Periodic Reporting for period 1 - SO-CUP (Southern Ocean Carbon Uptake)
Reporting period: 2020-09-01 to 2022-08-31
The Southern Ocean is a disproportionately important region, relative to its size, for mitigating the consequences of the anthropogenic climate change, being responsible for 43% and 75% of the ocean uptake of anthropogenic CO2 and heat, respectively. The MSCA SO-CUP aims at shedding light on the processes underpinning the role of the Southern Ocean as a major sink of atmospheric carbon dioxide (CO2).
Most of the heat and carbon uptake in the Southern Ocean is carried out by a group of water masses termed Sub-Antarctic Mode Waters (SAMW), which originate in deep surface mixing layers to the North of the Antarctic Circumpolar Current (ACC). As for most of the oceans’ water masses, SAMW properties are determined during winter, when large volumes of water are exposed to the atmosphere. However, the dynamical processes that control the formation of SAMW and the associated drawdown of carbon are not well understood due to the scarce data in this remote region during winter, which represents a critical bottleneck for the understanding of the current state and future evolution of the Southern Ocean carbon sink.
The observational gap was significantly relieved in recent years, with the advent and large-scale deployment of autonomous technologies for ocean observation. Since 2014, the US SOCCOM program deployed in the Southern Ocean hundreds of autonomous profiling floats equipped with biogeochemical sensors, allowing for an unprecedentedly thorough observation of the Southern Ocean carbon cycle. The main scientific goal of the MSCA SO-CUP is to leverage the ongoing explosion in situ observations to identify and quantify the ventilation pathways and processes that govern the drawdown of atmospheric carbon by SAMW.
Most of the heat and carbon uptake in the Southern Ocean is carried out by a group of water masses termed Sub-Antarctic Mode Waters (SAMW), which originate in deep surface mixing layers to the North of the Antarctic Circumpolar Current (ACC). As for most of the oceans’ water masses, SAMW properties are determined during winter, when large volumes of water are exposed to the atmosphere. However, the dynamical processes that control the formation of SAMW and the associated drawdown of carbon are not well understood due to the scarce data in this remote region during winter, which represents a critical bottleneck for the understanding of the current state and future evolution of the Southern Ocean carbon sink.
The observational gap was significantly relieved in recent years, with the advent and large-scale deployment of autonomous technologies for ocean observation. Since 2014, the US SOCCOM program deployed in the Southern Ocean hundreds of autonomous profiling floats equipped with biogeochemical sensors, allowing for an unprecedentedly thorough observation of the Southern Ocean carbon cycle. The main scientific goal of the MSCA SO-CUP is to leverage the ongoing explosion in situ observations to identify and quantify the ventilation pathways and processes that govern the drawdown of atmospheric carbon by SAMW.
The research goals of SO-CUP were addressed with a twofold strategy combining the analysis of observational float data and model output. In situ observations by biogeochemical floats were analysed focusing on a subset of the data comprising the winter surface layers. The analysis revealed that the changes in carbon concentration in Southern Ocean’s water masses follow, to first order, thermodynamic equilibrium with the atmosphere, and are therefore dictated by the physical processes. However, an examination of the air-sea and biological carbon fluxes revealed that biological carbon uptake plays a leading role in setting the carbon concentration of SAMW, while air sea fluxes weakly adjust the carbon concentrations towards equilibrium.
The analysis of observations also revealed a surprising, unexpected feature of SAMW: seasonal variations of salinity in regions of SAMW formation identified in the float data indicate that SAMW are strongly influenced by the arrival of subsurface subtropical waters with high-salinity and low nutrients. This finding contradicts the prevailing view of the Southern Ocean circulation, in which SAMW originate from surface Antarctic waters moving northward across the ACC.
The second part of the project consisted of the analysis of output from a physical-biogeochemical model of the Southern Ocean (B-SOSE), which assimilates biogeochemical data from the SOCCOM floats. We performed a Lagrangian particle tracking experiment using B-SOSE output and an open-source particle tracking software (OceanParcels). Virtual particles were released in the SAMW formation regions and tracked backwards in time for 9 years to identify the sources of SAMW and quantify carbon fluxes along their trajectories. This approach allowed for the quantification of the subtropical contribution to SAMW, which was dominant (~75%) for the SAMW varieties of the Indian Ocean and less important (~40%) in the Pacific Ocean.
Carbon fluxes recorded along particle pathways provided insights into the processes controlling the amount of carbon subducted with SAMW. All SAMW varieties have a carbon content similar to their northern subtropical sources and smaller than their Antarctic sources, following thermodynamic equilibrium, consistent with the observations. However, particles following both pathways experienced a net uptake of carbon from the atmosphere, which was larger in the Antarctic pathway. This uptake against the thermodynamic driving forces is allowed by a significant biological uptake reducing surface carbon concentrations.
These findings have been communicated to the oceanography and, specifically, Southern Ocean research communities, through participation in 6 international conferences and workshops and in the regular SOCCOM project meetings. Results from the observational analysis were published as a research letter in a high-impact specialised journal. Two additional manuscripts describing the modelling results are under preparation for a high-impact open-access journal. The project was advertised to a broader public with three articles science dissemination journals (e.g. https://bit.ly/3sdN90u) posts in social networks, and virtual visits to secondary schools.
The analysis of observations also revealed a surprising, unexpected feature of SAMW: seasonal variations of salinity in regions of SAMW formation identified in the float data indicate that SAMW are strongly influenced by the arrival of subsurface subtropical waters with high-salinity and low nutrients. This finding contradicts the prevailing view of the Southern Ocean circulation, in which SAMW originate from surface Antarctic waters moving northward across the ACC.
