## 3 Results
### Effect of Wind Fields on Upwelling  
Figure 5 presents the monthly mean sea surface temperature and wind direction of the wind field in the upwelling waters of the west-central South China Sea from 2000 to 2018. The figure reveals that the monsoon season shifted from the northeast to the southwest monsoon in May, causing the wind direction to change accordingly. As a result, the average temperature of the sea area in the west-central South China Sea decreased to varying degrees. During the formation of upwelling, there was a general decreasing trend in the SST. The correlation coefficient between the sea surface temperature and wind direction was 0.352 during the upwelling months (May-September) in the west-central South China Sea. As the wind direction angle increased (decreased), the sea surface temperature increased (decreased) to different degrees. Moreover, the correlation coefficients between the sea surface temperature and wind speed, and wind stress were \(-0.612\) and \(-0.624\), respectively. This indicates that the sea surface temperature increased (decreased) as the wind speed and wind stress increased (decreased) in the central and western parts of the South China Sea. Therefore, the SST time series plot showed alternating local peaks and troughs with the dynamic changes in the wind field of the study area. Figure 6 presents the monthly average upwelling strength index and wind direction characteristics of the wind field. The average upwelling index of the OU_1 sea area was 1477.43, with an average wind speed of 7.12 m/s. The average upwelling index of the OU_2 sea area was 1965.43, with an average wind speed of 7.78 m/s. Meanwhile, the average upwelling index of the OU_3 sea area was 809.91, with an average wind speed of 6.37 m/s. Among them, the upwelling index of OU_2 was the highest, and the average wind speed was greater than the other two sub-regions (Figure 6b). The upwelling index of OU_1 was lower than that of OU_2 but much higher than that of OU_3, indicating that upwelling mainly occurred in OU_1 and OU_2 in the central and western South China Sea. The upwelling index was only one-half of the average in OU_1 and OU_2, and the monsoon turned southwest later in OU_3 than in OU_1 and OU_2 (Figure 6b). This may be the primary reason for the low upwelling index in OU_3. The findings presented here provide critical insights into the spatial and temporal dynamics of upwelling in the South China Sea, which is essential for understanding the biogeochemical processes and fisheries production in the region. Figure 5: Monthly mean sea surface temperature and wind field wind direction in the mid-western South China Sea upwelling waters from 2000 to 2018. (**a**) Monthly average sea surface temperature from 2000 to 2018. (**b**) Monthly average wind speed and direction from 2000 to 2018. The upwelling index, which is computed monthly, displays a trend line based on three sub-area ranges (OU_1, OU_2, OU_3), as depicted in Figure 7. It was observed that the upwelling intensity in mid- and low-latitude seas was greater than that in high-latitude seas. The trend line reveals that the upwelling index of the OU_1 region exhibited a pattern of being low in the middle and high at both ends throughout the entire study period. Between 2003 and 2015, the index displayed a gradual decline, followed by a subsequent recovery in the following years. The upwelling index in the OU_2 waters was generally stable, with an annual average upwelling index higher than that of the OU_1 and OU_3 seas, except for 2000, 2001, and 2018. Conversely, the upwelling index of the OU_3 waters was consistently low, revealing a stable low index state. Between 2000 and 2002, the upwelling index in the central and western parts of the South China Sea was high, indicating an intense upwelling event. However, from 2003 to 2015, the upwelling index remained generally low. After 2016-2018, the upwelling index began to rise.