## 3 Results
### Effect of Wind Fields on Upwelling
During the strong upwelling years, the upwelling intensity centers were primarily concentrated in low-latitude seas (OU_1), whereas during the ordinary upwelling years, the upwelling intensity centers were mainly distributed in the mid-latitude seas (OU_2). The upwelling center in the central and western parts of the South China Sea was primarily concentrated in the OU_1 and OU_2 seas. Its position change was mainly influenced by the southwest monsoon. During the southwest monsoon, the central position of the upwelling was related to the wind speed, wind direction, and wind stress. The correlation analysis between the wind speed and the upwelling center latitude and longitude is presented in Table 1. \begin{table}
\begin{tabular}{l c c c c c} \hline  & **Wind Direction** & **Wind Speed** & **Wind Stress** & **Radial Wind Speed** & **Latitudinal Wind Speed** \\ \hline Center longitude & \(-0.175\)** & 0.102 ** & 0.130 ** & \(-0.037\) & 0.248 ** \\ of the upwelling & & & & & \\ Central latitude of & & & & & \\ the upwelling & & & & & \\ \hline \end{tabular} \(p\)-value, ** significantly correlated at the 0.01 level (bilateral).  
\end{table} 
Table 1: Correlation analysis of wind field characteristics with annual mean SST and upwelling center coordinates. Figure 6: Monthly average upwelling index and wind direction map of upwelling in the central and western parts of the South China Sea from 2000 to 2018. (**a**) Monthly Mean Upwelling Index from 2000 to 2018 (Subregion). (**b**) Monthly average wind speed and direction from 2000 to 2018 (Subregion). The wind speed was negatively correlated with the latitude of the upwelling center and positively correlated with the longitude of the upwelling center. The average wind speed of the wind farm in the study area increased, the annual average upwelling intensity center moved to the offshore low latitude, and, when the wind speed decreased, the annual average upwelling center moved to the nearshore high latitude. The correlation coefficients between the radial wind speed and the latitude of the upwelling center, the correlation coefficient between the latitudinal wind speed and the upwelling center latitude, and the correlation coefficient with the longitude of the upwelling center were \(-0.309\), \(0.181\), and \(0.248\), respectively, all of which showed weak positive correlations. The influence of wind direction on the coordinates of the upwelling intensity center was estimated to be carried out within the range of 0-180\({}^{\circ}\). In the range of 0-90\({}^{\circ}\), as the wind direction of the wind field increased, the radial wind speed decreased, the latitudinal wind speed increased, and the upwelling intensity center moved offshore towards high latitudes. In the range of 90-180\({}^{\circ}\), as the wind direction of the wind field increased, the radial wind speed increased, the latitudinal wind speed decreased, and the upwelling intensity center moved towards the low latitude nearshore direction. The intensity and movement of upwelling in the central and western parts of the South China Sea were affected by various local wind field elements [25]. The surge rate and duration of upwelling were determined by the wind field elements, indicating a significant lag in the impact of the southwest monsoon on the movement of upwelling in the central and western South China Sea [3; 26; 27]. The wind stress curl, which promotes wind-induced upwelling, was a crucial factor in this process. To explore the lag effect of upwelling on the local wind field changes, the interrelationship between the upwelling intensity and the corresponding wind stress curl of the same period was studied. The response time of the  
Figure 7: Monthly average upwelling index and trend direction for 2000–2018 (by latitude range). upwelling currents to the change of local wind field was found to vary with latitude in the central and western South China Sea.