Shelf Bathymetric Roughness Controls Mixing of the Persian/Arabian Gulf Outflow and Arabian Sea OMZ Ventilation

Abstract. Hypersaline overflows from Marginal seas contribute to global ocean circulation and climate variability. The Persian/Arabian Gulf (P/AG) contributes dense, saline, oxygenated deep outflow into the Arabian Sea’s Oxygen Minimum Zone (OMZ). Here, we present high-resolution hydrographic mapping of the UAE-Oman margin in the northwestern Arabian Sea, integrating CTD profiles, multibeam bathymetry and ROV imagery. The P/AG outflow forms a thin bottom-attached layer that thickens dramatically over the rough, irregular seafloor of the unstable shelf, where dilution and loss of bottom contact occur. Near-bottom salinity, oxygen, and pH decrease systematically downstream in association with enhanced mixing. Distinct benthic assemblages coincide with attached versus detached flow sectors, consistent with differing near-bed hydrodynamic regimes. These observations indicate that bathymetric roughness controls the near-field transformation of the outflow. Because outflow density depends on Gulf warming and evaporation, modest climatic shifts may alter detachment depth, pathway geometry, and intermediate-depth ventilation of the Arabian Sea OMZ.