Ventilation of the Arabian Sea Oxygen Minimum Zone

Abstract

Ocean deoxygenation, driven by climate-induced warming, stratification, and circulation changes, threatens marine ecosystems globally. The Arabian Sea hosts the thickest and most intense oxygen minimum zone (OMZ), maintained by an interplay between physical oxygen supply and biological consumption. This thesis combines observations from ocean gliders, autonomous floats, and numerical models to quantify how surface, mode, and dense marginal-sea waters ventilate the upper OMZ boundary across spatiotemporal scales.

The surface mixed layer forms the interface between the ocean and atmosphere and plays a central role in oxygen ventilation through wind-driven and buoyancy-driven mixing. In the Sea of Oman, intraseasonal surface mixed layer variability is forced by Shamal-driven latent heat loss and submesoscale fronts, with the latter accounting for nearly 70% of the wintertime restratifying buoyancy flux and contributing to the timing of spring restratification.

Mode waters (MWs) mediate the vertical transport of oxygen between the surface and ocean interior, acting as an oxygen reservoir and a buffer for oxygen demand in the OMZ. MW formation occurs annually in the northern Arabian Sea and biannually in the south, linked to the summer and winter monsoons, with regional variability shaped by advection and bio-optical modulation of heating absorption. Seasonal MW transformation is predominantly isopycnal, but diapycnal transformation becomes important at shorter timescales. We find that physical mixing - split nearly equally between isopycnal and diapycnal processes - accounts for about half of the observed oxygen variability in MW, while biological consumption contributes roughly one-fourth. Mesoscale eddies further amplify the physically mediated oxygen fluxes, nearly doubling them relative to non-eddy conditions. The estimated physical fluxes, together with high biological respiration within MW, underscore MW's central role in supplying oxygen to the OMZ and mitigating the biological oxygen demand that drives OMZ intensification.

Dense water formation from marginal seas, such as the Persian Gulf Water (PGW) outflow, is another important pathway in supplying oxygen to the Arabian Sea OMZ. Intermittent shear-driven mixing enhances dissipation from double-diffusive processes below PGW. These conditions are co-located with large oxygen gradients at the upper OMZ oxycline, resulting in increased oxygen ventilation towards the ocean interior on short timescales.

Altogether, this work demonstrates that small-scale processes are critical in controlling upper OMZ oxycline variability. These findings advance our understanding of key processes influencing oxygen dynamics within the OMZ, providing insights for estimates of water mass transformation and oxygen exchange, which can ultimately improve modeling and prediction efforts.