The Southern Ocean is a key region for the vertical and lateral exchanges of heat, carbon, and nutrients, with significant past and potential future impacts on the global climate system. However, the role of the Southern Ocean as a sink of anthropogenic carbon and heat, and as a source of natural carbon remains uncertain. Indeed, observations of many aspects of this system are still sparse and the ability to model the complex dynamics governing the air-sea exchange, export and storage of heat and carbon is limited, resulting in large climate projection uncertainties.
To address these knowledge gaps the Southern Ocean has been the subject of recent large-scale observational, theoretical and modelling investigations by several national and international programmes, including SOCCOM, the UK ORCHESTRA and RoSES, and the H2020 programme SO-CHIC, complimented by the IODP and other drilling programmes. These and other large scale efforts such as the CMIP6 simulations have provided insight into the processes governing the Southern Ocean heat and carbon exchanges, their spatial patterns and trends on subannual, multi-decadal and millennial timescales, as well as their potential future modifications under a changing climate.
This session welcomes contributions dealing with the physical, biogeochemical and ecological processes driving the air-sea exchange, export, and storage of heat and carbon in the Southern Ocean under past, present, and future climates. These include (but are not limited to) interior ocean mixing, water mass transformation and transport pathways, the cycling of carbon and nutrients, as well as ocean-ice-atmosphere interactions and fluxes. The session will also discuss the wider implications of changing Southern Ocean heat and carbon exchanges for the lower latitudes and for the global climate.
In addition to the official vEGU sessions, we are planning a joint discussion session (with OS1.7: Under cover: The Southern Ocean’s connection to sea ice and ice shelves) on Wednesday 28 April, at 13:30-15:30 CEST, where we will discuss in more depth topics arising from the sessions, such as the role of Southern Ocean circulation for the uptake and storage of heat and carbon, ecological and biogeochemical exchanges, ocean-ice interactions and the role of the Southern Ocean in wider climate. The discussion session is structured as a 1-hour plenary using zoom and moderated by the convenors, followed by a 1-hour informal exchange in break out groups using “wonder.me”. We especially encourage Early Career Researchers to participate and to use these informal sessions as an opportunity for networking with fellow Southern Ocean enthusiasts. The links to the rooms be displayed to attendees at the vPICO session. If you are a registered attendee but cannot attend the vPICO session please directly contact a convener for breakout session details.
vPICO presentations: Wed, 28 Apr
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 (AABW) supplies the lower limb of the global overturning circulation, ventilates the abyssal ocean and sequesters heat and carbon on multidecadal to millennial timescales. AABW originates on the Antarctic continental shelf, where strong winter cooling and brine released during sea ice formation produce Dense Shelf Water, which sinks to the deep ocean. The salinity, density and volume of AABW have decreased over the last 50 years, with the most marked changes observed in the Ross Sea. These changes have been attributed to increased melting of the Antarctic Ice Sheet. Here we use in situ observations to document a recovery in the salinity, density and thickness (that is, depth range) of AABW formed in the Ross Sea, with properties in 2018–2019 similar to those observed in the 1990s. The recovery was caused by increased sea ice formation on the continental shelf. Increased sea ice formation was triggered by anomalous wind forcing associated with the unusual combination of positive Southern Annular Mode and extreme El Niño conditions between 2015 and 2018. Our study highlights the sensitivity of AABW formation to remote forcing and shows that climate anomalies can drive episodic increases in local sea ice formation that counter the tendency for increased ice-sheet melt to reduce AABW formation.
How to cite: Silvano, A., Foppert, A., Rintoul, S., Holland, P., Tamura, T., Kimura, N., Castagno, P., Falco, P., Budillon, G., Haumann, A., Naveira Garabato, A., and Macdonald, A.: Recent recovery of Antarctic Bottom Water formation in the Ross Sea driven by climate anomalies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8371, https://doi.org/10.5194/egusphere-egu21-8371, 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.
