Interannual to decadal variation of spring sea level anomaly in the western South China Sea

Interannual to decadal variation of spring sea level

anomaly in the western South China Sea*

QIU Fuwen (丘福文) 1,**, FANG Wendong (方文东) 2, PAN Aijun (潘爱军) 1,CHA Jing (查晶) 1, ZHANG Shanwu (张善武) 1, HUANG Jiang (黄奖) 1 【摘 要】Abstract Satellite observations of sea level anomalies (SLA) from January 1993 to December 2012 are used to investigate the interannual to decadal changes of the boreal spring high SLA in the western South China Sea (SCS) using the Empirical Orthogonal Function (EOF) method. We find that the SLA variability has two dominant modes. The Sea Level Changing Mode (SLCM) occurs mainly during La Ni?a years,with high SLA extension from west of Luzon to the eastern coast of Vietnam along the central basin of the SCS, and is likely induced by the increment of the ocean heat content. The Anticyclonic Eddy Mode (AEM)occurs mainly during El Ni?o years and appears to be triggered by the negative wind curl anomalies within the central SCS. In addition, the spring high SLA in the western SCS experienced a quasi-decadal change during 1993–2012; in other words, the AEM predominated during 1993–1998 and 2002–2005, while the La Ni?a-related SLCM prevailed during 1999–2001 and 2006–2012. Moreover, we suggest that the accelerated sea level rise in the SCS during 2005–2012 makes the SLCM the leading mode over the past two decades. 【期刊名称】中国海洋湖沼学报（英文版）

【年(卷),期】2017(035)001 【总页数】10

【关键词】Keyword: variability; sea level anomalies; spring; western South China Sea

1 INTRODUCTION

The South China Sea (SCS) is one of the largest tropical marginal seas in the world, which connects with the western Pacific via Luzon Strait, and with the Sulu Sea through the Mindoro Strait (Fig.1). The remaining connections with external bodies of water are over shallow continental shelf regions. The SCS current system is characterized by strong seasonality associated with wind forcing, and the seasonal cycle over most of the SCS basin is determined predominantly by the regional dynamics within the SCS (Liu et al., 2001). The dominant basin-wide circulation in the upper layer is cyclonic (anticyclonic), driven by the northeasterly (southwesterly)boreal winter (summer) monsoon. The monsoon transition seasons appear during boreal spring and fall, with an anticyclonic and a cyclonic gyre in the central SCS, respectively (Qiu et al., 2012).

Considering the importance of the variability in the circulation, numerous studies have concentrated on the seasonal circulation, coastal jet separation, and abundant mesoscale eddies in the SCS (Qu et al.,2000; Gan and Qu, 2008; Xiu et al., 2010). In summer,the northward

coastal jet separates at approximately 12°N and extends to the east coast of central Vietnam,with an anticyclonic eddy to its south and a cyclonic eddy to its north (Fang et al., 2002; Xie et al., 2003;Wang et al., 2006). The eastward jet, and associated eddy formation, is primarily governed by the wind driven circulation in the basin interior (Xie et al.,2003; Wang et al., 2006; Cai et al., 2007; Bayler and Liu, 2008). Furthermore, it has been demonstrated that the boundary layer dynamics over the narrow shelf topography may also have an effect on the jet and eddy formation (Gan and Qu, 2008). In winter,the western boundary current flows southwestward along the South China shelf and slope, then turns to the south along the Vietnam coast, and partially exits the SCS through the Karimata Strait (Fang et al.,2012). The southward western boundary current transports the cold northern water to the south, which plays an important role in the SCS climate (Liu et al.,2004).

One of the major transition patterns occurs in spring, of which the most dominant feature is the spring warm pool. This warm pool develops and peaks in May over the central SCS and decays in June(Chu and Chang, 1997; Qu, 2001; Wang and Wang,2006). It has been suggested that the surface heatflux plays a dominant role in the formation of the spring warm pool (Wang and Wang, 2006); while the surface wind stress curl associated with the bottom topography of the SCS may be responsible

for its evolution (Chu and Chang, 1997). Correspondingly, a single high sea level peak centered at 14°N, 114°E becomes a dominant feature during spring (March and April)(Ho et al., 2000), namely, the spring warm-core eddy(Chi et al., 1998) or the spring mesoscale high (He et al., 2013). The 20-year (1993–2012) mean of the sea level anomaly (SLA) and geostrophic current for March–May (MAM) shows a noticeable anticyclonic eddy in the western SCS during spring (Fig.2). Using numerical experiments, Chi et al., (1998) highlighted the importance of wind in generating the warm-core eddy in the spring-to-summer transition season. He et al., (2013) suggested that the lateral transport in the western SCS also contributes to the generation of the high SLA during spring. The spring warm pool and high SLA in the SCS play an important role on the climate and weather, such as, rainfall and typhoons over the SCS. However, the interannual to decadal variation of these issues is not well understood and deserves further investigation. Previous studies have revealed that the monsoon wind in the SCS displays a remarkable interannual variation associated with ENSO (Wang and Zhang,2002). The weaker monsoon winds during El Ni?o events eventually influence the circulation, with weaker gyres and upwelling in the SCS (Chao et al.,1996; Qu et al., 2000; Xie et al., 2003; Fang et al.,2006). As described above, the spring high SLA in the western SCS is closely associated with the monsoon winds. Therefore, the spring high

SLA may have a large signal on an interannual time scale, which may have a potential further impact on the summer circulation. So far, few studies have reported on the interannual variability of this feature. In addition, the transition of the circulation patterns from spring to summer may play a significant role in the variability of the coastal jet (Wu et al., 1998). During normal years, the spring high SLA decays in May, and a cyclonic gyre appears and persists from summer well into January. In 1995, a late transition from anticyclonic to cyclonic in the western SCS allowed the northward coastal jet to penetrate into the Gulf of Tonkin. Except for the summer, the winter circulation in the SCS is also modulated by the transition of the spring to summer circulation pattern. For example,the early reversal of the spring to summer circulation resulted in a weak winter gyre during 1992 and 1994(Wu et al., 1998).

As a consequence, the variation of the circulation pattern in the western SCS during the spring transition season is an important academic issue to improve our understanding of the SCS ocean dynamics. In this study, using long continuous satellite altimeter data,the major objective was to investigate the interannual to decadal variability of the spring high SLA in the western SCS, and reveal its potential impact on the variability of the SCS circulation.

2 DATA AND METHOD