Different Responses of Phytoplankton to the ENSO in Two Upwelling Systems of the South China Sea

This study investigated the different responses of phytoplankton to El Nino-Southern Oscillation (ENSO) events in two major upwelling systems (Yuedong (YD) and Vietnam coastal (VC)) in the South China Sea (SCS). Results showed that in the YD system, chlorophyll-a (Chl-a) concentration was basically unaffected by the ENSO. Pearl River runoff and coastal current velocity (Cv) were the main controlling factors for Chl-a variations, and offshore Ekman transport (ETu) also played an important role. In the VC system, El Nino and La Nina reduced and enhanced the Chl-a, respectively. Specifically, the decreases in offshore current velocity (Cu), ETu, and Ekman pumping velocity (EPV) and Mekong River plume after El Nino events resulted in the weakening of the upwelling and reductions in the Chl-a. In contrast, enhanced Cu and Mekong River runoff increased Chl-a after La Nina events. However, due to the effects of the Madden–Julian Oscillation (MJO), the Chl-a may not always be negative after El Nino years in the VC system, as the MJO may reduce the weakening effect of the ENSO on upwelling strength. Overall, our results showed markedly different phytoplankton responses to ENSO events in different upwelling systems within the same marginal sea.