OS1.10

EDI
The Southern Ocean in a changing climate: open-ocean physical and biogeochemical processes

The Southern Ocean around the latitudes of the Antarctic Circumpolar Current is vital to our understanding of the climate system. It is a key region for vertical and lateral exchanges of heat, carbon, and nutrients, with significant past and potential future global climate implications. The role of the Southern Ocean as a dominant player in heat and carbon exchanges in present and future climate conditions remains uncertain. Indeed, the lack of observations of this system and its inherent sensitivity to small-scale physical processes, not fully represented in current Earth system models, result in large climate projection uncertainties. To address these knowledge gaps, the Southern Ocean has been the subject of recent observational, theoretical, and numerical modelling investigations. These efforts are providing deeper insight into the three-dimensional patterns of Southern Ocean change on sub-annual, multi-decadal and millennial timescales. In this session, we will discuss the current state of knowledge and novel findings concerning the role of the Southern Ocean in past, present, and future climates. These include (but are not limited to) small-scale physics and mixing, water mass transformation, gyre-scale processes, nutrient and carbon cycling, ocean productivity, climate-carbon feedbacks, and ocean-ice-atmosphere interactions. We will also discuss how changes in Southern Ocean heat and carbon transport affect lower latitudes and global climate more generally.

Convener: Lavinia Patara | Co-conveners: Camille AkhoudasECSECS, Alexander HaumannECSECS, Dan(i) Jones, Christian Turney
Presentations
| Tue, 24 May, 17:00–18:30 (CEST)
 
Room L3, Wed, 25 May, 08:30–11:50 (CEST), 13:20–14:50 (CEST)
 
Room L3

Presentations: Tue, 24 May | Room L3

Chairpersons: Alexander Haumann, Mark Hague
Weddell Sea & Water Masses
17:00–17:07
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EGU22-3914
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ECS
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Highlight
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On-site presentation
Natasha Lucas, Alexander Brearley, Povl Abrahamsen, Michael Meredith, Katherine Hendry, Clara Manno, Cecilia Liszka, Laura Gerrish, Andrew Shepherd, Anne Braakmann-Folgmann, Andrew Fleming, Norman Ratcliffe, Martin Collins, Eugene Murphy, David Barnes, and Geraint Tarling

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.

17:07–17:14
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EGU22-9100
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ECS
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On-site presentation
Aditya Narayanan, Birte Gülk, Fabien Roquet, and Alberto Garabato

Maud Rise polynyas are rare events in the Weddell Sea (Atlantic sector of the Southern Ocean) that cause deep vertical mixing within the ocean column and large surface fluxes of heat with large impacts on  the local Weddell gyre circulation and on the Antarctic bottom water properties. Here we use a 1/12o ocean reanalysis product to assess the dominant drivers of ocean stratification leading up to the polynya event of 2016 and 2017 in Maud Rise, Weddell Sea. We carry out a potential vorticity (PV) budget to identify the dynamical components of the regional circulation responsible for changes in ocean stratification that culminated in the formation of the 2017 polynya. During 2015, an exceptionally strong (about 2x that of the previous three years) buoyancy-driven destratification led to a shoaling of the pycnocline, and the restratification at the end of 2015 remained weak. During 2016 and 2017, the buoyancy-driven destratification decreased in strength, becoming weakest during the polynya of 2017. The destratification was once again strong in 2018, but this was balanced by a stratifying forcing from the surface stress and advective components, the latter of which was associated with a transport of denser (more saline and cooler) subsurface waters from the flanks of Maud Rise. These denser subsurface waters maintained a strong stratification through 2018. These results show how interannual anomalies in local sea ice production and regional circulation can promote or inhibit the formation of polynyas in the region. Furthermore, it suggests that the Maud Rise polynya opened in 2017 following a chain of perturbations that started at least back in 2015, contrary to the common view that the polynya was initiated solely  by a series of short-lived storms in 2017.

How to cite: Narayanan, A., Gülk, B., Roquet, F., and Garabato, A.: The oceanic drivers of the 2017 Maud Rise Polynya, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9100, https://doi.org/10.5194/egusphere-egu22-9100, 2022.

