OS1.6

The Southern Ocean south of 30°S, occupying about a third of global surface ocean area, accounts for approximately 40% of the past anthropogenic carbon uptake and about 75% of excess heat uptake by the ocean. However, Earth system models have large difficulties in reproducing the Southern Ocean circulation, and therefore its historical and future anthropogenic carbon and excess heat uptake. In the first part of the talk, we show that there exists a tight relation across two Earth system model ensembles (CMIP5 and CMIP6) between present-day sea surface salinity in the subtropical-polar frontal zone, the formation region of mode and intermediate waters, and the past and future anthropogenic carbon uptake in the Southern Ocean. By using observations and Earth system model results, we constrain the projected cumulative Southern Ocean anthropogenic carbon uptake over 1850-2100 by the CMIP6 model ensemble to 158 ± 6 Pg C under the low-emissions scenario SSP1-2.6 and to 279 ± 14 Pg C under the high emissions scenario SSP5-8.5. Our results suggest that the Southern Ocean anthropogenic carbon sink is 14-18% larger and 46-54% less uncertain than estimated by the unconstrained CMIP6 Earth system model results. The identified constraint demonstrated the importance of the freshwater cycle for the Southern Ocean circulation and carbon cycle. In the second part of the talk, potential emergent constraints for the Southern Ocean excess heat uptake will be discussed.

How to cite: Frölicher, T., Terhaar, J., and Joos, F.: Emergent constraints on the Southern Ocean anthropogenic carbon and heat uptake, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5336, https://doi.org/10.5194/egusphere-egu21-5336, 2021.

Projected changes in ocean heat and carbon storage are assessed in terms of the added and 
redistributed tracer using a transport-based framework for 6 CMIP5 Earth system models following an annual 1% rise in atmospheric 
CO2. Heat and carbon budgets for the added and redistributed tracer are used to compare the reasons for the relatively-reduced storage of heat and carbon within the Southern Ocean. Here the added tracer takes 
 account of the net tracer source and the advection of the added tracer, while the redistributed tracer takes account of the time-varying advection of the pre-industrial tracer  distribution. The added heat and carbon are nearly always positive over the Southern Ocean with the net source acting to supply tracer. However, there is a relatively-reduced local storage of heat and carbon in the Southern Ocean due to the passive northward transport of heat and carbon by the overturning, which is augmented by a passive northward carbon transport for the gyre circulation. In contrast, the redistributed heat is usually negative and the redistributed carbon is positive over the Southern Ocean due to the transport effects of a strengthening residual circulation and the opposing gradients in the pre-industrial temperature and 
carbon. Hence, climate projections for the Southern Ocean are expected to have heat anomalies of a variable sign and carbon anomalies of a consistently positive  sign, since the effects of added and redistribution heat are opposing in sign, while the effects of added and redistributed 
carbon reinforce each other.


How to cite: Roussenov, V., Williams, R., and Katavouta, A.: Added and redistributed heat and carbon in climate model projections for the Southern Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3153, https://doi.org/10.5194/egusphere-egu21-3153, 2021.

We examine the representation of Southern Ocean water mass properties, circulation and transformation in an ensemble of CMIP6 models, under historical climate forcing conditions and under a range of future climate scenarios. By using a dynamically defined water mass classification scheme based on physical characteristics (salinity minimum, potential vorticity minimum etc) rather than fixed water mass properties, we are able to compare water masses across a range of models, often with significant water mass property differences, as well as within single models where water mass properties change under climate forcing. We find that under strong climate forcing scenarios (ssp585) the heat content of SubAntarctic Mode Water (SAMW), Antarctic Intermediate Water (AAIW) and Circumpolar Deep Water (CDW) all increase consistently across models, while Antarctic Bottom Water (AABW) does not change significantly. Importantly this change is strongly modulated by using dynamic definitions. Both SAMW and AAIW lighten significantly in density, and using time varying definitions their volumes remain relatively constant, whereas using a time invariant definition both experience extremely significant increases in volume and heat content. We show that dynamically it is the ocean interior, CDW and AAIW, that dominate heat uptake under strong forcing. Similarly, dissolved inorganic carbon uptake occurs predominantly in the CDW. In contrast AABW volumes decrease significantly.

