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 the shelf of 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.

High-resolution underwater glider data collected in the Gulf of Oman (2015-16), combined with reanalysis datasets, 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 dataset 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.

Carbon export from the ocean surface to depth is an important component of the biological carbon pump, a key regulator of the world’s climate. The Antarctic Marginal Ice Zone accounts for 15% of Southern Ocean primary production, however, limited observations mean that the variability and drivers of primary production and its link to export are poorly constrained. Using a combination of gliders, biogeochemical argo floats and satellite observations, we show that sea-ice impacts both primary production and export through its influence on the upper ocean vertical density structure, light availability, and nutrient supply. Resultant changes in community composition, coupled with variations in vertical stratification, appear to be important determinants of carbon transfer to depth. The response of primary production and carbon export to sea-ice indicates that the biological carbon pump in this region is sensitive to ongoing climate change and predictions of reduced sea-ice cover in the future.