## 3 Results
### Spatial and Temporal Characteristics of Upwelling in the West-Central South China Sea
The identification results indicated that the primary life cycle of upwelling events in the central and western South China Sea occurred in May each year, peaked in July to August, and ended around the end of September to the beginning of October. Therefore, this paper identified the months of upwelling in the central and western South China Sea as May-September. The upwelling phenomenon in the central and western South China Sea is directly influenced by the sea surface temperature (SST), which displays prominent spatial and temporal distribution characteristics during upwelling events. To further investigate the spatiotemporal characteristics of the upwelling events, an EOF decomposition of the SST field in the central and western South China Sea was conducted to obtain the spatial and temporal modes of the SST variance variation. The eigenvalues and their errors were calculated for each mode, and the eigenvalues and errors of the first three modes of the EOF analysis were 2480.88 \(\pm\) 232.36, 297.87 \(\pm\) 27.89, and 182.19 \(\pm\) 17.06, respectively. This calculation result indicated that there was no information overlap among the modes. The variance contribution rates of the first three modes were 69.46%, 8.34%, and 5.1%, respectively. The cumulative explained variance of the first three modes was 82.9%. The variance contribution rates of the subsequent modes were low and could not reflect the sample characteristics; hence, they were not analyzed in this study. In order to further investigate the spatial and temporal characteristics of upwelling events in the central and  
Figure 3: Heat map of upwelling by month and latitude (2000–2018). western South China Sea, an EOF decomposition of the multi-year monthly mean SST field was performed, yielding spatial and temporal modes of SST variance variation. Figure 4 displays the spatial and temporal distribution of each mode. The first mode, which explains 69.46% of the total variance, was decisive. As shown in Figure 4a, the temperature trend of the first mode was positive in the coastal waters of the Central South Peninsula, with zero contours located near the 109.5\({}^{\circ}\) E diameter line and shifted toward the eastern waters with the increasing latitude. The negative area was located in the east-central part of the study area, with a negative central area in the sea near 10.5-14.3\({}^{\circ}\) N, 111.2-113.8\({}^{\circ}\) E, indicating opposite trends of the sea surface temperature changes in the two regions. Figure 4b displays the time coefficient of the first mode, and a positive time coefficient demonstrates an increase in the sea surface temperature in the coastal waters of the Central South Peninsula and a decrease in the sea surface temperature in the central-eastern part. A negative time coefficient indicates a decrease in the sea surface temperature in the coastal waters of the South China Sea and an increase in the sea surface temperature in the east central Sea. In the summer, the time coefficients were predominantly positive, indicating the presence of upwelling events in the range of 10.5-14.3\({}^{\circ}\) N, 111.2-113.8\({}^{\circ}\) E, resulting in lower sea surface temperatures in the upwelling waters in the South China Sea during the summer, when the overall sea surface temperature is increasing. The time coefficients of the first mode maintained an interannual variability cycle of 2-2.5 years, with oscillations generally showing an increased trend, which may be related to global warming factors. The second mode, explaining 8.34% of the total variance, exhibited a spatial distribution similar to that of the first mode. The temporal coefficients were opposite to the first mode, reflecting the latitudinal distribution of temperature field variation characteristics in the central and western South China Sea, subject to latitudinal and seasonal variations. The sea surface temperature tended to increase with decreasing latitude.