There is a consistent ‘fingerprint’ of temperature change in density space across all models under strong forcing scenarios, with CDW experiencing surface intensified warming as it shoals to the south, and SAMW/AAIW demonstrating cooling and freshening in their subducted layers and a uniform warming in the surface layers. We show that the upper cell of the residual overturning circulation (calculated with the new availability of eddy parametrisation terms in CMIP6) consistently increases across all models evaluated, by 10-50% (up to 10 Sv in some models), while the lower cell is dramatically decreased in strength, declining by up to 70% in some models. We provide evidence that surface warming may be modulated by increased eddy driven upwelling, as well as surface freshening driving the shutdown of AABW formation. Finally we compute a Walin water mass budget, balancing surface forcing, interior storage and meridional export and inferring interior mixing between water masses, and contrast all findings with similar analyses in CMIP5.

How to cite: Meijers, A., Munday, D., Roy, T., and Sallée, J.-B.: Southern Ocean water mass properties and circulation under CMIP6 climate forcing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8222, https://doi.org/10.5194/egusphere-egu21-8222, 2021.

The Southern Ocean features ventilation pathways that transport surface waters into the subsurface thermocline on timescales from decades to centuries, sequestering anomalies of heat and carbon away from the atmosphere and thereby regulating the rate of surface warming. Despite its importance for climate sensitivity, the factors that control the distribution of heat along these pathways are not well understood. In this study, we use an observationally-constrained, physically-consistent global ocean state estimate (i.e. ECCOv4) to examine how changes in ocean properties can affect the heat content both in the mixed layer and in the recently ventilated subsurface, focusing on the Southeast Pacific. First, we carry out a comprehensive adjoint sensitivity study using near-surface heat content as the objective function, highlighting the locations and timescales with the largest potential to affect the properties of relevant subduction regions. Next, we use a set of numerical tracer release experiments to identify the subduction and export pathways from the surface into the subsurface thermocline, thereby defining the recently ventilated interior. Using the tracer distribution to define our objective function, we employ an adjoint method to calculate temporally-evolving sensitivity maps that highlight the processes, locations, and timescales that are potentially most relevant for changing the heat content of the recently ventilated Pacific. In order to examine the full nonlinear response, we use the adjoint sensitivity fields to design a set of forward, nonlinear perturbation experiments. We find surprisingly weak sensitivities to high latitude wind stress and heat flux, and relatively high sensitivities to wind stress curl in subpolar latitudes. Despite the localized nature of mode water subduction hotspots, changes in basin-scale density gradients are an important controlling factor on heat distribution in the Southeast Pacific.

How to cite: Jones, D., Boland, E., Meijers, A., Forget, G., Josey, S., Pimm, C., Sallée, J.-B., and Shuckburgh, E.: The sensitivity of Southeast Pacific heat content to changes in ocean structure, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3016, https://doi.org/10.5194/egusphere-egu21-3016, 2021.

Surface heat loss leads to thick winter mixed layers over the Southern Ocean, which feeds the formation of subsurface mode water pools through subduction. One such water class is Subantarctic Mode Water (SAMW), which is characterised by its low absolute potential vorticity. SAMW occurs in several regions of the Southern Ocean on the northern side of the Antarctic circumpolar current and it extends into the subtropics below the surface on different density surfaces. Using the ECCOv4 global ocean circulation model, we conduct a series of adjoint sensitivity experiments and forward perturbation experiments at key Southern Ocean SAMW formation sites, focusing on how different surface forcing affects potential vorticity. This adjoint approach produces time-evolving sensitivity maps that identify where and when surface heat loss potentially impacts the formation of mode waters. Over the first year in lead time, we find that greater surface heat loss leads to stronger convection and lower SAMW potential vorticity. On lead times longer than one year, in some regions of high sensitivity, the sensitivity reverses its sign, such that more surface heat loss ultimately leads to higher values of potential vorticity in the subduction regions. This reversal of sign of the sensitivity can be attributed to a shift from local convective forcing to upstream advective forcing and the associated redistribution of potential temperature and salinity. Surface adjustment also plays a role in the upstream sensitivities due to the tendency for temperature anomalies to be weakened through compensating salinity before reaching the subduction zone. We use the adjoint sensitivity fields to design a set of forward, non-linear perturbation experiments to provide physical insight into how ventilation affects the uptake of heat and carbon. This physical insight is important for identifying which physical mechanisms affect the subducted properties in the Southern Ocean, especially as the ocean warms through climate change.

How to cite: Pimm, C., Williams, R., Jones, D., and Meijers, A.: How does heat flux affect potential vorticity in the Southern Ocean?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7799, https://doi.org/10.5194/egusphere-egu21-7799, 2021.

