Biophysical response of the central South China sea to autumn typhoons revealed by tropical storm Nakri.

This study examines the biophysical response of the central South China Sea to autumn typhoons using multi-satellite observations and reanalysis. These typhoons frequently interact with cyclonic eddies (CE), as exemplified by Tropical Storm (TS) Nakri in November 2019, which traversed a CE twice in a 'Looped' path, intensifying it. Nakri induced significant sea surface cooling (maximum of 6.52°C) and a phytoplankton bloom (maximum of 5.61 mg/m3) lasting three weeks. The hybrid coordinate ocean model effectively simulated the upper ocean response, revealing near-inertial oscillations in vertical temperature variation. Slow-moving Nakri, combined with CE intensification, drove strong upwelling, uplifting the 24 ℃ isoline to the surface. The prolonged forcing (60 hours) enhanced vertical mixing and energy input. The most pronounced bloom occurred in the northeastern 'Looped' region, likely due to strong northward geostrophic transport and subsurface nutrient supply. Temperature diagnostics indicate that mixed-layer entrainment and vertical advection primarily drive upper-ocean cooling, outweighing horizontal advection, especially during TS intensification. Biogeochemical model analysis supports these findings, highlighting TS-induced mixing as a key driver of nutrient transport into the euphotic zone, sustaining phytoplankton growth.