EGU General Assembly 2022  

Based on new observations and on improved ocean and climate modeling, the last decade was marked by a seminal progress in our understanding of the North Atlantic circulation.  The presentation will summarize recent insights and discuss future challenges. Based on new observations and on improved ocean and climate modeling, the last decade was marked by a seminal progress in our understanding of the North Atlantic circulation.  The presentation will summarize recent insights and discuss future challenges. Based on new observations and on improved ocean and climate modeling, the last decade was marked by a seminal progress in our understanding of the North Atlantic circulation.  The presentation will summarize recent insights and discuss future challenges.  

How to cite: Rhein, M.: On the North Atlantic Circulation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1488, https://doi.org/10.5194/egusphere-egu22-1488, 2022.

Deep water masses are the driver of the global ocean circulation, critical for transporting oxygen and nutrients throughout the water column, and a crucial mitigator of current climate change. They are also notoriously hard to observe: they form in winter in ice-infested areas, and then travel around the globe too deep for most autonomous instruments to monitor them. Therefore, although they represent at least half of the ocean volume, we still know very little about their circulation and variability.

What we do know is that they are already changing, much faster than expected.

From a ship in the Southern Ocean to models in the Arctic, I will share with you my obsession for these fascinating deep waters; highlight the blind spots that remain; and describe recent and upcoming deep-water-targeting projects that get me excited.

How to cite: Heuzé, C.: Global deep waters: what we know, what we know we do not know, and what we should do about it, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1695, https://doi.org/10.5194/egusphere-egu22-1695, 2022.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 (20^th 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.

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.

            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.

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 (1/12^ohorizontal 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 20^th 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-20^th 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 20^th 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.

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.

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.

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.

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.

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.

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.

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

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

Ocean vertical velocities are several orders of magnitude smaller than the horizontal velocity field when looking at patterns larger than the sub-mesoscales, and for this reason, direct measurement in the ocean has not yet been possible. One method for estimating in-situ vertical velocities (w) in the real ocean is through a theoretical approach using observation-based fields. In this work, the Geostrophic Linear Vorticity Balance (GLVB: βv_g = 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

[1] Feraco et al., EPL, 2018

[2] Feraco et al., EPL, 2021

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

[4] Camporeale et al., PRL, 2018

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

References:

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

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

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

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

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

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

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

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

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

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

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

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

One of the most important phenomena in the Arctic seas, in which all cascades of the scale of variability of oceanological processes are observed, are climatic and seasonal frontal zones. However, despite the climate changes noted by many researchers, so far, the ideas about the long-term dynamics and characteristics of the surface layer in the frontal zones in the Arctic region are fragmentary.

In our work, we considered seasonal and long-term variability of the Polar Frontal Zone (PFZ), the River Plumes Frontal Zone (RPFZ) and the Marginal Ice Zone (MIZ) in the Barents and Kara Seas. The authors evaluated their relationship with eddies structures and atmospheric oscillations. We used satellite data of temperature, salinity and sea level for the period from 2002 to 2020, which we processed using cluster analysis. To isolate the manifestations of eddies structures on the surface, we used radar images of the Envisat ASAR and Sentinel-1A/B. To analyze the relationship between the characteristics of the frontal zones and atmospheric oscillations, we used correlation analysis.

We have shown that the intensity of interannual and seasonal estimates of the SST gradient and the area of the PFZ and RPFZ in the first decade was an order of magnitude higher than in the period from 2011 to 2020. We observe the opposite pattern for the characteristics of the MIZ – in the second decade, the magnitude of the estimates of the SST gradient and area increases. We observe the maximum number of eddies structures in PFZ and RPFZ against the background of a general weakening of the SST gradients. We assume that this is due to the development of intense baroclinic instability in the frontal zones. In our opinion, the intensity of winter meridional transport over Northern Europe affects the growth of summer SST gradients and a decrease in the area of the PFZ and a decrease in SST in the RPFZ. The magnitude of the winter Arctic zonal transfer may increase the characteristics of SST in the RPFZ region. The value of the average seasonal gradient of the SST of the climatic surface PFZ is lower than that of the seasonal RPFZ and MIZ.

The analysis of frontal zone and eddies in this work was supported by RFBR grant 20-35-90053.

How to cite: Konik, A. and Zimin, A.: Seasonal and long-term variability of the characteristics surface frontal zones of the Barents and Kara seas, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-60, https://doi.org/10.5194/egusphere-egu22-60, 2022.

This contribution focuses on the Arctic water budget, including its atmospheric, terrestrial, and oceanic components. Oceanic volume fluxes through the main Arctic gateways are calculated, using data from the CMEMS Global Reanalysis Ensemble Product (GREP), and compared to water input to the ocean from atmosphere and land. For this purpose, we use various state-of-the-art reanalyses, including the European Centre for Medium Range Weather Forecast's (ECMWF) latest products ERA5 and ERA5-Land and evaluate them against available satellite (e.g., GRACE) and in-situ river discharge observations.

To obtain a consistent estimate of all physical terms, we combine the most credible estimates of the individual budget terms and perform a variational optimization to obtain closed water budgets on annual and seasonal scales. This up-to-date estimate of the Arctic water cycle is subsequently used to validate historical runs from the Coupled Model Intercomparison Project Phase 6 (CMIP6). Modelled water budget components are analyzed concerning their annual means, seasonal cycles and trends and compared to our observationally constrained data. Results suggest that there remain large uncertainties in the simulation of the Arctic water cycle of the recent decades.

Furthermore, we choose a similar approach to validate the coupled energy budget in CMIP6 models, including oceanic heat transports through the Arctic gateways (where mooring-derived oceanic heat transports are available), atmospheric energy transports and vertical energy fluxes at the surface and top-of-the-atmosphere, as well as Arctic Ocean heat storage.

This assessment helps to understand model biases in typically analyzed quantities such as sea ice extent or volume. It also provides physically based metrics for detecting outliers from the model ensemble which can help to reduce spread in future projections of Arctic change.

How to cite: Winkelbauer, S., Mayer, M., and Haimberger, L.: Validation of the Arctic water and energy cycles in CMIP6 with consistent observation-based estimates, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-571, https://doi.org/10.5194/egusphere-egu22-571, 2022.

The Arctic atmosphere is strongly affected by anthropogenic warming leading to long-term trends in surface temperature and sea ice extent. In addition, it exhibits strong variability on time scales from days to seasons. While recent research elucidated processes causing long-term trends as well as synoptic extreme conditions in the Arctic, we investigate unusual atmospheric conditions on the seasonal time scale. We introduce a method to identify extreme seasons – deviating strongly from a running-mean climatology – based on a principal component analysis in the phase space spanned by the seasonal-mean values of surface temperature, precipitation, and the atmospheric components of the surface energy balance. Given the strongly varying surface conditions in the Arctic, this analysis is done separately in Arctic sub-regions that are climatologically characterized by either sea ice, open ocean, or mixed conditions.

Using ERA5 reanalyses for the years 1979-2018, our approach identifies 2-3 extreme seasons for each of winter, spring, summer, and autumn, with strongly differing characteristics and affecting different Arctic sub-regions. Results will be shown for two contrasting extreme winters affecting the Kara and Barents Seas, including their substructure, the role of synoptic-scale weather systems, and potential preconditioning by anomalous sea ice extent and/or sea surface temperature at the beginning of the season.

To statistically quantify and confirm these results, we further apply our method to large ensemble simulations of the CESM climate model, using roughly 1000 years of data in present-day (1990-2000) and end-of-century (2091-2100) climate, respectively. Results show a strong similarity between the characteristics of extreme seasons in ERA5 and CESM for the present-day period. The identified seasons predominantly show the most extreme seasonal-mean anomalies of the applied surface parameters, confirming that our approach captures seasons with extraordinary conditions. Preliminary results will also be shown about our current investigation of possible changes in the characteristics and driving mechanisms of Arctic extreme seasons in the warmer end-of-century climate.

The framework developed in this study and the insight gained from analyzing both, reanalysis and climate model data, will be insightful for better understanding the effects of global warming on Arctic extreme seasons.

How to cite: Hartmuth, K., Boettcher, M., Wernli, H., and Papritz, L.: Identification, characteristics, and dynamics of Arctic extreme seasons in ERA5 and CESM climate simulations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1414, https://doi.org/10.5194/egusphere-egu22-1414, 2022.

The interaction between North Atlantic and Arctic Ocean waters plays a key role in climate variability and in
driving the global thermohaline circulation. In the past decades, an increased heat input to the Arctic has
occurred which is considered of high climatic relevance as, e.g., it contributes to enhancing sea ice melting.
In this frame, the progressive northward extension of the Atlantic signal within the Arctic domain known as
Arctic Atlantification is one of the most dramatic environmental local changes of the last decades.
In this study we used in situ data and the Copernicus Marine Environment Monitoring Service (CMEMS)
reanalysis dataset to explore spatial and temporal variability of water masses on different time-scales and
depths in the eastern Fram Strait. In that area, warm and salty Atlantic Water (AW) enters the Arctic Ocean
through the West Spitsbergen Current (WSC). Time series of potential temperature, salinity and potential
density obtained from CMEMS reanalysis in the surface, upper-intermediate and deep layers referring to the
period 1991-2019 have been considered. High-frequency observations gathered from an oceanographic
mooring maintained by the National Institute of Oceanography and Applied Geophysics (OGS) in
collaboration with the Italian National Research Council - Institute of Polar Science (CNR-ISP) have been
used to assess the reliability of CMEMS data in reproducing ocean dynamics in the deep layer (ca 900-1000
m depth) of the SW offshore Svalbard area. The mooring system has been collecting data since June 2014.
In this contribution, we will show how the CMEMS data compared with in situ measurements as far as
seasonal and interannual variations as well as long-term trends are concerned. We will also discuss how
CMEMS reanalyses show differences in resolving ocean dynamics at different depths. Particularly, the severe
limitations in reproducing thermohaline variability at depths greater than 700 m. Finally, we will illustrate how
our results highlight strengths and limitations of CMEMS reanalyses, underscoring the importance of
optimizing measurements in a strategic area for studying climate change impacts in the Arctic and sub-Arctic
regions.

How to cite: Dentico, C., Bensi, M., Kovačević, V., Zanchettin, D., and Rubino, A.: Water masses variability in the eastern Fram Strait explored through oceanographic mooring data and the CMEMS dataset, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1715, https://doi.org/10.5194/egusphere-egu22-1715, 2022.

Climate models are our best tools to quantify ongoing changes caused by the climate crisis, but they are not perfect. The Arctic Ocean is particularly challenging to simulate: complex circulation flowing through narrow gateways and around tortuous bathymetry, dense water cascading off the steep shelf break, slow exchanges in canyons, along with known biases in sea ice and neighbouring seas.

We investigate the Arctic Ocean in the historical run of 14 distinct models that participated to the latest Climate Model Intercomparison Project phase 6 (CMIP6) and find large biases in temperature, salinity, density, and depth of critical water masses, both on the shelves and in the deep basins. The biases are consistent throughout the water column and throughout the Arctic, with correlations often exceeding 0.9. However, no significant trend is observed in these biases, suggesting that the deep basins of the Arctic are not correctly ventilated already at the level of the Atlantic Water.

Using the subset of models that submitted the age of water output, we confirm this absence of ventilation by dense water overflows: the overflows occur at too few locations and are diluted at shallow depths.   

Work is ongoing to relate these biases to the relevant processes, the upper water column, and fluxes through the various Arctic Ocean gateways.

How to cite: Heuzé, C., Zanowski, H., Karam, S., and Muilwijk, M.: Large biases in hydrography and circulation of the Arctic Ocean in CMIP6 models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1760, https://doi.org/10.5194/egusphere-egu22-1760, 2022.

The Arctic Ocean is of central importance for the global climate and ecosystems. It is undergoing rapid climate change, with a dramatic decrease in sea ice cover over recent decades. Surface advective pathways connect the transport of nutrients, freshwater, carbon and contaminants with their sources and sinks. Pathways of drifting material are deformed under velocity strain, due to atmosphere-ocean-ice coupling. Deformation is largest at fine space- and time-scales and is associated with a loss of potential predictability, analogous to weather often becoming unpredictable as synoptic-scale eddies interact and deform. However, neither satellite observations nor climate model projections resolve fine-scale ocean velocity structure. Here, we use a high-resolution ocean model hindcast and coarser satellite-derived ice velocities, to show: that ensemble-mean pathways within the Transpolar Drift during 2004–14 have large interannual variability and that both saddle-like flow structures and the presence of fine-scale velocity gradients are important for basin-wide connectivity and crossing time, pathway bifurcation, and also for predictability and dispersion (the latter are covered in an associated paper [1].

The saddle-points in the flow and their neighbouring streamlines define flow separatrices, which partition the surface Arctic into separate regions of connected transport properties. The separatrix streamlines vary interannually and identify periods when the East Siberian Arctic Shelf, an important source of terragenic minerals, carbon and nutrients, is either connected or disconnected with Fram Strait and the North Atlantic. We explore the implications of this transport connectivity, with our new metric - the Separatrix Curvature Index – which in this context is arguably more informative than either the Arctic Oscillation or Arctic Ocean Oscillation indices.

This work resulted from the Advective Pathways of nutrients and key Ecological sub- stances in the Arctic (APEAR) project (NE/R012865/1, NE/R012865/2, #03V01461), part of the Changing Arctic Ocean programme, jointly funded by the UKRI Natural Environment Research Council (NERC) and the German Federal Ministry of Education and Research (BMBF). This work has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement no. 820989 (project COMFORT). The work reflects only the authors' view; the European Commission and their executive agency are not responsible for any use that may be made of the information the work contains. This work also used the ARCHER UK National Supercomputing Service and JASMIN, the UK collaborative data analysis facility. Satellitebased sea ice tracking was carried out as part of the Russian-German Research Cooperation QUARCCS funded by the German Ministry for Education and Research (BMBF) under grant 03F0777A. This study was carried out as part of the international Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) with the tag MOSAiC20192020 (AWI_PS122_1 and AF-MOSAiC-1_00) and the NERC Project “PRE-MELT” (Grant NE/T000546/1). We also acknowledge funding support received from the NERC National Capability programmes LTS-M ACSIS (North Atlantic climate system integrated study, grant NE/N018044/1) and LTS-S CLASS (Climate–Linked Atlantic Sector Science, grant NE/R015953/1). The authors would like to acknowledge the contribution of Maria Luneva to the discussions about the initial idea of the study and for highlighting the historical importance of observations from the Russian North Pole drifting stations. Sadly, Maria passed away suddenly in 2020 before the draft of the reported paper was written.

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

How to cite: Aksenov, Y., Wilson, C., Rynders, S., Kelly, S., Krumpen, T., and Coward, A. C.: Variability of surface transport pathways and how they affect Arctic basin-wide connectivity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1782, https://doi.org/10.5194/egusphere-egu22-1782, 2022.

The dynamics of the Arctic Ocean are changing significantly with increasing global greenhouse gas emissions. Under the current warming scenario, the thinning of sea ice could affect Arctic thermohaline dynamics for the foreseeable future, which would affect the development of the energy cascade. Here, we analyze in situ Lagrangian measurements of the wintertime upper-ocean thermohaline field that were taken during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. Horizontal wavenumber spectra of density are examined from 13 approximately 100-km long transects from October 2019 – May 2020 to determine the steepness of spectra for different spatial scales. Unlike the relatively well-defined frequency spectra, horizontal wavenumber spectra yield variable patterns depending on the region of observations. This issue motivates us to investigate the current state of horizontal wavenumber spectra in the multiyear ice zone of the central Arctic. Our preliminary results show that the wavenumber spectra are not consistent in space and time, implying an interplay of stratification, mixed layer depth, and external forcing, such as ice dynamics.

How to cite: Wu, W.-C., Fang, Y.-C., and Rabe, B.: Variability of the Upper Ocean Energy Field in the Amundsen Basin, Arctic Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2125, https://doi.org/10.5194/egusphere-egu22-2125, 2022.

Mesoscale eddies are important for many aspects of the dynamics of the Arctic Ocean. These include the maintenance of the halocline and the Atlantic Water boundary current through lateral eddy fluxes, shelf-basin exchanges, transport of biological material and sea ice, and the modification of the sea-ice distribution. Here we review what is known about the mesoscale variability and its impacts in the Arctic Ocean in the context of an Arctic Ocean responding rapidly to climate change. In addition, we present the first quantification of eddy kinetic energy (EKE) from moored observations across the entire Arctic Ocean, which we compare to output from an eddy resolving numerical model. We show that EKE is largest in the northern Nordic Seas/Fram Strait and it is also elevated along the shelfbreak of the Arctic Circumpolar Boundary Current, especially in the Beaufort Sea. In the central basins it is 100-1000 times lower. Except for the region affected by southward sea-ice export south of Fram Strait, EKE is stronger when sea-ice concentration is low compared to dense ice cover. Areas where conditions typical in the Atlantic and Pacific prevail will increase. Hence, we conclude that the future Arctic Ocean will feature more energetic mesoscale variability.

How to cite: von Appen, W.-J., Baumann, T., Janout, M., Koldunov, N., Lenn, Y.-D., Pickart, R., Scott, R., and Wang, Q.: Eddies and the distribution of eddy kinetic energy in the Arctic Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2274, https://doi.org/10.5194/egusphere-egu22-2274, 2022.

The average rate of coastal change in the Arctic Ocean is -0.5 m/yr, despite significant local and regional variations, with large areas well above -3 m/yr. Recent data suggest an acceleration of coastal retreat in specific areas due to an increasingly shorter sea ice season, higher storminess, warmer ocean waters and sea-level rise. Moreover, climate warming is inducing the subaerial degradation of permafrost and increasing land to sea sediment transportation. This work consists of the characterization and analysis of the main controlling factors influencing recent coastline change in the Tuktoyaktuk Peninsula, Northwest Territories, Canada. The specific objectives are I. mapping Tuktoyaktuk Peninsula’s coastline at different time-steps using remote sensing imagery, II. quantifying the recent coastal change rates, III., characterizing the coastal morphology, IV. identifying the main controlling factors of the coastal change rates. A very high-resolution Pleiades survey from 2020, aerial photos from 1985 and the ArcticDEM were used. Results have shown an average coastline change rate of -1.06 m/yr between 1985 and 2020. While this number is higher than the Arctic average rate, it neglects to show the significance of extreme cases occurring in specific areas. Tundra cliffs are the main coastal setting, occupying c. 56% of the Tuktoyaktuk Peninsula coast and foreshore beaches represent 51%. The results display an influence of coastal geomorphology on change rates. The coastal retreat was higher in backshore tundra flats (-1.74 m/yr), whereas more aggradation cases exist in barrier beaches and sandspits (-0.81 m/yr). The presence of ice-wedge polygons contributes to increasing cliff retreat. Foreshore assessment may be crucial, as beaches present a hindering impact on coastal retreat (-0.76 m/yr), whereas foreshore tundra flats promote it (-1.74 m/yr). There are 48 areas with retreat rates higher than -4 m/yr, most being submersion cases.

How to cite: Costa, B., Vieira, G., and Whalen, D.: The fast-changing coast of Tuktoyaktuk Peninsula (Beaufort Sea, Canada): geomorphological controls on changes between 1985 and 2020, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2426, https://doi.org/10.5194/egusphere-egu22-2426, 2022.

The marginal ice zone in Fram Strait is a highly variable environment, in which dense Atlantic Water and lighter Polar Water meet and create numerous mesoscale and submesoscale fronts. This makes it a model region for researching ocean frontal dynamics in the Arctic, as the interaction between Atlantic Water and the marginal ice zone is becoming increasingly important in an "atlantifying" Arctic Ocean. Here we present the first results of a front study conducted near the ice edge in central Fram Strait, where Atlantic Water subducted below Polar Water. We posit that the frontal dynamics associated with the sea ice edge also apply beyond, both to the open and the ice-covered ocean in the vicinity. They, in turn, can affect the structure of the marginal ice zone. The study comprises a total of 54 high resolution transects, most of which were oriented across the front. They were taken over the course of a week during July 2020 and include current velocity measurements from a vessel-mounted ADCP. Most of the transects also include either temperature and salinity measurements from an underway CTD, or temperature and salinity measurements and various biogeochemical properties from a TRIAXUS towed vehicle. Additionally, 22 CTD stations were conducted, and 31 surface drifters were deployed. This wealth of measurements gives us the opportunity to follow the temporal and spatial development of the density fronts present at the time. We discuss the dynamics of the frontal development, including the associated geostrophic motion, and the induced secondary ageostrophic circulation with subsequent subduction of Atlantic Water and biological material in a highly stratified region. Beneath the stratified upper ocean, subduction is clearly visible in the biogeochemical properties, and water samples indicate a substantial vertical transport of smaller particles. Surface drifters accumulated in locations of subduction, where sea ice, if present, would likely also accumulate. Our study thus demonstrates the importance of frontal dynamics for the vertical transport of water properties and biological material, and the highly variable development of the marginal ice zone in Fram Strait.

How to cite: Hofmann, Z., von Appen, W.-J., Iversen, M., and Hufnagel, L.: Subduction as Observed at a Submesoscale Front in the Marginal Ice Zone in Fram Strait, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2717, https://doi.org/10.5194/egusphere-egu22-2717, 2022.

The Atlantic Water inflow to the Arctic Ocean is transformed and modified in the ocean areas north of Svalbard, and influences the Arctic Ocean heat and salt budget. As the Atlantic Water layer advances into the Arctic, its core deepens from about 250 m depth around the Yermak Plateau to 350 m in the Laptev Sea, and gets colder and less saline due to mixing with surrounding waters. The complex topography in the region facilitates vertical and horizontal exchanges between the water masses and, together with strong shear and tidal forcing driving increased mixing rates, impacts the heat and salt content of the Atlantic Water layer that will circulate around the Arctic Ocean.

In September 2018, 6 moorings organized in 2 arrays were deployed across the Atlantic Water Boundary current for more than one year (until November 2019), within the framework of the Nansen Legacy project to investigate the seasonal variations of this current and the transformation of the Atlantic Water North of Svalbard. The Atlantic Water inflow exhibits a large seasonal signal, with maxima in core temperature and along-isobath velocities in fall and minima in spring. Volume transport of the Atlantic Water inflow varies from 0.7 Sv in spring to 3 Sv in fall. An empirical orthogonal function analysis of the daily cross-isobath temperature sections reveals that the first mode of variation (explained variance ~80%) is the seasonal cycle with an on/off mode in the temperature core. This first mode of variation is linked to the first mode of variation of the current. The second mode (explained variance ~ 15%) corresponds to a shorter time scale (6-7 days) variability in the onshore/offshore displacement of the temperature core linked to the mesoscale variability. On the shelf, a counter-current flowing westward is observed in spring, which transports colder (~ 1°C) and fresher (~ 34.85 g kg^-1) water than Atlantic Water (θ > 2°C and S_A > 34.9 g kg^-1). This counter-current is driven by Ekman dynamics. At greater depth (~1000 m) on the offshore part of the slope, a bottom-intensified current is detected, partly correlated with the wind stress curl. Heat loss of the Atlantic Water between the two mooring arrays is maximum in winter, estimated to 300-400 W m^-2 when the current speed and the heat loss to the atmosphere are the largest.

How to cite: Koenig, Z., Kalhagen, K., Kolås, E., Fer, I., Nilsen, F., and Cottier, F.: Atlantic Water properties, transport, and water mass transformation from mooring observations north of Svalbard, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3069, https://doi.org/10.5194/egusphere-egu22-3069, 2022.

SODA (Simple Ocean Data Assimilation) is one of the ocean reanalysis data widely used in oceanographic research. The SODA3 dataset provides multiple ocean reanalysis data sets driven by different atmospheric forcing fields. The differences between their arctic sea ice simulations are assessed and compared with observational data from different sources. We find that in the simulation of arctic sea ice concentration, the differences between SODA3 reanalysis data sets driven by different forcing fields are small, showing a low concentration of thick ice and a high concentration of thin ice. In terms of sea ice extent, different forced field model data can well simulate the decline trend of observed data, but the overall arctic sea ice extent is overestimated, which is related to more simulated sea ice in the sea ice margin. In terms of the simulation of arctic sea ice thickness, the results of different forcing fields show that the simulation of arctic sea ice thickness by SODA data set is relatively thin on the whole, especially in the thick ice region. The results of different models differ greatly in the Beaufort Sea, the Fram Strait, and the Central Arctic Sea. The above differences may be related to the differences between the model-driven field and the actual wind field, which leads to the inaccurate simulation of arctic sea ice transport and ultimately to the different thickness distribution simulation. In addition, differences in heat flux may also lead to differences in arctic sea ice between models and observations. In this paper, the differences between the results of arctic sea ice driven by different SODA3 forcing fields are studied, which provides a reference for the use of SODA3 data in the study of arctic sea ice and guidance for the selection of SODA data in the study of sea ice in different arctic seas.

How to cite: Ge, Z., Wang, X., and Wang, X.: Differences in Arctic sea ice simulations from various SODA3 data sets, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3289, https://doi.org/10.5194/egusphere-egu22-3289, 2022.

Mixing along the pathway of Atlantic Water in the Arctic Ocean is crucial for the distribution of heat in the Arctic Ocean. The warm boundary current typically flows along the upper continental slope where energy conversion from tides to turbulence and tidally driven mixing can be important; however, observations -and thus understanding- of these spatiotemporally highly variable processes are limited.

Here we analyze yearlong observations from three moorings (W1, W2 and W3) spanning the continental slope North of Svalbard at 18.5°E over 16 km from 400 m to 1200 m isobaths, deployed between September 2018 and October 2019. Full-depth current records show strong barotropic diurnal (i.e., sub-inertial) tidal currents, dominated by the K₁ constituent. These tidal currents are strongest at mooring W2 over the continental slope (~700 m isobath) likely due to topographic trapping far north of their critical latitude (30°N). The diurnal tide undergoes a seasonal cycle with amplitudes reaching minima of ~4 cm/s in March/April and maxima of ~11 cm/s in June/July. Associated with the diurnal tide peak at W2 in summer 2019 is a strong baroclinic semidiurnal signal up to 15 cm/s around 4.5 km further offshore at W3 between 500 m and 1000 m depth. This semidiurnal current signal exhibits a fortnightly modulation and is characterized by upward energy propagation, indicative of generation at the bottom rather than the surface.

We hypothesize that the semidiurnal baroclinic waves are generated by the barotropic diurnal tide about 15 km upstream. There, the slope is oriented approximately normal to the major axis of the tidal current ellipses, maximizing the cross-isobath flow and thus the tidal energy conversion potential. The topographic slope angle approaches criticality for frequencies close to the second harmonic of K₁ (2K₁, with a semidiurnal period of 11.965 h) around the 620 m isobath and may thus facilitate an efficient generation of second harmonic internal waves. Linear superposition of a 2K₁ wave with the rather weak (~5 cm/s) ambient M₂ tide would explain the observed fortnightly modulation. The super-inertial wave (w_2K1>f) propagates freely and its pathway is presently not known.

Although further research on the generation mechanism is needed, the strong baroclinic semidiurnal currents observed at the continental slope have direct implications for deep mixing. Furthermore, energetic diurnal tidal currents impinging on a steep continental slope are also known to generate non-linear internal lee-waves that can also lead to substantial turbulence and consequent mixing.

How to cite: Baumann, T. M. and Fer, I.: Vigorous Internal Wave Generation at the Continental Slope North of Svalbard, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3494, https://doi.org/10.5194/egusphere-egu22-3494, 2022.

The recent retreat of Arctic sea ice area is overlaid by strong internal variability on all timescales. In winter, the variability is currently dominated by the Barents Sea, where it has been primarily driven by variable ocean heat transport from the Atlantic. As the loss of winter Arctic sea ice is projected to accelerate and the sea ice edge retreats deeper into the Arctic Ocean, other regions will see increased sea-ice variability. The question thus arises how the influence of the ocean heat transport will change. To answer this question, we analyze and contrast the present and future regional impact of ocean heat transport on the winter Arctic sea ice cover using a combination of observations and simulations from several single model large ensembles from CMIP5 and CMIP6. For the recent past we find a strong influence of the heat transport through the Barents Sea and the Bering Strait on the sea ice cover on the Pacific and Atlantic side of the Arctic Ocean, respectively. There is strong model agreement for an expanding influence of ocean heat transport through these two gateways for high and low warming scenarios. This highlights the future importance of the Pacific and Atlantic water inflows.

How to cite: Dörr, J., Årthun, M., and Eldevik, T.: Present and future influence of ocean heat transport on winter Arctic sea-ice variability, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3595, https://doi.org/10.5194/egusphere-egu22-3595, 2022.

The presence of liquid or ice as cloud phase determines the climate radiative effect of Arctic clouds, and thus, their contribution to surface warming. Biogenic aerosols from phytoplankton production localized in leads or open water were shown to act as cloud condensation nuclei (liquid phase) or ice nuclei (ice phase) in remote regions. As extensive measurements of biogenic aerosol precursors are still scarce, we conduct a modelling study and use acidic polysaccharides (PCHO) and transparent exopolymer particles (TEP) as tracers. In this study, we integrate processes of algal PCHO excretion during phytoplankton growth or under nutrient limitation and processes of TEP formation, aggregation and also remineralization into the ecosystem model REcoM2. The biogeochemical processes are described by two functional phytoplankton and two zooplankton classes, along with sinking detritus and several (in)organic carbon and nutrient classes. REcoM2 is coupled to the finite-volume sea ice ocean circulation model FESOM2 with a high resolution of up to 4.5 km in the Arctic. We will present the first results of simulated TEP distribution and seasonality patterns at pan-Arctic scale over the last decades. We will elucidate drivers of the seasonal cycle and will identify regional hotspots of TEP production and its decay. We will also address possible impacts of global warming and Arctic amplification of the last decades in our evaluation, as we expect a strong effect of global warming on microbial metabolic rates, phytoplankton growth, and composition of phytoplankton functional types. The results will be evaluated by comparison to a set of in-situ measurements (PASCAL, FRAM, MOSAiC). It is further planned that an atmospheric aerosol-climate model will build on the modeled biogenic aerosol precursors as input to quantify the net aerosol radiative effects. This work is part of the DFG TR 172 Arctic Amplification.

How to cite: Zeising, M., Oziel, L., Gürses, Ö., Hauck, J., Heinold, B., Losa, S., Thoms, S., and Bracher, A.: High-resolution modelling of marine biogenic aerosol precursors in the Arctic realm, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3652, https://doi.org/10.5194/egusphere-egu22-3652, 2022.

