Vertical Transport by Frontal Subduction Alleviating Hypoxia in a Highly Stratified Estuary

Coastal hypoxia often develops in stratified estuaries, primarily caused by the coexistence of river plume expansion and shelf seawater intrusion. Surface fronts generated by tidal plumes are associated with enhanced vertical transport. While previous studies have explored the near-bed impacts of plume fronts, the frontal processes in influencing hypoxic zones caused by the barrier layer are not straightforward. Here, we use remote sensing, shipboard, and mooring observations, aiming to explore the physical dynamics of frontal subduction for alleviating bottom hypoxia. In situ observations were conducted in the highly stratified Pearl River Estuary, a region where surface fronts frequently occur, and significant bottom hypoxia appears during summer. We show that bottom hypoxia can be temporarily alleviated by the rapid water transport driven by frontal subductions. Specifically, the critical mid-field front, trapped by tidal flow reversal, leads to the convergence in the surface layer. A downwelling process is driven by this convergence at the front, subducting surface river-born buoyant material to the bottom. Both dissolved oxygen ($\text{DO}$) and large particles from the river plume are transported to the bottom layer through this subduction, effectively increasing bottom $\text{DO}$ concentrations. This bottom-attached subduction, characterized by weak mixing, moves downward along the sloping isopycnals, a pathway supported by local weak stratification and weak advective forcing during the low tide. Our findings reveal a unique mechanism of the subduction at mid-field fronts, and the specific tidal conditions support this slantwise vertical transport, which further impact coastal hypoxia and particle transport.