Antarctic Bottom Water formed in the Weddell Sea is transported by the Weddell Gyre (WG) into the Antarctic Circumpolar Current (ACC). From here, this water is exported to the world ocean and influences the global overturning circulation. Studying the dynamics of the WG could therefore improve our understanding of the Southern Ocean carbon and energy budget.

The dynamics of the WG in a NEMO global model is investigated at various resolutions. The WG transport is largest at intermediate resolution (R4) and only the low-resolution model (R1) has a transport close to observations. We attempt to identify the physical processes responsible for this difference by studying the vorticity diagnostics. These physical processes include (but are not limited to) wind stress curl, lateral friction and bottom pressure torques.  

A textbook understanding of gyres relies on the idea of vorticity balance and this idea is extended to identify the physical processes spinning the WG up and down. We integrate the vorticity diagnostics outputted by NEMO over the area enclosed by the WG streamlines. These integrations are equal to the work done by separate forces on fluid parcels circulating around the gyre.

In the future we also hope to apply this analysis to an idealised model representing the Weddell Sea. This model will also use NEMO but have analytic forcing, bathymetry and a prescribed ACC.

How to cite: Styles, A., Marshall, D., and Bell, M.: Diagnosing the vorticity balances of the Weddell Gyre, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1306, https://doi.org/10.5194/egusphere-egu21-1306, 2021.

In this study, a high-resolution eddy resolving regional ocean + sea ice coupled model (MITgcm) is used to study the effects of increasing westerlies along the Southern Ocean. Previous studies only focused on increasing wind stress, thus not taking into account of atmosphere-to-ocean heat and freshwater fluxes. Here, we conduct two concurrent simulations; i) 1.5 times increased wind stress (i.e. increased only mechanical forcing) ii) 1.2247 times increased wind speed (i.e. both mechanical and thermal flux forcing). Model domain covers whole Southern Hemisphere with lateral open boundary conditions from ECCOv2 ocean reanalysis and surface boundary conditions from ECMWF ERA-5 atmospheric reanalysis. In both sensitivity scenarios, due to the increase in the wind stress, the Ekman transport towards Equator towards north is increased. This caused increased upwelling of warmer North Atlantic Deep Water (NADW) near the Antarctic ice sheet. Both scenarios show reduced sea ice formation with up to 2 million km² in the austral summer and up to 4 million km² during the austral winter. Sea ice extent is reduced more in the mechanical forcing simulation than the mechanical+thermal forcing one. This is a clear result that increased wind anomalies should be studied with increased wind rather than increased stress. The reduction in the sea ice coverage that is attributed to the warmer water mass can also be observed through the Sea Surface Temperature (SST) values. The first case shows up to 1 – 1.5 °C very close to the Antarctica, whereas the second case shows a much limited SST change around 0.5 °C.

Both sensitivity scenarios show an increase of the transport along Drake Passage. However, the mechanical+thermal case shows larger increase in the Drake transport compared to the mechanical case. This indicates that a change in the Antarctic Circumpolar Circulation also modifies the meridional density gradient along with the upwelling characteristics. Finally, overturning transport in the density space shows that Subtropical Cell and ACC upper Cell strengthen in the mechanical+thermal case, while there are no significant changes in the thermal case. In both simulations, Subpolar Cell increases and Lower Cell decreases. We conclude that studying increased westerlies with two different approaches show significant changes in the surface and deep circulation. Previous studies which taken into only mechanical forcing part are missing thermal component of the wind effects.

How to cite: Tutak, B., Ilicak, M., and Mazloff, M.: Investigation of Mechanical and Thermal Wind Sensitivity on the Mesoscale Eddies in the Southern Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14755, https://doi.org/10.5194/egusphere-egu21-14755, 2021.

We present estimates of Southern Ocean air-sea CO₂ fluxes for the period 2000-2018 derived with the GEOSChem-LETKF (GCL) inverse analysis system in conjunction with the NOAA surface CO₂ monitoring network (ObsPack, Cooperative Global Atmospheric Data Integration Project, 2018).  We assess the impact of alternative representations of the ocean prior flux distribution and its associated uncertainties on derived flux estimates.  Ocean flux distributions assessed include the surface pCO₂-based representation of Landschutzer et al. 2016 and the more recent pCO₂-based estimates of Watson et al. 2020.  We present GCL estimates of the long-term trend and interannual variability of air-sea CO₂ fluxes in the Southern Ocean (south of 45˚S). These results are assessed against independent estimates from atmospheric inverse analyses and ocean biogeochemical models taken from the Global Carbon Budget 2020 synthesis (Friedlingstein et al. 2020).

