The Contribution of Local Wind and Ocean Circulation to the Interannual Variability in Coastal Upwelling Intensity in the Northern South China Sea

Plain Language Summary Using in situ data, satellite observations, and model outputs, we analyzed the interannual variability in coastal upwelling intensity in the northern South China Sea. Comparing coastal upwelling observed from three cruises during the summers of 2008 and 2016, we found that coastal upwelling was stronger during 2016 compared to 2008, although the local upwelling favorable wind was stronger in 2008. The stronger near-bottom cross-shelf current and shallower thermocline in the slope resulted in stronger upwelling intensity during the summer of 2016. The topographic position index (TPI), which is defined by the sea surface temperature difference between one center cell and its neighbors, was used to quantify the interannual variability in upwelling. Stronger (weaker) upwelling intensity occurred during the summers of 2007, 2008, 2011, 2015, and 2016 (2004, 2009, 2012, and 2014) when the local wind was more favorable (less favorable) to coastal upwelling. The correlation coefficient between the area-weighted TPI and alongshore wind speed was -0.60, thereby confirming that local wind is the primary dynamical factor controlling the interannual variability in upwelling intensity. The correlation coefficient between the area-weighted TPI and the eastward boundary current transport averaged between the 75- and 100-m isobaths on the shelf was -0.42, indicating that the interannual variability in large-scale circulation in the northern South China Sea also contributes to the interannual variability in upwelling intensity. The anomalously shallow thermocline in the summer of 2016 was likely associated with the strong 2015-2016 El Nino event through planetary wave propagations. Coastal upwelling, transporting deep, cold, saline, and nutrient-rich water to the surface result in high levels of primary production and fishery production. The coast of the northern South China Sea (NSCS) features seasonal coastal upwelling during the boreal summer. The summer southwesterly wind is widely regarded as the main driving mechanism for coastal upwelling in the NSCS. Previous studies have shown that the interannual variability in coastal upwelling intensity is largely controlled by the interannual variability in local winds in the NSCS, which is closely related to the El Nino-Southern Oscillation. Based on in situ data, we found that the upwelling was much stronger during 2016 than during 2008, though the local wind was more favorable to upwelling during 2008, which indicated that local wind is not the sole factor controlling the interannual variability in upwelling intensity in the NSCS. Further studies showed that, beside the locale wind, the interannual variability in shelf circulation could also contribute to the interannual variability in coastal upwelling intensity due to the topographically induced upwelling (induced by the interaction between northeastward large-scale current and alongshore variable topography) in the NSCS. In addition, thermocline depth variation on interannual time scale likely influences the coastal upwelling intensity in the NSCS.