Decoupling of surface and subsurface chlorophyll-a in response to tropical cyclones in the South China Sea

Phytoplankton, the primary producers in the ocean, contribute to over half of the global oxygen supply. Tropical cyclones (TCs) can enhance vertical mixing and upwelling, transporting nutrients into the euphotic layer and stimulating the enhancement of chlorophyll-a concentration (Chla) and new primary production. However, previous studies on phytoplankton responses to TCs have largely relied on ocean-color satellite-derived Chla data, limiting their focus to surface Chla dynamics. As a result, the responses of subsurface and depth-integrated Chla to TCs remain poorly understood. In this study, we analyze the responses of surface and subsurface Chla to TCs in the South China Sea using daily gridded data from the physical-biogeochemical coupling model NEMO-PISCES. Our key findings include: Surface and subsurface Chla exhibited strong decoupling during TC influence, primarily due to vertical redistribution driven by mixing and upwelling, as well as growth limitation under low-light conditions; Depth-integrated Chla was dominated by subsurface biomass, with blooms lagging several days behind surface blooms; Surface blooms depend primarily on TC intensity through rapid mixing processes, whereas subsurface blooms are more sensitive to translation speed via sustained upwelling effects. This study highlights the decoupling of surface and subsurface Chla under TC influence and advances the understanding of vertical Chla dynamics in response to TCs.