Ventilation of the Arabian Sea Oxygen Minimum Zone by Persian Gulf Water

Abstract

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 over the observational period, with dramatic inter- and intra-annual variability (±1.6 Tmol yr). 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.