In this work we investigate the intensity of eddy generation and their properties in the marginal ice zone (MIZ) of Fram Strait and around Svalbard using spaceborne synthetic aperture radar (SAR) data from Envisat ASAR and Sentinel-1 in winter 2007 and 2018. Analysis of 2039 SAR images allowed identifying 4619 eddy signatures in the MIZ. While the overall length and the area of MIZ are different in 2007 and 2018, the number of eddies detected per image per kilometer of MIZ length is similar for both years.
Eddy diameters range from 1 to 68 km with mean values of 6 km and 12 km over shallow and deep water, respectively, suggesting that submesoscale and small mesoscale eddies prevail in the record. At eddy diameter scales of 1-15 km, cyclones strongly dominate over anticyclones. However, in the range of 15-30 km this difference is gradually vanishing, and for diameter values above 30 km anticyclones start to dominate slightly.
Mean eddy size grows with increasing ice concentration in the MIZ, yet most eddies are detected at the ice edge and where the ice concentration is below 20%. The fraction of sea ice trapped in cyclones (53%) is slightly higher than that in anticyclones (48%). The amount of sea ice trapped by a single ‘mean’ eddy is about 40 km². Here we also attempt to give a first-order estimate of the eddy-induced horizontal sea ice retreat using observed values of eddy radii and amount of sea ice trapped in the eddies, and empirical mean values of ice bottom ablation and ice thickness. The obtained average horizontal ice retreat is about 0.2-0.5 km·d^–1 ± 0.02 km·d^–1. The spatial patterns of the eddy-induced horizontal sea ice retreat derived from SAR data suggest a pronounced decrease in MIZ area and a shift in the edge location that agrees with the observations.
The analysis of the spatial correlation between eddies, currents and winds shows that the intensity of eddy generation/observations and their detectability in the MIZ, and the width of eddy bands correlate with the intensity of northern and northeasterly winds. In some regions, e.g. along the Greenland Sea shelf break, in Fram Strait and over the Spitsbergen Bank the probability values of eddy occurrence in the MIZ seem to correlate with stronger boundary currents, while north of Svalbard and over Yermak Plateau higher eddy probability values are observed under low/moderate currents and winds.
This study was supported by the Russian Science Foundation grant # 21-17-00278 (analysis of sea ice conditions, ice trapping and melting by eddies) and by the Ministry of Science and Higher Education of the Russian Federation state assignment # 075-00429-21-03 (data acquisition & processing).

How to cite: Kozlov, I., Atadzhanova, O., and Pryakhin, S.: Eddies in the marginal ice zone of Fram Strait and Svalbard from spaceborne SAR observations in winter, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3711, https://doi.org/10.5194/egusphere-egu22-3711, 2022.

Mesoscale eddies are believed to play a substantial role for the dynamics of the Arctic Ocean, influencing the interaction of the ocean with the atmosphere and sea-ice as well as the transport and mixing of water masses. Especially their effects on the thermohaline structure and stratification could be crucial for better understanding future changes in the Arctic and the ongoing ‘atlantification’ of the Arctic Ocean water masses. Better understanding of Arctic eddy dynamics also allows the improvement of parametrization of eddy processes in models, which is critical for a realistic representation of the Arctic in climate models and understanding the role of the Arctic Ocean in the global climate. However, simulating Arctic Ocean mesoscale eddies in ocean circulation models presents a great challenge due to their small size at high latitudes and adequately resolving mesoscale processes in the Arctic requires very high resolution, making simulations very computationally expensive.
Here, we use the new unstructured‐mesh Finite volumE Sea ice-Ocean Model (FESOM2) with 1-km horizontal resolution in the Arctic Ocean to evaluate properties of mesoscale eddies. This very high-resolution model setup can be considered eddy resolving in the Arctic Ocean and has recently been used to investigate the distribution of eddy kinetic energy in the Arctic. The analysis here is based on automatically identifying and tracking eddies using a vector geometry-based algorithm and focuses on the model’s representation of eddy properties and dynamics. In-situ observations from the year-long MOSAiC expedition give us the unique possibility to assess the model’s representation of eddy properties against direct observations, both in the Arctic summer and winter seasons.

How to cite: Müller, V. and Wang, Q.: Properties of mesoscale eddies in the Arctic Icean from a very high-resolution model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4360, https://doi.org/10.5194/egusphere-egu22-4360, 2022.

The Arctic Ocean is strongly stratified by salinity gradients in the uppermost layers. This stratification is a key attribute of the region as it acts as an effective barrier for the vertical exchanges of Atlantic Water (AW) heat, nutrients, and CO2 between  intermediate depths and the surface of the deep Eurasian and Amerasian Basins (EB and AB). Observations show that from 1970 to 2017, the stratification in the AB has strengthened, whereas, in parts of the EB, the stratification has weakened. The strengthening of the stratification in the AB is linked to a freshening and deepening of the halocline. The weakened stratification in parts of the EB is linked to a shoaling, warming, and lack of freshening of the halocline (Atlantification). Future simulations from a suite of CMIP6 models project that under a strong greenhouse-gas forcing scenario (SSP585), the AB and EB surface freshening and AW warming continues. To meaningfully compare hydrographic changes in the simulations, we present a new indicator of stratification. We find that within the AB, there is agreement among the models that the upper layers will become more stratified in the future. However, within the EB models  diverge regarding future stratification. We discuss and detail some mechanisms responsible for these simulated discrepancies.

How to cite: Muilwijk, M., Smedsrud, L. H., Polyakov, I. V., Nummelin, A., Heuzé, C., and Zanowski, H.: Divergence in CMIP6 projections of future Arctic Ocean stratification, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5299, https://doi.org/10.5194/egusphere-egu22-5299, 2022.

Atlantic Water is the most significant source of oceanic heat in the Arctic Ocean, isolated from the surface by a strong halocline across much of the region. However, an increase in Atlantic Water temperatures and a decrease in eastern Arctic stratification are thought to have contributed to Arctic sea-ice loss in recent decades. Investigating how Atlantic Water heat is likely to change and affect the upper ocean during the coming decades is therefore an important part of understanding the future Arctic. In this study, data from the Community Earth System Model (CESM) large ensemble are used to investigate forced trends and natural variability in the Atlantic Water layer properties and heat fluxes over the period 1920-2100, under an RCP 8.5 scenario from 2006.

How to cite: Richards, A., Johnson, H., and Lique, C.: Studying Atlantic Water heat in the Arctic Ocean using the CESM Large Ensemble, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5601, https://doi.org/10.5194/egusphere-egu22-5601, 2022.

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

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

Submesoscale features with profound impact on ocean dynamics and climate-relevant fluxes are frequently observed in the upper ocean including Arctic region. Yet, modelling these features remains a challenge due to the difficulties in the parameterization of submesoscale processes and high resolution required, in particular, in the polar regions. The most effective way to study such phenomena is joint modelling and observational work. Several autonomous observation platforms have been deployed as part of Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) experiment within an approximately 50 km radius around the central observatory. Data from these buoys in combination with data from the central observatory provide a unique opportunity to reconstruct 3D water properties and velocity by constraining a numerical model that resolves the dynamics of the (sub-)mesoscale. It turns out that a minimum root mean square error between results of an optimal interpolation and observations indicates a characteristic length scale of about 7.5 km, corresponding approximately the first-mode barolinic Rossby radius in the area of investigation. However, results of the interpolation are questionable at the sub-mesoscale due to the distribution of the buoy observations in time and horizontal space. In order to describe the in-situ data to achieve a better characterization and understanding of (sub-)mesoscale dynamics we developed and applied a modification of the 3D regional model FESOM-C. The observed temperature and salinity were used to nudge the model to obtain an optimized solution at the resolution of the models. A series of simulations with different horizontal resolutions and model parameters make it possible to analyze the ability of models of this type to reproduce the observed dynamics, to estimate eddy kinetic energy and power spectra, and to compare findings with the observations used to nudge the model. We will show the eddy-induced fluxes and characteristics of eddies along the track of the beginning winter MOSAiC drift.

How to cite: Kuznetsov, I., Rabe, B., Fang, Y.-C., Androsov, A., Zurita, A. Q., Hoppmann, M., Mohrholz, V., Tippenhauer, S., Schulz, K., Fofonova, V., Janout, M., Fer, I., Baumann, T., Liu, H., and Mallet, M. P.: Submesoscale dynamics in the central Arctic Ocean during MOSAiC: optimising the use of observations and high-resolution modelling., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6164, https://doi.org/10.5194/egusphere-egu22-6164, 2022.

Climate change is affecting all the Svalbard fjords, which are more or less subject to global warming.  In situ observations in the Hornsund fjord indicate that more and more warm Atlantic water is reaching the fjord as well, and this may influence the rate of melting of sea ice and glaciers, which is likely to increase.  

More freshwater enters the fjord in several different ways. Melting glaciers bring freshwater in the form of surface inflows from freshwater sources, in the form of submarine meltwater at the interface between ocean and ice, and in the form of calving icebergs.  Retreating glaciers and melting sea ice allow the warm Atlantic waters to reach increasingly inland fjord basins and more heat stored in the fjords causes increased melting of the inner fjord glaciers.  The increasing amounts of freshwater in the fjord can change the local ecosystem.

Estimates of the heat and the salt fluxes will give a better understanding of how the ocean interacts with the glaciers through submarine melting and vice versa, how glaciers interact with the ocean through freshwater supply.  Budgetary conditions will be calculated from the high resolution model results (HRM) of velocity, temperature and salinity for the interior of the Hornsund fjord.

Calculations were carried out at the Academic Computer Centre in Gdańsk

How to cite: Przyborska, A., Strzelewicz, A., Muzyka, M., and Jakacki, J.: Heat and salt budgets in the Hornsund fjord, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6176, https://doi.org/10.5194/egusphere-egu22-6176, 2022.

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

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

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

The unprecedented warming in the Arctic opens broad prospects for connecting the Northern Sea Route (NSR) to the Maritime Silk Road. Such a "docking" will significantly impact the global economy. The main problems of the Northern Sea Route are the harsh environmental conditions of the North and, most importantly, the presence of sea ice. While, on average, the ice-free period lasts from June to November, the dates of start and end of ice season vary from year to year within a month or even more. Such variability is impossible to capture by numerical weather prediction, limiting predictability for five days. Therefore, currently, there is no specific timeframe when the waterway is free of ice.

Here I show that a long-range forecast for the navigation season is possible for specific locations in Bering and Okhotsk Seas. The approach is fundamentally different from the numerical weather and climate models; it is based on statistical physics principles and recently discovered spatial-temporal regularities in the Asian-Pacific monsoon system [1]. The regularities appear in the form of spatially organized critical transitions in the near-surface atmosphere over the see. The specific locations mean critical areas - tipping elements of the spatial-temporal structure of ice formation, which are identified via data analysis. I rely on the distribution of near-surface air temperature and wind data (NCEP/NCAR re-analyses data set) to reveal conditions for ice formation [2]. I show that a transition from open water to ice season begins when the near-surface air temperature crosses a critical threshold, it is a starting point for forecasting the ice season's start date. The approach provides long-term predictions of the ice season's start in critical areas 30 days in advance.

Furthermore, the transition from water to ice in the Bering and Okhotsk Seas is driven by the Asian-Pacific monsoon air movements. It has the following implications. First, there is a linkage between the onset of ice formation in the northern part of the Bering Sea and the western part of the Sea of Okhotsk. Second, Asian Monsoon, including the Indian monsoon [3], is driven by the same Asian-Pacific system [4]. As a result, the timing of the monsoon is linked with the ice season. These findings show that it is essential to consider these connections to overcome regional forecast limitations. The system approach applied on a continental scale will be relevant for improving the long-term monsoon and ice season forecasts, which we desperately need for climate adaptation.

How to cite: Surovyatkina, E.: Long-Range Forecast for the Navigation Season: linking the Northern Sea Route and Maritime Silk Road, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6572, https://doi.org/10.5194/egusphere-egu22-6572, 2022.

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

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

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

The Barents Sea is one of the main pathways by which Atlantic Water (AW) enters the Arctic Ocean and is an important region for key water mass transformation and production. As AW enters the shallow (< 400 m) Barents Sea, it propagates as a topographically steered current along a series of shallow troughs and ridges, while being transformed through atmospheric heat fluxes and exchanges with surrounding water masses. To the north, the warm and salty AW is separated from the cold and fresh Polar Water (PW) by a distinct dynamic thermohaline front (the Barents Sea Polar Front), often less than 15 km in width.

Two cruises were conducted in October 2020 and February 2021 within the Nansen Legacy project, focusing on the AW pathways and ocean mixing processes in the Barents Sea. Here we present data from CTD (Conductivity, Temperature, Depth), ADCP (Acoustic Doppler Current Profiler) and microstructure sensors obtained during seven ship transects and two repeated stations across and on top of a 200 m deep sill (77°18’N, 30°E) at the location of the Polar Front between AW and PW. The ship transects are complemented by five underwater glider missions, two equipped with microstructure sensors. On the sill, we observe warm (>2°C) and salty (>34.8) AW intruding below the colder (<0°C) and fresher (34.4) PW setting up a geostrophic balance where currents exceed 20 cm/s. We observe anomalous warm and cold-water patches on the cold and warm side of the front, respectively, collocated with enhanced turbulence, where dissipation rates range between 10^-8 and 10^-7 W/kg. In addition, tidal currents on the sill reach 15 cm/s. The variable currents affect the front structure differently in the vertical. While the mid-depth location of the front is shifted by several kilometers, the location of the front near the bottom remains stationary.  The frontal dynamics on the sill result in transformation and mixing of AW, manifested in the troughs north of the sill as modified AW.

How to cite: Hugaas Kolås, E., Baumann, T., Fer, I., and Koenig, Z.: Barents Sea Polar Front dynamics during fall and winter 2020-2021, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6934, https://doi.org/10.5194/egusphere-egu22-6934, 2022.

The Sea Ice Drift Forecast Experiment (SIDFEx) database comprises more than 180,000 forecasts for trajectories of single sea-ice buoys in the Arctic and Antarctic, collected since 2017. SIDFEx is a community effort originating from the Year Of Polar Prediction. Forecasts are provided by various forecast centres and collected, and archived by the Alfred Wegener Institute (AWI). AWI provides a dedicated software package and an interactive online platform for analysing the forecasts. Their lead times range from daily to seasonal scales. Among the buoys targeted by SIDFEx are the buoys of the Distributed Network (DN) array which was deployed during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. In this contribution, we show to what extent the deformation (divergence, shear and vorticity) of the DN can be forecasted by the SIDFEx forecasts. We investigate the performance of single models as well as a consensus forecast which merges the single forecasts to a seamless best-guess forecast. 

How to cite: Ludwig, V. and Goessling, H. and the SIDFEx Team: Sea-ice deformation forecasts for the MOSAiC Arctic drift campaign in the SIDFEx database, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7237, https://doi.org/10.5194/egusphere-egu22-7237, 2022.

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

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

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

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

One of the major branches of the warm and saline Atlantic Water supply is the current along the west coast of Spitsbergen in Fram Strait. The Yermak Plateau is a topographic obstacle in the path of this current. The diverging isobaths of the Plateau split the current, with an outer branch following the 1000-1500 m isobaths along the rim of the Yermak Plateau (the Yermak branch). Observation based estimates of the volume transport, structure and variability of the Yermak branch are scarce.

Here we present observations from an array of three moorings on the southern flank of the Yermak Plateau, covering the AW boundary current along the slope, between the 800 m to 1600 m isobaths over 40 km distance, from 11 September 2014 to 13 August 2015. The aim is to estimate the volume transport in temperature classes to quantify the contribution of the Yermak branch, to document the observed mesoscale variability, and identify the role of barotropic and baroclinic instabilities on the variability.

All three moorings show depth- and time-averaged currents directed along isobaths, with the middle mooring in the core of the boundary current. Depth-averaged current speeds in the core, averaged over monthly time scale, reach 20 cm s^-1 in March. Temperatures are always greater than 0°C in the upper 800 m, or than 2°C in the upper 500 m. Seasonal averaged volume transport estimates of Atlantic Water defined as temperature above 2°C, are maximum in autumn (1.4 ± 0.2 Sv) and decrease to 0.8 ± 0.1 Sv in summer. The annual average AW transport is 1.1 ± 0.2 Sv, below which there is bottom-intensified current, particularly strong in winter, leading to a substantial transport of cold water (<0°C) with an annual average of 1.1 ± 0.2 Sv.

Mesoscale variability and energy conversion rates are estimated using fluctuations of velocity and stratification in the 35 h to 14-days band and averaging over a monthly time scale.  Time-averaged profiles of horizontal kinetic energy (HKE) show a near-surface maximum in the outer and middle (core) moorings decreasing to negligible values below 700 m depth. HKE averaged between 100-500 m depth increases from about 3×10^-3 m² s^-2 in fall to (6-9)×10^-3 m² s^-2 in winter and early spring.  Temperature and cross-isobath velocity covariances show substantial mid-depth temperature fluxes in winter. Divergence of temperature flux between the core and outer moorings suggests that heat is extracted by eddies. Depth-averaged energy conversion rates show typically small barotropic conversion, not significantly different from zero, and highly variable baroclinic conversion rates with alternating sign at 1-2 month time scales. Observations suggest that the boundary current is characterized by baroclinic instabilities, which particularly dominate in winter months. 

How to cite: Fer, I. and Peterson, A. K.: Atlantic Water boundary current along the southern Yermak Plateau, Arctic Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8055, https://doi.org/10.5194/egusphere-egu22-8055, 2022.

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

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

The North Water Polynya (NOW) in northern Baffin Bay contains nutrient-rich waters which are essential to the biodiversity of the region and the native Inuit people. Over the observational period the size and duration of the NOW in spring has varied considerably, and recent studies suggest the NOW may fail to form in the future. Even small changes to the polynya have the potential to impact local ocean circulation and nutrient cycling. 

To assess the projected changes to the NOW, we look at CMIP5 large ensembles under multiple forcing scenarios. Initial results from CESM1 LE suggest that global temperatures greater than 2.5ºC above pre-industrial levels shift the peak polynya area from June to May. Work is ongoing to assess biogenic and physical impacts of such changes. Implications for climate change are that to avoid large changes to the NOW, warming should be limited.

Additionally, the Polynya area fluctuates with time but decreases as a whole throughout the 21st century.

How to cite: Patel, R., Ugrinow, P., Jahn, A., and Wyburn-Powell, C.: North Water Polynya Sensitivity to Arctic Warming, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8941, https://doi.org/10.5194/egusphere-egu22-8941, 2022.

The paucity of observations over the Arctic Ocean prevents us from fully understanding the interaction between sea ice and mesoscale dynamics. Previous studies on this interplay have documented the interaction between surface eddies and sea ice, omitting the subsurface eddies. This work focuses on the possible role of these subsurface eddies in shaping the sea ice distribution. First, we perform an extensive eddy census over the period 2004-2020 over the Arctic Basin, based on data from Ice Tethered Profilers (ITP) and moorings from the Beaufort Gyre Exploration Project. About 500 subsurface eddies are detected, including both submesoscale (radius between 2-10 km) and mesoscale (up to 80 km) structures. Second, we investigate the dynamical or thermodynamical signature that these eddies may imprint at the surface. On average, these eddies do not cause significant variations in either the temperature of the mixed layer or the melting of sea ice. However, we estimate that subsurface eddies induce a dynamic height anomaly of the order of a few centimetres, leading to a surface vorticity anomaly of O(10^{-5} - 10^{-4}) s^{-1}, suggesting that they may be a significant local forcing for the sea ice momentum balance. Our results suggest that there is no link between the sea ice evolution and the energy level associated with the presence of subsurface eddies. It suggests that once formed, these structures may evolve at depth independently of the presence of sea ice. 

How to cite: Cassianides, A., Lique, C., Treguier, A. M., Meneghello, G., and Demarez, C.: Interplay between subsurface eddies and sea ice over the Arctic Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9569, https://doi.org/10.5194/egusphere-egu22-9569, 2022.

In this work, we will show the main ideas for studying how the (sub-)mesoscale processes impact the flux of nutrients and dissolved inorganic and organic carbon (DIC/DOC) in the upper layers of the central Arctic Ocean. These fluxes are essential since they are one of the primary mechanisms to connect the deeper layers of the ocean with the upper part: nutrients stored deeper can go to the surface mixed-layer and be used for primary production. On the other side, the Arctic Ocean is considered a carbon sink and contributes to the biological pump. For doing this, we are using the high-resolution numerical model FESOM-C to assimilate the hydrographic observations from the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition (2019-2020) to describe the (sub-)mesoscale dynamics (eddies, fronts). We will make use of the OMEGA equation to disentangle the vertical fluxes due to diabatic and adiabatic processes in the model output. Finally, we will analyse those results with in-situ observations of nutrients and DIC/DOC to estimate associated mass fluxes.

How to cite: Quintanilla Zurita, A., Rabe, B., and Kuznetsov, I.: (Sub-)mesoscale Dynamics in the Arctic and its Impact on the Flux of Nutrients and Carbon: a case study from the MOSAiC expedition, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9777, https://doi.org/10.5194/egusphere-egu22-9777, 2022.

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

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

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

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

The Arctic Ocean's Beaufort Gyre is a wind-driven reservoir of relatively fresh seawater, situated beneath time-mean anticyclonic atmospheric circulation, and is covered by mobile pack ice for most of the year. Liquid freshwater accumulation in and expulsion from this gyre is of critical interest to the climate modelling community, due to its potential to affect the Atlantic meridional overturning circulation (AMOC). In this presentation, we investigate the hypothesis that wind-driven sea ice import to/export from the BG region influences the freshwater content of the gyre and its variability. To test this hypothesis, we use the results of a coordinated climate response function (CRF) experiment with four ice-ocean models, in combination with targeted experiments using a regional setup of the MITgcm, in which we apply angular changes to the wind field. Our results show that, via an effect on the net thermodynamic growth rate, anomalies in sea ice import into the BG affect liquid freshwater adjustment. Specifically, increased ice import increases freshwater retention in the gyre, whereas ice export decreases freshwater in the gyre. Our results demonstrate that uncertainty in the cross-isobaric angle of surface winds, and in the dynamic sea ice response to these winds, has important implications for ice thermodynamics and freshwater. This mechanism may explain some of the observed inter-model spread in simulations of Beaufort Gyre freshwater and its adjustment in response to wind forcing.

How to cite: Cornish, S., Muilwijk, M., Scott, J., Marson, J., Myers, P., Zhang, W., Wang, Q., Kostov, Y., and Johnson, H.: Sea ice import affects Beaufort Gyre freshwater adjustment, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10044, https://doi.org/10.5194/egusphere-egu22-10044, 2022.

One of the fastest changing environments of the Arctic is the Barents Sea (BS), located north of Norway between Svalbard, Franz Josef Land and Novaya Zemlja. Although covering only about 10% of the Arctic Ocean area, the BS is of Arctic-wide importance,  as the warm water advected from the North Atlantic cause massive heat fluxes in the atmosphere and sea ice melt, ultimately driving major water mass modifications relevant for the Arctic Ocean circulation  downstream.

We focus on the question whether the observed retreat in sea-ice extent in the BS over the past four decades has enhanced the inflow of warm Atlantic water (AW) into the BS via an ocean-sea-ice-atmosphere feedback contributing to Arctic Amplification, as follows. We start by presenting evidence that the retreating winter sea-ice cover of the Barents Sea has been associated with an increase in ocean-to-atmosphere heat flux that can be observed in a strong rise in near surface air temperature - spatially coinciding with the regions of strong sea-ice retreat. Furthermore, the rising air temperature and the associated convective processes in the atmosphere create a local low sea level pressure (SLP) system over the northern BS that results in additional westerly winds in the vicinity of the Barents Sea Opening (BSO), where the warm and saline AW enters the BS. In case these additional winds enhance the AW inflow into the BS a positive feedback is likely as more heat is available for melting further ice, amplifiying the negative SLP anomaly.

In a set of ocean sensitivity experiments using the sea-ice and ocean model FESOM2.1, we investigate the impact of sea ice-related SLP anomalies and their associated anomalous atmospheric circulation patterns on volume transport through the BSO. The simulations rely on a horizontal grid resolution of approx. 4.5 km in the Arctic and Nordic Seas allowing precise modeling of the BS hydrography and circulation. The model is initially driven with a repeated normal year forcing (CORE1) to isolate the impact of the wind anomalies from high frequency atmospheric variability. After a spin-up phase, the model is perturbed by anomalous cyclones over the BS derived from long term SLP differences in reanalysis datasets associated with the observed sea-ice retreat. The results point indeed to a slight increase in net volume transport into the BS across the BSO. This increase, however, is not caused by an increase in the inflow of AW, but rather a decrease of the outflow of modified AW recirculating back towards Fram Strait. In terms of the feedback, our results indicate that the BS AW inflow is not sensitive to cyclonic wind anomalies caused by the sea-ice retreat. The additional volume and heat transport in the modified AW range may not be sufficient to provide enough heat to melt further sea-ice and hence likely does not close the proposed feedback mechanism in the BS.

How to cite: Heukamp, F. and Kanzow, T.: Investigations on the coupling of the Barents Sea sea-ice retreat on the Atlantic Water inflow via an ocean-ice-wind feedback in the context of Arctic Amplification, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10191, https://doi.org/10.5194/egusphere-egu22-10191, 2022.

Surface heat and momentum fluxes between the atmosphere and ocean are mitigated by sea ice cover, resulting in an effective net forcing that can be very different in character from the wind stress alone. The effective stress is often expressed as a weighted sum of air-sea and ice-sea stresses. This is appropriate for levitating ice. Allowing instead for floating ice, one can rewrite the effective forcing in a way that makes no explicit mention of the ice-ocean stress. Instead, the net forcing becomes a linear sum of air-sea and internal ice stresses. These differences are explored in the context of the Beaufort Gyre. Previous studies have introduced the ice-ocean governor as a regulating mechanism for the gyre, and in this limit, the ice-ocean stress is assumed to vanish. For floating ice, the governor limit can be thought of instead as a balance between the wind stress and the internal ice stress. Note that this balance would seem to be unlikely in that the internal stress is associated with small-scale linear kinetic features, which are very different in character from the mesoscale and synoptic features that determine the wind stress. High-resolution ECCO data will be used to examine the instantaneous and time-averaged spatial structure of the various terms that drive the Beaufort Gyre. Future work will also examine the air-sea-ice interface in different wind and ice regimes, as well as the role of eddy fluxes in the gyre dynamics. 

How to cite: Webb, E., Straub, D., Tremblay, B., and Nadeau, L.-P.: Air-Sea, Ice-Sea, and Effective Wind Forcing of the Beaufort Gyre, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10689, https://doi.org/10.5194/egusphere-egu22-10689, 2022.

The Arctic Ocean is characterized by complex processes coupling the atmosphere, cryosphere, ocean and land and undergoes remarkable environmental changes due to global warming. To better understand this system of unique physical, biogeochemical and ecosystem processes and their recent changes, the year-round ice drift experiment Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) was conducted from autumn 2019 to autumn 2020.

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

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

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

How to cite: Ruiz-Castillo, E., Janout, M., Kanzow, T., Hoelmann, J., Schulz, K., and Ivanov, V.: Structure and seasonal variability of the Arctic Boundary Current north of Severnaya Zemlya, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11472, https://doi.org/10.5194/egusphere-egu22-11472, 2022.

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

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

In September-October 2021 during NABOS-2021 expedition specialized shipborne ice observations were carried following methodological principles developed in AARI. The overall research area for the cruise included Arctic basin area toward north of Laptev and East Siberian seas within 73-82°N 125°E-170°W. Ice conditions were generalized and analyzed along the oceanographic cross-sections in accordance with the ice conditions homogeneousness. Hard ice conditions were unforeseen during the planning period, which made adjustments to the initial expedition plans and several minor northern cross-sections were canceled.

The route fragment with the hardest ice conditions was observed within 78-82°N 160°-172°E. Sea ice concentration was 10 tenths totally, concentration of residual ice varied from 5-7 to 10 tenths directly on the route of the vesse. Prevailing forms of the sea ice were big (500m-2000m) and often vast (2000-10000m) floes with strongly smoothed hummock formations covered with snow 10-15 cm high. The thickness of the residual ice on the route was mainly 50-70 cm (17%), often over 100 cm (6%), in hummocks over 2-3 meters. The water area between the ice fields was captured by young ice, grey and grey-white (3-4 up to 9 tenths).

Several areas were crossed by vessel twice in a time difference of one month. Sea ice formation process during the month long was fixed and analyzed by changes in distribution of ice with different stages of development. In general, 66% of the ship track within the ice during expedition had sea ice concentration of 10 tenths, the residual ice on the route accounted for 26%, young ice was observed for 38%, nilas and new ice 36%.The residual ice thickness varied from 30-50 cm to 160 cm and above, in some cases (hummock formations) over 300 cm. Ice thickness of 30-50 and 50-70 cm accounts for 9% each, thicknesses over 70 cm account for 8% of all thickness ranges observed throughout the entire route of the vessel in the ice.

Key words: shipborne observations, ice conditions of navigation, ice thickness, ice concentration, stage of development of ice.

How to cite: Timofeeva, A.: Navigation in the ice conditions in Arctic basin in September-October 2021, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13087, https://doi.org/10.5194/egusphere-egu22-13087, 2022.

The early study of eddy properties in the Marginal Ice Zone (MIZ) and of their influence on the ice regime in the Greenland Sea, based on the results of the MIZEX project (Johannessen et al., 1987), revealed that eddies may capture and transport a significant amount of ice, enhancing its ablation. Estimates suggest that eddies may provoke the ice edge retreat as fast as 1–2 km per day during summer. However, up to present, the mesoscale dynamics in polar regions, as well as the effect of eddies on ice edge ablation are poorly understood. This is due to sparse in situ observations and to an insufficient spatial resolution of numerical models, typically not resolving the mesoscale processes due to a relatively small Rossby deformation radius in polar regions.
This study aims to better understand the ways eddies affect the sea ice edge and their relative effect on the MIZ position in the East Greenland Current (75-78°N and 20°W-10°E). Pronounced local water temperature gradients and the importance of thermodynamics ablation in the ice dynamics in the Greenland Sea, derived in previous studies (Selyuzhenok et al., 2020), suggest a possibly strong eddy effect on the MIZ. This effect was noted in several case studies, when eddies were observed to trap and transport a significant amount of ice away from the MIZ (see, for example, von Appen et al., 2018). 
We base our results on the output of the very high-resolution Finite Element Sea ice-Ocean Model (FESOM), tested against the remote sensing observations from ENVISAT. We investigate only the warm period of 2007, when ice is actively melting and during which period a data on eddies, detected in SAR data, is available. Comparison of the location and dynamics of the ice edge in FESOM, AMSR-E-based ice concentration products and ENVISAT ASAR data, as well as of eddy properties in FESOM and in SAR satellite images, suggest that the model is in good agreement with the observations and can be used to study mesoscale dynamics of the MIZ in the region.
The analysis showed that eddies affect the ice edge position through an enhanced horizontal exchange across the MIZ. The sea-ice is trapped by eddies and transported east, in the area of a warmer water, while the warmer water is entrained by eddies and transported west, towards the MIZ. Both effects contribute to the accelerated sea ice melt and destruction. The highest temperature gradients, as well as the largest concentration of eddies in the MIZ were detected in the northern part of the study area, adjacent to the Fram Strait. Here eddies were found to play a particular important role in the MIZ dynamics.
This research was financed by the Russian Science Foundation (RSF) project N 21-17-00278.

