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EGU22-4132
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ECS
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Virtual presentation
Hao Liu

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.

15:55–16:01
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EGU22-7908
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ECS
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Virtual presentation
Anna Olivé Abelló et al.

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.

16:01–16:07
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EGU22-414
Xiaoqing Chen et al.

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.

16:07–16:13
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EGU22-5416
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ECS
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Virtual presentation
Jon Baker et al.

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.

16:13–16:40
Discussion