17:14–17:21
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EGU22-13147
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ECS
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On-site presentation
Julia Neme, Matthew England, and Andrew McC Hogg

The Weddell Gyre, located in the Weddell Sea is one of the southernmost open ocean reaches in the world and largest feature of the ocean circulation south of the Antarctic Circumpolar Current. It is adjacent to a major site of bottom water production in the southwestern Weddell Sea and participates in poleward heat transport via its cyclonic circulation that brings relatively warm waters south towards the Antarctic continent. The region is covered by sea ice through most of the year, which has historically prevented long, continuous observational efforts, both in situ and remote. As a result, ocean circulation models offer perhaps the best means of estimating the Weddell Gyre's variability. Using a coupled ocean/sea ice high-resolution global model, ACCESS-OM2 at 0.1∘ horizontal resolution, we assess the variability of the Weddell Gyre on seasonal - multi-decadal timescales and explore possible drivers of this variability. The simulations suggest that the gyre is exhibits large variability in its circulation that is not captured by summer-biased or short-term observations. Anomalous strong and weak periods of the gyre's circulation are linked to changes in sea ice concentration and other oceanic features of the region. We further explore the link to possible driving mechanisms, including surface stress forcing and surface buoyancy fluxes.

How to cite: Neme, J., England, M., and Hogg, A. M.: Variability of the Weddell Gyre in a global high-resolution numerical model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13147, https://doi.org/10.5194/egusphere-egu22-13147, 2022.

17:21–17:28
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EGU22-9990
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ECS
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On-site presentation
Krissy Reeve, Mario Hoppema, Torsten Kanzow, Olaf Boebel, Walter Geibert, and Volker Strass

The large-scale mean horizontal circulation of the Weddell Gyre was determined solely from Argo floats drifting throughout the gyre since 2002. The circulation describes an elongated, double-celled gyre, where the eastern sub-gyre is stronger and subject to mesoscale variability in comparison to the considerably weaker western sub-gyre. Since positive long-term nutrient trends across the western sub-gyre have been associated with an increase in upwelling, this study aims to compare long-term nutrient trends in the western sub-gyre, from Kapp Norvegia to Joineville Island, to those along the Prime Meridian section, spanning the eastern sub-gyre. We find the strongest trends in surface Silicate in the central part of the western sub-gyre, where the horizontal circulation is weakest. Across the eastern sub-gyre, along the Prime Meridian, the strongest Silicate trends occur in the westward flowing southern limb, south of Maud Rise. This suggests that there are different dynamical causes of the nutrient trends in the east versus the west, since the strongest upwelling at the Prime Meridian occurs north of Maud Rise, where some of the lowest long term trends in nutrients were observed. We hypothesise that while increased upwelling may be the cause of positive long-term nutrient trends in the western Weddell Gyre, mesoscale variability and convection associated with Maud Rise in the eastern Weddell Gyre have a larger impact on nutrient concentrations, making long-term trends more challenging to extract.

How to cite: Reeve, K., Hoppema, M., Kanzow, T., Boebel, O., Geibert, W., and Strass, V.: How does the Weddell Gyre circulation influence long-term trends in nutrient concentrations?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9990, https://doi.org/10.5194/egusphere-egu22-9990, 2022.

17:28–17:35
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EGU22-10528
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Virtual presentation
Dan(i) Jones, Maike Sonnewald, Isabella Rosso, Shenjie Zhou, and Lars Boehme

The Weddell Gyre is a dominant feature of the Southern Ocean and an important component of the climate system; it regulates air-sea exchanges, controls the formation of deep and bottom water, and hosts upwelling of relatively warm subsurface waters. It is characterized by extremely low sea surface temperatures, active sea ice formation, and widespread salt stratification that stabilizes the water column. Studying the Weddell Gyre is difficult, as it is extremely remote and largely covered with sea ice; at present, it is one of the most poorly-sampled regions of the global ocean, highlighting the need to extract as much value as possible from existing observations. Thanks to recent efforts of the EU SO-CHIC project, much of the existing Weddell Gyre data, including ship-based CTD, seal tag, and Argo float profiles, has been assembled into a coherent framework, enabling new comprehensive studies. Here, we apply unsupervised classification techniques (e.g. Gaussian Mixture Modeling) to the new comprehensive Weddell Gyre dataset to look for coherent regimes in temperature and salinity. We find that, despite not being given any latitude or longitude information, unsupervised classification algorithms identify spatially coherent thermohaline domains. The highlighted features include the Antarctic Circumpolar Current, the central Weddell Gyre, and the Weddell-Scotia confluence waters; we also find potential signatures of the inflow of Weddell Deep Water, the intrusion of Circumpolar Deep Water into the gyre, and export pathways of Antarctic Bottom Water. We show how varying the statistical, machine learning derived representations of the data can reveal different physical structures and circulation pathways that are relevant to the delivery of relatively warm waters to the higher-latitude seas and their associated ice shelves.

How to cite: Jones, D., Sonnewald, M., Rosso, I., Zhou, S., and Boehme, L.: Unsupervised classification identifies coherent thermohaline structures in the Weddell Gyre, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10528, https://doi.org/10.5194/egusphere-egu22-10528, 2022.