How to cite: Pryakhin, S., Bashmachnikov, I., Kozlov, I., and Wekerle, C.: An effect of mesoscale and submesoscale eddies on sea ice processes in the Marginal Ice Zone, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13088, https://doi.org/10.5194/egusphere-egu22-13088, 2022.

The characteristics of Mediterranean Outflow Water (MOW) in the Northeast Atlantic were obtained during the 43^rd cruise of the R/V Akademik Nikolaj Strakhov (October 2019) and the 59^th cruise of the R/V Akademik Ioffe (September 2021) using CTD measurements. MOW is transformed Mediterranean Sea Water flowing down the slopes of the Strait of Gibraltar into the Gulf of Cadiz, where it mixes with underlying North Atlantic Central Water. MOW spreads at water depths between 500–1500 m in the eastern North Atlantic and is characterized by higher temperatures and salinities than other ambient water masses. In 2019 and 2021 MOW was located at depths of about 700–1500 m. The temperature in the core of MOW was in the range of 9.5–11.5 °C, while in 2019 both temperature and salinity were higher than in 2021. The salinity in the core was 36.15 psu in 2019 and 36.08 psu in 2021. The comparison of MOW characteristics obtained in 2019 and 2021 with data obtained in cruises in 1993, 2001 and 2005 from the CLIVAR and Carbon Hydrographic Data Office (https://cchdo.ucsd.edu/) showed that the maximum salinity values were observed in September 1993 and reached 36.17 psu. The minimum value of this parameter in the core of MOW was recorded in April 2001 and was 36.03 psu. According to the data of the 1993–2019 expeditions, the maximum salinity was noted at a depth of 1000–1100 m. In 2021, the core of MOW was slightly deeper — about 1150 m. The temperature in the MOW core in all studied years was in the range of 11.1–11.3 °C, with the exception of 2001, when the maximum temperature in the core was about 10.9 °C.

Acknowledgements

The financing of the expedition and the primary processing of the data obtained on the 59^th cruise of the R/V Akademik Ioffe were carried out at the expense of the state assignment of IO RAS № 0128–2021–0012. The analysis and interpretation of the data were supported by the Russian Science Foundation (project no. 21–77–20004).

How to cite: Bocherikova, I., Krechik, V., Kapustina, M., and Dvoeglazova, N.: Mediterranean Outflow Water characteristics in the Northeast Atlantic in 2019 and 2021., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-355, https://doi.org/10.5194/egusphere-egu22-355, 2022.

Based on the data obtained during the 59th cruise of the R/V "Akademik Ioffe", the comprehensive study of modern hydrological and hydrochemical conditions in the near-bottom layer of the Western Gap of the Azores-Gibraltar Fracture Zone was made for the first time. Eleven stations were performed in the study area. They were located to the south of the gap, at the entrance and exit sills, in the central part of the gap basin, as well as in the Iberian abyssal plain. Thermohaline parameters, characteristics of currents, the content of dissolved oxygen and nutrients (silicon, phosphorus) were measured at the stations.

There was water with a potential temperature less than 2°C, high in oxygen, silicon, and phosphorus deeper than 4558 m south to the gap. The current in this layer had a predominantly northeasterly direction with velocities ranging from 8 cm/s at the upper boundary to 2–3 cm/s near the bottom.

Water with θ <2 °С was found in the central part of the entrance sill —  in the bottom layer of 20–85 m thick and in the northeastern part at the depth of 4450–4560 m. The current flowed inside the gap and had high velocities: 10–20 cm/s in the central part and 27–30 cm/s in the northeastern part of the sill. The transport of water with θ<2°С through the transect was 0.097 Sv. Hydrochemical parameters in this section had elevated concentrations.

The near-bottom videorecording performed at the southern slope of the gap basin showed pronounced signs of erosion, which suggested a constant strong AABW flow directed along the slope into the Western Gap. Direct measurements showed that in the 200 m thick bottom layer, the current was directed northward, and its average velocity was 29 cm/s. The water in this layer had an average potential temperature of 1.998 °C and was rich in oxygen, silicon and phosphorus.

There was no water with θ<2 °С detected at the stations in the central part of the gap, at the exit sill and in the Iberian Abyssal Plain

Thus, the AABW corresponding to the classical definition crosses the entrance sill and moves along the southern slope of the Western Gap basin. However, this water does not enter the central part of the gap and does not propagate further. It can be assumed that the flow on the southern slope of the basin under the action of the Coriolis force turns to the right and mixes up, recirculating in the eastern part of the basin or propagating further to the east.

Acknowledgements

The expedition financing and the primary processing of the data obtained on the 59^th cruise of the R/V "Akademik Ioffe" were carried out at the expense of State Assignment of the Shirshov Institute of Oceanology, project № 0128-2021-0012. The analysis and interpretation of the data were supported by the Russian Science Foundation (project no. 21-77-20004).

How to cite: Muratova, A., Krechik, V., and Krivoshlyk, P.: Distribution of Antarctic Bottom Water  in the Western Gap (Northeast Atlantic), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-403, https://doi.org/10.5194/egusphere-egu22-403, 2022.

Hydrological and hydrochemical characteristics of the Discovery Gap bottom water (deeper than 4000 m) were obtained during the 59^th cruise of the R/V Akademik Ioffe (October 2021). Discovery Gap is the narrow gap of 150 km long, 10–50 km wide, oriented from southwest to northeast in Azores–Gibraltar Fracture Zone (Northeast Atlantic). Water with a potential temperature of less than 2 °C (modified Antarctic Bottom Water (AABW)) and high silicon concentrations was detected in the Discovery Gap. The terminal point of propagation of modified AABW in the exit sill of the Gap (depth more than 4700 m). There was cyclonic circulation in the Discovery Gap Narrows (the narrowest point of the Discovery Gap, 10 km wide, located in the northeastern part of it): in the northeast direction of more than 14 cm/s speed and with the high phosphate concentrations (1.46-1.54 μmol/l), in the southwest direction of 6-8 cm/s speed and with a low phosphate content (1.39-1.40 μmol/l). The localization of the extrema of phosphorus concentrations correlates with the maximum flow velocities, which may be associated with advective processes.

Acknowledgments:

The expedition and the hydrochemical processing of the data received during the 59^th cruise of the R/V Akademik Ioffe was carried out with a support of the state assignment of the IO RAS (No. 0128-2021-0012), the hydrophysical measurements were supported by the Russian Science Foundation (project no. 21-77-20004).

We thank the crew of the R/V Akademik Ioffe for assistance, B.V. Chubarenko for valuable comments and E.I. Gmyrya for preparing the map.

How to cite: Dvoeglazova, N., Kapustina, M., Krechik, V., and Bocherikova, I.: Hydrological and hydrochemical haracteristics of the modified AABW in the Discovery Gap (Northeast Atlantic) in 2021., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-478, https://doi.org/10.5194/egusphere-egu22-478, 2022.

Marine Heatwaves (MHWs) are ocean extreme events, characterized by anomalously high temperatures, which can have drastic ecological impacts. The Northeast U.S. continental shelf is of great economical importance being home to a highly productive ecosystem. Local warming rates exceed the global average and the region experienced multiple MHWs in the last decade with severe consequences for regional fisheries. Due to the lack of subsurface observations, the depth-extent of MHWs is not well known, which however hampers assessing impacts on pelagic and benthic ecosystems. This study utilizes a global ocean circulation model with a high-resolution (1/20°) nest in the Atlantic to investigate the depth structure of MHWs and associated drivers on the Northeast U.S. continental shelf. It is shown that MHWs exhibit varying spatial extents, with some only appearing at depth. Highest intensities are found around 100m depth with temperatures exceeding the climatological mean by up to 7°C, while surface intensities are typically smaller around 3°C. Distinct vertical structures are associated with different spatial patterns and drivers. Investigation of the co-variability of temperature and salinity revealed that over 80% of MHWs at depth (>50m) coincide with extreme salinity anomalies. Two case studies provide insight into opposing MHW patterns at the surface and at depth, being forced by anomalous air-sea heat fluxes and Gulf Stream warm core ring interaction, respectively, the latter hinting at the importance of local ocean dynamics. The results highlight the relevance of subsurface/deep MHWs, underlining the need of continuous subsurface measurements. Working towards a more quantitative assessment of WCRs, their interaction with the shelf break and impact on the shelf's hydrography, an eddy-tracking algorithm will be applied on the model output. This will also allow to further investigate the model's skill in representing mesoscale features in the Gulf Stream region.

How to cite: Grosselindemann, H., Ryan, S., Ummenhofer, C., Martin, T., and Biastoch, A.: Marine Heatwaves and their Depth Structures on the Northeast US Continental Shelf, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1015, https://doi.org/10.5194/egusphere-egu22-1015, 2022.

The Atlantic Meridional Overturning Circulation (AMOC) is thought to exist in multiple states of equilibria. In the present climate, the AMOC is believed to be in a relatively strong state, bringing warm waters into the North Atlantic and contributing to mild winters over Europe. However, proxy data show evidence of abrupt declines in the strength of the AMOC, often associated with the initiation of ice ages. The abrupt shifts in the strength of the AMOC are usually referred to as ‘tipping points’. Presently, state-of-the-art climate models are unable to spontaneously reproduce tipping points in the AMOC, preventing an accurate study of the climate impacts of an abrupt AMOC shutdown. Contextually, although it is deemed unlikely that the AMOC will collapse in response to climate change, it is expected to further slow down into the 21^st century. The impacts of this weakening, relative to those of global warming, are poorly understood, especially on daily timescales.

            To address this question, we run water hosing experiments with the EC-Earth3 earth system model to investigate the impacts of an AMOC abrupt weakening on the winter climate variability focusing on the North Atlantic and Europe. We confirm results from previous studies showing a large decrease in temperature, precipitation, and an increase in the jet stream over Europe. However, we further investigate the moisture budget and the impacts on daily weather regimes and blocking. In contrast to previous hypotheses, we find that the reduction in precipitation over Europe is due to changes in the storm tracks rather than thermodynamic effects. Further, we find a significant increase in the frequency and persistence of NAO+ days. Finally, we show precipitation and temperature extremes that are expected in response to the AMOC weakening.

            Our results show the climate impacts on weather events that can be expected from an AMOC weakening alone, and are relevant to understanding the relative roles of greenhouse gas forcing and AMOC weakening on the European climate in simulations of future climate change.

How to cite: Bellomo, K., Meccia, V., D'Agostino, R., Fabiano, F., von Hardenberg, J., and Corti, S.: The climate impacts of an abrupt AMOC weakening on the European winters, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1023, https://doi.org/10.5194/egusphere-egu22-1023, 2022.

A significant fraction of the Eulerian downwelling feeding the lower limb of the Atlantic Meridional Overturning Circulation (AMOC) has been proposed to occur around the subpolar North Atlantic's continental slopes. While this downwelling ultimately takes place in a thin boundary layer where relative vorticity can be dissipated via friction, it is maintained by a large-scale geostrophic balance and an along-shore densification of the boundary current. We here use modern hydrography data (Argo and shipboard hydrography mainly) to map the long-term mean density field along the continental slope via an optimal interpolation method specifically adapted to the length scales of the boundary current. The overall downstream densification of the boundary region implies a Eulerian-mean downwelling of 2.12 ± 0.43 Sv at 1100 m depth between Denmark Strait and Flemish Cap. While seasonal variations appear to be relatively limited, a clear regional pattern emerges with Eulerian-mean downwelling in the Irminger Sea and western Labrador Sea and upwelling along Greenland western continental slope. Comparisons with independent cross-basin estimates confirm that overturning transport across the marginal seas of the subpolar North Atlantic is mainly explained by vertical volume fluxes along the continental slopes, and suggest the usefulness of hydrographic data alone to estimate the regional pattern of the sinking branch of the AMOC. 

How to cite: liu, Y., Desbruyeres, D., Mercier, H., and Spall, M.: Observation-based estimates of Eulerian-mean boundary downwelling in the western subpolar North Atlantic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1130, https://doi.org/10.5194/egusphere-egu22-1130, 2022.

Since the industrial revolution, human activities have emitted large amount of anthropogenic carbon (C_ant) into the atmosphere through the burning of fossil fuel, the production of cement and land-use change. Via air-sea gas exchange, the ocean absorbs roughly a third of C_ant, meaning that C_ant is an additional source of carbon for the ocean. In particular, the North Atlantic is known to be a region with a high storage capacity of C_ant. Whereas the distribution of C_ant in the upper layers of the North Atlantic is well documented, its transport to the abyssal ocean and the mechanisms behind its deep redistribution remain scarcely described. To shed light on this research gap, we use a database provided by ~70 Deep-Argo floats equipped with oxygen sensors and located in the North Atlantic that allow us to explore the deep pathways of C_ant. First, the macronutrients and carbon variables (pH, total alkalinity, total inorganic carbon and pCO₂) are estimated with bayesian neural networks (CANYON-B and CONTENT) from the temperature, salinity and oxygen data of the floats. Second, C_ant concentrations in the water column are then estimated with back-calculation methods. Here we present the first results of our study.    

How to cite: Asselot, R., Bajon, R., López Mozos, M., Thierry, V., Mercier, H., Pérez, F., and Carracedo, L.: Mechanisms controlling the abyssal transport of anthropogenic carbon in the North Atlantic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1206, https://doi.org/10.5194/egusphere-egu22-1206, 2022.

Record low surface temperatures were observed in the subpolar North Atlantic during 2015, despite the majority of the global ocean experiencing higher than average surface temperatures. We compute mixed layer temperature budgets in the ECCO Version 4 state estimate to further understand the processes responsible for the North Atlantic cold anomaly. We show that surface forcing was the cause of approximately 75% of the initial cooling in the winter of 2013/14, after which the cold anomaly was sequestered beneath the deep winter mixed layer. Re-emergence of the cold anomaly during the summer/autumn of 2014 was primarily driven by a strong temperature gradient across the base of the mixed layer. Vertical diffusion resulted in approximately 70% of the re-emergence, with entrainment of deeper water driving the remaining 30%. In the summer of 2015, surface warming of the mixed layer was then anomalously low, resulting in the most negative temperature anomalies. Spatial patterns in the budgets show that the initial surface cooling was strongest in the south of the region, due to strong westerly winds related to the positive phase of the East Atlantic Pattern. Subsequent anomalies in surface fluxes associated with the North Atlantic Oscillation were stronger in the north, but the impact on the average temperature of the mixed layer was largely masked by anomalously high winter mixed layer depths.

How to cite: Sanders, R., Jones, D., Josey, S., Sinha, B., and Forget, G.: Using mixed layer heat budgets to determine the drivers of the 2015 North Atlantic cold anomaly in ocean state estimates, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1682, https://doi.org/10.5194/egusphere-egu22-1682, 2022.

We employ multi-ensemble Met Office Decadal Prediction System hindcasts to analyse the impact of atmospheric winds and North Atlantic Oscillation (NAO) phases on the overturning circulation in the North Atlantic Ocean. A positive NAO phase is generally associated with an anomalously strong and/or northward shifted jet stream in the North Atlantic, and the vice-versa is true for a negative NAO phase. As a consequence of relatively strong winds, oceans tend to lose more heat to the atmosphere in winter in many parts of the subpolar North Atlantic Ocean. This process is expected to create negative anomalies in sea surface temperature and generate more dense water on the ocean surface at high latitudes resulting in a strengthening in the overturning circulation. Here, we examine the sensitivity of the overturning circulation to NAO phases in multi-ensemble decadal hindcasts to understand how the interior ocean responds to different NAO phases. For this purpose, we analyse the changes in east-west density contrasts, upper ocean heat content, mixed-layer depth, meridional heat and salt transport in different oceanic regions, i.e. Labrador Sea, Irminger Sea and Nordic Seas. In particular, we perform a linear regression analysis for the above-mentioned diagnostics and NAO indices to assess how sensitive the upper ocean is to changes in the atmospheric state. We further compare our results against reanalysis data and in-situ observations.

How to cite: Khatri, H., Williams, R., Woollings, T., and Smith, D.: Inter-annual Variability in the Subpolar Overturning Circulation: A Sensitivity Analysis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1741, https://doi.org/10.5194/egusphere-egu22-1741, 2022.

The Atlantic Meridional Overturning Circulation redistributes heat across the Atlantic and is therefore a critical element of the climate system. Increased freshwater fluxes to the subpolar north Atlantic from the Greenland ice sheet and from the Arctic could lead to a strengthening of stratification in deep convection regions, and impact deep water formation and the overturning circulation. However, this additional freshwater first enters the boundary current on the Greenland shelf, and freshwater pathways from the shelf to deep convection regions are still unclear. In this study, we investigate the possible role of winds in driving short-lived freshwater export events from the south-east Greenland shelf to the deep convection region of the Irminger Sea.

Along the south-eastern shelf, strong and consistent north-easterly winds tend to restrain fresh surface waters over the shelf. This wind pattern changes at Cape Farewell, where strong westerly winds could lead to across-shelf export. Using a high-resolution model, we identify strong wind events and investigate their impact on freshwater export. The strongest westerly winds, westerly tip jets, are associated with the strongest and deepest freshwater export across the shelfbreak, with a mean of 40.7 mSv of freshwater in the first 100 m (with reference salinity 34.9). These wind events tilt isohalines and extend the front offshore, especially over Eirik Ridge. Moderate westerly events are associated with weaker export across the shelfbreak (mean of 17 mSv) but overall contribute to more freshwater export throughout the year, including in summer, when the shelf is particularly fresh. Particle tracking shows that half of the surface waters crossing the shelfbreak during tip jet events are exported away from the shelf, either entering the Irminger Gyre, or being driven over Eirik Ridge. During strong westerly wind events, sea-ice detaches from the coast and veers towards the Irminger Sea, but the contribution of sea-ice to freshwater export at the shelfbreak is minimal compared to liquid freshwater export.

How to cite: Duyck, E., Gelderloos, R., and De Jong, F.: Wind-driven freshwater export at Cape Farewell, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1786, https://doi.org/10.5194/egusphere-egu22-1786, 2022.

The Atlantic meridional overturning circulation (AMOC) is an important part of our climate system, which keeps the North Atlantic relatively warm. It is predicted to weaken under climate change. The AMOC may have a tipping point beyond which recovery is difficult, hence showing quasi-irreversibility (hysteresis). Although hysteresis has been seen in simple models, it has been difficult to demonstrate in comprehensive global climate models.

We present initial results from the North Atlantic hosing model intercomparison project, where we applied an idealised forcing of a freshwater flux over the North Atlantic in 9 CMIP6 models. The AMOC weakens in all models from the freshening, but once the freshening ceases, the AMOC recovers in some models, and in others it stays in a weakened state. We discuss how differences in feedbacks affect the AMOC response.  

How to cite: Jackson, L., Alastrue-De-Asenjo, E., Bellomo, K., Danabasoglu, G., Hu, A., Jungclaus, J., Meccia, V., Saenko, O., Shao, A., and Swingedouw, D.: AMOC thresholds in CMIP6 models: NAHosMIP, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2778, https://doi.org/10.5194/egusphere-egu22-2778, 2022.

Freshwater plays a key role in the Arctic - North Atlantic climate system, linking ice, ocean and atmospheric dynamics. In particular, large freshwater releases into the subpolar region drive extreme cold anomalies, create sharp sea surface temperature fronts, destabilise the overlying atmosphere, and trigger shifts in major ocean currents. Considering the expected increased freshwater fluxes in future due to more melt, it is critical to understand the resulting climate feedbacks.

Combining observations and models, we present evidence that past changes in Arctic freshwater outflow paced transitions between North Atlantic cold and warm anomalies. This circulation-driven freshwater cycle explained over 50% of the sea surface temperature variability in the subpolar North Atlantic and was particularly pronounced on decadal timescales. However, new findings indicate that the recent freshwater input due to more melting has increased the amplitude and frequency of freshwater variations in the North Atlantic, leading to a shift of power in the North Atlantic climate variability from decadal to interannual timescales. In addition, the interference of the circulation-driven freshwater cycle by melting has contributed to the storage of freshwater in the Arctic Ocean, where it now poses the possible risk of rapid climate change if the freshwater were released. In light of newly identified, Arctic feedbacks to melt-driven freshwater events in the North Atlantic, we suggest that an Arctic freshwater release is becoming increasingly likely.

How to cite: Oltmanns, M. and Moat, B.: Arctic pacing of North Atlantic climate variability through freshwater exports, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2821, https://doi.org/10.5194/egusphere-egu22-2821, 2022.

The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the global climate. Recent observations have highlighted the dominant role of the buoyancy forcing in the transformation of surface waters to the AMOC lower limb at subpolar latitudes. The short (4 years) length of the OSNAP timeseries, however, limits conclusions over longer time scales. To investigate a wide range of temporal scales, we use three 100-years long coupled simulations of HadGEM3-GC3.1, at resolutions ranging from ~130 km atmosphere and 1° ocean to 25 km atmosphere and 1/12° ocean. In line with observations, the models show that the mean overturning and buoyancy-induced transformation are concentrated in the eastern subpolar gyre rather than in the Labrador Sea.

However, the horizontal resolution of the models impacts the formation of dense water over the subpolar gyre. An unrealistically large sea ice extent induces a weak buoyancy-induced transformation over the western subpolar gyre at low resolution, while a bias in surface density produces too dense water at high resolution. These biases are associated with a shift in the location of dense water formation. The transformation is mainly localized in the interior of the Irminger and Labrador seas at low resolution, and over the boundary current at high resolution. The interannual variability of the transformation is thus driven by different mechanisms between the simulations. In contrast with observations, the interannual variance in air-sea fluxes plays a more prominent role in the variance of transformation along the boundary current at high resolution.

How to cite: Petit, T., Robson, J., and Ferreira, D.: Formation of dense water over the North Atlantic subpolar gyre in a hierarchy of climate models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3068, https://doi.org/10.5194/egusphere-egu22-3068, 2022.

Subpolar Mode Water (SPMW) represents a variety of near-surface waters that occupy a large volume in the upper 1000 m of the water column of the Subpolar North Atlantic (SPNA). Originating in the eastern and northeastern SPNA through late winter water mass formation, SPMW acts as a precursor to the formation of the North Atlantic Deep Water, which is an important ingredient of the Atlantic Meridional Overturning Circulation (AMOC). In this study we address spatial and temporal changes in the SPMW layer thickness and volume. We relate these changes to variability in the water mass formation estimated through the net subduction/obduction rates along predefined isopycnal bins between σ_θ = 27.05 kg m^-3 and σ_θ = 27.55 kg m^-3 with 0.1 kg m^-3 interval. We use two observation-based gridded 3D products from the Copernicus Marine Environmental Monitoring Service (CMEMS), i.e., the ARMOR3D and the OMEGA3D datasets. The first one provides 3D temperature and salinity fields and is available on a weekly 0.25° regular grid from 1993 to present. The second one provides observation-based quasi-geostrophic vertical and horizontal velocity fields with the same temporal and spatial resolution as ARMOR3D, but for the period 1993 to 2018. Throughout this period of 27 years of observations, the analysis reveals not only pronounced interannual variability in the SPMW formation and volume but also a strong spatial variability, which is caused by spatial changes of the main SPMW formation area within the northeastern SPNA.

How to cite: Stendardo, I., Buongiorno Nardelli, B., Durante, S., Iudicone, D., and Kieke, D.: Variability of Subpolar Mode Water Volume and Formation in the North Atlantic during 1993-2018, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3502, https://doi.org/10.5194/egusphere-egu22-3502, 2022.

The Labrador Sea in the subpolar North Atlantic is one of the few special regions, where strong wintertime buoyancy loss, consecutive substantially reduced vertical stratification, and the prevailing circulation facilitate the transfer of water mass properties from the surface to depths exceeding 1500 m through deep convective mixing. Hence, impacting the characteristics of the intermediate and deep waters in the entire Atlantic basin. Despite ever-growing evidence of the freshwater and atmospheric gas contents of these waters being directly affected by the strength of wintertime mixing in the Labrador Sea, the relative importance of the Labrador Sea convection for the strength of the overall Atlantic meridional overturning is still under debate, often leading to contradicting conclusions. This ongoing debate highlights the need for an in-depth all-inclusive investigation of the processes responsible for both occurrence and persistence of deep convective mixing events. Here, we make a first step in this direction by aligning multiplatform observations with model runs and quantifying the roles of the local atmospheric forcing (e.g., cumulative wintertime air-sea flux), the remote oceanic forcing (e.g., horizontal advection) and the ocean’s own memory of the past convective events (e.g., weak stratification resulting from convective preconditioning).

These three key factors, fully responsible for initiation and undergoing of winter convection, and both seasonal and interannual heat content changes in the Labrador Sea, are analyzed based on long time series. These are comprised from all available thoroughly quality-controlled ship, profiling float and mooring measurements in the central Labrador Sea and state-of the-art ocean models. The resulting variables compared between the observations and models include time series of the characteristic ocean state variables, such as temperature, salinity and density over the entire water column. Additionally, the variables quantifying specific outcomes of each winter convection, such as depth, density and volume of the newly mixed intermediate-depth water in the Labrador Sea are considered. 

We show that the seasonal evolution of the deep winter convective mixed layer is a result of the sum of the surface cooling and the overall multiyear inertia in density changes and variations in the heat, freshwater and salt imports from the neighboring North Atlantic and Arctic regions. This, in turn means that not forcibly the strongest surface cooling induces the deepest convection with maximum density water, but rather a combination of the three factors. Through the combined analyses of observations and model-based time series we are able to properly assess the relative contribution of these three factors to the development of deep convective mixing in the Labrador Sea.

How to cite: Handmann, P., Yashayaev, I., and Schwarzkopf, F.: Relative roles of different key forcing and preconditioning factors for recurrent deep convection in the Labrador Sea from observations and ocean models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3522, https://doi.org/10.5194/egusphere-egu22-3522, 2022.

Dominant Euro-Atlantic climate modes such as the North Atlantic Oscillation (NAO), the Eastern Atlantic pattern (EA), the Eastern Atlantic Western Russian pattern (EAWR), and the Scandinavian pattern (SCA) significantly affect interannual-to-decadal Euro-Mediterranean climate fluctuations, especially in winter.

In this contribution, we will present and discuss results from a CMIP6 multi-model analysis performed to investigate the robustness of historical and projected state and variability of such modes under the historical and ssp585 future scenario of anthropogenic forcing (fossil-fueled development with 8.5W/m² forcing level) simulations, focusing on the winter season.

Toward this goal, we first search for a reliable box-based index definition for each of the abovementioned observed climate modes and, then, we perform a comparative assessment of the temporal, spectral and distributional properties of the so-defined indices during the historical (1850-2014) and ssp585 future scenario (2015-2099) time periods, with a special focus on the two interdecadal periods 1960-1999 and 2060-2099.

Results show overall good skills of the historical ensemble to reproduce the observed temporal, spectral and distributional properties of all considered modes. At the end of the 21st Century the ssp585 ensemble yields non-significant distributional changes for NAO, EAWR, and SCA indices and a transition to a stronger baroclinic structure for EA, with persistent positive anomalies in the mid-troposphere enhancing globally-driven warming over the Euro-Mediterranean region. The hemispheric spatial correlation patterns with temperature and precipitation significantly change for all modes, that is, we observe a significant modulation of the teleconnections associated with each index.

How to cite: Cusinato, E., Rubino, A., and Zanchettin, D.: Winter Euro-Atlantic Climate Modes: Future Scenarios From a CMIP6 Multi-Model Ensemble, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3596, https://doi.org/10.5194/egusphere-egu22-3596, 2022.

The Irminger Current (IC) is known to be an important contributor to the northward volume transport associated with the Atlantic Meridional Overturning Circulation (AMOC). The IC has a two-core structure with surface intensified velocities and transports warm and saline waters originating from the North Atlantic Current further north. The strength of the subpolar AMOC is continuously measured by the Overturning in the Subpolar North Atlantic Program (OSNAP) since 2014. Recent results highlight that most of the overturning in density space occurs in the array east of Greenland, in the Irminger and Iceland Basins. In previous work we looked into the transport variability of the IC on decadal to interannual time scales and could identify long-term trend related to basin-wide density changes which have the potential to impact AMOC variability. However, the impact of mesoscale variability on northward transport variability in the Irminger Sea has not been studied yet.

In this study, we explore the mesoscale variability in the IC and its impact on northward transport variability.

Previous studies showed that the western flank of the Reykjanes Ridge, where the IC is located, is a region of enhanced eddy kinetic energy. We used high resolution mooring data from 2014 – 2020 from the IC mooring array to investigate its transport variability. The mean volume transport obtained for the IC is 10.4 Sv but it strongly varies on time scales from days to months (std. dev. of 4.3 Sv). The mooring data reveals a seasonal cycle in the eddy kinetic energy with the strongest activity in winter. However, this does not coincide with a seasonal cycle in volume transport. We found the strongest EKE in the western core of the IC. In 2019, an exceptional 6-month intensification of the IC led to exceptionally strong volume transport of the IC of 19.9 Sv in August. Using sea level anomaly maps from satellite altimetry, the intensification was attributed to the presence of a mesoscale eddy in the vicinity of the moorings.  At this time, altimetry shows an anticyclone lingering next to a cyclone in the mooring array, which intensified northward velocities within the IC. We thus conclude that mesoscale variability can directly impact both the transport and the variability of the IC.

Considering the potential importance of mesoscale variability along the Reykjanes Ridge, further research will focus on estimating the mean properties of the eddies, their formation region and their faith using a high-resolution model.

How to cite: Fried, N., Katsman, C. A., and de Jong, M. F.: The impact of mesoscale variability on northward volume transport in the Irminger Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3602, https://doi.org/10.5194/egusphere-egu22-3602, 2022.

Where earlier generations of ocean models with resolution of 1° or coarser tended to represent wintertime dense water formation in the North Atlantic mainly as a process of open water convection in the Labrador Sea and Nordic Seas, more recent models with higher resolution, in conjunction with observational programmes such as OSNAP, have presented us with a new, more complex, picture. Watermasses are progressively ventilated and lose buoyancy as they propagate cyclonically westward around the gyre, starting with the formation of Subpolar Mode Water close to the eastern boundary, and eventually leading to Labrador Sea Water, which forms part of the lower limb of the Atlantic meridional overturning circulation (AMOC).

We present a set of hindcast integrations of a global 1/4° NEMO ocean configuration from 1958 until nearly the present day, forced with three standard surface forcing datasets. We use the surface-forced streamfunction, estimated from surface buoyancy fluxes, along with the overturning streamfunction, similarly defined in potential density space, to investigate the causal link between surface forcing and decadal variability in the strength of the AMOC. We confirm that surface heat loss from the Irminger Sea is the dominant mechanism for decadal AMOC variability, while that from the Labrador Sea has about half the amplitude. The AMOC variability is shown to be related to that of the North Atlantic Oscillation, primarily through the surface heat flux, itself dominated by the air-sea temperature difference, and we show that a metric based on the surface-forced streamfunction has predictive value for AMOC variability on interannual to decadal time scales.

How to cite: Megann, A., Blaker, A., Josey, S., New, A., and Sinha, B.: Mechanisms for Late 20th and Early 21st Century Decadal AMOC Variability, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4010, https://doi.org/10.5194/egusphere-egu22-4010, 2022.

