Impact of ENSO Events on the Intensity and Phytoplankton in the Northeastern Hainan Island Upwelling System

To address the problem that previous studies have combined the phytoplankton biomass of two upwelling systems in eastern and northeastern Hainan Island, which are indistinguishable in location but occur by different mechanisms, into one study. This study used multi-source remote sensing data from 2002 to 2019 to investigate the response of the intensity and chlorophyll-a (Chla) concentration of the northeastern Hainan Island upwelling (QNE) system to El Niño-Southern Oscillation (ENSO) and the primary controlling factors. Unlike previous studies, this study adopted the temperature difference method to extract the location and range of the strongest month of QNE (July) in a specific and detailed manner, avoiding the variation of phytoplankton biomass affected by the variable location of the QNE system. The results showed that the intensity and Chla concentration in the QNE system were almost unaffected by ENSO. The most important factor of the long-term variation of the intensity of the QNE system was the local wind speed (r = − 0.480), and the wind speed variation generated by ENSO had almost no effect on the region. Chla concentration in the QNE system was not upwelling dominated, nor was it dominated by any single physical factor. Overall, the variability of Chla concentration in the QNE system is complex, and the current study is based mainly on satellite observations, which only directly reflect the surface Chl-a variability under ENSO events. The response of the entire water column phytoplankton to ENSO events is an important scientific question that deserves subsequent attention.