Strong enhancement of chlorophyll a concentration by a weak typhoon

Recent studies demonstrate that chlorophyll a (chl a) concentrations in ocean surface waters can be significantly enhanced due to typhoons. The present study investigated chl a concentrations in the middle of the South China Sea (SCS) from 1997 to 2007. Only the Category 1 (minimal) Typhoon Hagibis (2007) had a notable effect on chl a concentrations. Typhoon Hagibis had a strong upwelling potential due to its location near the equator, and the forcing time of the typhoon (>82 h) was much longer than the geostrophic adjustment time (similar to 63 h). The higher upwelling velocity and the longer forcing time increased the depth of the mixed-layer, which consequently induced a strong phytoplankton bloom that accounted for about 30% of the total annual chl a concentration in the middle of the SCS. Induction of significant upper ocean responses can be expected if the forcing time of a typhoon is long enough to establish strong upwelling.