17:35–17:42
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EGU22-13275
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ECS
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On-site presentation
Christopher Auckland, Povl Abrahamsen, Michael Meredith, Alberto Naveira-Garabato, and Eleanor Frajka-Williams

Antarctic Bottom Water has experienced a marked contraction and warming, particularly in the Atlantic sector, in the past three decades. Much of the global abyssal waters are composed of this bottom water and these changes have seen concomitant ocean heating and global sea level rise via thermal expansion. This warming has been linked to a contraction in export of the densest classes of bottom water from the Weddell Gyre due to processes that are not well determined but potentially including changes in wind forcing or source water formation. With regard to wind forcing, several mechanisms have been suggested, however their relative scale and whether they occur concurrently remains unclear. Using two mooring sites within the Weddell Sea, we estimate lag times between temperature anomalies at 3000m depth finding changes in the strength of the boundary current connecting the two sites. These changes in flow speed are synchronous with changes in wind forcing and bottom water transport. In particular, bottom water temperatures increased in response to anomalously strong wind forcing in 2015 and to a lesser extent in 2018 over a period of six months, indicating a contraction in export of the most dense water classes from the Antarctic. These findings reaffirm the importance of wind forcing in driving changes in the export of dense water to the lower limb of the Atlantic overturning circulation, with potential consequences for long-period climate evolution.

How to cite: Auckland, C., Abrahamsen, P., Meredith, M., Naveira-Garabato, A., and Frajka-Williams, E.: What controls the warming of the Antarctic Bottom Water supply to the Atlantic Ocean?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13275, https://doi.org/10.5194/egusphere-egu22-13275, 2022.

17:42–17:49
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EGU22-13277
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Virtual presentation
Ivana Cerovecki and Alexander Haumann

Subantarctic Mode Water (SAMW) is a voluminous water mass that forms on the equatorward side of the Antarctic Circumpolar Current. The subduction and export of SAMW plays an important role in the global redistribution of heat, freshwater, nutrients, and dissolved gases such as oxygen and carbon dioxide. Recently the global Argo program of profiling floats provided for the first time near-global coverage of temperature and salinity in the upper 2000 m, revealing basin-wide spatial patterns of strong interannual SAMW variability. The same observations also indicate variations on decadal time scales. Combined with the output from an ocean state estimate, we investigate the mechanisms that drive the regional distribution of decadal variability of the SAMW.  We show that the decadal variability of SAMW volume and formation rate is strongly correlated with the decadal variability in the atmospheric circulation, in particular the zonal sea-level pressure gradients, governing the meridional wind component and meridional heat and moisture redistribution. These findings imply that strong quasi-decadal variability of surface heat and freshwater fluxes also governs the regional uptake of anthropogenic heat and carbon dioxide by SAMW.

How to cite: Cerovecki, I. and Haumann, A.: Mechanisms for decadal Subantarctic Mode Water variability, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13277, https://doi.org/10.5194/egusphere-egu22-13277, 2022.

17:49–17:56
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EGU22-2316
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ECS
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On-site presentation
Ciara Pimm, Andrew Meijers, Dan Jones, and Ric Williams

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.

17:56–18:04
Past Climates
18:04–18:11
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EGU22-6923
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ECS
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Virtual presentation
Michael Bollen, Samuel Jaccard, Marcus Gutjahr, Juliane Müller, and Patrick Blaser

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.

18:11–18:18
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EGU22-2047
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Virtual presentation
Francois Fripiat, Daniel Sigman, Xuyuan Ai, Anja Studer, Preston Kemeny, Mathis Hain, Xingchen Wang, Haojia Ren, Gerald Haug, and Alfredo Martinez-Garcia

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.

18:18–18:25
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EGU22-358
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ECS
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On-site presentation
Madison Shankle, Molly Trudgill, Romain Euverte, Elisabeth Michel, William Gray, and James Rae

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.

18:25–18:30

Presentations: Wed, 25 May | Room L3

Chairpersons: Lavinia Patara, Mark Hague
Ocean Dynamics (small-scale)
08:30–08:37
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EGU22-320
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ECS
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Highlight
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Virtual presentation
Hannah Joy-Warren, Isabelle Giddy, Hanna Rosenthal, Marcel du Plessis, and Sebastiaan Swart

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.

08:37–08:44
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EGU22-892
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ECS
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Virtual presentation
Hanna Rosenthal, Louise C. Biddle, Sebastiaan Swart, Sarah T. Gille, Matthew R. Mazloff, and Marcel du Plessis

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.

08:44–08:51
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EGU22-411
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ECS
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Virtual presentation
Tobias Ehmen, Katy Sheen, Andrew Watson, Alexander Brearley, Matthew Palmer, Daniel Roper, and Andrew Thompson

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.