Cold dense waters flowing south from the Nordic Seas and the Arctic Ocean form strong bottom intensified gravity currents at the Denmark Strait, Iceland-Faroe ridge, and Faroe-Scotland channel. Such overflows generate water-masses with specific hydrographic features which form the lower limb of the thermohaline circulation, responsible for a large fraction of the ocean heat transport on the Globe.

Gravity current representation in ocean models is sensitive to the choice of the vertical coordinate system. Typically, global ocean models use geopotential z-level coordinates, representing the bottom topography as a series of step-like structures. However, this choice results in excessive entrainment and mixing when simulating gravity currents, even when the partial steps parametrization is employed. Conversely, terrain-following coordinates offers a natural representation of overflows but introduce errors in the computation of the pressure gradient force, making their use in global configurations challenging.

To improve the representation of Nordic overflows in global models, Colombo (2018) proposed the use of a local-sigma vertical coordinate, where model surfaces are terrain-following only in the proximity of the Greenland-Scotland ridge, whilst standard z-level coordinates (with partial steps) are used everywhere else. However, the development of such a mesh is not trivial, especially when defining the transition zone between the two vertical coordinates.

Similarly, to improve the representation of cross-shelf exchange in regional configurations Harle et al. (2013) developed a hybrid vertical coordinate (SZT) where terrain-following computational surfaces smoothly transition to z-level with partial steps below a user defined depth.

Recently, Bruciaferri et al. (2018) introduced the Multi-Envelope (ME) s-coordinate system, where computational levels are curved and adjusted to multiple arbitrarily defined surfaces (aka envelopes) rather than following geopotential levels or the actual bathymetry. This allows the optimisation of model levels in order to best represent different physical processes within sub-domains of the model.

In order to overcome the complexities of the local-sigma method, we propose combining this approach with the flexibility of the SZT and ME methods to generate localised versions of these vertical coordinates. We test this new methodology in the region of the Nordic Sea overflows in a ¼° global NEMO configuration. At first, a series of idealised numerical experiments is conducted to assess the ability of the local-SZT and local-ME grids to minimise both horizontal pressure gradient errors and spurious entrainment of overflow waters. Finally, the skill of the new local-ME and local-SZT systems in reproducing observed properties of the Nordic overflows is assessed and compared with the traditional approach of employing geopotential coordinates with partial steps.

Bruciaferri, D., Shapiro, G.I. & Wobus, F. A multi-envelope vertical coordinate system for numerical ocean modelling. Ocean Dynamics 68, 1239–1258 (2018). https://doi.org/10.1007/s10236-018-1189-x

Harle, J.D. et al. 2013. Report on role of biophysical interactions on basin-scale C and N budgets. Deliverable 6.5, European Basin-scale Analysis, Synthesis and Integration (EURO-BASIN) Project, http://eurobasin.dtuaqua.dk/eurobasin/documents/deliverables/D6.5%20Report%20on%20role%20of%20biophysical%20interactions%20on%20C%20N%20budget.pdf

Pedro Colombo. Modélisation des écoulements d’eaux denses à travers des seuils topographiques dans les modèles réalistes de circulation océanique: une démonstration du potentiel que représente l’hybridation d’une coordonnée géopotentielle et d’une coordonnée suivant le terrain. Sciences de la Terre. Université Grenoble Alpes, 2018.

How to cite: Bruciaferri, D., Guiavarc'h, C., Hewitt, H., Harle, J., Almansi, M., and Mathiot, P.: Improving nordic overflows representation in global ocean models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4072, https://doi.org/10.5194/egusphere-egu22-4072, 2022.

Global warming since the industrial revolution has led to a series of changes in the atmosphere and ocean. As a key indicator of global ocean circulation, AMOC has shown a weakening in recent decades from both the observed and simulated results. This process which is not only affected by the local variation of the Arctic, but also by the ocean and atmosphere circulation changes in the middle and lower latitudes, might have important implications for future global climate changes. We employ the Alfred Wegener Institute Climate Model (AWI-CM 1.1 LR) and a method of perturbing coupled models to quantify and understand the impact of anthropogenic warming on the slowdown of AMOC. Conducted one control (CTRL) experiment and three sensitivity experiments (60N, 60NS, and GLOB) in which CO2 concentration were abruptly quadrupled either regionally (60N-north of 60°N, 60NS-south of 60°N) or globally (GLOB). The goal of our research is to identify the response of AMOC weakening to the quadrupling of CO2 concentration in different regions and provide future insight into ocean circulation changes in the context of climate warming. Our results show that CO2 forcing outside the Arctic dominates the weakening of AMOC. In a warming climate, the poleward heat transport increased due to the extra-Arctic CO2 forcing, which enhanced the upper ocean average stratification within the mixed-layer depth over Nordic Seas and Labrador Sea and thus weakens the AMOC to a large extent. The warming in upper-layer also lead to the dominant role of temperature contribution to stratification. However, in both the deep convection regions, the mechanism resulting in the strengthening of stratification might be quite different.

How to cite: Chen, J., Wang, X., and Wang, X.: The weakening of AMOC highly linked to climate warming outside the Arctic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4440, https://doi.org/10.5194/egusphere-egu22-4440, 2022.

The Atlantic Meridional Overturning Circulation (AMOC) is an important component of the climate system. Results from the OSNAP (Overturning in the Subpolar North Atlantic Program) moored array show that the largest contribution to both the total overturning and its variability originates from the Irminger Sea and Iceland Basin. Deep convection in the Irminger Sea strongly impacts the transformation of buoyant to dense waters. Additionally, its localization in the center of the basin directly affects the basin’s horizontal density gradients that drive transport. However, the strength of convection varies greatly from winter to winter and is expected to weaken as a result of strengthening stratification forced by climate change. How exactly the Irminger Sea convection responds to stratification versus forcing is not known.

The LOCO (Long-term Ocean Circulation Observations) mooring recorded convection in the central Irminger Sea from 2003 through 2018. This record is now continued by the OOI (Ocean Observatory Initiative) mooring, deployed nearby in 2014. The combined record of the two moorings showcase the variability of Irminger Sea convection through this 17-year period. This includes the deepest (>1600 m) convection observed in the basin, forced by the exceptionally strong winter of 2014-2015, as well as several winters (in 2010-2011 and 2019-2020) where convection was inhibited by strong upper ocean stratification. The Irminger Sea hydrography changed as a result. The basin warmed and became more saline and stratified during the initial period with weak convection. This trend was halted during the intermittent convection in the mid-2010s. After 2014-2015, the upper 1500 m of the basin cooled and became fresher as a result of stronger convection in the subsequent winter, which led to denser water classes and weaker upper to mid-ocean stratification in the center of the basin. These hydrographic changes and their impact on the cross-basin density gradients are reflected in the Irminger Current transport.

The long record of the Irminger Sea hydrography shows the respective influence of atmospheric buoyancy forcing versus stratification on deep convection. In terms of stratification, we see the effects of both ocean memory in the upper 1500 m of the water column, during prolonged periods of weak or strong convection, and more sudden changes in the uppermost (~100 m) ocean. These insights will help to better predict how Irminger Sea convection will respond to future stratification changes.

How to cite: de Jong, F., Le Bras, I., Trafford McRaven, L., Sterl, M., Duyck, E., and Fried, N.: Variability in Irminger Sea convection and hydrography from 2003 through 2020, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4731, https://doi.org/10.5194/egusphere-egu22-4731, 2022.

Over the past years the North Atlantic has been the main scene of three interesting phenomena: a long-term warming hole (i.e. Drijfhout et al., 2012), a reoccurring cold blob (i.e. Duchez et al., 2016) and an unusual freshening in recent years (Holliday et al., 2020).

All three have been linked to either changes in ocean circulation causing i.e., anomalous heat transports, atmospheric circulation changes that, i.e., lead to enhanced surface heat loss or changes in precipitation patterns, – or a combination of the two. While it appears that the main drivers of these phenomena have been identified, the relative importance of them as well as the connections between the three are still unclear.

To assess this, we study the correlation of the main atmospheric and oceanic drivers in the North Atlantic region and the upper ocean heat (OHC) and freshwater content (FWC). By looking at OHC and FWC we remove some of the noise visible in the sea surface data, and it further enables us to remove the direct influence of the atmosphere by subtracting the heat and freshwater air-sea fluxes from the data.

The results indicate that long-term changes in the western subpolar North Atlantic are caused by the direct effects of changes in the atmosphere, while the eastern subpolar North Atlantic is more strongly influenced by changes in the ocean circulation causing a simultaneous cooling/freshening or warming/salinification, respectively. This has e.g., implications for the definition of temperature or salinity based AMOC indices (as used in e.g., Boers, 2021; Caesar et al., 2018) that often average quantities over the whole or even just the western subpolar North Atlantic. These should be redefined focusing on the eastern part.  

References

Boers, N. (2021). Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation. Nature Climate Change, 11(8), 680-688. https://doi.org/10.1038/s41558-021-01097-4

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. https://doi.org/10.1038/s41586-018-0006-5

Drijfhout, S., van Oldenborgh, G. J., & Cimatoribus, A. (2012). Is a Decline of AMOC Causing the Warming Hole above the North Atlantic in Observed and Modeled Warming Patterns? Journal of Climate, 25(24), 8373-8379. https://doi.org/10.1175/jcli-d-12-00490.1

Duchez, A., Frajka-Williams, E., Josey, S. A., Evans, D. G., Grist, J. P., Marsh, R., . . . Hirschi, J. J. M. (2016). Drivers of exceptionally cold North Atlantic Ocean temperatures and their link to the 2015 European heat wave. Environmental Research Letters, 11(7), 074004. https://doi.org/10.1088/1748-9326/11/7/074004

Holliday, N. P., Bersch, M., Berx, B., Chafik, L., Cunningham, S., Florindo-López, C., . . . Yashayaev, I. (2020). Ocean circulation causes the largest freshening event for 120 years in eastern subpolar North Atlantic. Nature Communications, 11(1), 585. https://doi.org/10.1038/s41467-020-14474-y

How to cite: Caesar, L. and McCarthy, G.: Drivers of heat and freshwater content changes in the North Atlantic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5025, https://doi.org/10.5194/egusphere-egu22-5025, 2022.

The 20th century “early warming” (1910-1940) and cooling (1940-1970) of the Northern Hemisphere offer an interesting contrast of periods with opposite temperature trends, similar hemispheric temperature anomalies, yet very different temperature anomaly patterns. These contrasts are particularly clear in the North Atlantic sector, which exhibits large climate variability over a range of time scales, from short (weather regimes) to long (Atlantic Multidecadal Variability). In this study, we explore the role of the atmospheric circulation (North Atlantic jet stream) in determining the temperature anomaly patterns over the 20^th century. While different jet configurations are associated with distinct synoptic temperature patterns in the North Atlantic sector, only some are found to contribute substantially to longer term temperature trends. Notably, the southern jet configuration has the strongest temperature anomalies, with a dipole signal that is opposite from the one under the tilted jet configuration. At the same time, these two jet configurations exhibit relatively large decadal variations in frequency (days of occurrence in given winter seasons), with trends that are almost the opposite. In fact, changes in the frequency of southern and tilted jet “days” alone account for much of the North Atlantic and Arctic temperature variability on decadal time scales, including the differences between the early warming and cooling periods (e.g., the flipped warming versus cooling patterns are associated with fewer southern jet days and more tilted jet days). However, the reconstruction skill of the 30-year mean temperature anomaly in the North Atlantic sector using jet frequency exhibits decadal variability, with high skill scores interestingly coinciding with the positive phases of the Atlantic Multidecadal Variability. The lower reconstruction skill especially during the global warming period from the1980s onwards is likely due to the impact from the warming hole in the North Atlantic, which dominates the temperature patterns in the North Atlantic. Overall, the evolution of Northern Hemisphere surface temperature over the 20^th century is found to be influenced by North Atlantic jet variability, with lower frequency ocean effects contributing more in recent decades.

How to cite: Tao, D., Madonna, E., and Li, C.: Using atmospheric variability to understand the wintertime regional warming and cooling patterns in the North Atlantic Sector, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5057, https://doi.org/10.5194/egusphere-egu22-5057, 2022.

Since 2011, Caribbean beaches have been regularly swamped by large quantities of a floating seaweed called Sargassum. Blooms of Sargasssum form large mats in the equatorial Atlantic and at their peak can span from the Gulf of Mexico to west coast of Africa, forming the Great Atlantic Sargassum Belt (GASB). Sargassum beaching events have significant environmental and socio-economic impacts, including impacts on fisheries, tourism, nesting marine animals, and coral reefs. Prior to 2011, Sargassum was predominantly found entrained within the currents of the North Atlantic Subtropical Gyre in the Sargasso Sea. It is thought that an extreme negative phase of the North Atlantic Oscillation (NAO) in 2010/2011 may have produced conditions in the Sargasso Sea that allowed Sargassum to escape and populate further south. The NAO impacts the strength and direction of winds over the Atlantic and modulates ocean properties such as sea surface temperature (SST) and mixed layer depth. Could a change in wind and ocean circulation in 2010 and 2011 explain how Sargassum escaped the ocean gyre as an extreme one-off event? In this study, Lagrangian particle tracking simulations are used to investigate the likelihood of Sargassum leaving the Sargasso Sea between 2009 and 2021, using a velocity field from the Copernicus Marine Environment Monitoring Service (CMEMS) GLORYS12V1 reanalysis. The study’s results show interannual variability in the escape of particles eastwards from the Sargasso Sea into the equatorial Atlantic and Caribbean Sea.

How to cite: Durston, S., Holt, J., Wolf, J., Gommenginger, C., Grosvenor, D., and Lavender, S.: Interannual variability in Sargassum seaweed transport from the Sargasso Sea to the equatorial Atlantic and Caribbean Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5088, https://doi.org/10.5194/egusphere-egu22-5088, 2022.

Accelerated melting of the Greenland Ice Sheet is considered to become a tipping point in the freshwater balance of the subpolar North Atlantic (SPNA). The ramifications of increased freshwater input have been projected to reduce deep convection in neighboring Labrador and Irminger Seas. The East Greenland Current is a primary pathway for transporting Arctic-sourced freshwater and Greenland glacial meltwater into the SPNA. Understanding the variability of the East Greenland (Coastal) Current (EGC/EGCC) is of high importance, as it contains the first imprint of ice melt which flows directly into the current when entering the open ocean. 

We performed a cross sectional analysis of salinity and temperature along the eastern Greenland shelf using output from an eddy-rich (1/20^o) ocean model (VIKING20X), which is forced with time-varying Greenland freshwater fluxes (Bamber et al., 2018), and the observational-based reanalysis product (GLORYS12V1 [1/12^o]) from 1993 to 2019. A time varying mask referenced to a salinity threshold of ≤ 34.8 psu was used to isolate the EGC close to the shelf at five locations for both winter (JFM) and summer (JAS) months. Selected locations are major ocean gateways, glacier outlets/fjords, and observing arrays: Fram Strait, Denmark Strait, just south of 66^oN (Helheim/~Sermilik) and 63.5^oN (Bernstorff), and OSNAP East extending up to the central Irminger Sea. Export of polar water from the Arctic Ocean through Fram Strait sets the initial, low salinity signature in the EGC, which mixes with Atlantic water further downstream and increases in salinity. However, in our simulation, we find lower salinity values again south of Denmark Strait in summer with some notable fresh imprints of extreme meltwater runoff in individual years, such as 2010 and 2012. Furthermore, we observe that for all the cross sections, excluding Fram Strait, there is a negative trend in salinity from 1993 to 2010 followed by a decade in which the salinity trend at Denmark Strait and further south decouples from that in Fram Strait in winter and summer. We explore the reasons for the temporal variations in salinity (and temperature) along the East Greenland Shelf and the potential of different data products to show early imprints of enhanced meltwater runoff into the EGC.

How to cite: Schiller-Weiss, I., Martin, T., Biastoch, A., and Karstensen, J.: Do salinity variations along the East Greenland shelf show imprints of increasing meltwater runoff?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5135, https://doi.org/10.5194/egusphere-egu22-5135, 2022.

Climate models exhibit large differences in the mean state and variability of the Atlantic meridional overturning circulation (AMOC), including in AMOC strength and the characteristic amplitude and frequency of its variability. Across different GCMs, AMOC long-term variability ranges from decadal to multi-centennial and its magnitude from a fraction of to several Sverdrups (Sv). In this study, we conduct ensemble experiments, using the latest coupled model from Institut Pierre Simon Laplace (IPSL-CM6A-LR), to investigate systematically how AMOC variability depends on the AMOC mean state. In the control simulations of this model AMOC mean volume transport is about 12Sv, while AMOC variability is dominated by two distinct modes – a multi-decadal mode with periodicity between 20-30 years and a centennial mode with periods of 100-200 years. The former mode is weaker and driven by temperature variations, while the latter is stronger and driven by salinity anomalies. To modify the mean state of the AMOC in the model we use an indirect method based on robust atmospheric teleconnections from the tropical Indian ocean (TIO) to the Atlantic as described in two recent studies (Hu and Fedorov, 2019; Ferster et al., 2021). Both studies have shown that warming the TIO results in an increased AMOC strength, while cooling the TIO results in a weakened AMOC. To change the Indian ocean temperature in our perturbation experiments we nudge TIO SST by -2°C, -1°C, +1°C, and +2°C; and the experiments last for approximately 1000 years. This allows us to go from a nearly collapsed AMOC state below 3Sv to a more realistic mean state of about 16Sv. We find that both modes of AMOC variability persist throughout the experiments while their amplitude increases almost linearly with AMOC mean strength, yielding linear relationships between the amplitude of variability (standard deviation) and AMOC mean strength of +0.04 Sv per 1 Sv and +0.07 Sv per 1 Sv, respectively. In the experiments that generate 16Sv of AMOC transport, the amplitudes of the two modes reach nearly 0.7 and 1.4Sv. Lastly, we compare the dynamical mechanisms of the two modes and their climate impacts. A corollary of this study is that in this model, a stronger AMOC would lead to stronger climate variability.

How to cite: Fedorov, A., Ferster, B., Mignot, J., and Guilyardi, E.: Multi-decadal and centennial modes of AMOC variability and their dependence on mean state in a climate model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5162, https://doi.org/10.5194/egusphere-egu22-5162, 2022.

How to cite: Hellmich, L., Matei, D., Suarez-Gutierrez, L., and Müller, W. A.: Contribution of the Atlantic Ocean to European Heat Extremes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5694, https://doi.org/10.5194/egusphere-egu22-5694, 2022.

The North Atlantic Jet Stream is well known to leave an imprint on the North Atlantic SST in the form of a tri-polar pattern.  The majority of the existing research has focused on the winter jet stream position or strength of the jet stream.  Here we look at the response of the North Atlantic SSTs to the strength and position of the North Atlantic Jet Stream across all seasons in the CMIP6 piControl simulations.  For the case of both the strength and position of the jet stream the multi-model mean response is a tripolar SST pattern, with the response to the changes in strength showing a slight horseshoe pattern with the northern and southern most anomalies connected on the east and most evident in the summer.  The SST response to winter and spring jet stream changes persist the longest with the northern most imprint on the SSTs lasting up to 2 years.  The response to changes in the jet stream in the summer and fall leave an imprint on the SSTs lasting atmost into the following year.   Furthermore, we investigate at how these responses vary among the CMIP6 models and potential mechanisms leading to the persistence.

How to cite: Mecking, J., Sinha, B., Harvey, B., Robson, J., and Bracegirdle, T.: Seasonal differences in the persistence of SST’s Response to the North Atlantic Jet Stream, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5829, https://doi.org/10.5194/egusphere-egu22-5829, 2022.

Even in models with vertical sidewalls, bottom pressure torques balance the wind stress curl in a zonal integral, with local modification from nonlinear terms. This can be seen explicitly in Stommel's classic 1948 solution in which, unusually, the sea level was calculated as well as the barotropic streamfunction. Here, I explore what this and other idealised solutions tell us about how coastal sea level relates to gyre circulations, western boundary currents, and simple overturning circulations. I show that the coastal sea level signal related to the gyre (or, particularly, to changes in the gyre) need not be stronger at the western boundary. I also show that, although details of where dissipation occurs can be very important for coastal sea level when sloping sidewalls are accounted for, they are much less important for the boundary bottom pressure torque (in the vertical sidewall case, sea level and torque are closely related, so the influence of dissipation on sea level is diminished). Although the real ocean will inevitably be more complex than these ideal cases, consideration of them does alter common assumptions about how coastal sea level is likely to respond to changing circulation patterns, in response to changing climatic forcing.

How to cite: Hughes, C. W.: Sea level, bottom pressure, gyres and overturning: lessons from classical models., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5908, https://doi.org/10.5194/egusphere-egu22-5908, 2022.

Climate model biases in the North Atlantic (NA) low-level tropospheric westerly jet are a major impediment to reliably representing variability of the NA climate system and its wider influence, in particular over western Europe. We highlight an early-winter equatorward jet bias in Coupled Model Inter-comparison Project (CMIP) models and assess whether this bias is reduced in the CMIP6 models in comparison to the CMIP5 models. Historical simulations from the CMIP5 and CMIP6  are further compared against reanalysis data over the period 1862-2005.  

The results show that an equatorward bias remains significant in CMIP6 models in early winter. Almost all CMIP5 and CMIP6 model realizations exhibit equatorward climatological jet latitude biases with ensemble mean biases of 3.0° (November) and 3.0° (December) for CMIP5 and 2.5° and 2.2° for CMIP6. This represents an approximately one-fifth reduction for CMIP6 compared to CMIP5. The equatorward jet latitude bias is mainly associated with a weaker-than-observed frequency of poleward daily-weekly excursions of the jet to its northern position. A potential explanation is provided.  Our results indicate a strong link between NA jet latitude bias and systematically too-weak model-simulated low-level baroclinicity over eastern North America in early-winter.  

Implications for model representation of NA atmosphere-ocean linkages will be presented. In particular CMIP models with larger equatorward jet biases tend to exhibit weaker correlations between temporal variability in jet speed and sea surface conditions over the NA sub-polar gyre (SPG). This has implications for the ability of climate models to represent key aspects of atmospheric variability and predictability that are associated with atmosphere-ocean interactions in the SPG region.  

How to cite: Bracegirdle, T., Lu, H., and Robson, J.: Equatorward North Atlantic jet biases in CMIP models and implications for simulated regional atmosphere-ocean linkages, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6401, https://doi.org/10.5194/egusphere-egu22-6401, 2022.

Using the climate model MIROC4m, we simulate self-sustained oscillations of millennial-scale periodicity in the climate and Atlantic meridional overturning circulation under glacial conditions. We show two cases of extreme climatic precession and examine the mechanism of these oscillations. When the climatic precession corresponds to strong (weak) boreal seasonality, the period of the oscillation is about 1,500 (3,000) years. During the stadial, hot (cool) summer conditions in the Northern Hemisphere contribute to thin (thick) sea ice, which covers the deep convection sites, triggering early (late) abrupt climate change. During the interstadial, as sea ice is thin (thick), cold deep-water forms and cools the subsurface quickly (slowly), which influences the stratification of the North Atlantic Ocean. We show that the oscillations are explained by the internal feedbacks of the atmosphere-sea ice-ocean system, especially subsurface ocean temperature change and salt advection feedback with a positive feedback between the subpolar gyre and deep convection.

How to cite: Kuniyoshi, Y., Abe-Ouchi, A., Sherriff-Tadano, S., Chan, W.-L., and Saito, F.: Effect of Climatic Precession on Dansgaard-Oeschger-like oscillations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6750, https://doi.org/10.5194/egusphere-egu22-6750, 2022.

The steady supply of warm Gulf Stream water to subpolar latitudes is crucial for maintaining a mild, maritime climate in north-western Europe. Ongoing anthropogenic climate change has prompted the oceanographic community to ask whether a slowdown of the North Atlantic circulation has occurred as a response to changes in heat and freshwater fluxes. The question has also caught the attention of policy makers and the media. However, climate models, ocean transport measurements, and paleo and proxy reconstructions show large discrepancies regarding the ‘state’ of the North Atlantic circulation over the historical period. Here, we use available measurements of North Atlantic and Nordic Seas circulation strength to discuss and reflect on potential circulation slowdown. The measurements indicate a stable circulation, but the short record makes distinguishing potential long-term trends from interannual and decadal variability difficult. The sensitivity seen in literature to methodology, data type, region, and time period over which trends are evaluated, demonstrates the lack of robust evidence for a circulation slowdown. The findings warrant caution and nuance in terms of interpreting and communicating research on past and future changes in North Atlantic circulation strength.  

How to cite: Asbjørnsen, H., Eldevik, T., and L. Johnson, H.: Observed changes and coherence in the Gulf Stream system, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7216, https://doi.org/10.5194/egusphere-egu22-7216, 2022.

Changes in the Atlantic Meridional Overturning Circulation (AMOC) are often assumed to lead to equivalent changes in poleward ocean heat transport. Such an assumption leaves only a small role for the ocean gyres in transporting heat poleward. Here, the structure and sensitivity of the North Atlantic thermohaline circulation are investigated with a focus on the comparative role of the horizontal and the vertical circulation components. We use the ECCOv4-r4 ocean state estimate for the period 1992-2017 to evaluate the gyre and overturning contribution in terms of northward volume transport, poleward heat transport, and freshwater transport. The total poleward heat transport increases from the equatorial region northward with a maximum of about 1 PW around 15N, followed by a gradual decrease northward disrupted by another maximum of about 0.5 PW at 50-60N. An important contribution from both the gyre and overturning components is seen at subtropical latitudes, though the components are notably not independent of each other. From about 50N, the gyre component is found to be the dominant contributor to poleward heat transport and equatorward freshwater transport. The results indicate that the gyre circulation in the North Atlantic cannot be ignored in the discussion of mechanisms behind poleward ocean heat transport. 

How to cite: Skrefsrud, J., Eldevik, T., Årthun, M., and Asbjørnsen, H.: On the structure and sensitivity of North Atlantic thermohaline circulation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7301, https://doi.org/10.5194/egusphere-egu22-7301, 2022.

The variations of the winter climate in Europe are influenced by the North Atlantic Oscillation (NAO). Therefore, the ability to predict the NAO is of great value. Predictability of the NAO can be enabled through oceanic processes that are characterized by relatively long time scales, for example interannual to decadal. An important variable for the interannual to (multi-)decadal variability in the North Atlantic is the Atlantic Meridional Overturning Circulation (AMOC). The NAO and the AMOC are known to interact, but observational records of the AMOC are short and the details of this interaction are unknown. Thus, our understanding largely relies on climate model simulations. However, the interaction of NAO and AMOC is very model dependent.

Here, we present the diversity across CMIP6 models in pre-industrial control experiments. The focus lies on simulations of the NAO, the AMOC, their interaction, and related variables on interannual to decadal timescales. Regarding the NAO-AMOC interaction, there are large differences in the strength of their relationship, in the location (like the latitude of the AMOC), its periodicity and in the time-lag between both variables.

Furthermore, we propose hypotheses of the causes for this diversity in the models. Specific processes involved in NAO-AMOC interaction might be of varying relative importance from model to model, for example, NAO-related buoyancy versus wind-forcing affecting the AMOC. Also, mean state difference like in the North Atlantic sea surface temperature might play an important role for causing differences in the variability across models.

How to cite: Reintges, A., Robson, J., Sutton, R., and Yeager, S.: Diversity in NAO-AMOC interaction on interannual to decadal timescales across CMIP6 models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7402, https://doi.org/10.5194/egusphere-egu22-7402, 2022.

Centennial-scale climate variability in the North Atlantic is characterized by the absence of a clear external forcing. Hence, identifying mechanisms of internal variability at these timescales is crucial to understand low-frequency climate variations. For this task, long control simulations with coupled climate models represent a key tool.

Although significant spectral peaks in centennial variability in the Atlantic Meridional Overturning Circulation (AMOC) were found among some state-of-the-art models, CMIP6 models disagree on the amplitude, periodicity and even existence of centennial AMOC variability. This disagreement motivates the use of models of reduced complexity with idealized setups and perturbed physics ensembles to elucidate the mechanisms of AMOC variability at long timescales.

Here, we investigate multi-millennial piControl simulations of PlaSim-LSG, an earth system model intermediate complexity (EMIC). For a range of vertical oceanic diffusion parameters, PlaSim-LSG exhibits strong oscillations of AMOC strength, as well as of salinity and surface temperatures in the North Atlantic, with a period of about 270 years.

Lag correlation analysis shows that a positive feedback involving the interplay of surface salinity, freshwater flux and sea ice concentration in the Norwegian Sea and the Arctic Ocean is the key driver behind these oscillations. In contrast to previous studies with other models, interhemispheric coupling only plays a minor role. We discuss preliminary results of sensitivity experiments for testing the proposed mechanism, and compare our results with previously proposed mechanisms of AMOC oscillations in CMIP6 models.

How to cite: Mehling, O., Angeloni, M., Bellomo, K., and von Hardenberg, J.: Mechanisms of centennial AMOC variability in a climate model of intermediate complexity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7464, https://doi.org/10.5194/egusphere-egu22-7464, 2022.

The Northwest Atlantic continental shelf is home to one of the richest ecosystems in the world, however it is also among the fastest warming regions globally and experienced multiple temperature extreme events, termed marine heatwaves, in the recent decade. These ongoing changes pose a large challenge for the highly valuable fishing industry in the Northeast U.S.. The generally cooler and fresher shelf water is supplied by subpolar waters via the Labrador current, while offshore waters in the Slope Sea, that is the continental slope region bounded by the Gulf Stream and the shelfbreak, are of subtropical origin. Warm core rings shedding of the Gulf Stream transport warm and saline water but also nutrients shoreward and frequently cause cross-shelf intrusions when interacting with bathymetry. The boundary of the two water masses is the Shelfbreak Front and the foot of the front is climatologically found over the 100\,m isobath in the northern Middle Atlantic Bight. While marking a transition of physical properties, the front and its position has also large implications for fisheries as temperate species are found shoreward of the front and more tropical species remain offshore of the front in the warm, saline waters. Monitoring the frontal position is challenging and requires high-resolution sampling, however large and persistent diversions may be detectable in coarser and more sporadic observations. Using data from the Coastal Pioneer Array by the Ocean Observative Initiative along with recent observations obtained during research cruises on the continental shelf and satellite-based sea surface salinity, we assess indicators of the frontal position in recent years. In 2021 the front migrated tens of km inshore for multiple months resulting in irregularities for the regional fishermen. This migration was likely connected to the presence of multiple warm core rings in the Slope Sea, driving record temperatures over the slope and shelf. We address the question whether similar frontal shifts occur more frequently and discuss how these maybe connected to larger scale forcing such as a shifting Gulf Stream, a slowing Atlantic Meridional Overturning Circulation or changes in the supply of subpolar water to the shelf.

How to cite: Ryan, S. and Gawarkiewicz, G.: Shoreward Migration of the Shelfbreak Front in the Middle Atlantic Bight, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8330, https://doi.org/10.5194/egusphere-egu22-8330, 2022.