The Southern Ocean is the main gateway for anthropogenic CO2 into the ocean owing to the upwelling of old water masses with low anthropogenic CO2 concentration, and the transport of the newly equilibrated surface waters into the ocean interior through intermediate, deep and bottom water formation. Here we present first results of the Southern Ocean chapter of RECCAP2, which is the Global Carbon Project’s second systematic study on Regional Carbon Cycle Assessment and Processes. In the Southern Ocean chapter, we aim to assess the Southern Ocean carbon sink 1985-2018 from a wide range of available models and data sets, and to identify patterns of regional and temporal variability, model limitations and future challenges.
We gathered global and regional estimates of the air-sea CO2 flux over the period 1985-2018 from global ocean biogeochemical models, surface pCO2-based data products, and data-assimilated models. The analysis on the Southern Ocean quantified geographical patterns in the annual mean and seasonal amplitude of air-sea CO2 flux, with results presented here aggregated to the level of large-scale ocean biomes.
Considering the suite of observed and modelled estimates, we found that the subtropical seasonally stratified (STSS) biome stands out with the largest air-sea CO2 flux per area and a seasonal cycle with largest ocean uptake of CO2 in winter, whereas the ice (ICE) biome is characterized by a large ensemble spread and a pronounced seasonal cycle with the largest ocean uptake of CO2 in summer. Connecting these two, the subpolar seasonally stratified (SPSS) biome has intermediate flux densities (flux per area), and most models have difficulties simulating the seasonal cycle with strongest uptake during the summer months.
Our analysis also reveals distinct differences between the Atlantic, Pacific and Indian sectors of the aforementioned biomes. In the STSS, the Indian sector contributes most to the ocean carbon sink, followed by the Atlantic and then Pacific sectors. This hierarchy is less pronounced in the models than in the data-products. In the SPSS, only the Atlantic sector exhibits net CO2 uptake in all years, likely linked to strong biological production. In the ICE biome, the Atlantic and Pacific sectors take up more CO2 than the Indian sector, suggesting a potential role of the Weddell and Ross Gyres.
These first results confirm the global relevance of the Southern Ocean carbon sink and highlight the strong regional and interannual variability of the Southern Ocean carbon uptake in connection to physical and biogeochemical processes.
How to cite: Hauck, J., Gregor, L., Nissen, C., Mortenson, E., Bushinsky, S., Doney, S., Gruber, N., Lenton, A., LeQuere, C., Mazloff, M., Monteiro, P. M. S., and Patara, L.: The Southern Ocean carbon sink 1985-2018: first results of the RECCAP2 project, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1834, https://doi.org/10.5194/egusphere-egu21-1834, 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.
Net primary production is a major contributor to carbon export in the Southern Ocean and supports rich marine ecosystems [Henley et al., 2020], driven in part by high macronutrient availability and summertime light levels, but ultimately constrained by seasonal changes in light and scarce supply of the essential micronutrient iron [Martin et al., 1990; Boyd, 2002; Tagliabue et al., 2016]. Although changing iron stress is a component of climate-driven trends in model projections of net primary production [Bopp et al., 2013; Laufkotter et al., 2015; Kwiatkowski et al., 2020], our confidence in the accuracy of their predictions is undermined by a lack of in situ constraints at appropriate spatial and temporal scales [Tagliabue et al., 2016; Tagliabue et al., 2020]. Earth System Models tend to predict increased Southern Ocean net primary production by the end of the 21st century, but are characterized by significant inter-model disagreement [Bopp et al., 2013; Kwiatkowski et al., 2020 Biogeosciences]. We show a significant multi-decadal increase in in situ iron stress from 1996 to 2020 that is positively correlated to the Southern Annular Mode and reflected by diminishing in situ net primary production over the last five years. It is not possible to directly infer Fe stress from observed concentrations, which necessitate experimental approaches (in situ open ocean fertilization / bottle nutrient addition experiments or proteomics). These experimental methods cannot be easily applied at appropriate spatial and temporal scales across the Southern Ocean that are required to assess trends in ecosystem status linked to climate drivers. Our novel proxy for in situ iron stress is based on the degree of non-photochemical quenching in relation to available light as a measurable photophysiological response to iron availability [Alderkamp et al., 2019; Schuback & Tortell, 2019; Schallenberg et al., 2020; Ryan-Keogh & Thomalla, 2020]. The proxy was able to reproduce expected variations in iron stress that occur seasonally [Boyd, 2002] and from natural and artificial fertilization [Boyd et al., 2000; Coale et al., 2004; Blain et al., 2008]. A particular strength of this iron stress proxy is that it can be retrospectively applied to data from ships and autonomous platforms with coincident measurements of fluorescence, photosynthetically active radiation and backscatter or beam attenuation to deliver a long-term time series. An iron stress trend of this magnitude in the Southern Ocean, where the primary constraint on net primary production is known to be iron limitation, is likely to have significant implications for the effectiveness of the biological carbon pump globally and may impact the trajectory of climate. The progressive in situ trend of increasing iron stress is however much stronger than net primary production trends from a suite of remote sensing and earth system models, indicating hitherto potential underestimation of ongoing Southern Ocean change.