How to cite: Chen, Z., Suntharalingam, P., Le Quere, C., Watson, A., and Shutler, J.: Estimates of Southern Ocean carbon uptake from atmospheric inverse analyses, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12411, https://doi.org/10.5194/egusphere-egu21-12411, 2021.

The Southern Ocean modulates the climate system by exchanging heat and carbon dioxide (CO₂) between the atmosphere and deep ocean. While this region plays an outsized role in the global oceanic anthropogenic carbon uptake, CO₂ is released into the atmosphere across large swaths of the Antarctic Circumpolar Current (ACC). Southern Ocean outgassing has long been attributed to remineralized carbon from upwelled deep water, but the precise mechanisms by which this water reaches the surface are not well known. Using data from a novel array of autonomous biogeochemical profiling floats, we estimate Southern Ocean air-sea CO₂ fluxes at unprecedented spatial resolution and determine the pathways that transfer carbon from the ocean interior into the mixed layer where air-sea exchange occurs. Float-based flux estimates suggest that carbon outgassing occurs predominantly in the Indo-Pacific sector of the ACC due to variations in the mean surface ocean partial pressure of CO₂ (pCO₂). Within the Polar Frontal Zone and Antarctic Southern Zone of the ACC, the annual mean pCO₂ difference between the Indo-Pacific and Atlantic is 40.1 ± 12.9 μatm and 17.9 ± 12.4 μatm, respectively. We show that this zonal asymmetry in surface pCO₂ and consequently air-sea carbon fluxes stems from regional variability in the mixed-layer entrainment of carbon-rich deep water. These results suggest that long-term trends of the Southern Ocean carbon sink inferred from sparse shipboard data may depend on the fraction of measurements from each basin in a given year. Furthermore, sampling these different air-sea flux regimes is necessary to monitor future changes in oceanic carbon release and uptake.

How to cite: Prend, C., Gray, A., Talley, L., Gille, S., Haumann, A., Johnson, K., Riser, S., Rosso, I., Sauvé, J., and Sarmiento, J.: Zonal asymmetry of Southern Ocean air-sea carbon fluxes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2155, https://doi.org/10.5194/egusphere-egu21-2155, 2021.

Uncertainty in the CO₂ gas transfer velocity (K₆₆₀) severely limits the accuracy of air-sea CO₂ flux calculations and hence hinders our ability to produce realistic climate projections.  Recent field observations have suggested substantial variability in K₆₆₀, especially at low and high wind speeds.  Laboratory experiments have shown that naturally occurring surface active organic materials, or surfactants, can suppress gas transfer.  Here we provide direct open ocean evidence of gas transfer suppression due to surfactants from a ~11,000 km long research expedition by making measurements of the gas transfer efficiency (GTE) along with direct observation of K₆₆₀.  GTE varied by 20% during the Southern Ocean transect and was distinct in different watermasses.  Furthermore GTE correlated with and can explain about 9% of the scatter in K₆₆₀, suggesting that surfactants exert a measurable influence on air-sea CO₂ flux.  Relative gas transfer suppression due to surfactants was ~30% at a global mean wind speed of 7 m s^-1 and was more important at lower wind speeds.  Neglecting surfactant suppression may result in substantial spatial and temporal biases in the computed air-sea CO₂ fluxes.

How to cite: Yang, M., Smyth, T., Kitidis, V., Brown, I., Wohl, C., Yelland, M., and Bell, T.: Suppression of air-sea CO2 transfer by surfactants – direct evidence from the Southern Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4601, https://doi.org/10.5194/egusphere-egu21-4601, 2021.

The Southern Ocean plays an important role in the uptake, transport and storage of carbon by the global oceans. These properties are dominated by the response to 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 on ocean carbon uptake and storage in the Southern Ocean. We assess the extent to which climate change may be distinguishable from the anthropogenic CO₂ signal and from the natural background variability. We use a combination of biogeochemical ocean modelling and observations from the GLODAPv2020 database to detect climate fingerprints in dissolved inorganic carbon (DIC).

