Multiple mechanisms for chlorophyll-a concentration variations in coastal upwelling regions: A case study east of Hainan Island in the South China Sea

Abstract. Using satellite observations from 2003 to 2020 and cruise observations in 2019 and 2021, this study reveals an unexpected minor role of upwelling in seasonal and interannual variations in chlorophyll-a (Chl-a) concentrations in the coastal upwelling region east of Hainan Island (UEH) in the northwestern South China Sea (NWSCS). The results show strong seasonal and interannual variability in the Chl-a concentration in the core upwelling area of the UEH. Different from the strongest upwelling in summer, the Chl-a concentration in the UEH area reaches a maximum of 1.18 mg m⁻³ in autumn and winter, with a minimum value of 0.74 mg m⁻³ in summer. The summer Chl-a concentration increases to as high as 1.0 mg m⁻³ with weak upwelling during El Niño years, whereas the maximum Chl-a concentration in October increases to 2.5 mg m⁻³ during La Niña years. The analysis of environmental factors shows that compared to the limited effects of upwelling, the along-shelf coastal current from the northern shelf and the increased precipitation are crucially important to the Chl-a concentration variation in the study area. These results provide new insights for predicting marine productivity in upwelling areas, i.e., multiple mechanisms, especially horizontal advection, should be considered in addition to the upwelling process.