The Atlantic meridional overturning circulation (AMOC) is key in regulating the global climate system through a large-scale system of currents transporting warm waters northward and cooler waters southward. The Overturning in the Subpolar North Atlantic Program (OSNAP) has been measuring the AMOC directly since 2014, demonstrating that water mass transformation within the eastern subpolar North Atlantic and Nordic Seas dominate AMOC variability in the subpolar North Atlantic. Here, we use OSNAP data to further analyse the AMOC in this region. We find that the North Atlantic Current (NAC) accounts for over 72% of the variability in the upper limb of the AMOC. The easternmost branches of the NAC (over the Rockall Plateau and Trough) account for the majority of the AMOC variability (~38%), even though the westernmost branches account for more than half the mean transport (~10 Sv). The lower limb of the AMOC is found to have a statistically meaningful connection to the circulation in the interior of the Irminger basin, i.e. the Irminger Gyre, accounting for ~38% of the AMOC variability. During the OSNAP time period, a prominent feature of the Irminger basin is a layer of low potential vorticity (PV) in the intermediate water density classes. Further observations (ARMOR3D) show that changes in intermediate water thickness in the Irminger basin are connected to AMOC variability (r = 0.60). We hypothesise a buoyancy-driven mechanism connecting the Irminger Gyre with AMOC variability, where an increase in intermediate water layer thickness in the Irminger basin inhibits the northward recirculation of the Irminger Gyre, leading to a strengthening of the subpolar AMOC.

How to cite: Sanchez-Franks, A. and Holliday, P.: The Irminger Gyre as a key driver of AMOC variability in the subpolar North Atlantic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8436, https://doi.org/10.5194/egusphere-egu22-8436, 2022.

The largest sea surface temperature (SST) anomalies associated with Atlantic Multidecadal Variability (AMV) occur over the Atlantic subpolar gyre, yet it is the tropical Atlantic from where the global impacts of AMV originate. Processes that communicate SST change from the subpolar Atlantic gyre to the tropical North Atlantic thus comprise a crucial mechanism of AMV. Here we use idealized model experiments to show that such communication is accomplished by an “atmospheric bridge.” Our results demonstrate an unexpected asymmetry: the atmosphere is effective in communicating cold subpolar SSTs to the north tropical Atlantic, via an immediate extratropical atmospheric circulation change that invokes slower wind-driven evaporative cooling along the Eastern Atlantic Basin and into the tropics. Warm subpolar SST anomalies do not elicit a robust tropical Atlantic response. Our results highlight a key dynamical feature of AMV for which warm and cold phases are not opposites.

How to cite: Baek, S. H., Kushnir, Y., Robinson, W., Lora, J., Lee, D. E., and Ting, M.: An Atmospheric Bridge Between the Subpolar and Tropical Atlantic Regions:A Perplexing Asymmetric Teleconnection, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8888, https://doi.org/10.5194/egusphere-egu22-8888, 2022.

The Labrador Sea is one of two major sites of the subpolar North Atlantic where deep convection occurs in wintertime as the ambient stratification is weakened through surface cooling and the water column homogenized to up to 2000 m depth. Deep convection has important biogeochemical implications, for example, the ventilation of the deep ocean through the formation of Labrador Sea Water, when convective mixing brings deep-water, undersaturated in oxygen, in contact with the atmosphere. Oceanic eddies in the Labrador Sea, in particular Irminger Rings, are known to transport heat and freshwater from the boundary current towards the central basin. This process regulates the strength of convection by influencing the preconditioning and restratification processes, hence modulating the production of Labrador Sea Water. However, the impact of these eddies on lateral biogeochemical fluxes between the coastal and open Labrador Sea, including the regions where deep convection is most pronounced, remains elusive. In this study, a high-resolution (1/12°) coupled biogeochemical-physical model of the northwest North Atlantic is employed to investigate the role of these eddies for lateral transport of biogeochemical constituents in the three distinct regions: eastern and western boundary and the central Labrador Sea. Oxygen, nutrient, and carbon budgets for these regions will be presented with an emphasis on horizontal and vertical transports, and mean and eddy-driven advection. The results of the biogeochemical budgets will be compared with those from the heat and freshwater budgets.

How to cite: Dilmahamod, A. F., Fennel, K., Laurent, A., and Karstensen, J.: The role of Irminger Rings for biogeochemical tracer advection in the Labrador Sea., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9395, https://doi.org/10.5194/egusphere-egu22-9395, 2022.

Understanding the causes of the variability of the North Atlantic and Mediterranean overturning circulations, and the possible correlation between them is important to disentangle the processes which link the two ocean basins. In this study, we hypothesize that the Gibraltar inflow transport is the main driver of the basin-mean sea surface height variability in the Mediterranean Sea and that they are both anti-correlated to the Atlantic Meridional Overturning Circulation (AMOC) in the North Atlantic.

We analyze here the AMOC and the Mediterranean mean sea surface height (SSH) in an ensemble of eddy-permitting global ocean reanalyses and the Gibraltar inflow transport using an eddy-resolving Mediterranean Reanalysis over the period 1993-2019. In this contribution, firstly we extend the results obtained in past literature with observations (2004-2017 period) and confirm the anti-correlation between the Mediterranean mean sea level and the upper branch of the AMOC at 26.5°N over the 1993-2019 period. Secondly, for the first time, we examine the correlation of the different components of the AMOC and the Gibraltar inflow transport and find significant anti-correlations at interannual time scales.

We show that during years of weaker/stronger AMOC and higher/lower SSH in the Mediterranean Sea, a stronger/weaker Azores Current results in stronger/weaker Gibraltar inflow transport. We argue that the anticorrelation between AMOC and the mean sea level of the Mediterranean Sea is explained by the anticorrelation between AMOC and the Gibraltar inflow transport which in turn is changed by the wind driven Azores current strength.

How to cite: Masina, S., Pinardi, N., Cipollone, A., Banerjee, D. S., Lyubartsev, V., von Schuckmann, K., Jackson, L., Escudier, R., Clementi, E., Aydogdu, A., and Iovino, D.: The Atlantic Meridional Overturning Circulation forcing the mean sea level in the Mediterranean Sea through the Gibraltar transport, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10212, https://doi.org/10.5194/egusphere-egu22-10212, 2022.

Observations between 2004 and 2020 at the RAPID array suggest a weakening trend in the Atlantic Meridional Overturning Circulation (AMOC). To assess the significance of this trend, trends that one might expect from natural variabilty in a time series of this length are assessed using CMIP6 pre-industrial simulations. The observed trend is not found to be statistically significant relative to this benchmark. Both the observed trend and the standard 
deviation of short-term model trends are found to decrease in magnitude with time. The rate of decrease, however, is faster for the observed trend, further calling into question its significance.

To clarify how variability in short-term model trends is related to power spectra of modelled AMOC strength, a conceptual model is developed. Essentially, trend variance is represented by a random walk in which there is one step for each frequency bin of the power spectrum (with step size determined by the frequency and variance of the bin in question). Most models are found underestimate interannual variability in AMOC strength; however, it is the variability at somewhat longer time scales that most influences model trends. This variability is represented quite differently between the various CMIP6 models. The conceptual model is also used to illustrate how the detectability threshold for trend detection (i.e., the 2 sigma level in a PDF of short-term model trends) is altered by the addition of noise added to make AMOC variance more in line with observations. 

How to cite: Straub, D., Kelson, R., and Dufour, C.: Using CMIP6 simulations to assess significance of an AMOC trend seen by the RAPID array, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10282, https://doi.org/10.5194/egusphere-egu22-10282, 2022.

The Northwest Atlantic (NWA) plays a critical role in the global ocean circulation and regulates the global climate system through meridional transport in western boundary currents as well as through deep convection. Global climate change is projected to significantly impact ocean circulation, vertical mixing, and sea ice dynamics in the NWA, with important implications for the area’s biological productivity and carbon export. These physical and biological features and their variabilities are challenging to numerical ocean models and often poorly represented in global climate models. This creates a difficulty in projecting future changes in nutrient dynamics, production, and carbon export. To address these challenges we have developed an advanced coupled circulation-sea ice-biogeochemistry modelling system for the NWA. This modelling system is based on the Regional Ocean Modeling System (ROMS), the Community Sea Ice Model (CICE), and a biogeochemical model including oxygen dynamics and carbon chemistry. The model domain spans the area from Cape Hatteras to Baffin Bay and from the east coast of North America to the central North Atlantic, with the horizontal grid resolution ranging from ~8 km in the south to ~2 km in the north. The circulation and sea ice models are forced by atmospheric and oceanic reanalysis data at the surface and lateral boundaries, respectively. The circulation model is additionally forced by tides, river discharge, and continental runoff. Preliminary model results are presented and compared to various types of observations, with a focus over coastal waters and the deep convection region of the Labrador Sea.

How to cite: Ohashi, K., Laurent, A., Renkl, C., Dilmahamod, F., Yang, S., Fennel, K., Oliver, E., and Sheng, J.: An advanced coupled modelling system to study interactions among the circulation, sea ice, and biogeochemistry in the Northwest Atlantic Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10294, https://doi.org/10.5194/egusphere-egu22-10294, 2022.

The North Atlantic Oscillation (NAO) represents an essential large-scale pattern of utmost importance in the understanding of the wintertime climate variability over North America and Eurasia. Despite a very large number of papers have disentangled the response of regional climate to its temporal changes, only recent works suggest that the role of spatial variability of NAO (NAO flavors) also demands attention (e.g. Rousi et al., 2020). These flavors are defined as the range of positions detected for the NAO action centers, which commonly locate over Iceland (Low) and Azores (High). This work analyses 1) the behaviour of NAO flavors (based on the first empirical orthogonal function -EOF- of Sea Level Pressure field, framed in -90W/40E/20N/80N and computed for chain 30-years periods) in the NOAA-CIRES Reanalysis, and 2) precipitation observations registered in Western Europe (Vicente-Serrano et al., 2021), across the period 1851-2015. One of the main objectives of this contribution is to assess the potential links between NAO flavors and regional wet/dry cycles in the recent past. Results reveal a physically coherent response between this spatial variability of NAO and European precipitation records. Significant positive/negative anomalies of precipitation are distinguished during different NAO flavors, ranged from -40% to +30% compared to the full period average. Likewise, the changes of mean wind direction/speed at mid/low levels have been identified as a potential physical cause. Also, the complex orography contributes to the spatial differences between wet/dry regimes. It should be highlighted that those changes of precipitation have affected European societies and ecosystems. In the case of the Iberian Peninsula, the drastic/strong reduction of winter precipitation and run-off records since 1980s (Halifa-Marín et al., 2021) is attributed to an abrupt shift eastward of NAO low action center. This work thus sheds some light on the lack of knowledge about how NAO flavors contribute to the European climate variability, meanwhile it might help understanding the abrupt shifts on regional precipitation regimes.

Acknowledgments

The authors acknowledge the ECCE project (PID2020-115693RB-I00) of Ministerio de Ciencia e Innovación (MCIN/AEI/10.13039/501100011033/) and the European Regional Development Fund (ERDF/ FEDER Una manera de hacer Europa). A.H-M thanks his predoctoral contract FPU18/00824 to the Ministerio de Ciencia, Innovación y Universidades of Spain. 

References

Halifa-Marín, A., Torres-Vázquez, M. Á., Pravia-Sarabia, E., Lemus-Cánovas, M., Montávez, J. P., and Jiménez-Guerrero, P.: Disentangling the scarcity of near-natural Iberian hydrological resources since 1980s: a multivariate-driven approach, Hydrol. Earth Syst. Sci. Discuss. [preprint], https://doi.org/10.5194/hess-2021-565, in review, 2021.

Rousi, E., Rust, H. W., Ulbrich, U., & Anagnostopoulou, C.: Implications of winter NAO flavors on present and future European climate. Climate, 8(1), 13, https://doi.org/10.3390/cli8010013, 2020.

Vicente-Serrano, S. M., Domínguez-Castro, F., Murphy, C., Hannaford, J., Reig, F., Peña-Angulo, D., ... & El Kenawy, A.: Long‐term variability and trends in meteorological droughts in Western Europe (1851–2018), International journal of climatology, 41, E690-E717, https://doi.org/10.1002/joc.6719, 2021.

How to cite: Halifa-Marín, A., Pravia-Sarabia, E., Vicente-Serrano, S. M., Jiménez-Guerrero, P., and Montávez, J. P.: Assessing the wintertime NAO flavors contribution to wet/dry cycles over Western Europe across the recent past, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10571, https://doi.org/10.5194/egusphere-egu22-10571, 2022.

Since 2014, the Overturning in the Subpolar North Atlantic Program (OSNAP) has maintained the first continuous Eulerian array across the North Atlantic Subpolar Gyre to monitor changes in the Atlantic Meridional Overturning Circulation (AMOC).  The deep limb of the AMOC – the Deep Western Boundary Current (DWBC) – forms in the North Atlantic subpolar gyre from the combination of cold, dense waters of Norwegian Sea origin with the ambient waters within the gyre.  Norwegian Sea Water enters the gyre by crossing southward over the Greenland Scotland Ridge as Denmark Strait Overflow Water to the west of Iceland and Iceland Scotland Overflow Water to the east.  As these waters descend into the Irminger and Iceland Basins (respectively), they entrain the surrounding waters, which are primarily comprised of Labrador Sea Water and Subpolar Mode Water, to increase their transport.  These waters mostly flow cyclonically along the bathymetry of the gyre before merging along the eastern flank of Greenland.  At the eastern tip of Greenland, near Cape Farewell, OSNAP maintains moorings consisting of current meters, acoustic doppler current profilers and temperature-salinity recorders to capture the transport of the DWBC.  This presentation will give new estimates of the DWBC from 6 years of OSNAP observations and shed new light into the current’s variability and long-term trend.  Previous observations at this location found 9-13 Sv of transport, including 10.8 Sv from the first two years of OSNAP data. 

How to cite: Koman, G., Bower, A., Furey, H., and Holliday, P.: Six years of continuous observations of the Deep Western Boundary Current from Cape Farewell, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10712, https://doi.org/10.5194/egusphere-egu22-10712, 2022.

The OVIDE section, composed of a hundred top-to-bottom stations from Portugal to Greenland, has been visited biennially since 2002. Collected data show a strong variability of both the Atlantic Meridional Overturning Circulation (AMOC) and of the water mass properties. The OVIDE-AMOC timeseries built upon the Argo array and altimetry has been updated and validated with the in-situ cruise estimates. It shows a strong seasonal variability and, on longer time scales, significant transition in 2014, from moderate (19 Sv) to strong (23 Sv) amplitude, along with the development of a fresh and cold anomaly in the upper 800m over the eastern subpolar Atlantic, discussed in the literature and observed at the OVIDE section. Through a composite analysis of both transport and property data, we compare the 2002-2012 OVIDE average with the 2014-2018 average and analyze the evolutions of the transports of the different water masses with special attention to LSW, which has been largely renewed since 2014 through deep convection in the western subpolar gyre. 

How to cite: Lherminier, P., Mercier, H., Carracedo, L., Pérez, F. F., Velo, A., Desbruyères, D., Lopez-Mozos, M., and Fontela, M.: Variability of the AMOC and water mass properties at the GO-SHIP OVIDE section over 2002-2018, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10730, https://doi.org/10.5194/egusphere-egu22-10730, 2022.

Following the recommendations of CMIP6, some climate models have for the first time started using a resolution of 1/4° for their oceanic component. This is significant, as it means that large eddies are resolved (so-called eddy-permitting models), introducing chaotic variability in oceanic models. Observational studies of the North Atlantic Subtropical Mode Water (STMW) have found that not all of its variability can be explained by atmospheric variability. The STMW is a water mass formed by ventilation over the winter and is the most abundant T,S class of water in the surface North Atlantic. Consequently it plays a key role in air-sea exchanges over the basin. These elements have motivated the present model investigation of the STMW's ocean-driven (intrinsic) chaotic variability using a NEMO-based, 1/4°, 50-member ensemble simulation of the Northern Atlantic ocean. Using this dataset, six STMW-wide integrated variables are defined and analysed: total volume, and averaged potential vorticity, depth, temperature, salinity and density. The model solution is assessed against the ARMOR3D ocean reanalysis, based on in situ data collected from ARGO floats and satellite observations. The water mass' chaotic variability is estimated from the time-averaged ensemble standard deviation, and is compared to the total variability estimated from the ensemble mean of the temporal standard deviations of all members. Initial results show that chaotic variability is significant for STMW properties at interannual timescales, representing almost half of the total variability of its average temperature. A spectral analysis indicates that chaotic variability remains significant at longer timescales. This suggests that as climate models move towards finer spatial resolution in the ocean, oceanic chaos can be expected to introduce more variability at interannual and longer timescales. This study also highlights the necessity of a good parametrisation of this oceanic chaos in non-eddying ocean models.

How to cite: Narinc, O., Thierry, P., Guillaume, M., and Stéphanie, L.: Chaotic variability of the North Atlantic Subtropical Mode Water, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11847, https://doi.org/10.5194/egusphere-egu22-11847, 2022.

The time series of the Atlantic Meridional Overturning Circulation at 26°N has been extended to March 2020 and is now almost 16 years long.    During the period from 2004 to 2008 the AMOC was c. 2.5 Sv stronger than in the following years.   Since then, there has been significant interannual variability, but the AMOC has remained relatively weak compared with the first four years of observations. The design of the array was changed in 2020 so that continuous measurements are no longer made over the mid-Atlantic Ridge.  In this presentation we examine the impact of this change on the accuracy of the RAPID timeseries. We find that, although the mid-Atlantic ridge measurements have been important in determining the mean structure of the overturning streamfunction, the impact upon the variability of the streamfunction maximum is very small.   

How to cite: Smeed, D., Moat, B., Frajka-Williams, E., Rayner, D., Volkov, D., and Johns, W.: Variability of the Atlantic Meridional Overturning Circulation (AMOC) at 26°N and the design of the RAPID observing array, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11850, https://doi.org/10.5194/egusphere-egu22-11850, 2022.

About 30% of the carbon dioxide derived from human activities (C_ANTH) has been absorbed by the ocean (DeVries, 2014; Gruber et al., 2019; Friedlingstein et al., 2021), with the North Atlantic (NA) being one of the largest C_ANTH sinks per unit area (Khatiwala et al., 2013; Sabine et al., 2004). In the NA, oceanic C_ANTH uptake strongly relies on the meridional overturning circulation and the associated regional winter deep convection. In fact, the formation and deep spreading of Labrador Sea Water stands as a critical C_ANTH gateway to intermediate and abyssal depths. The NA C_ANTH uptake has fluctuated over the years according to changes in the North Atlantic Oscillation. Biennial observation of the marine carbonate system along the Go-Ship A25-OVIDE section has allowed us assessing the decadal and interannual variability of the C_ANTH storage in the subpolar NA from 2002 to 2021. In this study, we investigate 1) the trend of C_ANTH and 2) the relationship between the C_ANTH saturation, the apparent oxygen utilization, and the ventilation of the water masses between the A25-OVIDE section and the Greenland-Iceland-Scotland sills during 2002-2021. We divided the A25-OVIDE section into three main basins (Irminger, Iceland, and Eastern NA). Our results show that the Irminger Basin presents a more homogenous C_ANTH profile and higher C_ANTH saturation values at depth than the other two basins, which is related to the pronounced convective activity in the Irminger Basin. In contrast, the Eastern NA Basin has higher C_ANTH values at the surface due to its higher surface temperature, but its deep water masses show the lowest C_ANTH values since they are the less ventilated in the section. Our analysis also reveals that, overall, the NA C_ANTH storage has increased during 2002-2021, but varied according to the ventilation changes. While the Eastern NA water masses experienced a relatively constant, although shallower, average ventilation, the Irminger and Iceland Basins underwent a less steady C_ANTH uptake pattern characterized by alternating periods of strong and weak C_ANTH storage.

How to cite: López Mozos, M., Velo, A., Fontela, M., de la Paz, M., Carracedo, L., Fajar, N., García-Ibáñez, M. I., Padín, X. A., Desbruyères, D., Mercier, H., Lherminier, P., and Pérez, F. F.: North Atlantic CO2 sink variability revealed by the Go-Ship A25-OVIDE section, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11978, https://doi.org/10.5194/egusphere-egu22-11978, 2022.

Understanding the mechanisms driving variability in the Atlantic Meridional Overturning Circulation (AMOC) on different timescales is essential for better predictions of our evolving climate. The newly updated time series (August 2014 to June 2020) from OSNAP (Overturning in the Subpolar North Atlantic Program) continues to reveal strong intra-annual and interannual variability. However, this six-year record allows us, for the first time, to examine the observation-based seasonal variability of the subpolar overturning circulation. We find that the overturning peaks in late spring from April through June and reaches the minimum in winter for both OSNAP West (a section from the coast of Labrador to West Greenland) and OSNAP East (a section from East Greenland to the Scottish shelf). An analysis of seasonality in the Labrador Sea (OSNAP West) suggests that the delay between wintertime transformation and the observed overturning peak in late spring is consistent with the advection and export of dense Labrador Sea Water along the western boundary. Further analysis is required to understand the mechanism driving seasonal overturning across OSNAP East.

How to cite: Fu, Y. and Lozier, M. S. and the OSNAP Team: Seasonal cycle of the overturning circulation in the subpolar North Atlantic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13044, https://doi.org/10.5194/egusphere-egu22-13044, 2022.

Climate models project an intensification of the wintertime North Atlantic storm track, over its downstream region, by the end of this century. Previous studies have suggested that ocean-atmosphere coupling plays a key role in this intensification, but the precise role of the different components of the coupling has not been explored and quantified. Here, using a hierarchy of ocean coupling experiments, we isolate and quantify the respective roles of thermodynamic (changes in surface heat fluxes) and dynamic (changes in ocean heat flux convergence) ocean coupling in the projected intensification of North Atlantic storm track. We show that dynamic coupling accounts for nearly all of the future strengthening of the storm track as it overcomes the much smaller effect of surface heat flux changes to weaken the storm track. We further show that by reducing the Arctic amplification in the North Atlantic, ocean heat flux convergence increases the meridional temperature gradient aloft, causing a larger eddy growth rate, and resulting in the strengthening of the North Atlantic storm track. Our results stress the importance of better monitoring and investigating the changes in ocean heat transport, for improving climate change adaptation strategies.

How to cite: Chemke, R., Zanna, L., Orbe, C., Zentman, L., and Polvani, L.: The future intensification of the North Atlantic winter storm track: the key role of dynamic ocean coupling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13094, https://doi.org/10.5194/egusphere-egu22-13094, 2022.

As part of the international Overturning in the Subpolar North Atlantic Program (OSNAP), 135 acoustically-tracked deep floats were deployed from 2014 to 2016 to track the spreading pathways of Iceland-Scotland Overflow Water (ISOW) and Denmark Strait Overflow Water (DSOW). These water masses, which originate in the Nordic Seas, compose the deepest branch of the Atlantic Meridional Overturning Circulation. The OSNAP floats provide the first directly observed, comprehensive Lagrangian view of ISOW and DSOW spreading pathways throughout the subpolar North Atlantic. Contrary to a decades-long expectation for how these deep water masses move equatorward, the collection of OSNAP float trajectories, complemented by model simulations, conclusively reveals that their pathways are (a) not restricted to western boundary currents, and (b) remarkably different from each other in character. The spread of DSOW from the Irminger Sea is primarily via the swift deep boundary currents of the Irminger and Labrador Seas, whereas the spread of ISOW out of the Iceland Basin is slower, more diffusive, and along multiple export pathways. The characterization of these overflow water pathways has important implications for our understanding of the Atlantic Meridional Overturning Circulation (AMOC) and its variability. Finally, reconstructions of AMOC variability from proxy data, involving either the strength of boundary currents and/or the property variability of deep waters, should account for the myriad pathways of DSOW and ISOW, but particularly so for the latter.

How to cite: Lozier, S., Bower, A., Furey, H., Drouin, K., Xu, X., and Zou, S.: Overflow Water Pathways in the North Atlantic: New Observations from the OSNAP Program, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13223, https://doi.org/10.5194/egusphere-egu22-13223, 2022.

The eastern North Atlantic subpolar gyre has become a focus of research in recent years, partly in response to the extreme cold anomaly (the 2015CA) that developed in winter 2013-14, peaked in 2015 and persisted in a weakened state for several years. The anomaly was evident both in sea surface temperature which exceeded 1.0 ^oC of cooling averaged over 2015 as a whole and in reduced temperatures at depth to of order 500 m. Here, we place it in a longer-term context by considering other anomalies in the observational record since 1980 and discuss its subsequent evolution through to 2022. We also explore the role played by large scale atmospheric modes of variability, particularly the East Atlantic Pattern (EAP) and North Atlantic Oscillation (NAO), in generating such anomalies. Furthermore, we draw a connection between the combined influence of these modes on both the eastern subpolar gyre and intense heat loss in the Irminger Sea which potentially leads to a coupling of mode and dense water formation processes in these two key North Atlantic regions.

How to cite: Josey, S. and Sinha, B.: Evolution of Cold Subpolar North Atlantic Conditions in the Past Decade, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13394, https://doi.org/10.5194/egusphere-egu22-13394, 2022.

Marine ecosystems are largely impacted by marine heat waves (MHWs). That includes coral reefs which are experiencing coral bleaching and subsequently loss of marine biodiversity because of MHWs. Such reefs are crucial habitat of fish stocks feeding the world’s population. As ocean temperatures increase, the occurrences of MHWs become more frequent. A further solid mechanistic understanding is therefore urgently required for adaptation and mitigation of future changes in MHWs. Importantly, this knowledge is needed on a local-scale.

Here we use a coupled ocean-atmosphere regional modelling system (COAWST), consisting of the atmospheric model WRF and the ocean model ROMS, to dynamically downscale an area over the Caribbean Sea and the Gulf of Mexico. Compared to a global model with coarser horizontal resolution, our 12 km grid spacing resolves smaller scale phenomena and ensures a skilled representation of the air-sea interactions which are important for a correct representation of MHWs. We show the results of a 20-year regional climate simulation and compare the output with two global climate model simulations (NorESM2-MM and NorESM2-MH) to address the added value of the regional simulation. Our high-resolution simulation represents the temporal distribution (frequency and duration) of MHWs well compared to the coarser global models which produce too few, but too long heatwaves in the area.

How to cite: Pontoppidan, M., De Falco, C., Mooney, P. A., and Tjiputra, J.: Marine heat waves: The added value of a high resolution, coupled atmosphere-ocean regional climate model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4629, https://doi.org/10.5194/egusphere-egu22-4629, 2022.

Tropical Atlantic interannual sea surface temperature (SST) variability has significantly weakened since 2000. Here, we use a coupled ocean-atmosphere model with an embedded high-resolution nest in the tropical Atlantic Ocean to investigate future changes in the southeastern tropical Atlantic SST variability in response to anthropogenic global warming. In the model, the Angola-Benguela Area (ABA) is among the regions in the tropical Atlantic that exhibit the largest surface warming. Relative to 1970-1999, the SST variability in the ABA during the peak season, May-June-July (MJJ), decreases by about 24% during 2070-2099 under the worst-case scenario of the Shared Socioeconomic Pathway 5-8.5 (SSP5-8.5). The MJJ interannual temperature variability weakens along the Angolan and Namibian coasts in the top 40 m of the ocean. This reduction appears to be due to a smaller temperature response to thermocline-depth variations, i.e. a weaker thermocline feedback. The weaker thermocline feedback is found where the thermocline deepens the most. Our model results suggest that the trend towards a weakening of the interannual SST variability in the ABA observed during the recent decades could persist in the future under a worst-case global warming scenario.

How to cite: Prigent, A., Imbol Koungue, R. A., Lübbecke, J. F., Brandt, P., Bayr, T., Harlaß, J., and Latif, M.: Future weakening of southeastern Tropical Atlantic Ocean interannual SST variability in a nested coupled model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5292, https://doi.org/10.5194/egusphere-egu22-5292, 2022.

The Angolan shelf system represents a highly productive ecosystem that exhibits pronounced seasonal variability. Productivity peaks in austral winter when seasonally prevailing upwelling favorable winds are weakest. Thus, other processes than local wind-driven upwelling contribute to the near-coastal cooling and nutrient supply during this season. Possible processes that lead to changes of the mixed-layer heat content does not only include local mechanism but also the passage of remotely forced coastally trapped waves. Understanding the driving mechanisms of changes in the mixed-layer heat content that may be locally or remotely forced is also vital for understanding of upward nutrient supply and biological productivity off Angola. Here, we investigate the seasonal mixed layer heat budget by analyzing atmospheric and oceanic causes for heat content variability. By using different satellite and in-situ data, we derive monthly estimates of surface heat fluxes, horizontal advection, diapycnal heat fluxes and local heat storage. The results show that the contribution of horizontal heat advection is small. When considering surface heat fluxes and horizontal heat advection only, the local mixed layer heat budget cannot be closed and the resulting residuum increases closer to the coast. Diapycnal heat fluxes at the base of the mixed layer and uncertainties of surface heat fluxes are suggested to explain the residuum. Our data suggests that the magnitude of diapycnal heat fluxes is controlled by stratification with stronger stratification reducing diapycnal heat fluxes. We conclude that local and remote impacts on stratification need to be examined in order to understand the mixed layer heat budget variability off Angola.

How to cite: Körner, M., Brandt, P., and Dengler, M.: Seasonal mixed layer heat budget in coastal waters off Angola, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5559, https://doi.org/10.5194/egusphere-egu22-5559, 2022.

Models, theory and observations suggest that symmetric instability is excited in the North Brazil Current after it crosses the equator. The instability is fuelled by the advection of waters with anomalous potential vorticity from the Southern to the Northern Hemisphere. There also exists a deep western boundary current which sits below the North Brazil Current. This current advects anomalous potential vorticity across the equator too, and so also becomes symmetrically unstable upon crossing it. Numerical models and scaling arguments will be used to predict the similarities and differences between the action of symmetric instability in the surface and deep currents. We will then explore how the excitement of the instability affects the structure of the deep western boundary current, and how this impacts the development of mesoscale features further down-stream.

How to cite: Goldsworth, F., Marshall, D., and Johnson, H.: Symmetric instability in the surface and deep components of the Atlantic Meridional Overturning Circulation close to the equator, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5802, https://doi.org/10.5194/egusphere-egu22-5802, 2022.

The scarcity of in-situ measurements and the variability among individual events has limited our understanding of the drivers and impacts of the tropical Atlantic Ocean circulation. Here we investigate the response of the surface and subsurface ocean circulation to the two main modes of tropical Atlantic Variability (TAV): the Meridional Mode (MM) and Equatorial Mode (EM). For this purpose, we use three oceanic reanalyses and an interannual forced-ocean simulation covering the period 1982-2018. The developing phase of the MM is associated with a spring intensification of the North Equatorial Countercurrent (NECC), the Equatorial Undercurrent (EUC) and the north South Equatorial Current (nSEC) in the eastern equatorial margin. It also triggers Rossby waves that reach the western boundary and are reflected as equatorial Kelvin waves that weaken the ocean surface and subsurface transports and cause anomalous warm equatorial conditions in boreal summer. During the developing spring-summer phase of the EM, the westward surface zonal transport is considerably reduced with no clear impact at subsurface levels. During the fall EM decaying phase, the reflected Kelvin wave reverses the zonal pressure-gradients at the equator and the westward equatorial nSEC is reinforced. This is accompanied by a weakening of the EUC that suggests an additional off-equatorial forcing. Our results reveal that the ocean circulation responds to both MM and EM, endorsing the key role played by the propagating zonal waves in connecting the tropical and equatorial ocean transports.