08:51–08:58
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EGU22-3614
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On-site presentation
Ria Oelerich, Karen J. Heywood, Gillian M. Damerell, Sebastiaan Swart, and Marcel du Plessis

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 20th of October 2019 to the 18th 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.

08:58–09:05
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EGU22-1404
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ECS
|
Virtual presentation
Kaihe Yamazaki, Kohei Mizobata, and Shigeru Aoki

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.

09:05–09:12
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EGU22-1611
|
On-site presentation
Julia Schmale and the ACE-DATA Team

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.

09:12–09:19
Ocean Dynamics (large-scale)
09:19–09:26
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EGU22-2229
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On-site presentation
Davide Zanchettin, Stefano Pierini, Stefano Aliani, Angelo Rubino, Enrico Zambianchi, Ricardo Viana Barreto, Paola de Ruggiero, and Alessio Colella

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.

09:26–09:33
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EGU22-1965
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Presentation form not yet defined
Roman Tarakanov

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.

09:33–09:40
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EGU22-13273
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ECS
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Virtual presentation
Jan-Erik Tesdal, Graeme A. MacGilchrist, Rebecca. L. Beadling, John P. Krasting, Stephen M. Griffies, and Paul J. Durack

The water mass transformation (WMT) framework provides a useful perspective on interior ocean circulation because it combines the influence of surface forcing and diapycnal mixing with overturning. Observational analyses, as well as the majority of ocean model diagnostics, only adequately resolve surface-forced transformation of water masses and rely on inference or rough approximations to quantify the effect of the interior transformation term due to mixing. Here we characterize the connection between surface-forced WMT and meridional overturning of the Southern Ocean in two state-of-the-art GFDL coupled climate models. We assess the mean state of the system as well as the transient response to changes in surface forcing. For the latter, we analyze a set of idealized perturbation experiments in which changes in Antarctic ice sheet melting and Southern Ocean wind stress are imposed. Assessment of the mean state in the two climate models is consistent with previous studies that identified overturning as a balance between surface and interior WMT, with the surface component being the dominant term. However, the perturbation runs in both models demonstrate important differences in the response of surface WMT and meridional overturning. Changes in overturning are consistent with surface WMT but are muted in terms of intensity, location, and the density at which they occur. This points to a crucial role for interior WMT associated with mixing, as well as changes in water mass volumes, which are important terms in characterizing anticipated shifts in overturning and ventilation in the Southern Ocean in response to anthropogenic forcing.

How to cite: Tesdal, J.-E., MacGilchrist, G. A., Beadling, R. L., Krasting, J. P., Griffies, S. M., and Durack, P. J.: Transient response of surface-forced water mass transformation over the Southern Ocean and its connection to overturning and ventilation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13273, https://doi.org/10.5194/egusphere-egu22-13273, 2022.

09:40–09:47
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EGU22-9044
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Virtual presentation
Agatha De Boer, David Hutchinson, Fabien Roquet, Louise Sime, and Natalie Burls

Southern Ocean bathymetry constrains the path of the Antarctic Circumpolar Current (ACC), but the bathymetric influence on the coupled ocean-atmosphere system is poorly understood. Here, we investigate this impact by respectively flattening large topographic barriers around the Kerguelen Plateau, Campbell Plateau, Mid-Atlantic Ridge, and Drake Passage in four simulations in a coupled climate model. The barriers impact both the barotropic and baroclinic forcing of the ACC, which increases by between 3% and 14% when barriers are removed individually and by 56% when all barriers are removed simultaneously. The removal of Kerguelen Plateau bathymetry increases convection south of the plateau and the removal of Drake Passage bathymetry reduces convection upstream in the Ross Sea, affecting the deep overturning cell. When the barriers are removed, zonal flattening of the currents leads to SST anomalies upstream and downstream of their locations. These SST anomalies strongly correlate to precipitation in the overlying atmosphere, with correlation coefficients ranging between r=0.92 and r=0.97 in the four experiments. Windspeed anomalies are also positively correlated to SST anomalies in some locations but other forcing factors obscure this correlation in general. The meridional variability in the wind stress curl contours over the Mid-Atlantic Ridge, the Kerguelen Plateau and the Campbell Plateau disappears when these barriers are removed, confirming the impact of bathymetry on overlying winds. However, bathymetry-induced wind changes are too small to affect the overall wave-3 asymmetry in the Southern Hemisphere Westerlies. Removal of Southern Hemisphere orography is also inconsequential to the wave-3 pattern, suggestion a remote control.

How to cite: De Boer, A., Hutchinson, D., Roquet, F., Sime, L., and Burls, N.: The impact of Southern Ocean bathymetry on the ocean circulation and the overlying atmosphere, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9044, https://doi.org/10.5194/egusphere-egu22-9044, 2022.

