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The sub‐seasonal CO2 flux (FCO2) variability across the Southern Ocean is poorly understood due to sparse observations at the required temporal and spatial scales. Twinned surface and profiling gliders experiments were used to investigate how storms influence FCO2 through the air‐sea gradient in partial pressure of CO2 (ΔpCO2) in the sub‐Antarctic zone. Winter‐spring storms caused ΔpCO2 to weaken (by 22–37 μatm) due to mixing/entrainment and weaker stratification. This weakening in ΔpCO2 was in phase with the increase in wind stress resulting in a reduction of the storm‐driven CO2 uptake by 6%–27%. During summer, stronger stratification explained the weaker sensitivity of ΔpCO2 to storms, instead temperature changes dominated the ΔpCO2 variability. These results highlight the importance of observing synoptic‐scale variability in ΔpCO2, the absence of which may propagate significant biases to the mean annual FCO2 estimates from large‐scale observing programmes and reconstructions. Plain Language Summary: The sub‐Antarctic zone of the Southern Ocean is a region that mostly experiences carbon dioxide (CO2) uptake because of its low temperature, strong winds and lower CO2 content. The wind can influence the CO2 uptake through two pathways: the speed of CO2 transfer between the air‐sea interface (kw) and the difference in CO2 concentration in the surface ocean and overlying atmosphere (ΔpCO2). Using autonomous robots that can measure hourly air and water conditions simultaneously, we show that not resolving ΔpCO2 during a storm event can lead to overestimating the CO2 uptake. This is particularly important during winter and spring when the ocean's surface layers are less stratified. The warmer temperatures during summer meant a more stratified surface layer resulting in a weaker and delayed impact of storms on the ΔpCO2. This study shows that the various annual CO2 uptake estimation methods used by the research community should not neglect ΔpCO2 responses during storms. Key Points: Hourly glider observations show that the impact of storms on both kw and ΔpCO2 simultaneously modulates the ocean CO2 uptake variabilityWinter‐spring storms weaken ΔpCO2 through enhanced entrainment and mixing, partially counteracting the increase in CO2 uptake due to kw aloneBy not accounting for the storm‐linked positive feedback in ΔpCO2, the cumulative seasonal CO2 uptake was found to be overestimated by ∼6% [ABSTRACT FROM AUTHOR]

We assess the Southern Ocean CO2 uptake (1985–2018) using data sets gathered in the REgional Carbon Cycle Assessment and Processes Project Phase 2. The Southern Ocean acted as a sink for CO2 with close agreement between simulation results from global ocean biogeochemistry models (GOBMs, 0.75 ± 0.28 PgC yr−1) and pCO2‐observation‐based products (0.73 ± 0.07 PgC yr−1). This sink is only half that reported by RECCAP1 for the same region and timeframe. The present‐day net uptake is to first order a response to rising atmospheric CO2, driving large amounts of anthropogenic CO2 (Cant) into the ocean, thereby overcompensating the loss of natural CO2 to the atmosphere. An apparent knowledge gap is the increase of the sink since 2000, with pCO2‐products suggesting a growth that is more than twice as strong and uncertain as that of GOBMs (0.26 ± 0.06 and 0.11 ± 0.03 Pg C yr−1 decade−1, respectively). This is despite nearly identical pCO2 trends in GOBMs and pCO2‐products when both products are compared only at the locations where pCO2 was measured. Seasonal analyses revealed agreement in driving processes in winter with uncertainty in the magnitude of outgassing, whereas discrepancies are more fundamental in summer, when GOBMs exhibit difficulties in simulating the effects of the non‐thermal processes of biology and mixing/circulation. Ocean interior accumulation of Cant points to an underestimate of Cant uptake and storage in GOBMs. Future work needs to link surface fluxes and interior ocean transport, build long overdue systematic observation networks and push toward better process understanding of drivers of the carbon cycle. Plain Language Summary: The ocean takes up CO2 from the atmosphere and thus slows climate change. The Southern Ocean has long known to be an important region for ocean CO2 uptake. Here, we bring together all available data sets that estimate the Southern Ocean CO2 uptake, from models that simulate ocean circulation and physical and biological processes that affect the ocean carbon cycle, from surface ocean observation‐based estimates, from atmospheric transport models that ingest atmospheric CO2 observations, and from interior ocean biogeochemical observations. With these data sets, we find good agreement on the mean Southern Ocean CO2 uptake 1985–2018, which is 50% smaller than previous estimates when recalculated for the time period and spatial extent used in the previous estimate. However, the estimates of the temporal change of the Southern Ocean CO2 uptake differ by a factor of two and thus are not in agreement. We further highlight that knowledge gaps exist not only in winter when observations are typically rare, but equally in summer when biology plays a larger role, which is typically represented too simplistically in the dynamic models. Key Points: Ocean models and machine learning estimates agree on the mean Southern Ocean CO2 sink, but the trend since 2000 differs by a factor of twoREgional Carbon Cycle Assessment and Processes Project Phase 2 estimates a 50% smaller Southern Ocean CO2 sink for the same region and timeframe as RECCAP1Large model spread in summer and winter indicates that sustained efforts are required to understand driving processes in all seasons [ABSTRACT FROM AUTHOR]