How to cite: Martín-Rey, M., Vallès-Casanova, I., and Pelegri, J. L.: Response of the upper ocean circulation to tropical Atlantic interannual modes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6900, https://doi.org/10.5194/egusphere-egu22-6900, 2022.

The equatorial Atlantic is subject to interannual variability that is centered in the eastern cold tongue region and is known as the Atlantic Zonal Mode (AZM). Previous studies have indicated that AZM variability has declined over the recent decades and this tendency is projected to continue based on climate change simulations. The period 2000 to mid-2019 was arguably most conspicuous in this regard, as it did not contain any major AZM event. In late 2019, however, the strongest event in more than 40 years developed. This was followed, in 2021, by an equally warm event. In the present work we examine the mechanisms behind these recent events. We show that while the accompanying wind stress forcing was strong, it cannot account for the exceptional strength of the two events. Analysis suggests that Ekman pumping north of the equator contributed to the strength of the events by generating downwelling Rossby waves that were reflected into downwelling Kelvin waves at the equator. In addition, an examination of observed sea-surface height and ocean temperature from reanalysis and PIRATA buoys suggests that there was a steady buildup of heat in the eastern equatorial region (20W-10E, 10S-5N) since about 2015. This excessive heat content was discharged during the 2019 and 2021 events and may have contributed to their exceptional strength. Our results highlight the need for a close monitoring of oceanic conditions in the region. This will not only have implications for seasonal prediction but also for the long-term development of AZM variability.

How to cite: Richter, I., Tokinaga, H., Okumura, Y., and Keenlyside, N.: Is equatorial Atlantic variability resurging?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11006, https://doi.org/10.5194/egusphere-egu22-11006, 2022.

The Angola Benguela Front (ABF), is a very dynamic area, characterized by a high-temperature gradient of up to 4°C per degree latitude. It fluctuates in position and intensity seasonally which strongly affects the local marine ecosystem. A lot of research, in the past decades, has focused on the SST variability at the interannual timescale in the ABF and the Angolan and Northern Namibian coast to the north and south of it in the contest of Benguela Niños and Niñas. A warming trend since the 1980’s in that region has been reported in the literature and was attributed to a decreasing trend in wind speed. In this study, we look at the processes responsible for the warming in the ABF region. The OGCM NEMO model is used for that matter. The results suggest that the warming is due to various processes acting during different seasons. In autumn, the modelled SST warming trend occurs along the Angolan sector and it is associated with a positive trend in net surface heat flux (Qnet) and with the weakening of the vertical flow associated with the upwelling of cooler water to the surface. In early summer (November-January), the modelled SST warming trend occurs along the Angolan and Namibian sector and it is primarily associated with the intensification of a coastal poleward flow bringing more warm water from the tropics into the ABF region and with the weakening of vertical flow, while locally, Qnet trend generates a cooling trend. The modelled SST cooling trend that occurred south of the ABF, especially in winter and early spring, is primarily associated with a northwards trend in the horizontal subsurface current that advects cooler water from the south and an intensification of the upwelling of cold water to the surface.

How to cite: Tomety, F. S., Rouault, M., Awo, F. M., and Keenlyside, N. S.: Origin of the recent warming along the Angola Namibia coast, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11894, https://doi.org/10.5194/egusphere-egu22-11894, 2022.

Tropical interbasin teleconnections at inter-annual time scales are receiving much attention in the last years. However, their controlling factors and long-term changes are still under debate. In this work, we investigate whether selected features in the climatology, the position of the ITCZ and strong tropical convection, can influence the teleconnections between the tropical Atlantic and Pacific basins at inter-annual timescales.

For investigation, we contrast a CGCM control simulation with an experiment in which the climatological position of the ITCZ is shifted in latitude by artificially reducing the shortwave radiation incident in a region of the south Atlantic sector. The perturbation magnitude and sign are such that the local model’s biases in Atlantic SST are reduced. The experiment shows stronger interannual variability over the tropical Atlantic and Pacific oceans, a westward extension of the Atlantic Niño pattern, and enhanced interannual teleconnections between equatorial Atlantic and Pacific.

We examine the mechanisms at work for these changes. We find several factors as major contributors to enhance the tropical interbasin teleconnections. One is the modified Walker circulation resulting from the westward extension of SST anomalies during the Atlantic Niño and concurrent westward displacement of convection. The other factors are the enhancement of the precipitation at the equator and the shallowing of thermocline in the Pacific, which make the latter basin more sensitive to both local and remote perturbations.

On the contrary, the North Tropical Atlantic – equatorial Pacific teleconnection is weakened in the experiment, despite the strongest impact of the NTA anomalies in the north tropical Pacific winds. due to the opposite effect on divergence exerted by the off equatorial winds related to NTA and the equatorial winds related to the concomitant warming in the eastern and central equatorial Pacific.

How to cite: Losada, T., Rodríguez-Fonseca, B., Mechoso, C. R., Mohino, E., and Castaño-Tierno, A.: Interhemispheric asymmetries, ITCZ location and interannual tropical Atlantic-Pacific interactions produced by South Atlantic cooling., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12308, https://doi.org/10.5194/egusphere-egu22-12308, 2022.

Tropical Instability Waves (TIWs) at the equatorial Atlantic Ocean lead to SST cooling due to enhanced mixing and heat fluxes above the EUC core. This phenomenon has been studied predominantly at the equator and to the north, where TIWs are most pronounced. However, a recent study has shown the presence of subsurface TIWs in the Atlantic Ocean, which frequently occur to the south of the equator. As TIW induced subsurface mixing leads to SST cooling at the equator, we suspect a similar cooling may occur in the Southern Hemisphere due to the presence of subsurface TIWs. Using one decade of high-resolution ICON ocean simulations, we investigate such effect of subsurface TIWs in the southern hemisphere on SST in the tropical Atlantic Ocean. The analysis of all terms of the mixed layer heat budget allows for the investigation and quantification of the processes involved in subsurface TIW induced SST changes.

How to cite: Specht, M. S., Jungclaus, J., and Bader, J.: Influence of subsurface tropical instability waves on sea surface temperature in the tropical Atlantic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12325, https://doi.org/10.5194/egusphere-egu22-12325, 2022.

Observational studies have reported that tropical Atlantic interannual variability impacts
on ENSO in different seasons and periods: Atlantic Ni ̃nos (AN) in boreal summer during
negative phases of the Atlantic Multidecadal Variability (AMV); and tropical north Atlantic
(TNA) in boreal spring during positive AMV. Nevertheless, this relation is not clear for the
whole observational record. This paper is an step forward towards understanding of tropical
Atlantic impacts on ENSO: how and when do they occur? Using observations and a pool of
preindustrial control simulations from the CMIP5 initiative we investigate the background
ocean and atmospheric conditions promoting these tropical interbasin connections.Periods
with a negative AN-ENSO connection appear characterized by a shallower thermocline
over the western Pacific and deeper in the east, together with an increase in interannual
SST variability over the tropics. Periods with a negative TNA-ENSO connection appear
characterized by a steeper thermocline in the Pacific and positive interhemispheric SST
gradient in the the Atlantic. A decrease in tropical Pacific atmospheric and ocean variability
characterizes these periods.

How to cite: Rodriguez-Fonseca, B., Polo Sanchez, I., Mohino Harris, E., Losada Doval, T., Martin del Rey, M., Keenlyside, N., and Mechoso, C. R.: Multidecadal Modulations of Tropical Atlantic impact on ENSO, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12602, https://doi.org/10.5194/egusphere-egu22-12602, 2022.

The Azores High shows a strong intra-seasonal variability that, transmitted to the whole North Atlantic by the Trade Wind, generates a multi-factor variability of the Canary Islands eastern edge upwelling system. In this work, we study the cold season (March to April), using satellite observations and numerical simulation, and how the variability of the wind at the equator, the Kelvin and coastal waves, and the local wind along the North-West African coast combine
to force upwelling variability. Composite analyses show how, in 80% of the cases, the pulsations of the anticyclone at 40 d excite equatorial waves that arrive in the Senegalese upwelling 15 d later, precisely at the time of the phase change of coastal wind anomalies.  These waves trapped at the coast, from upwelling or downwelling, reinforce the local wind anomaly. The intra-seasonal variability of the SST is thus the result of a double local and remote effect whose respective contributions we quantify 

How to cite: Sané, B., Lazar, A., Wade, M., and Gaye, A. T.: Pulsations of the Azores anticyclone at intra-seasonal scale: how oceanic waves and coastal wind anomalies combine constructively to force the variability of the north-eastern boundary upwelling system in winter-spring., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12733, https://doi.org/10.5194/egusphere-egu22-12733, 2022.

Climate variability in the Tropical Atlantic is complex and significantly different than that in the Pacific. A strong ocean-atmosphere coupling is present, sea surface temperature (SST) variability in this region impacts the hydroclimate of the surrounding continents and influences the meridional displacement of the Intertropical Convergence Zone (ITCZ).  We observe a decrease in the variability of the Tropical Atlantic after 1970 in both CMIP6 models and observations. Most of the Tropical Atlantic interannual variability is explained by equatorial and meridional modes. The Atlantic Zonal Mode (AZM) characterizes an equatorial cold tongue. The Atlantic Meridional Mode (AMM) represents an interhemispheric SST anomaly gradient.  Both modes respond to positive ocean-atmosphere feedback: the Bjerkens Feedback controls most of the dynamics underlying the AZM; and a thermodynamic feedback amplifies the AMM, the WES (wind-evaporation-SST) feedback .

            The observed winds relaxation after 1970 in both the equatorial Atlantic region and in the Tropical Northern Atlantic (TNA) plays a role in the decrease of Tropical Atlantic variability, for each mode predominant season. With respect to the AZM, a widespread warming trend is observed in the equatorial Atlantic accompanied by a weakening trend of the trade winds. This drives a weakening in the Bjerkens Feedback by deepening the thermocline in the eastern equatorial Atlantic and increasing the thermal damping. Even though individually the TNA and Tropical South Atlantic (TSA) show increased variability, the observed asymmetric warming in the Tropical Atlantic and relaxed northeast trade winds after the 70s play a role in decreasing the AMM variability. This configuration leads to positive WES feedback, increasing further the TNA SST, preventing AMM from changing phases as before 1970.

            Associated with SST, trade wind trends and decreased Tropical Atlantic variability, the African Sahel shows a positive precipitation trend. The southwest wind anomaly (trade wind relaxation) over the Tropical North Atlantic carries more humidity into the Sahel region, therefore increasing precipitation. As a consequence of the observed trends and decreased variability especially in the AMM, the ITCZ tends to shift northward, which acts on maintaining the increased precipitation over the Sahel.

How to cite: Sobral Verona, L., Silva, P., Wainer, I., and Khodri, M.: Variability Changes in the Tropical Atlantic in CMIP6, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13162, https://doi.org/10.5194/egusphere-egu22-13162, 2022.

The spatiotemporal evolutions of sea surface salinity measurements from the SMOS satellite reveal presence of a local salinity maximum in the northwestern tropical Atlantic beginning in September increasing with a Maximum in October and disappearing in January. Its structure and variability are analyzed through SMOS SSS daily products derived with advanced techniques developed at the Barcelona Expert Centre during 9 years. The results are compared with in situ data along the North Brazil Current (NBC) from the Prediction and Research moored Array in the Tropical Atlantic - PIRATA program. This seasonal tropical SSS maximum, produces the salty signature Northward of the NBC, which is seen as a localized salinity maximum on satellite imagery, in contrast to the fresh signature present in summer-early fall. These changes suggest a change in the composition of water masses that enter in the South Atlantic contributing to an alteration in the dynamics of global circulation.

How to cite: Castellanos, P., Olmedo, E., Campos, E., Santis, W., and Dias, J.: Surface salinity maximum in the western boundary of the Tropical Atlantic as observed from SMOS salinity maps, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13538, https://doi.org/10.5194/egusphere-egu22-13538, 2022.

In 2021 there was an exceptionally strong Atlantic Niño—stronger than the last major event in 1996. Positive SST anomalies developed in May and peaked in June-August. There was a build up of heat content in the spring in the western north Atlantic that could be related to local wind stress curl anomalies.  The event appears to have been triggered by zonal wind anomalies in April and May in the western equatorial Atlantic, when strong rainfall anomalies were also observed along the equator. The event terminated with rainfall anomalies shifting northward in late summer. Interestingly, there was also a strong Benguela Niño that developed already in April and has persisted into boreal summer. Furthermore, the event may have contributed to the La Niña event that developed later in the year in the Pacific.

How to cite: Keenlyside, N.: The Super Atlantic Niño of 2021, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13546, https://doi.org/10.5194/egusphere-egu22-13546, 2022.

High-resolution underwater glider data collected in the Gulf of Oman (2015-16), combined with reanalysis datasets, describe the spatial and temporal variability of the mixed layer during winter and spring. We assess the effect of surface forcing and submesoscale processes on upper ocean buoyancy and their effects on mixed layer stratification. Episodic strong and dry wind events from the northwest (Shamals) drive rapid latent heat loss events which lead to intraseasonal deepening of the mixed layer. Comparatively, the prevailing southeasterly winds in the region are more humid, and do not lead to significant heat loss, thereby reducing intraseasonal upper ocean variability in stratification. We use this unique dataset to investigate the presence and strength of submesoscale flows, particularly in winter, during deep mixed layers. These submesoscale instabilities act mainly to restratify the upper ocean during winter through mixed layer eddies. The timing of the spring restratification differs by three weeks between 2015 and 2016 and matches the sign change of the net heat flux entering the ocean and the presence of restratifying submesoscale fluxes. These findings describe key high temporal and spatial resolution drivers of upper ocean variability, with downstream effects on phytoplankton bloom dynamics and ventilation of the oxygen minimum zone.

How to cite: Font, E., Y. Queste, B., and Swart, S.: Seasonal to intraseasonal variability of the upper ocean mixed layer in the Gulf of Oman, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-109, https://doi.org/10.5194/egusphere-egu22-109, 2022.

According to Food and Agriculture Organization (FAO), the fisheries sector is a major contributor to coastal economy, ensuring nutritional security and generating employment opportunities is the central Indian Ocean covering Lakshadweep (India), Maldives and Sri Lanka. Harvesting of fish in this region happens mainly in coastal waters up to 100m depth. The fishing pressure on the stock in these waters has increased? Considerably and the deep-sea fishery has become an area for expansion in developed countries (FAO). However, fisheries in high seas pose scientific and technical challenges. High value fish are strongly influenced by the physical environment such as temperature, currents etc. Being able to predict this environment with high degree of accuracy is an invaluable tool for assisting on this expansion.

In order to help forecast the physical environment in the Lakshadweep Sea at medium to high resolutions we have developed two pre-operational data assimilating models at 1/20 (called LD20) and 1/60 (LD60) degrees of resolution based on with NEMO v3.6 as an engine. Both models have 50 geopotential computational levels with full steps in the vertical, they use Smagorinsky scheme for horizontal diffusion, bi-Laplacian viscosity for momentum, and k −epsilon turbulence scheme. The models use time-splitting algorithm with the ratio of baroclinic to barotropic time steps equal to 20. The Galperin parametrization is used to preserve the stratification. The models take initial and boundary conditions as well as data for assimilation from a global model at 1/12 degree resolution available from EU Copernicus Marine Service (CMEMS). The bathymetry is taken from GEBCO_2021. Meteorological forcing comes from the Met Office global model (NWPn768 and NWPn1280), and the tides are forced using OTIS tidal scheme (https://www.tpxo.net/otis). Both models run within Rose/Cylc software environment (https://metomi.github.io/rose/2019.01.2/html/index.html), a toolkit for orchestrating the running models that automatically executes tasks according to their schedules and dependencies.

The LD20 and LD60 models use a novel model-to-model data assimilation scheme (Shapiro and Ondina, 2021) by which the observations are assimilated indirectly, via a data assimilating parent model (CMEMS for LD20 and LD20 for LD60). The models have been run for 5 years from 01.01.2015. As expected, the models reveal more granularity of temperature, salinity and currents, particularly in the coastal areas. The model skill was assessed against The Operational Sea Surface Temperature and Ice Analysis (OSTIA) system. The results show improvement of the bias and Root-mean-square-error in the higher-resolution models compared to the lower-resolution ones. The model outputs can be helpful in the identification of small-scale ocean fronts which are linked to Potential Fishing Zones (Solanki et al, 2005)

References

Shapiro, GI. and Gonzalez-Ondina, JM., 2021. Model-to-model data assimilation method for fine resolution ocean modelling, Ocean Sci. Discuss. https://doi.org/10.5194/os-2021-77, in review.

Solanki HU, Mankodi PC, Nayak SR, Somvanshi VS. 2005. Evaluation of remote-sensing-based potential fishing zones (PFZs) forecast methodology. Continental Shelf Research. 25, (18):2163–2173

How to cite: Poovadiyil, M. S., Ondina, J. M. G., Tu, J., Asif, M., and Shapiro, G. I.: Pre-operational high-resolution ocean models of the Lakshadweep Sea (Indian Ocean), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-656, https://doi.org/10.5194/egusphere-egu22-656, 2022.

The Bay of Bengal is under the influence of the monsoon and has a highly contrasted and variable Sea Surface Salinity (SSS). In situ salinity data is however too sparse to reconstruct interannual SSS variability of the Bay of Bengal prior to synoptic SSS mapping of SMOS launched in 2009.

Previous studies have demonstrated the ability of X minus C-band measurements, such as those of AMSR-E (May 2002-Oct 2011), to track SSS changes in high-contrast regions and at high Sea Surface Temperature (SST). Here, we apply this approach to reconstruct the Bay of Bengal SSS before 2010. We remove the effects of other geophysical variables such as SST, surface wind, and atmospheric water content using an empirical approach. SSS is then retrieved based on another empirical fit, trained on the ESA Climate Change Initiative (CCI) SSS dataset, over the AMSR-E and CCI common period (Jan 2010 to Oct 2011). Our first results are encouraging: spatial contrast between the low post-monsoon SSS values close to estuaries and along the west coast of India are reproduced. Our algorithm, however, tends to overestimate low SSS and underestimate high SSS values, possibly due to data contamination near the coast and/or a suboptimal removal of the signals from other geophysical variables. Nevertheless, the first results show a correct representation of the recognizable Indian Ocean Dipole (IOD) phenomena. Furthermore, we are currently creating and studying the use of a neuronal network with the intention to include more parameters in the algorithm.

The long-term goal of this work is to merge the C-, X-, and L-band data with in-situ measurements thus providing a long-term reconstruction of monthly SSS in the Bay of Bengal with a ~50 km resolution This dataset will be used to explore the physical processes that drive interannual SSS variability in regions where it is strong, such as near major river estuaries or along the west coast of India.

How to cite: Montero, M., Reul, N., de Boyer Montégut, C., Vialard, J., and Tournadre, J.: Towards long-term (2002-present) reconstruction of northern Indian Ocean Sea Surface Salinity based on AMSR-E and L-band Radiometer data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1749, https://doi.org/10.5194/egusphere-egu22-1749, 2022.

This study finds that the winter (December–February) barrier layer (BL) in the Bay of Bengal (BoB) acts as a dynamical thermostat, modulating the subsequent summer BoB SST variability and potentially affecting the Indian summer monsoon (ISM) onset and associated rainfall variability. In the years when the prior winter BL is anomalously thick, anomalous sea surface cooling caused by intensified latent heat flux loss appears in the BoB starting in October and persists into the following year by positive cloud–SST feedback. During January–March, the vertical entrainment of warmer subsurface water induced by the anomalously thick BL acts to damp excessive cooling of the sea surface caused by atmospheric forcing and favors development of deep atmospheric convection over the BoB. During March–May, the thinner mixed layer linked to the anomalously thick BL allows more shortwave radiation to penetrate below the mixed layer. This tends to maintain existing cold SST anomalies, advancing the onset of ISM and enhancing June ISM precipitation through an increase in the land–sea tropospheric thermal contrast. We also find that most CMIP5 models fail to reproduce the observed relationship between June ISM rainfall and the prior winter BL thickness. This may be attributable to their difficulties in realistically simulating the winter BL in the BoB and ISM precipitation. The present results indicate that it is important to realistically capture the winter BL of the BoB in air–sea coupled models for improving the simulation and prediction of ISM.

How to cite: Pang, S., Wang, X., Foltz, G. R., and Fan, K.: Contribution of the Winter Salinity Barrier Layer to Summer Ocean–Atmosphere Variability in the Bay of Bengal, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2149, https://doi.org/10.5194/egusphere-egu22-2149, 2022.

The Middle Pleistocene Transition (MPT) represents a critical rearrangement in the Earth’s climate state, expressed as a switch from obliquity-dominated glacial/interglacial patterns towards the quasi-periodic 100 kyr cyclicity that characterized the Earth’s recent climatic history. This fundamental reorganization in the climatic response to orbital forcing occurred without comparable changes in the astronomical rhythms before or during the MPT. Although the MPT has been intensely studied, the triggering mechanisms still remain poorly understood.

High-resolution records from the equatorial to mid-latitude shelf areas are to date rarely considered. For this reason, we investigated an expanded MPT section from International Ocean Discovery Program (IODP) Expedition 356 Site U1460A (eastern Indian Ocean, 27°22.4949′S, 112°55.4296′E, 214.5 mbsl). At Site U1460A, we combine high-resolution records of shallow marine productivity and organic matter flux (Auer et al., 2021) with new benthic and planktonic foraminifera records. By implementing this multi-proxy approach, we aim to better define the response of the Leeuwin Current System over the MPT on tropical shelf regions.

We will investigate benthic foraminifera assemblages at Site U1460A to reconstruct the bottom water community response to the Leeuwin Current System variations during the MPT. At the same time, the benthos/plankton (B/P) ratio of U1460A will be used to constrain the local impact of sea-level changes. Presently work is in progress to generate a B/P ratio for the MPT interval to better assess the impact of sea-level changes on a highly dynamic shelf setting on the western coast of Australia. Shallow coastal areas are markedly sensitive to the glacial/interglacial connected sea-level oscillations. Monitoring the variation in the B/P ratio can provide a preliminary overview of local sea-level changes along the Australian shelf which could be linked to the glacial/interglacial changes of the MPT. Higher values in this ratio indicate lowstand phases, while lower values are characteristic of higher sea level phases. The foraminifera data will be compared to a multi-proxy dataset (Auer et al., 2021) to constrain the local sea-level-driven environmental change over the MPT. Using this we will be able to untangle the impact of local versus global climatic change over the MPT.

Taxonomic identifications are underway following an extensive washing procedure developed for the sample material. Benthic foraminifera show moderate to good preservation, while the planktonic assemblage exhibits moderate to very good preservation. Foraminiferal tests appear white, opaque with apertures, and pores moderately covered by sediment. Some individuals are chipped or partially broken. Specimen preservation (plankton and benthos) decreases during glacial intervals where the abundance of planktonic foraminifera is low.

Finally, we recorded the presence of Globorotalia tosaensis at the top of our study interval at a depth of 61.72 mbsf (corresponding to sample U1460A-14F-3W, 20-24 cm). The continuous presence of this taxon indicates an age older than 0.61 Ma (Wade et al., 2011) at the top of our study interval, and therefore supports the age model of Auer et al. (2021).

How to cite: Arrigoni, A., Auer, G., Petrick, B., Mamo, B., and Piller, W. E.: Multi-proxy study of the Leeuwin Current System evolution along the northwestern coast of Australia during the Middle Pleistocene Transition, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2632, https://doi.org/10.5194/egusphere-egu22-2632, 2022.

“Decadal influence” on the El Nino--Southern Oscillation-Indian summer monsoon (ENSO-ISM) teleconnection have been much studied but with plurality and ambiguity about the concept of influence. We provide formal definitions of the apparent influence of a specific factor which enable us to test them as null-hypotheses. Using the recently released Community Earth System Model v2 (CESM2) Large Ensemble (LE) data, we show that a 50% chance for the detection of the apparent Indian Ocean (IO) influence under stationary conditions might take 2000 years of data. However, we find that this influence is mostly apparent indeed, as it originates from fluctuations of the decadal apparent -- as opposed to climatological -- ENSO variability, which causally influences an IOD-like apparent mean state. We also show that no unattributed so-called “decadal influence”, reflected in a deviation from a linear regression model of the teleconnection as a null-hypothesis, can be detected in 20th c. observations even regionally.  Only the LE data is sizable enough to reveal this effect.

How to cite: Bodai, T., Sundaresan, A., Lee, J.-Y., and Lee, S.-S.: Indian Ocean influence on the ENSO-Indian monsoon teleconnection is mostly apparent, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3397, https://doi.org/10.5194/egusphere-egu22-3397, 2022.

Based on SODA reanalysis data set from 1980 to 2016, this paper combined with a variety of mathematical statistical methods to study the intraseasonal variability characteristics of barrier layer thickness and its physical correlation with climate modes in the Bay of Bengal, and quantitatively explored the dynamic mechanism of intraseasonal variability of barrier layer in different sea areas in the Bay of Bengal by means of Marine dynamic diagnosis method. The relative contributions of different physical processes, such as oceanic advection, Kelvin waves, Rossby waves and freshwater fluxes (rainfall and river runoff), to the barrier layer were evaluated. The physical relationship between the seasonal variation of barrier layer thickness and the Indian Ocean dipole (IOD) is also discussed. The results show that the thickness of the barrier layer varies most obviously in the northern coast of the bay of Bengal and the western coast of Sumatra, and the maximum value of the barrier layer occurs in November ~ December every year, while the variation of the barrier layer in the northern coast is more regular than that in the southern coast. Horizontal advance and entrainment affect the thickness of barrier layer by affecting the salinity of the mixed layer. However, the thickness of barrier layer is mainly caused by the change of isothermal layer due to the obvious stratification of sea surface salinity in the Bay of Bengal. In the southern part of the Bay of Bengal near the equator, during the positive IOD events, the isothermal layer shallowness was caused by the negative anomaly of equatorial zonal wind stress from October to December. In negative IOD events, the equatorial zonal wind stress appears positive abnormality after June, which leads to the increase of isothermal layer in this period. As a result, the thickness of barrier layer In positive IOD years is smaller than that in normal years from October to December, and that in negative IOD years is greater than that in normal years from June to September. However, in the northern Bay of Bengal, the seasonal variability of barrier layer caused by different IOD events was not obvious. At the same time, the net heat flux upward at the air-sea interface will lead to instability and deepen the local mixed layer.

How to cite: Li, Y. and Wang, X.: Intraseasonal Variability in Barrier Layer Thickness in the Bay of Bengal and its Causes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3453, https://doi.org/10.5194/egusphere-egu22-3453, 2022.

Marine heatwaves (MHWs) in the tropical Indian Ocean (TIO) showed remarkable increases in duration and frequency during the satellite observing era, responding to rising sea surface temperature. Long-lasting MHWs were found in three upwelling regions of the TIO in 2015–2016 and 2019–2020, closely related to persistent downwelling oceanic planetary waves. In 2015, a prolonged MHW (149 days) in the western TIO was initiated by the downwelling Rossby waves associated with the co-occurring super El Niño and positive Indian Ocean dipole (IOD) events. In the following year, the negative IOD sustained the longest MHW (372 days) in the southeastern TIO, prompted by the eastward-propagating equatorial Kelvin waves. In 2019–2020, the two longest MHWs recorded in the southwestern TIO (275 days in 2019 and 149 days in 2020) were maintained by the downwelling Rossby waves associated with the 2019 extreme IOD. This study revealed the importance of ocean dynamics in long-lasting MHWs in the TIO.

How to cite: Zhang, Y., Du, Y., Feng, M., and Hu, S.: Long-Lasting Marine Heatwaves Instigated by Ocean Planetary Waves in the Tropical Indian Ocean During 2015–2016 and 2019–2020, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4358, https://doi.org/10.5194/egusphere-egu22-4358, 2022.

The sea surface salinity (SSS) maximum of the South Indian Ocean (the SISSS-max) is a large, oblong, high-salinity feature centered at 30degS, 90degE, at the center of the South Indian subtropical gyre. It is located poleward of a region of strong evaporation and weak precipitation. Using a number of different satellite and in situ datasets, we track changes in this feature since the beginning of the Argo era in the early 2000's. The centroid of the SISSS-max moves seasonally north and south, furthest north in late winter and farthest south in late summer. Interannually, the SISSS-max has moved on a northeast-southwest path about 1500 km in length. The size and maximum SSS of the feature vary in tandem with this motion. It gets larger (smaller) and saltier (fresher) as it moves to the northeast (southwest) closer to (further from) the area of strongest surface freshwater flux. The area of the SISSS-max almost doubles from its smallest to largest extent. It was maximum in area in 2006, decreased steadily until it reached a minimum in 2013, and then increased again. The seasonal variability of the SISSS-max is controlled by the changes that occur on its poleward, or southern, side, whereas intereannual variability is controlled by changes on its equatorward side. The variations in the SISSS-max are a complex dance between changes in evaporation, precipitation, wind forcing, gyre-scale ocean circulation and downward Ekman pumping. Its motion correlated with SSS changes throughout the South Indian Ocean and is a sensitive indicator of changes in the basin's subtropical circulation.

How to cite: Bingham, F., Gordon, A., and Brodnitz, S.: Seasonal and Interannual Variability of the South Indian Ocean Sea Surface Salinity Maximum, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4578, https://doi.org/10.5194/egusphere-egu22-4578, 2022.

Upwelling along the western boundary of the Arabian Sea and the Indian summer monsoon rainfall are positively correlated in modern observations. Upwelling transports nutrients into the euphotic zone and thus controls primary productivity. Therefore, primary productivity in the region of upwelling has been used to reconstruct monsoons of the distant past. Such reconstructions suggest that monsoons lag insolation by about 9 kyrs (nearly out-of-phase), contrary to several speleothem-based reconstructions that indicate a more in-phase relation of monsoon with insolation. Using results from transient as well as time-slice experiments, we have shown that factors other than the Indian monsoon affect upwelling on the precession time scales. These factors modulate the spatial extent of upwelling, resulting in the precession-scale variability in primary production. This is in contrast with modern observations, where most of the variations in primary productivity are a result of changes in the intensity of upwelling. We find that the spatial extent of upwelling is nearly out-of-phase with insolation. Thus, primary productivity lags insolation. We conclude that primary productivity in the Arabian Sea is not a good proxy for the Indian summer monsoon rainfall.

