Changes In The Upwelling Characteristics And Mechanisms In The Central And Western South China Sea

Upwelling is an important natural phenomenon in the South China Sea,which has significant impacts on the fishery economy and marine environment.Understanding the temporal and spatial characteristics of upwelling in the central and western South China Sea and its response to climate change under global warming is of great significance for the development of marine resources,ecological environment protection,and maintenance of territorial sovereignty.In this paper,we defined the region of the upwelling area in the central and western South China Sea based on Sea Surface Temperature(SST),Chlorophyll-a concentration(Chl-a),Sea Surface Wind(SSW)data,and Ni?o3.4 Index(I Ni?o3.4),and explored its spatial and temporal variations using Ensemble Empirical Mode Decomposition(EEMD)method.Furthermore,we conducted cross-correlation analysis to investigate the impacts of wind and El Ni?o events on the strength and area of upwelling events in the central and western South China Sea.The research results are as follows:(1)During the period from June to September every year,an anomalous area of low Sea Surface Temperature(SST)appears in the central and western South China Sea,with a range of 10°N-15.5°N and 110°E-115°E,and an average temperature difference of about 0.8℃.The SST and Chl-a concentration in the upwelling area in the central and western South China Sea show a "∨" and "∧" type changes,respectively,throughout the upwelling event`s life cycle,which indicates that upwelling occurs in this area in June,matures in July and August,and begins to weaken in September.In the entire study area,the frequency of occurrence of upwelling events in June is 57.6%,and in July-August,the frequency of upwelling occurrence is higher than 85%,while in September,the frequency of occurrence drops to 59.8%.(2)The temperature thresholds of the upwelling in the middle and western South China Sea during June to September were 29.651℃,29.149℃,28.830℃ and 29.034℃,respectively.The upwelling in the middle and western South China Sea showed a "tongue-shaped" distribution,and extended to the eastern sea area with increasing intensity.The results of Ensemble Empirical Mode Decomposition(EEMD)showed that the main period of the upwelling intensity index variation was 4 years,and the upwelling intensity fluctuated cyclically with a trend of first decreasing and then increasing.The main period of the upwelling area variation was consistent with that of the upwelling intensity,and showed a decreasing trend overall.(3)The Southwest Monsoon prevails in the middle and western South China Sea,with the frequency of westerly and southerly winds exceeding 60%.There is a correlation between zonal wind and the intensity and area of the upwelling in the middle and western South China Sea,with correlation coefficients of 0.623 and 0.258,respectively.The stronger the zonal wind is,the stronger the intensity and the larger the area of the upwelling events are.The correlation coefficient between radial wind speed and upwelling events is 0.4,which has a relatively small impact on the upwelling area.The response time of the intensity and area of the upwelling in the middle and western South China Sea to changes in the wind field were 72.32 hours and52.14 hours,respectively.(4)The correlation coefficient between the NINO3.4 index and the intensity of the upwelling is 0.191,and the correlation coefficient with the upwelling area is-0.041.The lagged effect of the El Ni?o event on the intensity and area of the upwelling in the middle and western South China Sea usually appears after 9 months.The NINO3.4 index is positively correlated with wind speed,indicating that the El Ni?o phenomenon not only changes the sea water temperature but also strengthens the local wind field to enhance the intensity of upwelling events in the middle and western South China Sea.