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Dense overflows from marginal seas are critical pathways of oxygen supply to the Arabian Sea oxygen minimum zone (OMZ), yet these remain inadequately understood. Climate models struggle to accurately reproduce the observed extent and intensity of the Arabian Sea OMZ due to their limited ability to capture processes smaller than their grid scale, such as dense overflows. Multi‐month repeated sections by underwater gliders off the coast of Oman resolve the contribution of dense Persian Gulf Water (PGW) outflow to oxygen supply within the Arabian Sea OMZ. We characterize PGW properties, seasonality, transport and mixing mechanisms to explain local processes influencing water mass transformation and oxygen fluxes into the OMZ. Atmospheric forcing at the source region and eddy mesoscale activity in the Gulf of Oman control spatiotemporal variability of PGW as it flows along‐shelf off the northern Omani coast. Subseasonally, it is modulated by stirring and shear‐driven mixing driven by eddy‐topography interactions. The oxygen transport from PGW to the OMZ is estimated to be 1.3 Tmol yr−1 over the observational period, with dramatic inter‐ and intra‐annual variability (±1.6 Tmol yr−1). We show that this oxygen is supplied to the interior of the OMZ through the combined action of double‐diffusive and shear‐driven mixing. Intermittent shear‐driven mixing enhances double‐diffusive processes, with mechanical shear conditions (Ri < 0.25) prevailing 14% of the time at the oxycline. These findings enhance our understanding of fine‐scale processes influencing oxygen dynamics within the OMZ that can provide insights for improved modeling and prediction efforts. Plain Language Summary: The Arabian Sea hosts extremely low‐oxygenated waters at depth (oxygen minimum zone, OMZ). Understanding how the depth of this layer changes and what processes control its variability is vital to understanding how it will change under climate change and how it may affect the ecosystem. One of the ways the oxygen gets into this low‐oxygenated region is through the sinking of a water type called Persian Gulf Water (PGW). This water forms in the Persian Gulf and is hypersaline, warm, and well‐oxygenated. It leaves the Persian Gulf through the narrow Strait of Hormuz and flows around 200 m depth along the northern coast of Oman. High‐resolution ocean glider observations show the variability of its properties across different seasons and years. The amount of oxygen the PGW brings varies a lot over time, changing from year to year and between seasons. We estimate it to be 1.3 Tmol per year during the study period. We identify events, locations and mixing mechanisms that explain how the oxygen contained in PGW can mix with the surrounding low‐oxygen waters, hence increasing the OMZ's oxygen content. This knowledge can be useful for improving climate models and predicting how the OMZ might change in the future. Key Points: PGW is characterized by high spatiotemporal variability in its properties and transport that impacts the variability of the upper OMZOxygen contribution from PGW to the Arabian Sea OMZ is resolved for the first time as 1.3 Tmol yr−1Intermittent shear‐driven mixing at the PGW bottom boundary amplifies ventilation of the OMZ by salt fingering 14% of the time [ABSTRACT FROM AUTHOR]

Atmospheric rivers (ARs) dominate moisture transport globally; however, it is unknown what impact ARs have on surface ocean buoyancy. This study explores the surface buoyancy gained by ARs using high‐resolution surface observations from a Wave Glider deployed in the subpolar Southern Ocean (54°S, 0°E) between 19 December 2018 and 12 February 2019 (55 days). When ARs combine with storms, the associated precipitation is significantly enhanced (189%). In addition, the daily accumulation of AR‐induced precipitation provides a buoyancy gain to the surface ocean equivalent to warming by surface heat fluxes. Over the 55 days, ARs accounted for 47% of the total precipitation equating to 10% of the summer surface ocean buoyancy gain. This study indicates that ARs play an important role in the summer precipitation over the subpolar Southern Ocean and that they can alter the upper‐ocean buoyancy budget from synoptic to seasonal timescales. Plain Language Summary: Atmospheric rivers (ARs) are river‐like features in the sky that carry enormous amounts of water vapor from the tropics toward Antarctica and across the Southern Ocean. However, it is not well documented how this freshwater carried by ARs in the atmosphere impacts rainfall over the ocean. This study uses an uncrewed surface vehicle, called a Wave Glider, to measure the meteorology and surface ocean temperature and salinity. We show that ARs almost double rainfall, which directly impact the surface ocean buoyancy (i.e., rainfall adds freshwater and lightens the surface layer waters). These results provide evidence and highlight the importance of ARs in the Atlantic Southern Ocean when it comes to lightening the surface waters. Key Points: Atmospheric rivers (ARs) significantly increase precipitation (189%) in midlatitude cyclones over the Atlantic Southern OceanDuring intense AR‐driven rainfall events, precipitation contributes to daily buoyancy forcing equivalent to the surface heat fluxPrecipitation associated with ARs contributes cumulatively to 10% of the surface ocean buoyancy gain in summer [ABSTRACT FROM AUTHOR]

