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

Abstract

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 ) 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 À0.60, 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 À0.42, 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 Niño event through planetary wave propagations.Plain Language Summary 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 Niño-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.

Experimental evidence of long-term oceanic circulation reversals without wind influence in the North Ionian Sea

“…More importantly, they depict the NIG reversals as possessing different nature and scientific relevance: either being a basic wind-induced circulation or, instead, representing a proper mode of oceanic variability. Note that phenomena from which such alternative behavior could emerge have been observed also in other regions of the World Ocean, which puts our investigation in a context of more general relevance [27][28][29] . A key step forward in our understanding of the BiOS, particularly towards establishing it as a mode of variability, regards thus whether interior ocean processes alone are capable of triggering reversals in the NIG circulation.…”

Using TPI to Map Spatial and Temporal Variations of Significant Coastal Upwelling in the Northern South China Sea

“…However, in the MNCU and the YDCU, the temporal pattern of the upwelling index was different from that of SST_A or Chl-a increase, which may indicate that there were other factors contributing to the formation of these two coastal upwelling regions. It has been found that the coastal upwelling occurs off Shantou coast during wind relaxation [29], indicating that in addition to the local wind, there are other factors (e.g., topography, thermocline depth on the shelf) contributing to the formation of the MNCU or the YDCU [6,30]. Moreover, according to the in situ observations, the MNCU and the YDCU can uplift the deeper water to the layer of 10 m [31].…”

“…Generally speaking, the upwelling strength increases from May, reaches the maximum in July, and then decreases to a minimum in September. This is because the coastal upwellings in the NSCS are mainly wind-driven and thus follow the inter-monthly patterns of the summer monsoon, although there are still other factors driving the MNCU and the YDCU [6,29,30]. The correlation coefficients of the monthly characteristics (duration in Figure 6a, influence area in Figure 8a, SST_A in Figure 9a, Chl-a increase in Figure 10a, and upwelling index in Figure 11a) are shown in Tables 1-3 for the Minnan coastal upwelling, the Yuedong coastal upwelling, and the Qiongdong coastal upwelling, respectively.…”

“…They pointed out that the coastal upwelling in the NSCS mainly occurs off south coast of Fujian (called Minnan coastal upwelling hereafter), east coast of Guangdong (called Yuedong coastal upwelling hereafter), and east coast of Hainan Island (called Qiongdong coastal upwelling hereafter), and the coastal upwelling is mainly driven by the southwesterly monsoon in boreal summer. Since Hu and Wang [3], there have been some studies on the upwelling in the SCS using the remotely sensed data [4][5][6][7][8]. Combining the Advanced Very High Resolution Radiometer (AVHRR) sea surface temperature (SST) data with the European Center for Medium-Range Weather Forecasts (ECMWF) wind field data, Xie et al [4] studied the variability and long-term tendency of the Qiongdong coastal upwelling, showing that the Qiongdong coastal upwelling has a weakening trend in most recent 30 years.…”

Origin of the springtime South China Sea Warm Current in the southwestern Taiwan Strait: Evidence from seawater oxygen isotope

“…With a well-developed broad continental shelf and a steep continental slope, the southwestern Taiwan Strait is one of the most important fishing grounds in China (Figure 1a). Affected by factors such as topography, monsoon, and Kuroshio intrusion, the southwestern Taiwan Strait has a variable circulation structure with a complex interaction among different currents (Guan, 1998;Hu et al, 2010;Liu et al, 2008;Shu et al, 2016Shu et al, , 2018aShu et al, , 2018bShu et al, , 2019Su, 2004). Major circulation systems in the southwestern Taiwan Strait include the South China Sea Warm Current (SCSWC, also known as the counter-wind current in the northern SCS), the South China Sea Branch of Kuroshio (SCSBK, also known as slope current in the northern SCS), and the coastal current (Hu et al, 2010;Liu et al, 2008).…”