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'.
Atmospheric rivers (ARs) are narrow bands of elevated water vapor fluxes typically associated with the low-level atmospheric jet, located in the lower 3 km of the atmosphere (Ralph et al., 2017). They are transient features and despite covering only 10% of Earth's surface area, they account for 90% of poleward water vapor transport (Nash et al., 2018;Zhu & Newell, 1998). ARs often feature ahead of an extratropical cyclone's cold front and within the cyclone's warm conveyor belt. In the Eastern Pacific, 82% of ARs are connected to a midlatitude cyclone (storm), while only 45% of storms are paired with an AR (Zhang et al., 2019). AR presence could impact the storm dynamics due to the close connection between cold fronts and ARs over the Southern Ocean (Simmonds et al., 2012). Simmonds et al. (2012) found that these cold fronts strengthen the Subantarctic cyclone dynamics and that the fronts appear with the highest frequency between 40°S and 60°S with typical lengths of 2,000 km. The climatic importance of ARs, which stretch across the Atlantic Ocean from the Subtropics to the Antarctic continent, is starting to be realized. For instance, ARs have been shown to greatly impact precipitation patterns in the Western Cape of South Africa via a poleward migration of the moisture track (Blamey et al., 2018;Sousa et al., 2018), reduce high-latitude Antarctic sea-ice concentrations and expanse (Wille et al., 2021), and contribute to the opening of the Weddell Sea polynya (Francis et al., 2020). One area that remains unexplored to date is the potential impact ARs have on precipitation magnitudes over the ocean and the associated change in the surface ocean buoyancy. Southern Ocean surface buoyancy fluxes are crucial to understanding climate due to their role in ocean ventilation and water-mass modification (Abernathey et al., 2016;Marshall & Speer, 2012;Pellichero et al., 2018). South of the Polar Front, deep waters upwell to the surface where they are exposed to surface fluxes of heat (solar radiation) and freshwater (sea-ice melt and precipitation). Abernathey et al. (2016) showed that sea-ice formation/melt and freshwater flux from precipitation played a critical role in lowering the density of upwelled waters,
<p>Atmospheric rivers (ARs) dominate moisture transport globally, accounting for 90% of poleward atmospheric freshwater transport in the mid-to-high latitudes while only covering 10% of the surface. Yet, it is unknown what impact ARs have on the surface ocean buoyancy in the high latitudes. This is explored using high-resolution surface observations from a Wave glider deployed at a site in the Southern Ocean (54&#176;S, 0&#176;E) during austral summer. During this time (19 December 2018 - 12 February 2019, 55 days) we show that when ARs combine with storms over this area, the associated precipitation is enhanced significantly (162%). AR-induced precipitation events provided a major source of surface ocean buoyancy equivalent to the input of surface heat fluxes on a daily timescale. Cumulatively, ARs account for 44% of the summer precipitation equating to 9% of surface buoyancy gain. These results show that AR variability is a previously unaccounted driver of Southern Ocean surface buoyancy that may ultimately impact upper ocean water mass transformation and the dynamics of the ocean surface boundary layer.</p>
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