How to cite: Jalihal, C., Srinivasan, J., and Chakraborty, A.: Precession-scale variability of upwelling in the Arabian Sea and its implications for proxies of Indian summer monsoon, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4602, https://doi.org/10.5194/egusphere-egu22-4602, 2022.

The tropical Indian Ocean (TIO) basin-wide warming occurred in 2020, following an extreme positive Indian Ocean Dipole (IOD) event instead of an El Niño event, which is the first record since the 1960s. The extreme 2019 IOD induced the oceanic downwelling Rossby waves and thermocline warming in the southwest TIO, leading to sea surface warming via thermocline-SST feedback during late 2019 to early 2020. The southwest TIO warming triggered equatorially antisymmetric SST, precipitation, and surface wind patterns from spring to early summer. Subsequently, the cross-equatorial “C-shaped” wind anomaly, with northeasterly–northwesterly wind anomaly north–south of the equator, led to basin-wide warming through wind-evaporation-SST feedback in summer.

The TIO warming excited a strong and westward extend anomalous anticyclone on the western North Pacific (WNPAC). The WNPAC is usually associated with strong El Niño-Southern Oscillation (ENSO), except for the 2020 case. In 2020, the anomalous winds in the northwestern flank of the WNPAC bring excess water vapor into central China. The water vapor, mainly carried from the western tropical Pacific, converges in central China and result in heavy rainfall. Unlike extreme events in 1983, 1998, and 2016, the extreme rainfall in 2020 was the first and only event during 1979-2020 that followed an extreme positive IOD rather than a strong El Niño. A theory of regional ocean-atmosphere interaction can well explain the processes, called the Indo-Western Pacific Ocean Capacitor (IPOC) effect. This study reveals the importance of IOD in the IPOC effect, which can dramatically influence the East Asian climate even without involving the ENSO in the Pacific.

How to cite: Du, Y., Cai, Y., Chen, Z., and Zhang, Y.: Extreme IOD induced IOB warming and its impacts on western North Pacific anomalous anticyclonic circulation transport in early summer 2020: without significant El Nino influence, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5055, https://doi.org/10.5194/egusphere-egu22-5055, 2022.

In this study, the possible associations between the precipitation in the Southeastern Africa (SEAF, in this study area between 10°S to 25°S and 25°E to 53°E,) and the Antarctic Oscillation (AAO) in seasons from October to March (DJFM) was investigated. A statistically significant three-month lag correlation between them was found. After removing the El Niño/Southern Oscillation and Indian Ocean dipole signals, AAO from August to October (ASO-AAO) and DJFM-precipitation was significantly correlated, and the interannual correlation coefficients calculated by CMAP, GPCP, CRU, and GPCC were +0.63, +0.42, +0.59, and +0.53 (p<0.05), respectively. The positive correlation suggests that an enhancing (weakening) ASO‐AAO could be conducive to increases (decreases) of DJFM-precipitation in SEAF in austral summer. Further analyze the corresponding water vapor and circulation conditions. The responses of local and regional meteorological conditions to the ASO‐AAO support the AAO-precipitation links. During positive ASO-AAO years, in the troposphere low level is a cyclonic flow field in the high level is an anticyclonic circulation, accompanied by an enhanced ascending motion, and such a structure is favor to rainfall. A preliminary mechanism analysis shows that a positive ASO-AAO may induce a sea surface temperature warming tendency in Western Equatorial Indian Ocean.  This warming then enhances the regional ascending motion in SEAF and enhances the convection precipitation on the northwest SEAF. Moreover, the anomalous sensible and latent heating, in turn, intensifies the cyclone through a Gill-type response of the atmosphere. Through this positive feedback, the tropical atmosphere and SST patterns sustain their strength from spring to summer and eventually the SEAF precipitation.Note that’s for simplicity, the AAO index was multiplied by −1 throughout this study.

How to cite: Du, C. and Gong, D.: Influence of Antarctic Oscillation on the Southeastern Africa summer precipitation during 1979-2018, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5207, https://doi.org/10.5194/egusphere-egu22-5207, 2022.

In the current study, we analyzed the predictability of the tropical Indian Ocean precipitation anomalies and the North Atlantic European (NAE) circulation anomalies during the boreal early winter season using the ECMWF System-5 seasonal (SEAS5) prediction dataset. The observational analysis show that the boreal Autumn Indian Ocean dipole (IOD) conditions are the pre-courser for the early winter precipitation anomalies in the Tropical Western-Central Indian Ocean (TWCIO) region, which is well represented in the ECMWF-SEAS5 prediction system. Moreover, the ECMWF-SEAS5 skillfully predicts the Indian Ocean (IO) precipitation anomalies with some biases during the early winter. These biases tend to weaken the IO teleconnections to the NAE Region during the boreal early winter, mimicking the prediction skill of the NAE circulation anomalies. Furthermore, the positive TWCIO heating anomalies tend to favor the above normal Surface Air temperature (SAT) conditions over the NAE region, indicating to the mild early winter conditions over the region. The ECMWF-SEAS5 system shows a significant prediction skill of the surface temperature anomalies over the NAE region.

How to cite: Abid, M. A., Kucharski, F., and Molteni, F.: Predictability of the Indian Ocean and North Atlantic European circulation anomalies during early winter, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6567, https://doi.org/10.5194/egusphere-egu22-6567, 2022.

The El Niño-Southern Oscillation (ENSO) has great impacts on the Indian Ocean sea surface temperature (SST). In fact, two major modes of the Indian Ocean SST namely the Indian Ocean Basin (IOB) and Indian Ocean Dipole (IOD) modes, exerting strong influences on the IO rim countries, are both influenced by the ENSO. Based on a combined linear regression method, this study quantifies the ENSO impacts on the IOB and IOD during ENSO concurrent, developing, and decaying stages. After removing the ENSO impacts, the spring peak of the IOB disappears along with significant decrease in number of events, while the number of events is only slightly reduced and the autumn peak remains for the IOD. By isolating the ENSO impacts during each stage, this study reveals that the leading impacts of ENSO contribute to the IOD development, while the delayed impacts facilitate the IOD phase switch and prompt the IOB development. Besides, the decadal variations of ENSO impacts are various during each stage and over different regions. These imply that merely removing the concurrent ENSO impacts would not be sufficient to investigate intrinsic climate variability of the Indian Ocean, and the present method may be useful to study climate variabilities independent of ENSO.

How to cite: Zhang, L. and Du, Y.: Revisiting ENSO impacts on the Indian Ocean SST based on a combined linear regression method, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6704, https://doi.org/10.5194/egusphere-egu22-6704, 2022.

The weakening of zonal atmospheric circulation, a widely accepted projection of climate change in response to global warming, features a weakening of the Indian Ocean Walker circulation (IWC), with an anomalous ascending motion over the western and anomalous descending motion over the eastern Indian Ocean.  The projected IWC weakening has previously been attributed to slower warming in the east than the west, that is, to a positive Indian Ocean Dipole (IOD)-like warming pattern.  However, such a warming pattern can also be induced by IWC weakening. As a result, the cause-and-effect relationship cannot be easily determined, and the projected change is poorly constrained and highly uncertain. Here, using a suite of coupled climate model simulations under a high-emission scenario, we find that the IWC slowdown is accompanied by not only a positive IOD-like warming pattern but also anomalous meridional circulation that is associated with anomalous descending motion over the eastern Indian Ocean. We further show that the anomalous local meridional circulation is closely linked to enhanced land-sea thermal contrast and is unlikely to result from the positive IOD-like warming pattern, suggesting that the IWC weakening is in part driven by the anomalous local meridional circulation. Our findings underscore the important role of local meridional circulation changes in modulating future IWC changes. 

How to cite: Sharma, S., Ha, K. J., Cai, W., Chung, E.-S., and Bódai, T.: Local meridional circulation changes contribute to a projected slowdown of the Indian Ocean Walker circulation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6852, https://doi.org/10.5194/egusphere-egu22-6852, 2022.

Accurate forecasts of tropical Indian Ocean variability are crucial for skilful predictions of climate anomalies on a range of spatial and temporal scales. Here we assess the ability of ECMWF’s operational monthly and seasonal prediction systems to represent variability in the Eastern Equatorial Indian Ocean (EEIO), an important center of action especially for the Indian Ocean Dipole (IOD) Mode. Strong air-sea coupling is present in this region. In ECMWF’s currently operational seasonal prediction system, this leads to rapid amplification of a weak cold bias of the oceanic initial conditions in the EEIO, resulting in too frequent occurrences of positive IOD events. Diagnostics show that this is related to winds in the EEIO exhibiting a biased relationship with local and remote sea surface temperatures when compared to reanalysis. The impact of the forecast bias in the EEIO on the skill of ENSO predictions via interbasin interactions is evaluated. We furthermore present results from numerical experiments with, i.a., changed atmospheric model physics and oceanic initial conditions which help to better understand causes of the diagnosed forecast errors as well as mechanisms of interbasin interaction, and provide guidance for model development.

How to cite: Mayer, M., Alonso Balmaseda, M., and Johnson, S.: Understanding and reducing seasonal prediction errors of the ECMWF system in the tropical Indian Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7416, https://doi.org/10.5194/egusphere-egu22-7416, 2022.

The role of the Indian Ocean heating anomalies in the ENSO teleconnection to South Asia and North Atlantic/European regions are investigated in the early winter season. Using re-analysis data, CMIP5 simulations and idealized numerical model experiments it is shown that the ENSO teleconnections in early winter in these regions are dominated by an ENSO-induced heating dipole in the Indian Ocean region. The Indian Ocean heating dipole leads to a Gill-type response in the South Asian region through Sverdrup balance. For a warm ENSO event, this response is a cyclonic upper-level anomaly that shifts the subtropical South Asian jet southward and increases precipitation in the that region. The cyclonic anomaly is the starting point of a stationary Rossby wavetrain that traverses the North Pacific and North American region and eventually reaches the North Atlantic. Here transient eddy feedbacks are likely to strengthen a response that spatially projects on the positive phase of the NAO and negative phase of the Atlantic ridge patterns. For cold ENSO events these anomalies are roughly opposite. The importance of the Indian Ocean heating dipole decreases towards late Winter due to a southward shift of the Indian Ocean rainfall climatology and a more dominant direct wavetrain from the central Pacific region.

How to cite: Kucharski, F., Abid, M. A., Joshi, M. K., Ashfaq, M., and Evans, K. J.: Role of Indian Ocean heating anomalies in the early winter ENSO teleconnection to the South Asian and North Atlantic regions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9122, https://doi.org/10.5194/egusphere-egu22-9122, 2022.

In recent decades, worldwide marine heat wave events have become stronger and more frequent. Especially in the Indian Ocean, where occurs the most significant sea surface temperature warming trend. We use observation and reanalysis data to extract the Indian Ocean marine heatwave events since 1981. And then analyzing the temporal and spatial characteristics of marine heatwave events through feature indicators. According to the different period of the development of the marine heatwave, the sources of predictability from the atmospheric and ocean circulation anomaly are revealed. Then five representative heat wave events will be selected, and multi-member ensemble hindcast with different lead times will be conducted for each event with CESM2 model. Based on the hindcast results, we evaluate the prediction skills for the Indian Ocean marine heatwaves. The capability of models to simulate the sub-seasonal to seasonal signals that affect the heat wave event will be examined eventually.

How to cite: Yu, Y.: The subseasonal to seasoanl predictability of marine heatwave in Indian Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9751, https://doi.org/10.5194/egusphere-egu22-9751, 2022.

The Andaman Sea, located in the Indian Ocean's northeastern region, is well known for its large-amplitude internal waves. The Indian Ocean Dipole, according to recent research, has a significant impact on the interannual variability of density stratification and internal wave activity in this region. The global climate model CanESM5 has demonstrated a reasonable ability to capture the variability of the Indian Ocean Dipole in its historical simulations. As a result, the long-term variability of internal waves is investigated using the CanESM5 density stratification. The stratification showed an increasing trend in the upper 100 m since 1900 due to radiative forcing. Internal wave activity is expected to increase in the twenty-first century, altering the effects of climate change on coastal ecosystems. Additionally, model simulations utilizing the three-dimensional Massachusetts Institute of Technology general circulation model are conducted to investigate the impact of increasing stratification on internal tides. Variations in the generation, propagation, and dissipation of internal tides along with their basic characteristics are quantified.  

How to cite: Yadidya, B. and Devendra Rao, A.: The effect of climate change on internal wave activity in the Andaman Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10745, https://doi.org/10.5194/egusphere-egu22-10745, 2022.

Various intercomparison studies have demonstrated significant disagreements and biases in the Southern Ocean’s (SO) representation in Earth System Models (ESMs). Examples include discrepancies in the strength and location of westerly wind forcing, water mass properties, or the seasonal air-sea CO₂ flux phase and strength. To better understand these discrepancies, we investigate the influence of atmospheric forcing and coupling on the SO hydrodynamics based on the ocean component of the UKESM1, consisting of NEMO (ocean physics), MEDUSA (marine biogeochemistry) and CICE (sea ice). We compare a fully coupled historical UKESM1 run and an ocean-only run initialised on 01/01/1980 from the coupled run but forced with the ERA-Interim atmospheric reanalysis between 1980-2014. The first years after initialising the ocean-only run shows a strong loss in upper ocean buoyancy (top 1000 m), reducing the overly strong stratification present in the coupled run compared to observations. In response the horizontal circulation changes, for example the Antarctic Circumpolar Current (ACC) extends deeper and is more confined meridionally in the ocean-only run while maintaining a similar strength. Furthermore, stratification and circulation changes allow for deeper winter mixed layers in the mode water formation regions north of the ACC in the ocean-only run, better matching the observations. Thus, the representation of mode water properties improves in the ocean-only run, as well as the overall water column structure. These highlighted differences between the two runs further affect the SO’s export of water masses to the global ocean, and the local variability of biogeochemistry: for example, the seasonal cycle of air-sea CO₂ flux in mode water formation regions has the opposite phase in the coupled compared to ocean-only run. Our results highlight room for diverse improvements in the representation of SO dynamics in ESMs, ultimately improving global climate projections.

How to cite: Rochner, A., Ford, D., Sheen, K., and Watson, A.: Exploring the influence of atmospheric forcing on Sub-Antarctic Southern Ocean hydrography and air-sea CO2 flux in coupled and ocean-only simulations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-257, https://doi.org/10.5194/egusphere-egu22-257, 2022.

The first autonomous surface vehicle (Saildrone) circumnavigation of Antarctica revealed spatio-temporally variable chlorophyll with occasionally high concentrations in late autumn/early winter. Low light availability at this time of year makes high chlorophyll concentrations unexpected and the patterns of variability hint at physical processes, such as submesoscale fronts, controlling chlorophyll distributions. Here, we assess the physical drivers of spatio-temporal chlorophyll distribution measured by the Saildrone. Together with large-scale variability in surface heat and light, we identify submesoscale (0.1-10 km) frontal activity and regions of high eddy kinetic energy to characterize possible physical drivers of the observed variability in chlorophyll. Autonomous platforms measuring oceanic variables at fine spatial and temporal resolution are enabling new discoveries, such as this one, and open the door to understand the impact of submesoscale flows on the local ecosystem.

How to cite: Joy-Warren, H., Giddy, I., Rosenthal, H., du Plessis, M., and Swart, S.: Physical drivers of patterns in autumn—winter chlorophyll variability from Saildrone measurements in the Southern Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-320, https://doi.org/10.5194/egusphere-egu22-320, 2022.

Vertical and lateral exchanges of heat and carbon make the Southern Ocean a key player in regulating global climate, yet its role in future climate change remains uncertain. To address this knowledge gap, paleoceanographers study the state of the Southern Ocean under past climate states to better understand the processes governing its role in global climate. For instance, the Southern Ocean is widely thought to play a driving role in the atmospheric CO₂ fluctuations of the ice ages, ventilating carbon-rich deep waters to the atmosphere during interglacial periods and limiting this deep-surface exchange during glacial periods. However, direct evidence of these dynamics and of the Southern Ocean’s overall role in glacial CO₂ draw down remains limited.

Here we present a suite of geochemical data that provides new insights into Southern Ocean carbon cycling and circulation, evincing deep-ocean carbon storage over the last glacial cycle. Trace element and stable isotope (δ¹³C, δ¹⁸O) compositions of foraminiferal calcite from the high-latitude Indian Ocean demonstrate how carbon was sequestered in the deep ocean during glacial intensification and subsequently released to surface waters during deglaciation. These dynamics are captured by geochemical records reflecting temperature, pH, and circulation changes, providing key insights into the processes responsible for this carbon cycling. This observational data provides the foundation for developing a better mechanistic understanding of the Southern Ocean’s role in past and future climate change, including processes such as advection and mixing, ocean-ice interactions, and productivity.

How to cite: Shankle, M., Trudgill, M., Euverte, R., Michel, E., Gray, W., and Rae, J.: Southern Ocean CO2 draw down and release on glacial-interglacial timescales, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-358, https://doi.org/10.5194/egusphere-egu22-358, 2022.

The Southwest Atlantic section of the Southern Ocean is a highly energetic confluence zone where Pacific and Antarctic waters flow via the Antarctic Circumpolar Current (ACC) to merge with waters from the Atlantic Ocean and contribute to the global overturning circulation. However, there is an insufficient understanding of sub-mesoscale variability in the region. Such processes are known to play an important role in the vertical and lateral exchange of water masses, along with tracers such as carbon, atmosphere-ocean exchange, ocean productivity, and the mixing budget necessary to complete the overturning circulation. In particular, observations of the subsurface structure of frontal systems on high spatial scales are currently lacking, with typical hydrographic transects being too coarse to resolve sub-mesoscale processes and ACC filaments.

Here we present the first multichannel seismic images of ocean finestructure to the north of the North Scotia Ridge in the Southern Ocean, which cross several frontal systems and bathymetric features. High-resolution (O(10m)) sections of sub-surface thermohaline structure are revealed and analysed by combining the acoustic information with hydrographic (CTD, XBT and ARGO floats), current velocity (VMADCP) and satellite altimetry data. In addition, diapycnal mixing and potential vorticity estimates are generated from acoustic data. The sections reveal an intricate and complex pattern of oceanic finestructure: very high thermohaline gradients are present in shallow and intermediate waters of up to 700 m depth associated with Subantarctic Surface Water, Subantarctic Mode water and a mix of Antarctic Intermediate Water and Antarctic Surface Water; curving features, lenses and filaments with length scales of 100m-10km are found at the Polar Front and Southern ACC front; and steep continuous gradients with separate filaments are typically present in deeper sections (up to 2000 m) associated with Circumpolar Deep Water. Furthermore, acoustic reflections provide evidence that bathymetric features like the Maurice Ewing Bank or the Northeast Georgia Rise disrupt the flow of intermediate and deep water in the region and enhance diapycnal mixing.

How to cite: Ehmen, T., Sheen, K., Watson, A., Brearley, A., Palmer, M., Roper, D., and Thompson, A.: High resolution acoustic imaging of frontal dynamics and thermohaline finestructure in the Southwest Atlantic sector of the Southern Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-411, https://doi.org/10.5194/egusphere-egu22-411, 2022.

How to cite: Sallée, J.-B.: Southern Ocean Carbon and Heat Impact on Climate (SOCHIC): processes and long term change, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-764, https://doi.org/10.5194/egusphere-egu22-764, 2022.

The Southern Ocean is fundamental for our climate, accounting for 75% of the total oceanic heat uptake and absorbing 93% of excess heat arising from global warming. However, direct observations of air-sea heat fluxes are still scarce, particularly at small spatial and temporal scales. This study investigates the effect of fine-scale frontal activity (0.1 km–10 km) and sampling bias on measured turbulent heat fluxes in the Southern Ocean using high-resolution hydrographic and meteorological data collected by three autonomous surface vehicles (Saildrones) during their 2019 circumnavigation of Antarctica. We show that the uncertainty in observed air-sea heat fluxes increases with reduced sampling frequency. To have a  90% chance of capturing the mean turbulent heat flux within ±1 Wm², the required sampling resolution is less than 30 km in summer and less than 10 km in winter. Surface temperature-driven density fronts were found to be numerous throughout the in situ datasets and ranged in length-scales from sub-kilometer to mesoscale (order of 0.1 km–100 km). The magnitude and variability of the turbulent heat flux gradient over these fronts tends to decrease with increasing frontal length, suggesting a strong coupling between heat fluxes and front size, thereby underscoring the need to capture fine-scale oceanographic features to better resolve air-sea fluxes of heat.

How to cite: Rosenthal, H., Biddle, L. C., Swart, S., Gille, S. T., Mazloff, M. R., and du Plessis, M.: The impact of submesoscale fronts on turbulent air-sea heat fluxes in the Southern Ocean: Results from the first Saildrone circumnavigation of Antarctica, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-892, https://doi.org/10.5194/egusphere-egu22-892, 2022.

Warm, salty Circumpolar Deep Water (CDW) has long been regarded as the climatological driver for Antarctica, but the mechanism of how it can reach the continental shelf remains unsettled. Motivated by the absence of observational eddy flux estimation in the Antarctic margin, we quantify isopycnal diffusivity of CDW by hydrographic records and satellite altimetry under the mixing length framework. For comparison, spiciness and thickness are used as the isopycnal tracer, and two yield similar results. Over the extent of Antarctic Circumpolar Current (ACC), we find a general agreement with the mixing suppression theory and its exception in the lee of the topography as previously reported. In contrast, no mixing length’s dependency on mean flow is obtained to the pole, reflecting a stagnant flow regime in the Antarctic margin. Isopycnal diffusivity ranges 100–500 m² s^-1 to the south of the ACC. Eddy diffusion is likely enhanced where the CDW intrusion is localized by the recirculating gyres, mostly attributable to the small gradient of isopycnal thickness. Volume transport is then estimated by the layer thickness gradient. Thickness-diffusive onshore heat flux across the continental slope (~3.6/1.2 TW in the eastern/western Indian sectors) is quantitatively consistent with cryospheric heat sinks by sea ice formation and ice shelf basal melt, suggesting that the isopycnal eddy diffusion is the main cause of the onshore CDW intrusion. We emphasize that the thickness field is essential for determining the eddy fluxes in the Antarctic margin.

How to cite: Yamazaki, K., Mizobata, K., and Aoki, S.: Onshore diffusion of Circumpolar Deep Water, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1404, https://doi.org/10.5194/egusphere-egu22-1404, 2022.

The Southern Ocean is a critical component of Earth’s climate system, but its remoteness makes it challenging to develop a holistic understanding of its processes from the small scale to the large scale. The Antarctic Circumnavigation Expedition (ACE, austral summer 2016/2017) surveyed a large number of variables describing the state of the ocean and the atmosphere, the freshwater cycle, atmospheric chemistry, and ocean biogeochemistry and microbiology. This circumpolar cruise included visits to 12 remote islands, the marginal ice zone, and the Antarctic coast.

Here, we use 111 of the observed variables to study the latitudinal gradients, seasonality, shorter-term variations, geographic setting of environmental processes, and interactions between them over the duration of 90 days. To reduce the dimensionality and complexity of the dataset and make the relations between variables interpretable we applied an unsupervised machine learning method, the sparse principal component analysis (sPCA), which describes environmental processes through 14 latent variables.

Our results provide a proof of concept that sPCA with uncertainty analysis is able to identify temporal patterns from diurnal to seasonal cycles, as well as geographical gradients and “hotspots” of interaction between environmental compartments. Our analysis provides novel insights into the Southern Ocean water cycle, atmospheric trace gases, and microbial communities. More specifically, we

• identify the important role of the oceanic circulations, frontal zones, and islands in shaping the nutrient availability that controls biological community composition and productivity;
• find that sea ice controls sea water salinity, dampens the wave field, and is associated with increased phytoplankton growth and net community productivity possibly due to iron fertilisation and reduced light limitation;
• elucidate the clear regional patterns of aerosol characteristics, stressing the role of the sea state, atmospheric chemical processing, and source processes near hotspots for the availability of cloud condensation nuclei and hence cloud formation.

A set of key variables and their combinations, such as the difference between the air and sea surface temperature, atmospheric pressure, sea surface height, geostrophic currents, upper-ocean layer light intensity, surface wind speed and relative humidity played an important role in our analysis, highlighting the necessity for Earth system models to represent them adequately.

In conclusion, our study highlights the use of sPCA to identify key ocean–atmosphere interactions across physical, chemical, and biological processes and their associated spatio-temporal scales. It thereby fills an important gap between simple correlation analyses and complex Earth system models.

The paper and links to data are available here: https://esd.copernicus.org/articles/12/1295/2021/

How to cite: Schmale, J. and the ACE-DATA Team: Exploring the coupled ocean and atmosphere system with a data science approach applied to observations from the Antarctic Circumnavigation Expedition, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1611, https://doi.org/10.5194/egusphere-egu22-1611, 2022.

A technique has been developed for assessing the linear long-term change in the structure of the gradient field of the Absolute Dynamic Topography (ADT) based on satellite altimetry data distributed on the website https://marine.copernicus.eu. This structure is understood as the alternation in the meridional direction of the zones of increased values ​​of the absolute values of the ADT gradient (jets) and the zones of their reduced values ​​(interjet spaces). The technique uses linear regression analyzes and makes it possible to calculate the meridional shift in the structure of the gradient field of the ADT and the change in the absolute values of the gradient of the ADT, as well as to estimate the calculation error.

Two 26-year series of dependences of the mean annual absolute values ​​of the ADT gradient on latitude and on the ADT have been analyzed. Analysis of the dependence on latitude showed that in the ACC band (42°–57°S), there are three zones of increased gradients conventionally corresponding to the cores of the Subantarctic (SAC), South Polar (SPC) and South Antarctic (SthAC) currents. Analysis of the dependence on ADT showed that in the ACC band (-130–20 cm in ADT units) there are four such zones; an additional zone is observed in the SPC. In general, in the ACC band for 26 years of observations, a shift in the structure of the gradient field of the ADT in latitude by 0.05±0.10° to the north is noted. At the same time, in the zones of SAC, SPC, and SthAC the shifts on average are 0.16°±0.15 ° to the south, 0.30°±0.14° to the north and 0.03°±0.26° to the north, respectively. The extreme values ​​of the shift in the SAC and SPC zones reach 0.4° to the south and 1.4° to the north, respectively. In the ACC band relative to the ADT, a positive shift in the structure of the gradient field of the ADT is observed amounting to 8.3±1.0 cm. This shift is mainly due to the corresponding increase in the ocean level at geographic points. However, for separate zones within the ACC, the shift can differ significantly from the mentioned value due to the meridional shift of the structure of the ADT gradient in geographic coordinates. In particular, in the boundary zone between SAC and SPC it reaches 17 cm. In the ACC band, an increase in the absolute value of the ADT gradient is also observed, 1.9±2.7×10^-3 cm/km, which is equivalent to an increase in the ADT difference across the ACC by about 3 cm, which corresponds to the difference in the increase in ADT at geographical points on the southern and northern periphery of the current. At the same time, in the SAC zone, a decrease in the absolute value of the ADT gradient by 8.0±4.2×10^‑3 cm/km is observed, and in the SPC and SthAC, on the contrary, an increase by 10.6±4.3×10^-3 cm/km and 8.2±5.4×10^-3 cm/km, respectively.

How to cite: Tarakanov, R.: The long-term linear meridional shift of the jet structure of the Antarctic Circumpolar Current south of Africa, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1965, https://doi.org/10.5194/egusphere-egu22-1965, 2022.

The Southern Ocean is recognized as a potential cause of the lower atmospheric concentration of CO₂ during ice ages, but the mechanism is debated. In the ice age Antarctic Zone, biogeochemical paleoproxy data suggest a reduction in the exchange of nutrients (and thus water and carbon) between the surface and the deep ocean. We report simple calculations with those data indicating that the decline in the supply of nutrients during peak glacials was extreme, >50% of the interglacial rate. Weaker wind-driven upwelling is a prime candidate for such a large decline, and new, complementary aspects of this mechanism are identified here. First, reduced upwelling would have resulted in a “slumping” of the pycnocline into the AZ. Second, it would have allowed diapycnal mixing to “mine” nutrients out of the upper water column, possibly causing an even greater slumping of the vertical nutrient gradient (or “nutricline”). These mechanisms would have reduced shallow subsurface nutrient concentrations, decreasing wintertime resupply of nutrients to the surface mixed layer, beyond the reduction in upwelling alone. They would have complemented two changes previously proposed to accompany a decline in upwelling: (1) halocline strengthening and (2) reduced isopycnal mixing in the deep ocean. Together, the above changes would have encouraged declines in the nutrient content and/or the formation rate of new deep water in the AZ, enhancing CO₂ storage in the deep ocean.

How to cite: Fripiat, F., Sigman, D., Ai, X., Studer, A., Kemeny, P., Hain, M., Wang, X., Ren, H., Haug, G., and Martinez-Garcia, A.: The Southern Ocean during the ice ages: A slumped pycnocline from reduced wind-driven upwelling?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2047, https://doi.org/10.5194/egusphere-egu22-2047, 2022.

Since the 1970s, the ocean has absorbed almost all of the additional energy in the Earth system due to greenhouse warming. However, our knowledge of where ocean heat uptake (OHU) has occurred and where this heat is stored today is limited by sparse observations. Here, we use a global ocean-sea ice model forced by observationally constrained atmospheric fields to conduct hindcast simulations that are initialised from an equilibrated control simulation that improves on commonly used spin-up approaches. The hindcast with full interannual forcing captures the observed global ocean heat content evolution better than most previous ocean-sea ice model simulations. Applying trends in only surface winds or thermal properties reveals that each can explain ∼50% of the total ocean warming signal. These contributions, when restricted to the Southern Ocean, account for nearly all of the global OHU of 5.4 × 10²¹ J year^-1. Integrated over the Southern Ocean, the sensible heat flux drives 75% more OHU than the longwave radiative flux in the simulation with only surface wind trends, while it is the opposite in the simulation with only trends in thermal properties. Almost 50% of the additional Southern Ocean-sourced heat signal is exported into the Atlantic Ocean where two-thirds of this added heat is then lost to the atmosphere.

How to cite: Huguenin, M., Holmes, R., and England, M.: Drivers and distribution of global ocean heat uptake over the last half century, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2076, https://doi.org/10.5194/egusphere-egu22-2076, 2022.

The Southern Ocean (SO) dynamics, and the various fronts of the Antarctic Circumpolar Current in particular, are well known to display a very energetic variability covering a wide range of spatial and temporal scales. Since a substantial fraction of such variability is known to be intrinsic, and therefore basically chaotic, predictability in this part of the world ocean is particularly poor.

In this context, the YOPP-endorsed IPSODES project of the Italian “Programma Nazionale di Ricerche in Antartide” (PNRA) is aimed at improving process understanding concerning the predictability of the SO dynamics through ensemble simulation (ES) hindcasts analyzed by means of various statistical techniques supported by dynamical interpretations, with special focus on multiscale interactions linking high-frequency (up to seasonal) and low-frequency (interannual and larger) variability. IPSODES uses existing state-of-the-art eddy-permitting global ocean-sea-ice model ESs and coupled global atmosphere-ocean-sea-ice model ESs developed for decadal climate predictions. Moreover, new ESs performed with an ocean model specifically developed for IPSODES are carried out: sensitivity numerical experiments to assess model uncertainty are performed with these new simulations. The study of transport of marine debris provides an application of such modelling effort, and contributes also to model validation through the use of an available valuable data set.

