The Southern Ocean mixed layer and its boundary fluxes: fine-scale observational progress and future research priorities

Abstract

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'.

Sub‐Mesoscale Wind‐Front Interactions: The Combined Impact of Thermal and Current Feedback

“…The stronger sub‐mesoscale modification to wind stress curl in this study could potentially enhance vertical velocities in the upper ocean through Ekman dynamics (Gaube et al., 2015; Seo et al., 2016). The impact of wind‐front interactions on the transport of tracers, ventilation, and upper ocean stratification in the Southern Ocean are key directions for future study (Morrison et al., 2022; Swart et al., 2023).…”

Atmospheric Fronts Shaping the (Sub)Mesoscale SST‐Wind Coupling Over the Southern Ocean: Observational Case

“…Recently, both observations (Acquistapace et al, 2022;de Szoeke et al, 2021;Gaube et al, 2019;Iyer et al, 2022;Shao et al, 2019;Yang et al, 2024) and simulations (Conejero et al, 2024;Strobach et al, 2022;Sullivan & McWilliams, 2022;Sullivan et al, 2020;Wenegrat & Arthur, 2018) have shown that submesoscale SST variability can have direct impacts on the MABL, including surface wind speed, air-sea heat flux, and cloud formation (Acquistapace et al, 2022). The energetic submesoscale processes in the Southern Ocean also implied the strong submesoscale air-sea coupling (Siegelman et al, 2020;Su et al, 2018;Swart et al, 2023), but it remains largely unexplored, particularly from the observation perspective. These oceanic submesoscale processes are beyond the current remote sensing and reanalysis product (Klein et al, 2019), therefore, the field observations are currently the most valuable approach to study the submesoscale air-sea coupling processes.…”

Storm‐Driven pCO2 Feedback Weakens the Response of Air‐Sea CO2 Fluxes in the Sub‐Antarctic Southern Ocean

“…The intra‐seasonal mode of variability is thought to dominate the seasonality of pCO 2sea (dominant driver of FCO 2 variability across the SO), especially in the SAZ, due to the high frequency of storms interacting with mesoscale gradients (Swart et al., 2023). Consequently, large regions of the SAZ show a low seasonal cycle reproducibility (SCR) of pCO 2sea ( r 2 < 0.65, LSCR) (Figure 1b, Djeutchouang et al., 2022; Gregor, Lebehot, Kok, & Scheel Monteiro, 2019; Monteiro et al., 2015).…”