OS1.1

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.

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.

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.

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.

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.

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.

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.

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" (0.10^o resolution) that simulates a rich field of mesoscale features such as mesoscale eddies and fronts, "eddy-present" (0.25^o) that simulates mesoscale features to a lesser extent, and "eddy-param" (1^o 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 0.4^oC 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.

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.

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⁺, PO₄^3-), 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^-, SO₄^2-, 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.

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

Salma Elageed^1,3 , 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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

The 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.

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.

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.

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.