## 3 Results  
### Spatial and Temporal Characteristics of Upwelling in the West-Central South China Sea  
To study the overall variation of the upwelling events in the central and western South China Sea on a large time scale, monthly averaged sea surface temperature data and wind field data were selected to analyze the upwelling events in the central and western South China Sea. Figure 2 illustrates the distribution of SST and wind field for each month. In May, the monsoon alternation period in the west-central South China Sea, the southwest wind dominated the northern sea, but the wind speed remained low, with a monthly average wind speed ranging from 4.6 to 4.9 m/s. At this time, no apparent temperature anomaly area was observed. In June, the southwest monsoon began to prevail, and the frequency of westerly and southerly winds exceeded 60%, with a monthly average wind speed of 6.8 m/s. The temperature anomaly sea area appeared in the range of 10-12\({}^{\circ}\) N, 109-110\({}^{\circ}\) E, and spread to the eastern sea area. In July and August, the wind field intensity further increased, with the monthly average wind speed reaching 7.2 m/s. The sea surface temperature anomaly area expanded and further spread to the eastern sea area. In September, the north wind began to appear in the northern sea, while the southwest wind still dominated the southern sea. However, the overall wind speed decreased, with a monthly average wind speed of 6.1 m/s. The low-temperature center strength weakened and the SST anomaly was reduced. In October, the study area's northeast wind in the northern sea dominated, and the wind speed gradually increased. The southern sea wind speed remained minor. The SST anomalies in the central and western parts of the South China Sea occurred from June to September. These anomalies were characterized by significantly low temperatures, approximately one degree Celsius lower than the surface water temperature at the same latitude. The upwelling phenomenon brang deep low-temperature water to the surface layer, causing low-temperature areas on the sea surface. However, the existence of upwelling events could not be determined, and the identification of upwelling events in May and October are analyzed subsequently. Figure 2: Multi-year monthly mean SST (\({}^{\circ}\)C) distribution and wind speed and direction from January to December 2000–2018. Figure 3 demonstrates that the monthly mean upwelling frequency displays a general trend from low to high-low on the time scale. On the spatial scale, areas with high frequencies of monthly mean upwelling were mainly located in the middle and low latitude waters (OU_1, OU_2) and were less frequent in the high latitude waters (OU_3). The frequency of upwelling events in the west-central part of the South China Sea exhibited a low frequency from January to April and increased from May onwards. In May, the average frequency of the upwelling events was 35.6%, mainly in the OU_3 area, which may be attributed to the dominance of the southwest monsoon in the OU_3 area during the monsoon transition period in May. The average frequency of upwelling events gradually increased in June (57.6%), which may be associated with the prevalence of southwest monsoons during this period. The average frequency of upwelling in July and August was the highest (85%), indicating that the mature period of upwelling events occurred mainly from July to August. In September, the average upwelling frequency was 59.8%, with a decreasing trend compared to July and August, and was mainly concentrated in the OU_1 area. The average upwelling frequency in October was 29.3%, mainly concentrated in OU_3, which may be related to the dominance of northeasterly winds in OU_3 in October and the gradual increase in wind speed, which led to the diffusion of high-latitude low-temperature sea surface water to lower latitudes, resulting in an increase in the frequency of cold water masses in OU_3.