Upwelling Along the Coast of Vietnam
The upwelling region along the coast of Vietnam is demonstrated by a low sea surface temperature (SST) that is 1 ˚C cooler than the long-term average (Levitus, 1984). In boreal summer, the north-south orientation of the coast of Vietnam and the southwesterly monsoon winds parallel to the coast is an important driving factor for the coastal upwelling along the coast of Vietnam (Chu et al., 1998; Huang et al., 1994; Pohlmann, 1987; Shaw and Chao, 1994; Wyrtki, 1961) either by means of offshore Ekman transport due to the prevailing alongshore winds (Dippner et al., 2007; Wang et al., 2013) or by Ekman pumping due to wind stress curl (Wang et al., 2013; Xie et al., 2003). The upwelling starts in May in the western part of Vietnam and moves southward in June, reaching its peak intensity in August before disappearing in October (Kuo et al., 2000). The key feature of this upwelling area is the development of cold band water along the coast of Vietnam initially, and it migrates southward and gradually spread eastward, becoming a cold filament near 12 ˚N latitude (Figure 1) sandwiched between a cyclonic eddy in the central coast of Vietnam and an anticyclonic eddy in the southwestern part of the south China Sea (SCS) (Gan and Qu, 2008; Kuo et al., 2004; Zu et al., 2020). Remote sensing and modelling studies have shown that the upwelling region along the coast of Vietnam is the most productive area in the SCS, with high surface chlorophyll concentrations of close to 2 mg m-3 and close to 80 mmol C m-2 d-1 of modelled primary productivity (Liu et al., 2002; Tang et al., 2004a, 2004b). Previous studies have shown that the upwelling along the coast of Vietnam exhibited interannual variability that is closely linked to ENSO (Dippner et al., 2007; Kuo et al., 2004; Xie et al., 2003). The upwelling along the coast of Vietnam was enhanced during the El Niño year owning to the strengthening of the southwest monsoon wind (Kuo et al., 2004). A contradict situation occurred during the summer following El Niño where the upwelling along the coast of Vietnam was weakened owning to the weakening of the southwest monsoon wind (Dippner et al., 2007; Kuo et al., 2004; Xie et al., 2003), resulting in a sharp decline in chlorophyll-a concentration and primary productivity (Bombar et al., 2010; Zhao and Tang, 2007).
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