Spatiotemporal Development and Dissipation of Hypoxia Induced by Variable Wind-Driven Shelf Circulation off the Pearl River Estuary: Observational and Modeling Studies

Observations reveal the high spatiotemporal variability of the hypoxia in the western and eastern coastal transition zones (WCTZ and ECTZ) off the Pearl River Estuary (PRE). We utilized data from cruise observations, buoy mooring, and a three-dimensional model based on ROMS to investigate the hypoxia variability response to the typical synoptic variation of the summer monsoon, i.e., from prevailing upwelling-favorable wind (UFW), the ensuing episodic upwelling relaxation (URW), downwelling (DFW), to downwelling relaxation wind (DRW). During the UFW, we found that the northeastward shelf current converges with nutrient-rich river plume, forms strong stratification, and provides hubs for depositing detritus for hypoxia to develop in the CTZ. The ensuing URW and DFW forcing alters the transport to advect the plume and detritus westward, weakens vertical mixing, and enhances hypoxia in the WCTZ. Correspondingly, opposite conditions occur in the ECTZ as the plume veers westward during the URW and DFW. Contrarily, the weakening westward transport during the DRW forcing restores the biophysical conditions to their states when there is an UFW event. We found that varying wind-driven circulation interacts with the plume to jointly regulate the transport of nutrient and detritus, water vertical mixing, and residence time, and, thus, the hypoxia variability in the CTZ. The variability of hypoxia in the CTZ is less controlled by the river discharges once biogeochemical conditions become saturated. Our study illustrates the biophysical control of hypoxia variability that is frequently observed in estuaries and coastal oceans.

Automated summary

Observations show high spatiotemporal variability of hypoxia in the western and eastern coastal transition zones off the Pearl River Estuary. The study reveals how wind-driven circulation interacts with plume to regulate nutrient transport, detritus deposition, and hypoxia variability in the area. River discharges have lesser control over hypoxia once biogeochemical conditions become saturated, illustrating the biophysical control of hypoxia variability in estuaries and coastal oceans.