09:47–09:54
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EGU22-10363
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On-site presentation
Joakim Kjellsson, Sebastian Wahl, Torge Martin, and Wonsun Park

We examine the current surface biases in sea-surface temperature (SST), sea-ice fraction, and winds over the Southern Ocean in the FOCI climate model and demonstrate various methods to reduce them. We examine and tune biases in both atmosphere-only simulations with ECHAM6 and OpenIFS 43r3 and coupled models FOCI (ECHAM6+NEMO) and FOCI-OpenIFS (OpenIFS + NEMO). Over the Southern Ocean both coupled climate models suffer from a warm SST bias, low sea-ice fraction, and surface westerlies with a maximum too far north. We explore how modifying ocean mixing parameters, air-sea coupling frequency and ice model parameters impacts surface biases. 

Shortening coupling frequency in the FOCI model from 3-hourly to hourly reduces both the warm SST bias and the low sea-ice fraction bias, while the northward bias of the westerly wind maximum is largely unchanged. This suggests that the SST and sea-ice fraction biases are related to a lack of wind gustiness and not the biases in mean the winds. Similarly, reducing the horizontal tracer diffusion in the ocean from 600 m2/s to 300 m2/s also reduces the warm SST bias and the low sea-ice fraction bias. The cooling of the Southern Ocean surface is likely due to a reduced vertical heat transport by the tracer diffusion, which is along iso-neutral surfaces. Combined, both reducing the coupling frequency and re-tuning the horizontal mixing parameters acts to reduce the Southern Ocean surface biases more than either one alone. 

The two coupled models, FOCI and FOCI-OpenIFS, share identical ocean model configurations, NEMO ORCA05, but produces warm SST biases in different ways. OpenIFS suffers from a strong cloud radiative forcing bias which is not existent in ECHAM. Hence, reducing the SST and sea-ice fraction biases in FOCI-OpenIFS requires improvements in the cloud scheme rather than tuning oceanic mixing parameters. 

How to cite: Kjellsson, J., Wahl, S., Martin, T., and Park, W.: Understanding and reducing surface biases over the Southern Ocean in the FOCI climate model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10363, https://doi.org/10.5194/egusphere-egu22-10363, 2022.

09:54–10:00
Coffee break
Chairpersons: Alexander Haumann, Dan(i) Jones
Heat & Carbon Uptake
10:20–10:30
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EGU22-764
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solicited
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Virtual presentation
Jean-Baptiste Sallée

The Southern Ocean regulates the global climate by controlling heat and carbon exchanges between the atmosphere and the ocean. Rates of climate change on decadal time scales ultimately depend on oceanic processes taking place in the Southern Ocean, yet too little is known about the underlying processes. Limitations come both from the lack of observations in this extreme environment and its inherent sensitivity to intermittent small-scale processes that are not captured in current Earth system models. We address some of these limitations in the european consortium Southern Ocean Carbon and Heat Impact on Climate (SO-CHIC). In this talk, I will present an overview of the important advances we made in the first two years of the consortium, ranging from (i) new understanding of small-scale transcient processes, such as ocean (sub)mescale or atmopsheric storms, impact on upper ocean ventilation and air-sea fluxes, to (ii) long term change in Southern Ocean structure, from the surface to the abysses, and via (iii) investigation of processes controling Maud Rise polynya events, decadal variability of heat and carbon storage, and large-scale atmospheric feedback. 

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.

10:30–10:37
10:37–10:44
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EGU22-2076
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ECS
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On-site presentation
Maurice Huguenin, Ryan Holmes, and Matthew England

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.

10:44–10:51
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EGU22-4852
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ECS
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Virtual presentation
Mathias Zeller and Torge Martin

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.