The second part of the project consisted of the analysis of output from a physical-biogeochemical model of the Southern Ocean (B-SOSE), which assimilates biogeochemical data from the SOCCOM floats. We performed a Lagrangian particle tracking experiment using B-SOSE output and an open-source particle tracking software (OceanParcels). Virtual particles were released in the SAMW formation regions and tracked backwards in time for 9 years to identify the sources of SAMW and quantify carbon fluxes along their trajectories. This approach allowed for the quantification of the subtropical contribution to SAMW, which was dominant (~75%) for the SAMW varieties of the Indian Ocean and less important (~40%) in the Pacific Ocean.
Carbon fluxes recorded along particle pathways provided insights into the processes controlling the amount of carbon subducted with SAMW. All SAMW varieties have a carbon content similar to their northern subtropical sources and smaller than their Antarctic sources, following thermodynamic equilibrium, consistent with the observations. However, particles following both pathways experienced a net uptake of carbon from the atmosphere, which was larger in the Antarctic pathway. This uptake against the thermodynamic driving forces is allowed by a significant biological uptake reducing surface carbon concentrations.
These findings have been communicated to the oceanography and, specifically, Southern Ocean research communities, through participation in 6 international conferences and workshops and in the regular SOCCOM project meetings. Results from the observational analysis were published as a research letter in a high-impact specialised journal. Two additional manuscripts describing the modelling results are under preparation for a high-impact open-access journal. The project was advertised to a broader public with three articles science dissemination journals (e.g. https://bit.ly/3sdN90u) posts in social networks, and virtual visits to secondary schools.
SO-CUP represents a major push forward in our understanding of the mechanisms driving the Southern Ocean carbon sink, by expanding and re-drawing the current theoretical framework of the Southern Ocean circulation and carbon sink, improving our capacity to anticipate future changes in response to climate change.
The existing framework for interpreting the Southern Ocean carbon sink is based on a simplified, zonally- and time-averaged view of the Southern Ocean circulation. In this view, SAMW – main drivers of the carbon drawdown – originate from the old, deep Circumpolar Deep Waters (CDW) sourced in the North Atlantic, which upwell to the ocean surface around Antarctica and travel northward across the ACC. Instead, the SO-CUP findings show that the sources of SAMW follow complex 3-dimensional pathways and include a major, previously unaccounted, subtropical contribution.
Critically, in the existing framework, the Southern Ocean carbon sink is viewed as the result of thermodynamic forces imposing a net carbon uptake in upwelled CDW, which were last exposed to the atmosphere decades ago, when atmospheric CO2 was much lower. This interpretation is at odds with the lower carbon concentrations in SAMW with respect to CDW, and with CDW being oversaturated with respect to the present-day atmosphere, due microbial activity in the deep ocean. SO-CUP reconciles these facts by showing that biological uptake in surface waters drives CO2 concentrations below local equilibrium, allowing for a net intake from the atmosphere. SO-CUP thus places the biological carbon pump at the forefront of the processes driving the carbon sink and highlights the need of further research on the sensitivity and feedbacks of biological processes on climate.
Further, the existence of a large subtropical source of SAMW revealed by SO-CUP has implications for global ocean productivity. SAMW are believed to fertilise the global surface ocean by redistributing Antarctic nutrients across the World’s Oceans. SAMW nutrient concentrations, controlling global biological productivity, are thought to depend on the strength of biological nutrient drawdown in the Southern Ocean. Instead, our findings point out to a strong influence of the nutrient-poor subtropical sources of SAMW, suggesting that other processes (subtropical gyre circulation strength and across-ACC mixing processes) play also an unaccounted role.
The existing framework for interpreting the Southern Ocean carbon sink is based on a simplified, zonally- and time-averaged view of the Southern Ocean circulation. In this view, SAMW – main drivers of the carbon drawdown – originate from the old, deep Circumpolar Deep Waters (CDW) sourced in the North Atlantic, which upwell to the ocean surface around Antarctica and travel northward across the ACC. Instead, the SO-CUP findings show that the sources of SAMW follow complex 3-dimensional pathways and include a major, previously unaccounted, subtropical contribution.
Critically, in the existing framework, the Southern Ocean carbon sink is viewed as the result of thermodynamic forces imposing a net carbon uptake in upwelled CDW, which were last exposed to the atmosphere decades ago, when atmospheric CO2 was much lower. This interpretation is at odds with the lower carbon concentrations in SAMW with respect to CDW, and with CDW being oversaturated with respect to the present-day atmosphere, due microbial activity in the deep ocean. SO-CUP reconciles these facts by showing that biological uptake in surface waters drives CO2 concentrations below local equilibrium, allowing for a net intake from the atmosphere. SO-CUP thus places the biological carbon pump at the forefront of the processes driving the carbon sink and highlights the need of further research on the sensitivity and feedbacks of biological processes on climate.
Further, the existence of a large subtropical source of SAMW revealed by SO-CUP has implications for global ocean productivity. SAMW are believed to fertilise the global surface ocean by redistributing Antarctic nutrients across the World’s Oceans. SAMW nutrient concentrations, controlling global biological productivity, are thought to depend on the strength of biological nutrient drawdown in the Southern Ocean. Instead, our findings point out to a strong influence of the nutrient-poor subtropical sources of SAMW, suggesting that other processes (subtropical gyre circulation strength and across-ACC mixing processes) play also an unaccounted role.