How to cite: Thomalla, S., Ryan-Keogh, T., Tagliabue, A., and Monteiro, P.: Long term trend of increasing iron stress in Southern Ocean phytoplankton, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7488, https://doi.org/10.5194/egusphere-egu21-7488, 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.
Observational estimates point to pronounced changes of the Southern Ocean carbon uptake in the past decades, but the mechanisms are still not fully understood. In this study we assess physical drivers of the Southern Ocean carbon uptake variability in a suite of global ocean biogeochemistry models with 0.5º, 0.25º and 0.1º horizontal resolution as well as in a 3-member ensemble performed with an Earth System Model (ESM) sharing the same ocean biogeochemistry model. The ocean models show a positive trend of the Southern Ocean CO2 uptake in the past decades, with a weakening of its rate of increase in the 1990s. The 0.1º model exhibits the strongest trend in the Southern Ocean carbon uptake. Different physical drivers of the carbon uptake variability and of its trends (such as changes in stratification, ventilation, overturning circulation, and SST) are analyzed. A particular focus of this study is to assess the role of open-ocean polynyas in driving Southern Ocean carbon uptake. Open-ocean polynyas in the Southern Ocean have pronounced climate fingerprints, such as reduced sea-ice coverage, heat loss by the ocean and enhanced bottom water formation, but their role for the Southern Ocean carbon uptake has been as yet little studied. To this end we analyze conjunctly ESM simulations and an ocean-only sensitivity experiment where open-ocean polynyas are artificially created by perturbing the Antarctic freshwater runoff. We find that enhanced CO2 outgassing takes place during the polynya opening, because old carbon-rich waters come in contact with the atmosphere. The concomitant increased uptake of anthropogenic CO2 partially compensates the CO2 outgassing. When the polynya closes, the ocean CO2 uptake increases significantly, possibly fueled by abundant nutrients and higher alkalinity brought to the surface during the previous convective phase. Our results suggest that open-ocean polynyas could have a significant impact on the Southern Ocean CO2 uptake and could thus modulate its decadal variability.
How to cite: Patara, L., Martin, T., Frenger, I., Rieck, J. K., and Chien, C.-T.: Modeling the physical drivers of the decadal variability of the Southern Ocean carbon uptake, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8654, https://doi.org/10.5194/egusphere-egu21-8654, 2021.
