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The Southern Ocean plays a fundamental role in the global carbon cycle, dominating the oceanic uptake of heat and carbon added by anthropogenic activities and modulating atmospheric carbon concentrations in past, present, and future climates. However, the remote and extreme conditions found there make the Southern Ocean perpetually one of the most difficult places on the planet to observe and to model, resulting in significant and persistent uncertainties in our knowledge of the oceanic carbon cycle there. The flow of carbon in the Southern Ocean is traditionally understood using a zonal mean framework, in which the meridional overturning circulation drives the latitudinal variability observed in both air-sea flux and interior ocean carbon concentration. However, recent advances, based largely on expanded observation and modeling capabilities in the region, reveal the importance of processes acting at smaller scales, including basin-scale zonal asymmetries in mixed-layer depth, mesoscale eddies, and high-frequency atmospheric variability. Assessing the current state of knowledge and remaining gaps emphasizes the need to move beyond the zonal mean picture and embrace a four-dimensional understanding of the carbon cycle in the Southern Ocean.

Despite its importance for the global cycling of carbon, there are still large gaps in our understanding of the processes driving annual and seasonal carbon fluxes in the high‐latitude Southern Ocean. This is due in part to a historical paucity of observations in this remote, turbulent, and seasonally ice‐covered region. Here, we use autonomous biogeochemical float data spanning 6 full seasonal cycles and with circumpolar coverage of the Southern Ocean, complemented by atmospheric reanalysis, to construct a monthly climatology of the mixed layer budget of dissolved inorganic carbon (DIC). We investigate the processes that determine the annual mean and seasonal cycle of DIC fluxes in two different zones of the Southern Ocean—the Sea Ice Zone (SIZ) and Antarctic Southern Zone (ASZ). We find that, annually, mixing with carbon‐rich waters at the base of the mixed layer supplies DIC which is, in the ASZ, either used for net biological production or outgassed to the atmosphere. In contrast, in the SIZ, where carbon outgassing and the biological pump are weaker, the surplus of DIC is instead advected northward to the ASZ. In other words, carbon outgassing in the southern Antarctic Circumpolar Current (ACC), which has been attributed to remineralized carbon from deep water upwelled in the ACC, is also due to the wind‐driven transport of DIC from the SIZ. These results stem from the first observation‐based carbon budget of the circumpolar Southern Ocean and thus provide a useful benchmark to evaluate climate models, which have significant biases in this region. Plain Language Summary: The ocean surrounding the frozen continent of Antarctica plays an important role in the global cycling of carbon and is important for the climate of our planet. Despite its importance, there are gaps in our knowledge due to the difficulties involved in collecting data from a remote, seasonally ice‐covered ocean. In this study, we use year‐round data collected by autonomous instruments that can even measure under sea ice. We build a budget of carbon in the surface layer of the ocean, quantifying the different sources and sinks of inorganic carbon. We find that carbon mostly enters the surface layer through mixing with carbon‐rich waters below. In the more stormy, northern part of our study area, this carbon is then either consumed by photosynthesis in the ocean or it is transferred to the atmosphere. In the southernmost region, biological activity and gas transfer at the ocean‐atmosphere interface is hindered by the presence of sea ice and the surplus of carbon is instead transferred north by wind‐driven circulation. Our results show that year‐round measurements of carbon are necessary to understand carbon cycling in the region and we provide a useful product to compare to global simulations of the Earth system. Key Points: We build a Southern Ocean observation‐based mixed layer inorganic carbon budget with circumpolar coverage and a full seasonal cycleBiological activity and circulation dominate the seasonal variations in mixed layer dissolved inorganic carbonWind‐driven transport from the seasonal ice zone contributes to carbon outgassing in the Antarctic Circumpolar Current [ABSTRACT FROM AUTHOR]

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 also released to 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 constrained from observations. Using data from a novel array of autonomous biogeochemical profiling floats, we examine Southern Ocean air‐sea CO2 fluxes and the pathways that transfer carbon from the ocean interior into the mixed layer where air‐sea exchange occurs. These float‐based flux estimates of unprecedented spatial resolution indicate 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). We show that this zonal asymmetry in surface pCO2, and consequently air‐sea carbon fluxes, stems primarily from regional variability in the mixed‐layer entrainment of upwelled carbon‐rich deep water. These results suggest that a sustained circumpolar observing system is crucial to monitor future changes in oceanic carbon release and uptake. Plain Language Summary: The Southern Ocean, which surrounds Antarctica, helps regulate the global climate system by sequestering atmospheric heat and carbon dioxide (CO2) in the deep ocean. Although this region makes a significant contribution to the total oceanic carbon uptake, there are also locations in the Southern Ocean where CO2 is released back into the atmosphere. Understanding what controls these patterns of air‐sea carbon exchange is necessary to improve climate models and prepare for the effects of climate change. However, studying the Southern Ocean carbon cycle is difficult due to the lack of observations from this remote environment. Here, we use new data collected by autonomous biogeochemical floats to examine air‐sea carbon fluxes throughout the Southern Ocean. We find that ocean carbon release occurs preferentially in the Indo‐Pacific sector of the Southern Ocean due to systematic differences between basins in the transport of subsurface carbon from the deep ocean into the mixed layer—the portion of the upper ocean that is in direct contact with the atmosphere. These results suggest that collecting data from all sectors of the Southern Ocean is crucial to monitor future changes in the ocean carbon sink. Key Points: Autonomous biogeochemical floats reveal that carbon outgassing occurs predominantly in the Indo‐Pacific sector of the Southern OceanAsymmetry in carbon outgassing is driven by systematic interbasin variations in the mixed‐layer entrainment of carbon‐rich deep waterExpanded data coverage, particularly circumpolar sampling, is necessary to accurately capture Southern Ocean carbon sink variability [ABSTRACT FROM AUTHOR]