10:51–10:58
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EGU22-13431
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ECS
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On-site presentation
Mark Hague, Matthias Münnich, and Nicolas Gruber
The Southern Ocean is well recognised as globally the most important region for the uptake and storage of excess heat (Q`) and carbon (C`) resulting from anthropogenic CO2 emissions. Although the processes governing the transport and storage of Q` and C` are tightly connected, the near surface boundary conditions of the two perturbation tracers are very different. That is, the spatial distribution of C` in the atmosphere is rather homogenous, while the uptake of Q` is characterised by strong horizontal gradients and temporal variability ranging from seasonal to interannual and decadal. The effect this difference has on the uptake and storage patterns of Q` and C` has received relatively little attention, especially when compared to the role of ocean circulation changes. In order to address this, we utilise a regional ocean biogeochemical model (ROMS-BEC) forced with an atmospheric reanalysis (ERA5) and perform a suite of model experiments. A first set of experiments quantifies changes in C` and Q` over the period 1979 -2019, comparing the model results with observation-based estimates. Here we find that the model is able to reproduce the main features of the observed Q` and C` storage patters: a region of enhanced heat storage at ~40oS and down to 1200m, with carbon storage peaking in the surface layer (~200m) north of 40oS and decreasing poleward and with depth. A second set of experiments aims to isolate the role of spatial and temporal variability of net surface heat flux (SHF) in driving changes in Q`, with the resulting storage patterns then compared to those derived for C`. We find that for C` the storage pattern is driven largely by the uptake of anthropogenic CO2 , with a small contribution from circulation changes. In contrast, the storage pattern of Q` appears not to be strongly related to trends in SHF, suggesting that the mean SHF spatial distribution, as well as circulation changes may play a more prominent role. The SHF trends themselves are highly spatially heterogenous, and act to reduce the magnitude of the zonally integrated heat storage over the simulated period. However, we find that there are significant regional differences, with a modest increase in storage in the Pacific and Atlantic sectors being offset by a much stronger reduction in the Indian sector. Overall, we develop a conceptual framework for understanding the potential (de)coupling between oceanic uptake and storage of heat and carbon within the unique context of the Southern Ocean. 

How to cite: Hague, M., Münnich, M., and Gruber, N.: Coupling between Southern Ocean Heat and Carbon: The role of atmospheric boundary conditions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13431, https://doi.org/10.5194/egusphere-egu22-13431, 2022.

10:58–11:05
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EGU22-2761
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ECS
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Virtual presentation
Timothée Bourgeois, Nadine Goris, Jörg Schwinger, and Jerry F. Tjiputra

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.

11:05–11:12
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EGU22-7827
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ECS
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On-site presentation
Elif Yilmaz, Raffaele Bernardello, and Adrian P. Martin

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 1st 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.

11:12–11:19
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EGU22-12773
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ECS
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Virtual presentation
Nicolas Mayot, Corinne Le Quéré, Andrew Manning, David Willis, Nicolas Gruber, Jörg Schwinger, Roland Séférian, Tatiana Ilyina, Judith Hauck, Laure Resplandy, Laurent Bopp, Ralph Keeling, and Christian Rödenbeck

The Southern Ocean plays a major role in both the global oceanic uptake of anthropogenic CO2 and its interannual variations. The size and origin of the interannual variability in the Southern Ocean CO2 fluxes is debated. Observation-based estimates suggest a large variability (+/- 0.11 PgC/yr) while Global Ocean Biogeochemistry Models (GOBMs) simulate almost no variability. Studying the air-sea fluxes of O2 can provide independent information that help resolve this data-model inconsistency. Oceanic O2 is influenced by the same physical and biogeochemical processes as CO2, but unlike CO2, its variability is not masked by a large anthropogenic flux. Here, we used 26 years (1994-2019) of monthly O2 fluxes from 9 GOBMs. These model outputs were compared to air-sea O2 fluxes inferred from an atmospheric inversion of precisely quantified changes in atmospheric O2 and CO2 levels. The 26-year time series of air-sea O2 fluxes from all GOBMs and the atmospheric inversion exhibited similar temporal variations. This could be linked to the Southern Annular Mode and its influence on air-sea heat flux forcing that induced large-scale changes in observed wintertime Mixed Layer Depth (MLD). However, the amplitude of the interannual variability in air-sea O2 fluxes was two times higher in the atmospheric inversion than in GOBMs. It possible that this was induced by the general overestimation of the mean wintertime MLD by the GOBM and subsurface vertical gradients in oxygen saturation lower than observed. Implications of these results for the variability in air-sea fluxes of CO2 will be discussed.

How to cite: Mayot, N., Le Quéré, C., Manning, A., Willis, D., Gruber, N., Schwinger, J., Séférian, R., Ilyina, T., Hauck, J., Resplandy, L., Bopp, L., Keeling, R., and Rödenbeck, C.: Origin and magnitude of interannual variabilities in Southern Ocean air-sea O2 and CO2 fluxes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12773, https://doi.org/10.5194/egusphere-egu22-12773, 2022.

11:19–11:26
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EGU22-6326
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On-site presentation
Lavinia Patara, Jan Klaus Rieck, Toste Tanhua, Malin Ödalen, and Andreas Oschlies

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.

11:26–11:33
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EGU22-257
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ECS
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Virtual presentation
Andrea Rochner, David Ford, Katy Sheen, and Andrew Watson

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.

11:33–11:40
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EGU22-2239
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ECS
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On-site presentation
Rebecca Wright, Corinne Le Quéré, David Willis, Dorothee Bakker, and Nicolas Mayot

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.

