Hydrodynamic controls of hypoxia formation in a shallow inverse estuary: Insights from Kuwait Bay.

Kuwait Bay, a shallow inverse estuary in the north Arabian/Persian Gulf, has a documented history of recurrent summer hypoxia and associated fish-kill events under intense anthropogenic pressure, yet the physical mechanisms controlling these events remain poorly understood. This study investigates the hydrodynamic controls on hypoxia formation through analysis of in-situ environmental observations collected at a single near-bed station between June and August 2020, a period that included two hypoxic events (dissolved oxygen, DO < 2 mg L-1). Empirical Orthogonal Function (EOF) analysis of the subtidal current velocities identifies two dominant modes that together explain approximately 92% of the total flow variability. Mode 1 (∼78%) represents an east-west advective flow, while Mode 2 (∼14%) captures a vertically sheared, bidirectional circulation that enhances water-column stratification. Wavelet coherence analysis is consistent with eastward advection associated with Mode 1 transporting hypoxic water from its formation zone in the western bay, whereas the stratified flow represented by Mode 2 traps oxygen-depleted water locally and prolongs event duration. A lag of approximately 2-4 days between Shamal wind forcing and the near-bed DO response is identified, offering a potential early-warning window. Together, these results are consistent with an advection-trapping framework for near-bed hypoxia in inverse estuaries, in which the hypoxia observed at the station reflects the transport and retention of oxygen-depleted water generated in the western embayments rather than purely local production. This framework complements earlier modelling studies and provides a basis for developing and testing candidate early-warning indicators for hypoxic events in Kuwait Bay.