We conduct an ensemble of hindcast model simulations of the period 1920-2019, using a global ocean biogeochemical model which incorporates plankton ecosystem dynamics based on twelve plankton functional types. We use the model ensemble to isolate the changes in DIC due to rising anthropogenic CO₂ alone and the changes due to climatic drivers (both climate variability and climate change), to determine their relative roles in the emerging total DIC trends and patterns. We analyse these DIC trends for a climate fingerprint over the past four decades, across spatial scales from the Southern Ocean, to basin level and down to regional ship transects. Highly sampled ship transects were extracted from GLODAPv2020 to obtain locations with the maximum spatiotemporal coverage, to reduce the inherent biases in patchy observational data. Model results were sampled to the ship transects to compare the climate fingerprints directly to the observational data.

Model results show a substantial change in DIC over a 35-year period, with a range of more than +/- 30 µmol/L. 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 and the influence of climatic drivers dominating at higher latitudes. In the deep ocean, the anthropogenic CO₂ generally acts to increase DIC except in the subsurface waters at lower latitudes, while climatic drivers act to decrease DIC concentration. The combined fingerprint of anthropogenic CO₂ and climatic drivers on DIC concentration is for an increasing trend at the surface and decreasing trends in low latitude subsurface waters. Preliminary comparison of the model fingerprints to observational ship transects will also be presented.

How to cite: Wright, R., Le Quéré, C., Buitenhuis, E., and Bakker, D.: Detecting Climate Fingerprints in Southern Ocean Carbon using a Biogeochemical Ocean Model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11116, https://doi.org/10.5194/egusphere-egu21-11116, 2021.

Reconstructions of Antarctic surface temperature covering the past millennia display a large centennial variability that is not synchronous with fluctuations recorded on other continents and which is generally not well simulated by models. Many processes can be at the origin of these temperature variations such as teleconnections with tropical oceans and changes in the Southern Ocean. The focus here will be on the latter, in particular on the influence of westerly winds that have a large impact on the exchange of heat and carbon between the ocean and atmosphere. Changes in the Southern Ocean circulation and stratification also influence the carbon cycle at global scale. It is generally suggested that atmospheric CO₂ variations over the past two millennia were mainly controlled by land processes but the Southern Ocean might have also played a role. We will thus test whether the joint analysis of Antarctic temperature and atmospheric CO₂ concentration fluctuations can inform us on the origin of the observed changes over this period. In this purpose, we use the climate model LOVECLIM which includes a representation of the global carbon cycle. Experiments over the last two millennia will address the sensitivity to realistic perturbations of the wind stress. Finally, experiments with data assimilation will allow assessing what constraints are needed for model results to better reproduce the atmospheric CO₂ concentration and reconstructed temperature history.

How to cite: Goosse, H., Lyu, Z., Menviel, L., Meissner, K., and Mouchet, A.: Influence of Southern Ocean dynamics on Antarctic temperatures and on the global carbon cycle over the past two millennia., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1180, https://doi.org/10.5194/egusphere-egu21-1180, 2021.

The mechanisms of atmospheric CO2 draw-down by ~90 ppm during glacial cycles have been one of the most contentious questions in the past several decades. Processes in the Southern Ocean (SO) have been suggested to be at the heart, while the North Atlantic (NA) is recently proposed to be critical during glacial periods as well. However, in a full course of glacial cycles, the individual and synergic roles of these two regions remain enigmatic. Using a state-of-the-art biogeochemical model (MITgcm-REcoM2) associated with an interactive CO₂ module, we examined the impact of the onset of individual mechanisms and combinations of them on atmospheric CO₂. Here we show that SO controls carbon sequestration in both hemispheres. In sensitivity runs with respect to mechanisms happening during glacial inceptions, cooling in SO contributes to a larger portion of CO₂ draw-down than cooling in NA, by shortening the surface water exposure time, while the early sea ice expansion tends to weaken the carbon uptake. The efficiency of surface carbon storage in the North Atlantic depends on the volume of Antarctic bottom water and reaches its maximum when the glacial stratification is well developed during glacial maxima.  SO cooling and sea ice expansion strongly promote the formation of AABW and the full development of the glacial stratification. Furthermore, increased dust deposition during the glacial maxima raises the contribution of the Southern Ocean in the global biological carbon pump, leading to a higher efficiency of the biological carbon pump. And the maximal expanded sea ice suppresses local carbon leakage.

How to cite: Du, J., Zhang, X., Ye, Y., Völker, C., and Tian, J.: Southern control of interhemispheric synergy on marine carbon sequestration during glacial cycles  , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9623, https://doi.org/10.5194/egusphere-egu21-9623, 2021.