Effects of typhoon and upwelling on Chlorophyll-a distribution in the northeastern coast of Hainan during Summer.

Typhoons or upwelling are thought to promote higher phytoplankton chlorophyll-a (Chl-a) concentration in many previous studies. However, the combined effects of typhoons and upwelling have been less studied in the South China Sea. Based on satellite remote sensing data, we investigated potential contributions of temperature-characterizing upwelling and typhoon events to Chl-a changes in the Hainan northeast area. Results showed that the Chl-a concentration was 0.80 mg m-3 at the coastal upwelling index (CUI) of 1.7°C in the summer of 2020 when there were no typhoons crossing the area. The CUI (1.01°C) of typhoon-influenced period in 2019 was 0.21°C higher than that of typhoon-free period in 2019. And the Chl-a also increased from 0.70 mg m-3 to 0.99 mg m-3. In comparison, during the typhoon-free period, with the higher CUI, there was the higher concentration of Chl-a. In addition, the typhoon affected Chl-a concentration is significantly higher than that in the other two typhoon-free periods of 2019 and 2020. Though the typhoon has a limited effect on the upwelling intensity, the Chl-a concentration is much higher than when the upwelling acts alone. This is due to the combined effect of typhoon (vertical mixing and runoff) and upwelling. The above results indicate that upwelling dominated the changes in Chl-a concentration in the Hainan northeast upwelling area during the typhoon-free period. In contrast, strong vertical mixing and runoff dominated the changes of Chl-a concentration during the typhoon-influenced period in the above area.