Evolution of a coastal upwelling event during summer 2004 in the southern Taiwan Strait
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Evolution of a coastal upwelling event during summer 2004 in the southern Taiwan Strait ZHANG Caiyun1∗ , HONG Huasheng1 , HU Chuanmin2 , SHANG Shaoling1 1
2
State Key Laboratory for Marine Environmental Science, College of Oceanography and Environmental Sciences, Xiamen University, Xiamen 361005, China College of Marine Science, University of South Florida, St. Petersburg, USA
Received 18 January 2010; accepted 27 September 2010 ©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2011
Abstract A coastal upwelling event in the southern Taiwan Strait (STWS) was investigated using intensive cruise surveys (four repeated transects in a month) and satellite data in July and early August 2004. The extensive upwelling-associated surface cold water was first observed in early July (∼2.0×104 km2 ) along the STWS coast. Then, the cold surface water reduced in size by ∼50% with decreased chlorophyll concentrations after 15 days, indicating the weakening of the upwelling event. At the end of July, the cold surface water disappeared. The temporal variations of the surface cold water and the 3-D hydrography around Dongshan Island are thought to be mainly attributed to the weakened upwelling-favorable southwestern wind, the asymmetric spatial structure of the wind field and the intrusion of warm water from the northern South China Sea. Key words: coastal upwelling, evolution, SST, ocean color, remote sensing, Taiwan Strait
1 Introduction Upwelling is an important process in the ocean, by which deep, cold, nutrient-rich water is brought to the surface, stimulating new production and phytoplankton biomass. However, there is great variability in the coastal upwelling processes, mainly related to changes in wind strength and direction (Brink, 1983; Mann and Lazier, 1996; Chavez et al., 2002; Ramp et al., 2005). For example, within the 6-month season of upwelling off Oregon, there are four or five periods of strong upwelling separated by periods of little or no upwelling (Mann and Lazier, 1996). The Taiwan Strait (TWS) is located between the South China Sea (SCS) and the East China Sea (ECS) (Fig.1). During summer, southwest monsoon prevails. Following the wind pattern, the horizontal advection in the TWS mainly originates from the SCS, normally referred to as Yuedong coastal water and/or South China Sea shelf water. Meanwhile, such a wind pattern is favorable for inducing coastal upwelling along the mainland coast (see review by Hu et al., 2003; Shang et al., 2004; Tang et al., 2002; Gan et al., 2009). The coastal upwelling in the southern TWS has been investigated since the 70’s (Chen et al., 1982; Hong, et al., 1991; Xiao et al., 1988). It occurs in the shal-
low channel (∼40 m average water depth) between the Taiwan Bank and the East Guangdong-South Fujian coast, which has a complicated dynamic and frontal structure (Li et al., 2000; Wang et al., 2001) and thus becomes one of the main summer fishing grounds in China Sea. Furthermore, this upwelling was suggested to contain two sub-systems, Yuedong and Minna
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