114 / 2024-09-08 21:47:59

Impact of Tropical Cyclone Bavi (2020) on Hypoxic Zone Variation off the Yangtze River Estuary: A Case Study

Tropical Cyclone; coastal ecological environment; hypoxia

Coastal hypoxia, characterized by dissolved oxygen concentrations falling below a critical threshold, is a widespread phenomenon that places severe stress on marine ecosystems. Tropical cyclones (TCs), while primarily recognized for their impact on ocean dynamics, also play a complex role in modulating biogeochemical processes. TCs can disrupt stratification, alter nutrient distribution, resuspend organic matter and affect oxygen concentrations, influencing both the formation and persistence of hypoxic zones. This study investigates the influence of TC Bavi (2020) on the development and spatial migration of hypoxia off the Yangtze River estuary in the East China Sea. Using a three-dimensional coupled physical-biogeochemical model, validated with observational surveys conducted before and after Bavi's passage, we observed significant changes in hypoxia. Contrary to the expectation that typhoon-induced mixing would dissipate the hypoxic zone, our findings revealed that hypoxia migrated following Bavi's passage. Model simulations showed that this was primarily driven by typhoon-induced oceanic advection. Bavi generated a regional quasi-barotropic cyclonic loop circulation and coastal downwelling, reversing the typical summer circulation patterns. The onshore transport of warmer shelf water, coupled with coastal downwelling, led to coastal water warming, while the southward coastal current facilitated the migration of the hypoxic zone. Additionally, the resuspension of planktonic detritus from the steep inner shelf and the Changjiang bank contributed to the redistribution of organic matter. This case study highlights the significant role TCs play in shaping biogeochemical processes and hypoxia dynamics in coastal waters, emphasizing the need for further research into TC-ocean interactions to improve predictions of marine ecosystem variations across multiple timescales.