Tropical Cyclone-Induced Ecological Responses and Their Feedback on Physical Fields: A Case Study for Hurricane Fernanda (2017)

Abstract

Tropical cyclones (TCs) are known to trigger chlorophyll bloom and boost local primary productivity. The characteristics of ocean ecological responses to TCs and their feedback on physical fields in the northeastern Pacific are investigated using a coupled ocean general circulation model-ocean ecosystem model. A case study is carried out on Hurricane Fernanda (2017), which formed in the northeastern tropical Pacific. TC-induced mixing and upwelling act to transport subsurface cold and nutrient-rich waters into the mixed layer, leading to surface cooling and chlorophyll (CHL) bloom. The phytoplankton budget analyses indicate that the CHL bloom is dominated by small phytoplankton (P_S) growth term, which is modulated by biological consumption terms (i.e., small zooplankton grazing and P_S mortality) and physical processes-related terms (i.e., advection and vertical mixing); biological consumption terms and physical processes tend to mainly offset the contributions from the P_S growth term. Furthermore, CHL is found to exert feedback on physical fields. The CHL bloom mainly contributes to a decrease in surface temperature, thereby enhancing the temperature structure induced physically by TCs. This biofeedback on physical fields involves two mechanisms: a direct heating (OBH) due to ocean biology-induced effect, and an indirect cooling effect due to dynamic processes associated with vertical mixing and advection. In particular, the CHL bloom-induced sea surface temperature cooling is dominated by vertical mixing and modulated by the OBH effect and advection. These findings offer novel perspectives on TC-induced ecological responses, as well as the related mechanisms for biofeedback on physical fields.