Tropical cyclone-induced ocean response: A comparative study between the South China Sea and Northwest Pacific Ocean

Located on the two sides of the Kuroshio, the South China Sea (SCS) and tropical Northwest Pacific Ocean (NWP) differ significantly in the upper-ocean condition such as the thermocline depth. This study examines the effect of this difference on the response of sea surface temperature (SST) and chlorophyll a (chl-a) to tropical cyclones (TCs), using both satellite-derived measurements and three-dimensional numerical simulations. In both regions, TC-produced SST cooling strongly depends on both TC intensity and translation speed. When subject to identical TC forcing, the SST cooling in the SCS is more than 1.5 times that in the NWP, which may partially account for observed weaker mean TC intensity in the SCS. Both a shallower mixed layer and stronger subsurface thermal stratification in the SCS contribute to this regional difference in SST cooling. The first factor dominates when TCs are weak and/or moving fast, and for strong and slow-moving TCs these two factors are nearly equally important.

In both regions, TCs tend to elevate surface chl-a concentration with the amplitude of the increase also strongly depending on TC characteristics. For identical TC forcing, the chl-a increase in the SCS is around 10 times that in the NWP, a difference much stronger than that in SST cooling. This large regional difference in chl-a response is mainly due to a shallower nutricline and stronger vertical nutrient gradient in the nutricline in the SCS. The effect of upper-ocean density stratification on the nutrient response is negligible.