Quantifying the strength and efficiency of the Southern Ocean biological carbon pump (BCP) and its response to predicted changes in the Earth's climate is fundamental to our ability to predict long-term changes in the global carbon cycle and, by extension, the impact of continued anthropogenic perturbation of atmospheric CO2. There is little agreement, however, in climate model projections of the sensitivity of the Southern Ocean BCP to climate change, with a lack of consensus in even the direction of predicted change, highlighting a gap in our understanding of a major planetary carbon flux. In this review, we summarize relevant research that highlights the important role of fine-scale dynamics (both temporal and spatial) that link physical forcing mechanisms to biogeochemical responses that impact the characteristics of the seasonal cycle of phytoplankton and by extension the BCP. This approach highlights the potential for integrating autonomous and remote sensing observations of fine scale dynamics to derive regionally optimized biogeochemical parameterizations for Southern Ocean models. Ongoing development in both the observational and modelling fields will generate new insights into Southern Ocean ecosystem function for improved predictions of the sensitivity of the Southern Ocean BCP to climate change. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'. [ABSTRACT FROM AUTHOR]

[[12]] examine the role of the Southern Ocean in setting global ocean heat uptake in coupled climate models, as well as its impacts on climate metrics such as the transient climate response where it exerts an impact greater than its proportional area. [[7]] document how a new type of dataset coming from sailboats, particularly racing craft, navigating the Southern Ocean can help fill the observational gap and be useful in our understanding of Southern Ocean carbon budget. Crucially, the response of future Southern Ocean heat and carbon uptake to projected global warming scenarios, and the nature of the feedbacks that these may generate, are poorly understood and even more poorly constrained by observations. Keywords: oceanography; Southern Ocean; ocean carbon EN oceanography Southern Ocean ocean carbon 1 5 5 05/09/23 20230626 NES 230626 The Southern Ocean is an extreme environment. [Extracted from the article]

Interactions between the upper ocean and air-ice-ocean fluxes in the Southern Ocean play a critical role in global climate by impacting the overturning circulation and oceanic heat and carbon uptake. Remote and challenging conditions have led to sparse observational coverage, while ongoing field programmes often fail to collect sufficient information in the right place or at the time-space scales required to constrain the variability occurring in the coupled ocean-atmosphere system. Only within the last 10 years have we been able to directly observe and assess the role of the fine-scale ocean and rapidly evolving atmospheric marine boundary layer on the upper limb of the Southern Ocean's overturning circulation. This review summarizes advances in mechanistic understanding, arising in part from observational programmes using autonomous platforms, of the fine-scale processes (1–100 km, hours-seasons) influencing the Southern Ocean mixed layer and its variability. We also review progress in observing the ocean interior connections and the coupled interactions between the ocean, atmosphere and cryosphere that moderate air-sea fluxes of heat and carbon. Most examples provided are for the ice-free Southern Ocean, while major challenges remain for observing the ice-covered ocean. We attempt to elucidate contemporary research gaps and ongoing/future efforts needed to address them. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'. [ABSTRACT FROM AUTHOR]