New estimates of pCO2 from profiling floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project have demonstrated the importance of wintertime outgassing south of the Polar Front, challenging the accepted magnitude of Southern Ocean carbon uptake (Gray et al., 2018, https://doi:10.1029/2018GL078013). Here, we put 3.5 years of SOCCOM observations into broader context with the global surface carbon dioxide database (Surface Ocean CO2 Atlas, SOCAT) by using the two interpolation methods currently used to assess the ocean models in the Global Carbon Budget (Le Quéré et al., 2018, https://doi:10.5194/essd‐10‐2141‐2018) to create a ship‐only, a float‐weighted, and a combined estimate of Southern Ocean carbon fluxes (<35°S). In our ship‐only estimate, we calculate a mean uptake of −1.14 ± 0.19 Pg C/yr for 2015–2017, consistent with prior studies. The float‐weighted estimate yields a significantly lower Southern Ocean uptake of −0.35 ± 0.19 Pg C/yr. Subsampling of high‐resolution ocean biogeochemical process models indicates that some of the differences between float and ship‐only estimates of the Southern Ocean carbon flux can be explained by spatial and temporal sampling differences. The combined ship and float estimate minimizes the root‐mean‐square pCO2 difference between the mapped product and both data sets, giving a new Southern Ocean uptake of −0.75 ± 0.22 Pg C/yr, though with uncertainties that overlap the ship‐only estimate. An atmospheric inversion reveals that a shift of this magnitude in the contemporary Southern Ocean carbon flux must be compensated for by ocean or land sinks within the Southern Hemisphere. Plain Language Summary: The Southern Ocean is thought to take up a significant amount of carbon dioxide each year but is a difficult region to observe due to its remote location and harsh winter weather. Recently, autonomous robots deployed by the Southern Ocean Carbon and Climate Observations and Modeling project have been making year‐round measurements of ocean carbonate chemistry, from which we can estimate surface carbon dioxide. These provide new data at times and locations where we previously had very little. We found that combining the float observations with traditional shipboard data reduced our estimate for the amount carbon that the Southern Ocean takes up each year, though by less than had been previously estimated when considering float observations alone. We also show that some of the new signals is likely due to the differences in when and where floats make measurements. The magnitude of difference between prior estimates of the Southern Ocean carbon flux and our new approach is significant, ~20% of the contemporary global ocean carbon flux. It is therefore crucial to understand how this may impact the global carbon cycle, and we show that a compensating flux must be found somewhere within the Southern Hemisphere. Key Points: A combined ship and float carbon dioxide flux estimate for the Southern Ocean yields 0.4 Pg C/yr less uptake than a ship‐only estimateModel subsampling indicates that some of the differences between ship and float flux estimates may be due to sampling times and locationsAn atmospheric inversion using the new ocean fluxes indicates that any compensating flux must be found in land or ocean south of 5°S [ABSTRACT FROM AUTHOR]

The Southern Ocean is highly under-sampled for the purpose of assessing total carbon uptake and its variability. Since this region dominates the mean global ocean sink for anthropogenic carbon, understanding temporal change is critical. Underway measurements of pCO2 collected as part of the Drake Passage Time-series (DPT) program that began in 2002 inform our understanding of seasonally changing air-sea gradients in pCO2, and by inference the carbon flux in this region. Here, we utilize available pCO2 observations to evaluate how the seasonal cycle, interannual variability, and long-term trends in surface ocean pCO2 in the Drake Passage region compare to that of the broader subpolar Southern Ocean. Our results indicate that the Drake Passage is representative of the broader region in both seasonality and long-term pCO2 trends, as evident through the agreement of timing and amplitude of seasonal cycles as well as trend magnitudes both seasonally and annually. The high temporal density of sampling by the DPT is critical to constraining estimates of the seasonal cycle of surface pCO2 in this region, as winter data remain sparse in areas outside of the Drake Passage. An increase in winter data would aid in reduction of uncertainty levels. On average over the period 2002-2016, data show that carbon uptake has strengthened with annual surface ocean pCO2 trends in the Drake Passage and the broader subpolar Southern Ocean less than the global atmospheric trend. Analysis of spatial correlation shows Drake Passage pCO2 to be representative of pCO2 and its variability up to several hundred kilometers away from the region. We also compare DPT data from 2016 and 2017 to contemporaneous pCO2 estimates from autonomous biogeochemical floats deployed as part of the Southern Ocean Carbon and Climate Observations and Modeling project (SOCCOM) so as to highlight the opportunity for evaluating data collected on autonomous observational platforms. Though SOCCOM floats sparsely sample the Drake Passage region for 2016-2017 compared to the Drake Passage Time-series, their pCO2 estimates fall within the range of underway observations given the uncertainty on the estimates. Going forward, continuation of the Drake Passage Time-series will reduce uncertainties in Southern Ocean carbon uptake seasonality, variability, and trends, and provide an invaluable independent dataset for post-deployment assessment of sensors on autonomous floats. Together, these datasets will vastly increase our ability to monitor change in the ocean carbon sink. [ABSTRACT FROM AUTHOR]

