Phytoplankton Increases Induced by Tropical Cyclones in the South China Sea During 1998–2015

Abstract

AbstractTropical cyclones (TCs, with maximum sustained 10 m wind speed greater than 17.5 m s−1) are strong synoptic systems that form over tropical oceans and can exert important effects on the upper ocean by mixing and upwelling, and thus mixed‐layer cooling. However, hitherto, biological effects of TCs have not been systematically explored in the South China Sea (SCS) yet. In this study, we have examined 184 TCs and found 74 TCs with well‐matched satellite ocean color data (i.e., the number of valid pixels accounting for over 60% of the total pixels in the sampling area) in the SCS during 1998–2015. The ocean color data show a general post‐TC increase in surface chlorophyll‐a (Chl‐a) concentrations in the cold wakes of TCs, due to the injection of nutrients and/or biogenic pigments into the oligotrophic surface waters. The ratio of the post‐TC to the pre‐TC Chl‐a is significantly correlated with the sea surface temperature (SST), and TC translational speed and intensity. Results from the multiple correlation analyses in different seasons indicate that the response of phytoplankton to TCs also depends on the preexisting upper‐ocean conditions, including the mixed‐layer depth, vertical nutrients structure and light conditions. Results thus strongly suggest that the initial upper‐ocean conditions along with moving speed and intensity of TCs are key to phytoplankton increase after the passages of TCs.