11:40–11:50
Lunch break
Chairpersons: Lavinia Patara, Dan(i) Jones
Carbon & Biogeochemistry
13:20–13:27
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EGU22-8865
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ECS
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Virtual presentation
Lydia Keppler, Matthew Mazloff, Ariane Verdy, Sarah Gille, Lynne Talley, Nancy Williams, and Veronica Tamsitt

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

Landschützer, P., Gruber, N., Haumann, A., Rödenbeck, C., Bakker, D. C. E., van Heuven, S., Hoppema, M., Metzl, N., Sweeney, C., Takahashi, T., Tilbrook, B., & Wanninkhof, R. (2015). The reinvigoration of the Southern Ocean carbon sink. Science, 349(6253), 1221–1224. https://doi.org/10.1126/science.aab2620

Le Quéré, C., Rödenbeck, C., Buitenhuis, E. T., Conway, T. J., Langenfelds, R., Gomez, A., Labuschagne, C., Ramonet, M., Nakazawa, T., Metzl, N., Gillett, N., & Heimann, M. (2007). Saturation of the Southern Ocean CO2 sink due to recent climate change. Science, 316(5832), 1735–1738. https://doi.org/10.1126/science.1136188

Su, J., Strutton, P. G., & Schallenberg, C. (2021). The subsurface biological structure of Southern Ocean eddies revealed by BGC-Argo floats. Journal of Marine Systems, 220, 103569. https://doi.org/10.1016/j.jmarsys.2021.103569

How to cite: Keppler, L., Mazloff, M., Verdy, A., Gille, S., Talley, L., Williams, N., and Tamsitt, V.: Eddy-induced carbon pumping in the Southern Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8865, https://doi.org/10.5194/egusphere-egu22-8865, 2022.

13:27–13:34
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EGU22-3665
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ECS
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On-site presentation
Elizabeth Ellison

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.

13:34–13:41
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EGU22-2980
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Virtual presentation
Heiner Dietze and Ulrike Löptien

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.

13:41–13:48
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EGU22-9764
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ECS
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Virtual presentation
Neill Mackay

The Southern Ocean accounts for 40% of the uptake of anthropogenic CO2 by the global ocean, which in turn absorbs a quarter of all anthropogenic CO2 emissions, mitigating climate change. Despite its importance, sampling of the Southern Ocean is sparse and biased towards the summer months, and consequentially uncertainties in the carbon sink and its variability are largest there. Recently, autonomous platforms have begun to provide year-round coverage of the parameters needed to estimate the Southern Ocean carbon sink; however, these new observations cannot address the historical sparsity. We present a new estimate of the sink to address historically sparse wintertime sampling through interpretation of subsurface summertime observations to produce new ‘pseudo’ wintertime observations of surface fCO2, boosting the wintertime spatiotemporal coverage by 22% and improving the spatial distribution. We show through a commonly used machine learning technique mapping method, that enhanced wintertime coverage does not significantly alter estimates of the flux or its variability at the sub-basin scale. After adjusting for surface boundary layer temperature effects, we find a strong mean sink south of 35°S of 1.29 ± 0.29 PgC yr-1 for 2004-2018, consistent with recent independent estimates from atmospheric data.

How to cite: Mackay, N.: New wintertime observations allow re-examination of Southern Ocean carbon sink variability, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9764, https://doi.org/10.5194/egusphere-egu22-9764, 2022.

13:48–13:55
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EGU22-12483
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On-site presentation
Parvadha Suntharalingam, Zhaohui Chen, Anna Jones, and David Buchanan

The Southern Ocean plays a fundamental role in in the global carbon cycle and is estimated to absorb  ~40% of anthropogenic carbon-dioxide (CO2) emissions. Recent studies have highlighted  the potentially large  decadal variability of this uptake, and the uncertainties associated with estimates derived from different  ocean carbon measurement technologies. The majority of these estimates of Southern Ocean CO2 uptake are commonly derived from ‘bottom-up’ analyses of oceanic carbon measurements. An independent means of estimating air-sea CO2 fluxes is provided by ‘top-down’ analyses, which employ inverse methods or data assimilation techniques  combining atmospheric CO2 measurements with numerical transport model analyses. Robust regional flux estimates from such top-down methods require an atmospheric observational network with sufficient spatial coverage. At present, however, long-term measurements of atmospheric CO2 are only available at a limited number of sites in the Southern Ocean region. Given this sparse atmospheric sampling there is an urgent need for expansion of the current Southern Ocean atmospheric CO2 measurement network.