High‐resolution underwater glider data collected in the Gulf of Oman (2015–2016), combined with reanalysis data sets, describe the spatial and temporal variability of the mixed layer during winter and spring. We assess the effect of surface forcing and submesoscale processes on upper ocean buoyancy and their effects on mixed layer stratification. Episodic strong and dry wind events from the northwest (Shamals) drive rapid latent heat loss events which lead to intraseasonal deepening of the mixed layer. Comparatively, the prevailing southeasterly winds in the region are more humid, and do not lead to significant heat loss, thereby reducing intraseasonal upper ocean variability in stratification. We use this unique data set to investigate the presence and strength of submesoscale flows, particularly in winter, during deep mixed layers. These submesoscale instabilities act mainly to restratify the upper ocean during winter through mixed layer eddies. The timing of the spring restratification differs by three weeks between 2015 and 2016 and matches the sign change of the net heat flux entering the ocean and the presence of restratifying submesoscale fluxes. These findings describe key high temporal and spatial resolution drivers of upper ocean variability, with downstream effects on phytoplankton bloom dynamics and ventilation of the oxygen minimum zone. Plain Language Summary: Atmospheric forcings, such as wind and solar heating, and small‐scale ocean processes (1–10 km; e.g., eddies, fronts, filaments) modify the properties and the structure of the water column near the surface. These processes regulate the surface layer, creating a well‐mixed surface layer. The variation in these processes determine how the depth of this surface mixed layer changes through both time and space. This study investigates the variability of this layer during winter and spring in the Gulf of Oman using in situ observations and atmospheric data derived from models and observations. Episodic strong and dry winds from the northwest (Shamals) increase mixing and cause shorter‐term variability of the surface mixed layer. Concurrently, we find that the small‐scale ocean processes mainly shoal the mixed layer depth during winter. These processes are also important in determining the timing of the change from the deeper winter mixed layer to the shallower spring mixed layer, as we find a three‐week difference between the two observed years. The observations illustrate previously unquantified processes in the region that can impact coupling between the atmosphere, surface ocean, and deep ocean, with consequences for regional marine ecosystems. Key Points: Ocean glider observations reveal mixed layer variability that cannot be explained by seasonal warming aloneShamal winds dominate intraseasonal variability of the mixed layer in the Gulf of OmanSubmesoscale mixed layer eddies are responsible for 68% of the restratifying buoyancy flux in winter [ABSTRACT FROM AUTHOR]

Submesoscale flows in the ocean are energetic motions, O(1–10 km), that influence stratification and the distributions of properties, such as heat and carbon. They are believed to play an important role in sea‐ice‐impacted oceans by modulating air‐sea‐ice fluxes and sea‐ice extent. The intensity of these flows and their response to wind forcing are unobserved in the sea‐ice regions of the Southern Ocean. We present the first submesoscale‐resolving observations in the Antarctic marginal ice zone (MIZ) collected by surface and underwater autonomous vehicles, for >3 months in austral summer. We observe salinity‐dominated lateral density fronts occurring at sub‐kilometer scales. Surface winds are shown to modify the magnitude of the mixed‐layer density fronts, revealing strongly coupled atmosphere‐ocean processes. We posture that these wind‐front interactions occur as a continuous interplay between front slumping and vertical mixing, which leads to the dispersion of submesoscale fronts. Such processes are expected to be ubiquitous in the Southern Ocean MIZ. Plain Language Summary: Satellite radar imagery shows evidence of 1–10 km eddies and jets in the ocean adjacent to the sea‐ice edge around Antarctica. We use field observations of temperature, salinity, and wind speed from autonomous robotic platforms deployed in the sea‐ice zone for >3 months. These measurements provide estimates of the surface ocean density fronts which are controlled primarily by lateral variations in salinity. We show that, during high wind speeds, these surface fronts temporarily dissipate, indicating an atmosphere‐ocean coupling occurring at the submesoscale. The fronts strengthen again during low wind speed, which is thought to be because the stirring of the fresher surface layer by mesoscale eddies leads to the generation of the submesoscale fronts. Providing in situ observations of such features improves our understanding of the small‐scale ocean and climate processes at play, such as how heat and carbon may exchange between the atmosphere and the ocean. Key Points: Using combined surface and underwater robotic observations, we observe haline‐dominated submesoscale fronts in the Antarctic MIZEnhanced wind speeds reduce the magnitude of lateral submesoscale fronts within the surface mixed layerSubmesoscale wind‐front interactions cause a continuous interplay between front slumping and vertical mixing, arresting lateral shear [ABSTRACT FROM AUTHOR]