Biological processes in the subpolar Southern Ocean play a crucial role in the global carbon cycle, mediating CO2 exchange between the atmosphere and the densest waters of the global ocean. While historical perspectives have centred the importance of shelf-sea regions, recent reframing emphasises the role of the open ocean, and the cyclonic gyres. Here, we investigate the operation of the biological carbon pump (BCP) in the Weddell Gyre using satellite ocean colour and bio-Argo floats. We find first that a significant proportion (>54 %) of the inter-annual variability in NPP was explained by the area of open (ice-free) water. Spatial patterns suggest that peak productivity is associated with the ice edge. The seasonal decline in NPP occurs before ice cover returns, suggesting that other controls are limiting annual NPP (e.g. the exhaustion of iron). Comparing the shelf region to the open ocean, the shelf was seen to have higher rates of productivity, but NPP in the relatively less productive open ocean region accounted for ~95% of total carbon uptake each year. The total NPP in the Weddell Gyre (97-197 Tg C yr-1) is sufficient to supply the BCP-derived carbon that was previously observed to be exported from the region in Circumpolar Deep Water (~80 Tg C yr-1). NPP in the open ocean Weddell Gyre could thus provide the major source of carbon exported from the Weddell Gyre to the deep ocean via the horizontal circulation.
How to cite: Douglas, C., Brown, P., Briggs, N., MacGilchrist, G., and Naveira Garabato, A.: Assessing the Biological Carbon Pump in the Weddell Gyre , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12020, https://doi.org/10.5194/egusphere-egu21-12020, 2021.
Deep and bottom water formation regions have long been recognized to be efficient vectors for carbon transfer to depth, leading to carbon sequestration on time scales of centuries or more. Precursors of Antarctic Bottom Water (AABW) are formed on the Weddell Sea continental shelf as a consequence of buoyancy loss of surface waters at the ice-ocean or atmosphere-ocean interface, which suggests that any change in water mass transformation rates in this area affects global carbon cycling and hence climate. Many of the models previously used to assess AABW formation in present and future climates contained only crude representations of ocean-ice shelf interaction. Numerical simulations often featured spurious deep convection in the open ocean, and changes in carbon sequestration have not yet been assessed at all. Here, we present results from the global model FESOM-REcoM, which was run on a mesh with elevated grid resolution in the Weddell Sea and which includes an explicit representation of sea ice and ice shelves. Forcing this model with ssp585 scenario output from the AWI Climate Model, we assess changes over the 21st century in the formation and northward export of dense waters and the associated carbon fluxes within and out of the Weddell Sea. We find that the northward transport of dense deep waters (σ2>37.2 kg m-3 below 2000 m) across the SR4 transect, which connects the tip of the Antarctic Peninsula with the eastern Weddell Sea, declines from 4 Sv to 2.9 Sv by the year 2100. Concurrently, despite the simulated continuous increase in surface ocean CO2 uptake in the Weddell Sea over the 21st century, the carbon transported northward with dense deep waters declines from 3.5 Pg C yr-1 to 2.5 Pg C yr-1, demonstrating the dominant role of dense water formation rates for carbon sequestration. Using the water mass transformation framework, we find that south of SR4, the formation of downwelling dense waters declines from 3.5 Sv in the 1990s to 1.6 Sv in the 2090s, a direct result of the 18% lower sea-ice formation in the area, the increased presence of modified Warm Deep Water on the continental shelf, and 50% higher ice shelf basal melt rates. Given that the reduced formation of downwelling water masses additionally occurs at lighter densities in FESOM-REcoM in the 2090s, this will directly impact the depth at which any additional oceanic carbon uptake is stored, with consequences for long-term carbon sequestration.
How to cite: Nissen, C., Timmermann, R., Hoppema, M., and Hauck, J.: Attenuated carbon sequestration by Weddell Sea dense waters over the 21st century – an assessment with FESOM-REcoM, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6248, https://doi.org/10.5194/egusphere-egu21-6248, 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.
The Southern Ocean (SO) is a key region for ocean-atmosphere CO2 exchanges, as it witnesses significant changes in physical and biological pump dynamics. While numerous studies have highlighted the central role of reinvigorated SO upwelling behind rapid increases in atmospheric CO2 during glacial terminations, a very few studies have yet focused on the impact of the Biological Carbon Pump and more specifically of the Carbonate Counter Pump (CCP) that, contrary to the Soft Tissue Pump, participates to increase the concentration of dissolved CO2 in oceanic surface waters and thus, in the atmosphere.