The Southern Ocean is highly under-sampled for the purpose of assessing total carbon uptake and its variability. Since this region dominates the mean global ocean sink for anthropogenic carbon, understanding temporal change is critical. Underway measurements of pCO2 collected as part of the Drake Passage Time-series (DPT) program that began in 2002 inform our understanding of seasonally changing air-sea gradients in pCO2, and by inference the carbon flux in this region. Here, we utilize all available pCO2 observations collected in the subpolar Southern Ocean to evaluate how the seasonal cycle, interannual variability, and long-term trends in surface ocean pCO2 in the Drake Passage region compare to that of the broader subpolar Southern Ocean. Our results indicate that the Drake Passage is representative of the broader region in both seasonality and long term pCO2 trends shown through the agreement of timing and amplitude of seasonal cycles as well as trend magnitudes. The high temporal density of sampling by the DPT is critical to constraining estimates of the seasonal cycle of surface pCO2 in this region, as winter data remain sparse in areas outside of the Drake Passage. From 2002-2015, data show that carbon uptake has strengthened with surface ocean pCO2 trends less than the global atmospheric trend in the Drake Passage and the broader subpolar Southern Ocean. Analysis of spatial correlation shows Drake Passage pCO2 to be representative of pCO2 and its variability up to several hundred kilometers upstream of the region. We also compare DPT data from 2016 and early 2017 to contemporaneous pCO2 estimates from autonomous biogeochemical floats deployed as part of the Southern Ocean Carbon and Climate Observations and Modeling project (SOCCOM) so as to highlight the opportunity for evaluating data collected on autonomous observational platforms. Though SOCCOM floats sparsely sample the Drake Passage region for 2016-2017, their pCO2 estimates typically fall within the range of underway observations. Going forward, continuation of the Drake Passage Time-series will reduce uncertainties in Southern Ocean carbon uptake seasonality, variability, and trends, and provide an invaluable independent dataset for post-deployment quality control of sensors on autonomous floats. Together, these datasets will vastly increase our ability to monitor change in the ocean carbon sink. [ABSTRACT FROM AUTHOR]

• SOCCOM has deployed >260 BGC-Argo floats with O 2 , NO 3 , pH and bio-optical sensors. • SOCCOM floats operate throughout the Southern Ocean including the seasonal ice zone. • All SOCCOM float data is freely available in real-time. • A Biogeochemical Southern Ocean State Estimate (BSOSE) assimilates float data. • Float data and BSOSE are used with coupled models to better understand climate. The Southern Ocean serves as the primary gateway through which the intermediate, deep, and bottom waters of the ocean interact with the surface ocean (and thus the atmosphere), and it has a profound influence on the oceanic uptake of anthropogenic carbon and heat as well as nutrient resupply from the abyss to the surface. Yet it has been the least observed and understood region of the world ocean. The Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project was implemented in 2014 with a goal to help remedy this deficit in observations and understanding. The SOCCOM project is based on two major advances that have the potential to transform understanding of the Southern Ocean. The first is the development of new biogeochemical sensors mounted on autonomous profiling floats that allow sampling of ocean biogeochemistry in 3-dimensional space. Floats may detect processes with a temporal resolution that ranges from hours to years. The second is that the climate modeling community finally has the computational resources and physical understanding to develop fully coupled climate models that can represent crucial, mesoscale processes in the Southern Ocean, as well as corresponding models that assimilate observations to produce a state estimate. The observational component, based on deployment of profiling floats with oxygen, nitrate, pH and bio-optical sensors, is generating vast amounts of new biogeochemical data that provide a year-round view of the Southern Ocean from the surface to 2000 m. The modeling effort is applying these observations and enhancing our understanding of the current ocean, and reducing uncertainty in projections of future carbon and nutrient cycles and climate. After nine years of operation, including a project renewal in the sixth year, the SOCCOM project has deployed more than 260 profiling floats. These floats have collected over 27,000 vertical profiles throughout the Southern Ocean. A data-assimilating biogeochemical state estimate model has been implemented. Here, the design of the SOCCOM project is reviewed and the scientific results that have been obtained are described. The project's capability to help meet the observing system priorities outlined for a notional UN Decade for Ocean Sciences Southern Ocean observing system is assessed. [ABSTRACT FROM AUTHOR]