Upper-ocean physical and biogeochemical response to Hurricane Norbert in the eastern tropical Pacific

Abstract. Tropical cyclones can strongly perturb the upper ocean through intense wind stress, vertical mixing, Ekman pumping, sea surface cooling, and nutrient injection into the euphotic zone. These processes enhance phytoplankton biomass generation and produce delayed chlorophyll-a responses after storm passage. Here, we investigate the physical and biogeochemical response of the Eastern Tropical Pacific to Hurricane Norbert, a category 4 hurricane that crossed the Mexican Pacific in October 2008. We combine tropical cyclone best-track data from IBTrACS, ERA5 wind forcing, CROCO physical-biogeochemical simulations, satellite-derived sea surface temperature and chlorophyll-a products, and Argo float profiles located near the hurricane track. The results show that Norbert generated strong positive Ekman pumping, mixed-layer deepening, surface nitrate enrichment, and a delayed increase in chlorophyll-a concentration along portions of the storm trajectory. The model reproduced the main structure of sea surface temperature cooling more clearly than the chlorophyll-a response, indicating that the physical response was more directly controlled by hurricane forcing, whereas the biological response was modulated by nutrient availability, pre-existing stratification, light conditions, and regional circulation. Argo profiles confirmed post-storm changes in the vertical thermal structure and mixed-layer depth. These results indicate that major hurricanes in the eastern tropical Pacific can generate significant but spatially heterogeneous biogeochemical responses through the coupling between wind-driven mixing, nutrient entrainment, and phytoplankton growth.