The British Antarctic Survey has identified a number of locations (including the sub-Antarctic and South Atlantic Islands of Tristan da Cunha, South Georgia and the Falklands) where new systems for long-term observations of CO2 could be established. In this analysis we present results from a set of Observing System Sampling Experiments (OSSEs) using the GEOS-Chem atmospheric transport model, in combination with the Local Ensemble Transform Kalman Filter method (Chen et al. 2021) to identify the effectiveness of these locations towards providing improved constraints on Southern Ocean air-sea fluxes.  Our assessment of potential sampling sites is derived from metrics quantifying the uncertainty reduction of regional oceanic CO2 flux estimates.

References

Chen et al.  (2021) Variability of North Atlantic CO2 fluxes for the 2000–2017 period estimated from atmospheric inverse analyses. Biogeosciences, 18 (15). pp. 4549-4570. ISSN 1726-4189.

How to cite: Suntharalingam, P., Chen, Z., Jones, A., and Buchanan, D.: Assessing Potential Atmospheric CO2 Monitoring Sites for Improved  Estimation of Southern Ocean CO2 Uptake, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12483, https://doi.org/10.5194/egusphere-egu22-12483, 2022.

13:55–14:02
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EGU22-3858
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ECS
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Virtual presentation
Guillaume Barut, Corentin Clerc, Coraline Leseurre, Claire LoMonaco, and Nicolas Metzl

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 (Cant) 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 Cant 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 Cant 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 Cant. The increase in Cant 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 Cant 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 Cant, these results could significantly reduce the oceanic inventories of Cant in recent years at both the regional and global scales. The reduction in Cant 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.

14:02–14:09
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EGU22-3580
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ECS
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On-site presentation
Coraline Leseurre, Claire Lo Monaco, Gilles Reverdin, Nicolas Metzl, Jonathan Fin, Claude Mignon, and Léa Benito

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 (AT) and total carbon (CT) collected over the period 1998-2019 in the frame of the French monitoring program OISO. We used three datasets (underway fCO2, underway AT-CT and station AT-CT) to evaluate the trends of fCO2 and pH and their drivers, including the accumulation of anthropogenic CO2 (Cant). 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 AT-CT 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 CT, mainly due to the accumulation of Cant (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 AT (around +0.4 µmol kg-1 yr-1)that limited the impact of a more rapid accumulation of Cant 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 CT (up to +1.4 µmol kg-1 yr-1), mainly explained by the uptake of Cant.

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.

14:09–14:16
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EGU22-12068
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ECS
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Presentation form not yet defined
Lithogenic inputs to the Southern Ocean assessed with thorium and rare earth elements
(withdrawn)
Habacuc Perez Tribouillier, Taryn L. Noble, Ashley T. Townsend, Bruce L.A. Charlier, and Zanna Chase
14:16–14:23
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EGU22-6327
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ECS
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On-site presentation
Channing Prend, Madhavan Keerthi, Marina Lévy, Olivier Aumont, Sarah Gille, and Lynne Talley

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.

14:23–14:30
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EGU22-12662
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ECS
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On-site presentation
Pablo Trucco-Pignata, Peter Brown, Dorothee Bakker, Hugh Venables, Alberto Naveira-Garabato, Filipa Carvalho, Katsia Pabortsava, Maribel García-Ibáñez, Sheri White, Stephanie Henson, and Adrian Martin

In the Subantarctic Zone (SAZ) of the southeast Pacific, the densest, coolest, and freshest Subantarctic Mode Water (SAMW) is formed. There, water masses reset their physicochemical characteristics interchanging properties with the atmosphere, and play a critical role in global climate through their impact on the overturning circulation and oceanic heat and carbon uptake. We estimate the magnitude, variability and uncertainty of the air-sea flux of oxygen from five years of hourly observations around the Observatories Initiative (OOI) Southern Ocean mooring.

The magnitude of oxygen fluxes depends greatly on the parameterization used, particularly for high wind events. Hence, there is a need for validation of the high wind speed regime at high latitudes. Surface waters remain undersaturated from autumn to mid-spring, when most of the annual oxygen uptake occurs. We calculate a total annual flux into the ocean of -12.6 ± 3.4 mol m-2 yr-1, with a thermal component of -10.3 ± 2.6 mol m-2 yr-1 and a non-thermal component of -1.0 ± 0.3 mol m-2 yr-1. These results provide the first estimate of oxygen fluxes for the region from high-frequency observations, surpassing previous estimates for the entire SAZ by one order of magnitude.

How to cite: Trucco-Pignata, P., Brown, P., Bakker, D., Venables, H., Naveira-Garabato, A., Carvalho, F., Pabortsava, K., García-Ibáñez, M., White, S., Henson, S., and Martin, A.: Surface oxygen balance in the Subantarctic Mode Water Formation region., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12662, https://doi.org/10.5194/egusphere-egu22-12662, 2022.

14:30–14:50