Impacts of Marine Heatwave Events on Three Distinct Upwelling Systems and its Implication for Marine Ecosystems in the Northern South China Sea

Under global warming, the frequency and intensity of marine heatwaves are increasing. However, the inhibition of atmospheric forcing marine heatwaves (AMHW) on upwelling and its impact on marine ecosystems remain poorly understood. To address this issue, the satellite sea surface temperature and reanalysis data during 1998-2021 were analyzed in three distinct upwelling systems, northwestern South China Sea. The results showed that the coastal tide-induced upwelling in the west (W) of Hainan Island is primarily suppressed by enhanced stratification during the AMHW events, since the coastal tide-induced upwelling is insensitive to wind weakening. Contrarily, the wind-driven upwelling in the east (E) and northeast (NE) of Hainan Island are jointly regulated by wind and stratification during the AMHW. Specifically, the AMHW events have a stronger inhibitory effect in the upwelling and phytoplankton growth in the NE than that in the E. The causes could be the followings: (1) the background upwelling in the NE region is stronger than in the E, thus the NE region has a higher susceptibility to the wind weakening; (2) the wind-driven upwelling begins to be suppressed by AMHW when the high-pressure system is aligned with the coastline of the upwelling. In the NE region, the location of the high-pressure center during the occurrence of AMHW is positioned in closer proximity to the upwelling area. Moreover, the inhibitory effect of wind weakening and stratification enhancing on upwelling changes with the development of the AMHW. Before and during the mature phase of AMHW, stratification and wind jointly inhibit upwelling and phytoplankton growth, while it shifts to stratification dominated (>85%) during the decline phase. This study suggests that MHW has a great impact on the upwelling ecosystem, especially the wind-driven upwelling, which should be given high attention under global warming (with increasing MHW events in the future).