Amongst the last 9 interglacials, Marine Isotope Stage (MIS) 11 (~ 400 ka) is the longest interglacial of the past 800,000 years, characterised by a ~30 ka-long plateau with atmospheric CO2 hovering around 280 ppm. Reconstructions of past global biosphere productivity based on Δ17O of O2 measurements on air bubbles trapped in ice cores, show that MIS 11 registers the strongest global biosphere productivity (~ 20% higher) compared to the other 8 interglacials (Brandon et al., 2020; Yang et al., EGU21) Meanwhile, marine sedimentary records suggest strong carbonate production and export. Studying the detailed variations of the CCP during this specific period can therefore be useful to better understand its relationship with biospheric productivity changes and to better constraint its impacts on atmospheric CO2.
As calcifying organisms, coccolithophores and planktonic foraminifera represent the major producers of CaCO3 in pelagic environments and are therefore useful tools to reconstruct past variations in the CCP strength. Here, we calibrate CaXRF and CaCO3 signals from marine core MD04-2718 located in the Indian sector of the SO (48°53 S; 65°57 E) in terms of coccolith and planktonic carbonate production and export signals over the last 800 ka, with a focus on the interval MIS 12 to MIS 10. We compare our results with published micropaleontological and geochemical records from the subantarctic zone (SAZ) in order to reconstruct past changes in CCP efficiency and circulation at the SO scale and understand their relationships with atmospheric CO2 patterns.
We show an increase in CCP efficiency during interglacial periods, with an exceptional high carbonate export production during MIS 11. We demonstrate that enhanced CCP efficiency at the beginning of MIS 11 is likely the consequence of both higher SST conditions and nutrient contents in the upper water column of the SAZ, that increase coccolithophore and planktonic foraminifera productions, thanks to the southward migration of SO fronts and the reinvigoration of southern upwelling. While the sharp increase in atmospheric CO2 during Termination V seems correlated with the reinvigoration of the SO upwelling, enhanced CCP at the beginning of MIS 11 might have greatly reduced the efficiency of the biological pump, impacting the CO2 flux from the ocean to the atmosphere. The strong global biological productivity registered during this interval might have permitted to sustain the 30 ka-long plateau of atmospheric CO2 that characterize this time interval.
How to cite: Brandon, M., Duchamp-Alphonse, S., Michel, E., Landais, A., Isguder, G., Pige, N., Bassinot, F., Jaccard, S., Pont, S., and Bartolini, A.: Enhanced Carbonate Counter Pump efficiency during interglacials of the past 800 000 years in the Indian sector of the Southern Ocean and its impact on the carbon cycle , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8610, https://doi.org/10.5194/egusphere-egu21-8610, 2021.
The Antarctic Circumpolar Current (ACC) is a major driver of global climate. It connects all three ocean basins, integrating global climate variability, and its vertical water mass structure plays a key role in oceanic carbon storage. The Atlantic and Indian sectors of the ACC are well studied, but the Pacific sector lacks deep-sea drilling records. Therefore, past water mass transport through the Drake Passage and its effect on Atlantic Meridional Overturning Circulation are not well understood. To fill this gap, IODP Expedition 383 recovered sediments from three sites in the central South Pacific and three sites from the southern Chilean Margin.
Here we present the preliminary biostratigraphy developed during the expedition. The sediments contained abundant nannofossils, foraminifera, radiolarians, diatoms and silicoflagellates which produced age models that were in excellent agreement with the shipboard magnetostratigraphy. Two sites contain high-resolution Pleistocene records, one site goes back to the Pliocene, and two others reach back to the late Miocene. Post-cruise research will further refine these age models through high-resolution bio-, magneto- and oxygen isotope stratigraphies that are currently being generated.
How to cite: Saavedra-Pellitero, M., Brombacher, A., Esper, O., de Souza, A., Malinverno, E., Venancio, I., Riesselman, C., and Singh, R. K. and the Expedition 383 Scientists: Preliminary biostratigraphy of IODP Expedition 383 sites, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1818, https://doi.org/10.5194/egusphere-egu21-1818, 2021.
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