This contribution illustrates advances achieved so far in IPSODES towards improving our understanding of the predictability properties of oceanic variability of the SO dynamics.

How to cite: Zanchettin, D., Pierini, S., Aliani, S., Rubino, A., Zambianchi, E., Viana Barreto, R., de Ruggiero, P., and Colella, A.: Predictability of the Southern Ocean dynamics through ensemble simulation hindcasts, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2229, https://doi.org/10.5194/egusphere-egu22-2229, 2022.

The Southern Ocean (SO) plays an important role in the uptake, transport and storage of carbon by the global oceans. These properties are dominated by the rise in anthropogenic CO₂ in the atmosphere, but they are modulated by climate variability and climate change. Here we explore the effect of climate variability and climate change in the SO using a combination of modelling and observations to identify climate fingerprints in dissolved inorganic carbon (DIC). We conduct an ensemble of hindcast model simulations using the NEMO-PlankTOM12 global ocean biogeochemical model. We use the model to isolate the changes in DIC due to anthropogenic CO₂ alone and the changes due to climatic drivers (both climate variability and climate change) and determine their relative roles in the emerging total DIC trends and patterns. We analyse the DIC climate fingerprint since 1995, across spatial scales in the SO, and check the extent to which they are detectable in the GLODAPv2.2020 observations. Model results were subsampled to the observations to directly compare the climate fingerprints. Results show that in the surface ocean, both anthropogenic CO₂ and climatic drivers act to increase DIC concentration, with the influence of anthropogenic CO₂ dominating at lower latitudes (<55°S) and the influence of climatic drivers dominating at higher latitudes (>55°S). This pattern is present in all basins. In the subsurface ocean, climatic drivers act to decrease DIC concentration, opposing the influence of anthropogenic CO₂, with a stronger decrease at lower latitudes (<50°S). These patterns resulted in a climate fingerprint specific to SO change and were detectable in the observations. However, the model underestimates the surface DIC increase and the spatial and depth variability of the subsurface DIC decrease. We use the model to directly attribute the climate fingerprint to various climate drivers and discuss timescales for unambiguous detectability of the fingerprint in observations.

How to cite: Wright, R., Le Quéré, C., Willis, D., Bakker, D., and Mayot, N.: Detecting Climate Fingerprints in Southern Ocean Carbon Using a Global Ocean Biogeochemical Model and Observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2239, https://doi.org/10.5194/egusphere-egu22-2239, 2022.

Subantarctic mode water (SAMW) is a subsurface water mass which is formed through surface heat loss. This leads to thick winter mixed layers which are then subducted resulting in a low stratification subsurface watermass. SAMW formation regions are important for the storage and transport of heat and carbon. Recently, it was found that SAMW layers are getting thicker each year over much of the Southern Ocean. In the South Pacific mode water formation region, a central and eastern pool of mode water has been found to have winter thicknesses that vary strongly interannually and out of phase across the basin. This is associated with peaks in sea level pressure variability at a quasi-stationary anomaly situated between the two pools.

To investigate how this external forcing, as well as the propagation of upstream anomalies, affects these mode water pools, a set of adjoint sensitivity experiments are conducted. The traditional approach to adjoint sensitivity experiments in the ECCOv4 state estimate uses a vertical mask that is fixed at all times. Instead, the adjoint is developed so that a density following mask is employed, which more closely reflects how water masses preferentially spread along density surfaces.   

The ECCOv4 state estimate, with this new feature, is used to conduct a set of adjoint sensitivity experiments that directly quantify the role of local versus remote forcing in setting the variability in regional mode water properties raised in recent studies. Two separate adjoint sensitivity experiments are completed with horizontal masks in the two pools of mode water in the south east Pacific mode water formation region. The objective function used here is the yearly averaged heat content over the pool and the density surfaces. The analysis compares the effect of local versus remote forcing, identifying the separate effects of  the wind stress, heat flux, and freshwater flux. The sensitivities of the SAMW are then identified in terms of the different forcing components associated with  the atmospheric modes, ENSO and SAM.

How to cite: Pimm, C., Meijers, A., Jones, D., and Williams, R.: Identifying the drivers of Subantarctic mode water thickness across the south Pacific., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2316, https://doi.org/10.5194/egusphere-egu22-2316, 2022.

The Southern Ocean is a major sink of anthropogenic carbon and excess heat. The Earth system model projections of these sinks provided by the CMIP5 and CMIP6 scenario experiments show a large model spread contributing to the large uncertainties in climate sensitivity and remaining carbon budgets for ambitious climate targets. A recent study identified an emergent coupling between anthropogenic carbon and excess heat uptake, highlighting that the passive-tracer behavior of these two quantities is dominant under high-emission scenarios. This coupling indicates that the use of a single observational constraint might be sufficient to reduce projection uncertainties in both anthropogenic carbon and excess heat uptake. In the northern limb of the Southern Ocean (30°S-55°S) where the subduction of intermediate and mode water is known to drive carbon and heat uptake, we find that the variations in model´s contemporary water-column stability over the first 2000 m is highly correlated to both its future anthropogenic carbon uptake and excess heat uptake efficiency. Using observational data, we reduce the uncertainty of future estimates of (1) the cumulative anthropogenic carbon uptake by up to 53% and (2) the excess heat uptake efficiency by 28%. Our results show that improving the representation of water-column stratification in Earth system models should be prioritized to improve future anthropogenic climate change projections.

How to cite: Bourgeois, T., Goris, N., Schwinger, J., and Tjiputra, J. F.: Contemporary stratification constrains future anthropogenic carbon and excess heat uptake in the northern limb of the Southern Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2761, https://doi.org/10.5194/egusphere-egu22-2761, 2022.

Recent advances in the development of hardware have pushed the explicit resolution of mesoscale (and, using nesting approaches, even sub-mesoscale) processes in global coupled ocean-circulation biogeochemical models within reach. This adds realism to the models in that previously-parameterized processes can now be explicitly resolved. Showcasing examples of our modeling work in the Baltic Sea, the subtropical North Atlantic and the Southern Ocean we will put the relevance of this paradigm to the test. We report on surprisingly small effects of explicitly-resolved mesoscale and even submesoscale features on a variety of domain-averaged entities such as air-sea and carbon cluxes in Boussinesq-approximated general ocean circulation models.

How to cite: Dietze, H. and Löptien, U.: Eddies, Winds, and Carbon in coupled ocean-circulation biogeochemical models: from the Baltic Sea to the Southern Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2980, https://doi.org/10.5194/egusphere-egu22-2980, 2022.

The decadal changes of the fugacity of CO₂ (fCO₂) and pH in surface waters are investigated in the Southern Indian Ocean (45°S-57°S) using repeated summer observations, including measurements of fCO₂, total alkalinity (A_T) and total carbon (C_T) collected over the period 1998-2019 in the frame of the French monitoring program OISO. We used three datasets (underway fCO₂, underway A_T-C_T and station A_T-C_T) to evaluate the trends of fCO₂ and pH and their drivers, including the accumulation of anthropogenic CO₂ (C_ant). The study region is separated into three domains based on the frontal system and biogeochemical characteristics: (i) High Nutrients Low Chlorophyll (HNLC) waters in the Polar Front Zone (PFZ), (ii) HNLC waters south of the Polar Front (PF) and (iii) the highly productive zones in fertilized waters near Crozet and Kerguelen Islands. Almost everywhere, we obtained similar trends in surface fCO₂ and pH using the fCO₂ or A_T-C_T datasets. Over the period 1998-2019, we observed an increase in surface fCO₂ and a decrease in pH ranging from +1.0 to +4.0 µatm yr^-1 and from -0.0015 to -0.0043 yr^-1, respectively. South of the PF, the fCO₂ trend is close to the atmospheric CO₂ rise (+2.0 µatm yr^-1) and the decrease in pH is in the range of the mean trend for the global ocean (around -0.0020 yr^-1). These trends are driven by the warming of surface waters (up to +0.04°C yr^-1) and the increase in C_T, mainly due to the accumulation of C_ant (around +0.6 µmol kg^-1 yr^-1). In the PFZ, our data show slower fCO₂ and pH trends (around +1.3 µatm yr^-1 and -0.0013 yr^-1, respectively) associated with an increase in A_T (around +0.4 µmol kg^-1 yr^-1)that limited the impact of a more rapid accumulation of C_ant north of the PF (up to +1.1 µmol kg^-1 yr^-1)_. In the fertilized waters near Crozet and Kerguelen Islands, fCO₂ increased and pH decreased faster than in the other domains, between +2.2 and +4.0 µatm yr^-1 and between -0.0023 yr^-1 and -0.0043 yr^-1. The fastest trends of fCO₂ and pH are found around Kerguelen Island north and south of the PF. These trends result from both a significant warming (up to +0.07°C yr^-1) and a rapid increase in C_T (up to +1.4 µmol kg^-1 yr^-1), mainly explained by the uptake of C_ant.

How to cite: Leseurre, C., Lo Monaco, C., Reverdin, G., Metzl, N., Fin, J., Mignon, C., and Benito, L.: Trends and drivers of sea surface fCO2 an pH changes observed in the Southern Indian Ocean over the last two decades (1998-2019), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3580, https://doi.org/10.5194/egusphere-egu22-3580, 2022.

The southern boundary of the Antarctic Circumpolar Current (ACC) is often associated with the southern limit of Upper Circumpolar Deep Water, and so forms the boundary between warm ACC waters and colder waters within the marginal seas of Antarctica. Strong density gradients across the southern boundary constitute the frontal jet and are thought to modulate the heat transport across the southern boundary. It is well known that eddies cross the fronts of the ACC and are advected downstream, but how does an eddy interact with the southern boundary of the ACC? Does it change its frontal structure? Does it impact the intensity of the frontal jet? Can changes of the southern boundary’s frontal structure impact mixing? These are questions that we aim to discuss.

As part of the Robotic Observations And Modelling in the Marginal Ice Zone (ROAM-MIZ) project, profiling ocean glider observations at the Greenwich Meridian between 54-57 °S from the 20^th of October 2019 to the 18^th of February 2020 provide a unique data set of 5 highly resolved hydrographic transects that cross the southern boundary repeatedly. Θ/S diagrams from the hydrographic transects, maps of absolute dynamic topography and dive average currents are used to identify the location, properties and rotational direction of eddies crossing the meridional transects in close proximity to the southern boundary and the frontal jet. We demonstrate that a cyclonic eddy crossing the meridional transect significantly impacts the southern boundary's frontal structure. While the eddy is crossing the meridional transect, density gradients are strengthened and geostrophic velocities show a narrow and strong frontal jet (~50 km wide with velocities of ~80 cm/s). Shortly after the eddy has crossed the meridional transect, density gradients are weakened and geostrophic velocities show a broadened and weakened frontal jet (~75 km wide with velocities of ~60 cm/s). Mixing length scales (the length at which a water parcel can move before mixing laterally) are calculated for all transects with L_mix=Θ_rms/(∇_n Θ_m)  (Θ_rms a measure of temperature fluctuations , ∇_n the gradient along density surfaces  and Θ_m mean temperature field). Values of L_mix are near zero across the frontal jet while the eddy is crossing and near 40 km after the eddy has crossed the meridional transect. The increased mixing length scales indicate that the exchange of water parcels between ACC waters and waters further south is increased after the eddy has crossed the meridional transect.

How to cite: Oelerich, R., Heywood, K. J., Damerell, G. M., Swart, S., and du Plessis, M.: The Antarctic Circumpolar Current’s Southern Boundary at the Greenwich Meridian, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3614, https://doi.org/10.5194/egusphere-egu22-3614, 2022.

The Southern Ocean (SO) connects major ocean basins and hosts large air-sea carbon fluxes due to the resurfacing of deep nutrient and carbon rich waters, driven by strong surface winds. Strong vertical mixing in the SO is induced by breaking waves excited by strong surface winds and interaction of tides, jets and eddies with rough topography. Vertical mixing has primarily been considered of importance for biogeochemical cycles due to the role of mixing in setting the underlying dynamics of the meridional circulation on a centennial timescale. Using an eddy-permitting ocean model that assimilates an extensive array of observations, we show that altered mixing can cause up to a 40% change in SO air-sea fluxes in only a few years by altering the distribution of dissolved inorganic carbon, alkalinity, temperature and salinity. Biological productivity is also highly altered, with strong regional and seasonal variations in the sensitivity and response to enhanced mixing. This altered biological productivity could lead to alterations in the biological carbon pump over longer time scales. The high sensitivity of carbon fluxes and biological productivity shown over short time scales is due to high vertical gradients in nutrients, DIC, alkalinity and temperature found in the upper waters of the SO. Further carbon flux and other biogeochemical observations are to better constrain the rates of vertical mixing from observations. Vertical mixing processes are unresolved in climate models, yet essential for the modelling of SO carbon cycles.

How to cite: Ellison, E.: Hypersensitivity of Southern Ocean air-sea carbon fluxes and biological productivity to turbulent diapycnal fluxes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3665, https://doi.org/10.5194/egusphere-egu22-3665, 2022.

The Southern Ocean plays an important role in the climate system by regulating excess CO₂ in the atmosphere. A large part of the anthropogenic CO₂ (C_ant) absorbed in surface waters of the Southern Ocean is isolated from the atmosphere at mid-latitudes through the sinking of mode waters down to 1500m. Since the Southern Ocean CO₂ sink and mode waters formation vary from inter-annual to decadal scales, one would expect the C_ant content in these waters to be also variable. This has been suggested through modeling studies but it is challenging to detect these changes based on observations.  This study attempts to estimate the evolution of C_ant accumulation in mode waters of the Southern Indian Ocean over the period 1985-2019 based on observations from the Global Ocean Data Analysis Project (GLODAPv2_2020) and the long-term monitoring program OISO (Océan Indien Service d’Observations). The comparison of three data-based diagnostic approaches showed the strength of the eMLR(C*) method (Clement and Gruber, 2018) for estimating temporal variations in the accumulation of C_ant. The increase in C_ant estimated between 1985 and 2019 show a relatively good agreement for the three methods in the different types of mode waters identified in the Indian Ocean:  the mean trend is between +1.02 and +1.09 μmol kg^-1 yr^-1 in the Subtropical Mode Water (STMW), between +0.73 and +1.02 μmol kg^-1 yr^-1 in the Subantarctic Mode Water (SAMW) and between +0.25 and +0.51 μmol kg^-1 yr^-1 in the Antarctic Intermediate Water (AAIW). However, on shorter periods we found larger discrepancies between the eMLR(C*) method and the two other techniques (back-calculation and TrOCA), the latter showing larger uncertainties. The mean increase in C_ant between 1994 and 2007 estimated using the eMLR(C*) is +1.34 (± 0.18) μmol kg^-1 yr^-1 in the STMW, +1.05 (± 0.05) μmol kg^-1 yr^-1 in the SAMW and +0.60 (± 0.11) μmol kg^-1 yr^-1 in the AAIW, which is consistent with previous results obtained over the same time period using the same method (Gruber et al, 2019). Interestingly, the trends estimated with this method in recent years (between 2007 and 2017) weakened by about half in all mode waters, in STMW (+0.73 (± 0.20) μmol kg^-1 yr^-1), the SAMW (+0.49 (± 0.20) μmol kg^-1 yr^-1) and AAIW (+0.26 (± 0.42) μmol kg^-1 yr^-1). Due to the important contribution of mode waters in the storage of C_ant, these results could significantly reduce the oceanic inventories of C_ant in recent years at both the regional and global scales. The reduction in C_ant trends in mode waters of the Southern Indian Ocean raises questions on the external/internal processes that control mode waters formation and air-sea CO₂ exchanges in the Southern Ocean at a decadal scale.

How to cite: Barut, G., Clerc, C., Leseurre, C., LoMonaco, C., and Metzl, N.: Recent changes in the accumulation of anthropogenic carbon in mode waters of the Southern Indian Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3858, https://doi.org/10.5194/egusphere-egu22-3858, 2022.

Giant icebergs can greatly impact the mass, freshwater and nutrient budgets of the ocean. They can deposit large amounts of freshwater at great distances from their origins, impacting upper-ocean stratification and mixing, and they can be important vectors for micronutrient delivery with impacts on primary production and carbon drawdown. Their impacts on advection, productivity and blocking of flows can be critical for zooplankton and regional ecosystem functioning, with consequences for higher trophic levels and local fisheries. Their breakouts from ice shelves create new opportunities for biological colonisation and carbon sinks and their collisions with the seabed (iceberg scour) can shape local and regional benthic biodiversity patterns and influence carbon sequestration.

In 2017, the A68 iceberg (around 6000 km²) calved from the Larsen C Ice Shelf on the Antarctic Peninsula. It subsequently moved eastward and northward, crossing the Scotia Sea to move, virtually intact, to within 300 km of the island of South Georgia (SG) in late 2020. This caused concern, following the impact of a previous iceberg, A38, on the SG ecosystem in 2003-2004. Further, given the advances in observing technology since the time of the previous iceberg, it afforded an unparalleled opportunity to study in detail the impacts of giant bergs on the ocean physical, biogeochemical and biological systems.

Diverse datasets were collected in response to this event. A research cruise on RRS James Cook was mobilised, to study the iceberg as it approached SG and fragmented into multiple smaller pieces. These measurements included physical parameters (including oxygen isotopes to inform on freshwater sources), dissolved inorganic nutrients, biosilica concentration, and composition of the phytoplankton community to inform bloom dynamics and primary production by the input of terrigenous material. Ocean gliders, deployed from the ship, surveyed the largest iceberg fragment in extremely close proximity and followed this for the remainder of its life, deconvolving the iceberg influence from frontal dynamics and assisting in understanding meltwater influence. Concurrently, Earth Observation (EO) techniques were employed including Sentinel-1 SAR imagery, Planet Labs very high-resolution optical imagery, MODIS Aqua and Terra imagery and satellite radar and laser altimetry. A sediment trap deployed on a mooring downstream of SG will be utilised to investigate the carbon export from the cruise period to that of the previous 10 years while enhanced observations on higher predator colonies will compare their foraging paths and breeding performance to those of previous years.

This presentation will discuss preliminary findings from the study of A68, including EO-derived quantifications of changing iceberg morphology, ice loss from fragmentation and basal melting, and the significance of fractures in dictating collapse fissures. Physical oceanographic data from the ship and gliders are used to determine the impact on water column stability, mixing and circulation on a range of scales. Biogeochemical and biological data reveal the impact of interacting processes on phytoplankton community biomass and species composition. Ecosystem implications and future directions of investigation will be outlined.

How to cite: Lucas, N., Brearley, A., Abrahamsen, P., Meredith, M., Hendry, K., Manno, C., Liszka, C., Gerrish, L., Shepherd, A., Braakmann-Folgmann, A., Fleming, A., Ratcliffe, N., Collins, M., Murphy, E., Barnes, D., and Tarling, G.: Physical, biogeochemical and ecological impacts of giant icebergs: a multidisciplinary study of iceberg A68 near South Georgia, Southern Ocean , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3914, https://doi.org/10.5194/egusphere-egu22-3914, 2022.

Mesoscale eddies play a key role in Southern Ocean dynamics, upwelling and transformation of water masses, the surface heat flux and therefore in storage of heat at deeper layers. To better understand local processes but also basin-scale implications, we apply regional ocean grid refinement to the entire region south of 28˚S in the fully coupled climate model FOCI. This two-way nesting configuration (FOCI-ORION10X) enables us to resolve the entire Southern Ocean at 0.1˚ yielding an eddy-rich simulation in this region whereas eddies are parameterized in the remainder of the global ocean running on a 0.5˚ grid. We contrast our high-resolution simulation with the non-eddying pre-industrial control run of FOCI. Heat uptake and redistribution in mean states of three pre-industrial simulations with FOCI-ORION10X are investigated: one 100 years after starting the model from rest, and two branching off from two different FOCI reference runs without and with regular open ocean deep convection in the Weddell Sea.

Net surface heat fluxes are significantly enhanced by up to 50% in the eddy-rich nested simulations compared to the non-eddying reference simulations. In our simulations, eddy kinetic energy (EKE) is largest in the Brazil–Malvinas Confluence Zone and the Agulhas Current system, regions of large upward surface heat flux, i.e. ocean heat loss. Heat uptake occurs farther south in the region of the Antarctic Circumpolar Current associated with a broad band of enhanced EKE between 45˚S and 55˚S. Explicitly simulating instead of parameterizing eddies also impacts Southern Ocean upwelling and heat convergence at mid-latitudes. We explore and quantify the associated impact on heat storage in three mean states representing different states of Southern Ocean mean temperature and bottom water volume, which affects the meridional overturning circulation strength.

How to cite: Zeller, M. and Martin, T.: Role of mesoscale dynamics in Southern Ocean heat uptake and storage, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4852, https://doi.org/10.5194/egusphere-egu22-4852, 2022.

Recent studies point to pronounced decadal variability in the Southern Ocean carbon sink over the past decades, but the mechanisms are still not fully understood. In this study, the regional patterns and bio-physical drivers of the interannual-to-decadal variability of the air-sea CO₂ fluxes in the Antarctic Circumpolar Current (ACC) are investigated. A suite of global ocean biogeochemistry configurations (based on the NEMO-MOPS model) is used to perform hindcast experiments covering the period 1958-2018. The configurations include a non-eddying 0.5° model, an eddy-permitting 0.25° model, and a global 0.5° model featuring an eddy-rich 0.1° nest between 30°S and 68°S. The 0.25° model is also used to perform additional sensitivity experiments, where the variability of the wind stress or of the buoyancy forcing is suppressed on interannual time scales. All simulations show a positive trend in the air-sea CO₂ fluxes over ACC, with a weaker rate of increase in the 1970s and in the 1990s, and a stronger rate of increase in the 1980s and 2000s. The interannual and decadal variability of air-sea CO₂ fluxes is highest in frontal regions of the ACC, especially in the Southeast Pacific basin. Wind stress emerges as the dominant driver of the large interannual and decadal variability of air-sea CO₂ fluxes at subpolar latitudes. On the other hand, air-sea buoyancy fluxes gain more relevance at middle latitudes. The simulations highlight the relevant role of explicitly simulating ocean mesoscale eddies for the Southern Ocean carbon uptake. Indeed, the 0.1º model shows a steeper trend of the Southern Ocean carbon uptake with respect to the lower-resolution models, driven to a large extent by a higher uptake of anthropogenic carbon.

How to cite: Patara, L., Rieck, J. K., Tanhua, T., Ödalen, M., and Oschlies, A.: Interannual-to-decadal variability of the Southern Ocean carbon uptake in a high-resolution ocean biogeochemistry model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6326, https://doi.org/10.5194/egusphere-egu22-6326, 2022.

Primary productivity in the Southern Ocean plays a key role in global biogeochemical cycles. While much focus has been placed on phytoplankton seasonality, interannual fluctuations exceed the amplitude of the seasonal cycle across large swaths of the Antarctic Circumpolar Current. Interannual variability of surface chlorophyll, a proxy for phytoplankton biomass, is typically linked to changes in the ocean circulation associated with the Southern Annular Mode (SAM). However, it is important to note that variations in annual mean chlorophyll may reflect processes occurring across a broad range of timescales from sub-seasonal to multi-annual. Here, we apply a timeseries decomposition method to satellite-derived surface chlorophyll in order to separate the low-frequency and high-frequency contributions to the interannual variability. Throughout most of the Southern Ocean, interannual variations are dominated by the sub-seasonal component, which is not strongly correlated with the SAM. The multi-annual component, while correlated with the SAM, only accounts for about 10% of the total chlorophyll variance. This suggests that year-to-year variations in annual mean chlorophyll are related to high-frequency events driven by intermittent forcing at small scales, such as storms and eddies, rather than low-frequency climate variability. Consequently, interannual variations of primary productivity are highly localized and do not remain correlated over large regions.

How to cite: Prend, C., Keerthi, M., Lévy, M., Aumont, O., Gille, S., and Talley, L.: Year-to-year variations of Southern Ocean primary productivity driven by sub-seasonal forcing, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6327, https://doi.org/10.5194/egusphere-egu22-6327, 2022.

Water-mass transformation in the Weddell Sea is responsible for the generation of 50-70% of Antarctic Bottom Water exported to the global deep ocean, with effects for the deep marine sequestration of atmospheric CO₂. Uncertainties in the dynamics of this system urgently need to be addressed to assist with modelling the carbon cycle in the southern high latitudes, and identifying whether the Weddell Sea and the Southern Ocean may act as either a carbon source or sink as the global climate shifts.

In this study, we used sequential acid-reductive leaching and total digestion to obtain neodymium (Nd), lead (Pb), and uranium (U) concentrations and isotopic compositions from both the authigenic and the detrital fractions of sediments from a suite of long-cores and surface sediments around the Weddell Sea. Paired isotope analyses were carried out to reconstruct bottom water conditions during deposition, and determine the sedimentary provenance of lithogenic detritus. The combination allows us to observe the relationship between lithogenic and authigenic phases. Authigenic Nd and Pb isotope signatures were interpreted to reflect pore-water compositions, affected by a combination of bottom-water composition, lithology, and element release from sediments into the pore-water and overlying water column. We further assess whether authigenic U may serve as a proxy for bottom-water oxygenation and ocean productivity at our Southern Scotia Sea sites, giving insight to deep-ocean ventilation and bottom-water export rates from the Weddell Sea.

Our results suggest that detrital isotopic records indicate an increase in sediment delivery from the East Antarctic to the northwestern Weddell Sea during the deglacial. We hypothesize that this was the result of a strengthening in the Weddell Gyre or Antarctic Circumpolar Current at this time. Here, we present an updated dataset of new authigenic and detrital measurements from the Weddell Sea investigating this hypothesis, and we unveil new details of the dynamic nature of Weddell Sea circulation and ice-ocean interactions over the last 30 kyr.

How to cite: Bollen, M., Jaccard, S., Gutjahr, M., Müller, J., and Blaser, P.: Reconstructing Weddell Sea current variability since the LGM: insights from authigenic and detrital radioisotope analyses of marine sediments., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6923, https://doi.org/10.5194/egusphere-egu22-6923, 2022.

Anthropogenic activities during the past two centuries have caused an increase in atmospheric CO₂ which has driven a linear increase in oceanic CO₂ uptake. The Southern Ocean (SO, < 35ᵒS) is one of the major uptake areas for anthropogenic CO₂, responsible for ~40% of ocean CO₂ sink. Apart from the linear increase in the CO₂ sinking trend, in the SO pronounced variations have been observed in recent decades, driven by natural processes, but the exact mechanisms behind them are still debated. Aiming to fill this knowledge gap, we investigated the natural drivers of CO₂ flux variations in the SO using existing observation-based datasets between the years 1982-2019. We removed the long-term linear trend in the time series of CO₂ flux and other indexes to focus on decadal variations. We found that two mechanisms explain the interannual to decadal variations in the SO: Ekman upwelling and eddy kinetic energy, by their controls on different components of surface pCO₂ variations. The pattern of variability in Ekman upwelling during the time period studied was markedly circumpolar, and the time series of its 1^st principal component was strongly correlated with the detrended SAM Index (r=0.81, p<0.05). Similarly, leading EOF maps of CO₂ flux anomalies and the components of surface pCO₂ changes (i.e., nonthermal and thermal) show that their variations were dominantly symmetric. As previously shown, weakening of SO CO₂ sink in the 1990s coincides with intense positive SAM episodes. Following the late 1990s, the intensity of SAM decreased, which strengthened the CO₂ sink in the early 2000s. At the same time, the relative contribution of the thermal component grew south of the Polar Front, indicating positive temperature anomalies during this period. Such warming events, following intense and recursive SAM episodes were reported before and were attributed to the increased mesoscale eddy activity in the region. In agreement with these studies, our results show that eddy kinetic energy increased after intense SAM periods with a lagged response of ~2 years, and a positive temperature anomaly in low frequency was observed following these peaks. This warming prevented the CO₂ uptake rate from reaching immediately to its potential strength in the absence of strong westerlies, and explains the growing effect of the thermal pCO₂ component.

How to cite: Yilmaz, E., Bernardello, R., and Martin, A. P.: Natural processes behind the CO2 sink variability in the Southern Ocean during the last three decades, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7827, https://doi.org/10.5194/egusphere-egu22-7827, 2022.

Recent studies have shown that the air-sea carbon fluxes in the Southern Ocean display large signals of variability on interannual to decadal timescales (e.g., Le Quéré et al., 2007; Landschützer et al., 2015, Keppler & Landschützer, 2019). However, due to data sparsity, little attention has been paid to mesoscale processes affecting the Southern Ocean carbon fluxes. This region, dominated by zonal fronts and the Antarctic Circumpolar Current, is rich in highly dynamic eddies (Frenger et al., 2015). These eddies have the potential to significantly alter local air-sea carbon fluxes through eddy pumping, where anticyclonic eddies transport carbon downward, allowing for additional oceanic carbon uptake, and cyclonic eddies pump carbon stored at depth upward, resulting in outgassing. Additionally, the strong westerly winds could result in significant eddy-induced Ekman pumping that has the opposite direction and offsets the effect from eddy pumping (Su et al., 2021; Gaube et al., 2015). Thus, identifying the influence of eddies on the Southern Ocean carbon fluxes forms a crucial part in quantifying the global carbon cycle.

Although this region is historically under-sampled, we now have nearly a decade of biogeochemical (BGC) observations from Argo floats deployed as part of the Southern Ocean Carbon and Climate Observations and Modeling project (SOCCOM). Moreover, the Aviso database provides us with eddies detected from satellite altimetry measurements. Together, the two datasets allow us to investigate the vertical structure of the biogeochemistry in Southern Ocean eddies. Here, we co-locate the Southern Ocean eddies with BGC Argo floats to present the composite vertical structure of pH, oxygen, and nitrate inside anticyclonic and cyclonic eddies compared to the mean fields. We conduct this analysis in several subregions with different dominant processes. Our findings enable us to characterize and interpret the influence of mesoscale eddies on the overall Southern Ocean carbon fluxes, including the relative dominance of eddy pumping and eddy-induced Ekman pumping in different subregions of the Southern Ocean.

References

Frenger, I., Muennich, M., Gruber, N., & Knutti, R. (2015). Southern Ocean eddy phenomenology. Journal of Geophysical Research-Oceans, 120(11), 7413–7449. https://doi.org/10.1002/2015JC011047

Gaube, P., Chelton, D. B., Samelson, R. M., Schlax, M. G., & O’Neill, L. W. (2015). Satellite Observations of Mesoscale Eddy-Induced Ekman Pumping. Journal of Physical Oceanography, 45(1), 104–132. https://doi.org/10.1175/JPO-D-14-0032.1

Keppler, L., & Landschützer, P. (2019). Regional Wind Variability Modulates the Southern Ocean Carbon Sink. Scientific Reports, 9(1), 1–10. https://doi.org/10.1038/s41598-019-43826-y