The Southern Ocean accounts for a disproportionately large percentage of the total oceanic anthropogenic carbon uptake, which results from the region's unique role in the global overturning circulation (Frölicher et al., 2015;Khatiwala et al., 2009). In the southern portion of the Antarctic Circumpolar Current (ACC), deep water returns to the upper ocean via wind-driven upwelling and is transformed into northward-flowing intermediate and bottom waters (J. Marshall & Speer, 2012). These upwelled deep waters have been isolated from the atmosphere since preindustrial times, and thus are poised both to absorb anthropogenic CO 2 and to release natural carbon that has accumulated over centuries through the remineralization of organic matter (Lovenduski et al., 2008). The net air-sea carbon exchange in the Southern Ocean is, therefore, a balance between opposing processes of carbon uptake and outgassing (DeVries et al., 2017;Gruber et al., 2019).Both uptake of anthropogenic CO 2 and outgassing of natural carbon are fundamentally tied to the physical transport of water between the surface mixed layer and ocean interior. While the circulation of the Southern Ocean has

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 an 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 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 frontal zones of the Antarctic Circumpolar Current (ACC)—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 then, 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 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.

Background: Antarctic sea ice concentration has been observed to increase from 1978 to 2015, in contrast with the decrease that most climate models show. Here, we aim to examine the respective roles of natural variability and anthropogenic forcing in shaping Antarctic sea ice trend. Method: To do so, we use the GFDL-CM2 coupled climate model with varying horizontal resolutions in the ocean (1°, 0.25° and 0.10°) that displays a range of behaviours in natural variability with the representation of Weddell Sea polynyas, and different intensities in the decrease of sea ice under climate change. Results: In the 0.10° model, a sea ice trend of similar sign and magnitude to that observed over the satellite record is found between two occurrences of the Weddell Sea polynya. In the 1° and 0.25° models, which do not simulate any polynya, no equivalent trend of what the satellite record shows is found. Under increasing CO2 forcing, all models show a surface cooling on a short time scale (years) south of 50°S, followed by a warming on a longer time scale (decades), consistent with the delayed warming mechanism of Ferreira et al. (2015). Of all models, the higher resolution model shows the strongest surface warming and decrease in sea ice, suggesting an important role for mesoscale eddies in the response of Antarctic sea ice to climate change. Conclusion: We conclude that the Weddell Sea polynya is key to the representation of the sea ice trend and that the disagreement between models and observations might partly arise from a desynchronization of the polynya cycles or a too weak natural variability of sea ice in models compared to observations.

<p>The Southern Ocean modulates the climate system by exchanging heat and carbon dioxide (CO<sub>2</sub>) between the atmosphere and deep ocean. While this region plays an outsized role in the global oceanic anthropogenic carbon uptake, CO<sub>2</sub> 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 CO<sub>2</sub> 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 CO<sub>2</sub> (<em>p</em>CO<sub>2</sub>). Within the Polar Frontal Zone and Antarctic Southern Zone of the ACC, the annual mean <em>p</em>CO<sub>2</sub> 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 <em>p</em>CO<sub>2</sub> 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.</p>

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