One of the strongest El Niño events on record took place in the tropical Pacific in 1997–1998, causing remarkable changes in monsoon systems over the South China Sea (SCS) basin. This would induce prominent anomalies in the SCS upper layer circulation. The monsoon anomalies and the response of the upper layer circulation over the SCS during the 1997–1998 El Niño event are examined through data analysis and ocean numerical modeling, respectively. The winter monsoon and summer monsoon show southerly wind anomalies and northeasterly wind anomalies respectively during 1997/98 El Niño event, which was a reversal of the climatology. In contrast to climatology, the weakening of the winter monsoon does not change the upper layer circulation pattern, but causes a prominent change in the strength of the western boundary current. Corresponding to the anomalies of the summer monsoon, the upper layer circulation shows changes not only in magnitude, but also in structure from the climatology. The changes of the structure lie in the bifurcation of the western boundary current.

Introduction

The South China Sea (SCS) is a semi-enclosed ocean basin located between the Asian land mass to the north, the Philippine Islands to the east, the Palawan Island and the Kalimantan Island to the southeast, Indonesia to the south, and Vietnam to the west (Figure 1). The SCS extends from the equator to 23oN and from 99oE to 121oE, and embraces an area of about 3.5×106 km2, with a mean depth of 1800 m. The central part is a deep, elliptical shaped sea basin with a maximum depth of over 5000 m. The basin oriented northeast-southwest and numerous reef islands and underwater plateaus are scattered throughout.

The SCS experiences two monsoons, winter and summer, every year (Chu et al., 1999; Liu et al., 2001). During the winter monsoon, the strong and cold northeasterly wind blows over the SCS, while during the summer monsoon, the weak and warm southwesterly wind blows over the SCS. The winter monsoon is much stronger than the summer monsoon. Spring and fall are transitional seasons marked by highly variable winds. The upper layer circulation within the SCS is driven primarily by the SCS monsoon wind (Wyrtki, 1961). Corresponding to the winter and summer monsoon, the basin-scale circulation of the upper SCS shows a dramatic seasonal variability, e.g. generally cyclonic in the winter, reversing to largely anticyclonic in the summer (Wrytki, 1961; Shaw, 1989; Chu et al., 1997; Chu and Chang, 1997; Chu et al., 1998; Shaw and Fu, 1999; Liu et al., 2001).

Although El Niño occurs in the equatorial eastern and central Pacific, it affects the global ocean, including the SCS (Wang et al., 2002). The influence of El Niño-Southern Oscillation (ENSO) is transmitted through the ‘atmospheric bridges’ of atmospheric circulation changes (Wang C. et al., 2006,; Wang D. et al., 2006). Using satellite and ship observations and reanalysis fields, Klein et al. (1999) and Wang (2002, 2005) showed that the Walker and Hadley circulations could serve as an atmospheric bridge. These anomalous Walker and Hadley circulations result in variations in surface wind that in turn influence upper layer circulations over the SCS (Wang, C. et al., 2006; Wang, D. et al., 2006).

One of the strongest El Niños on record took place in the tropical Pacific in 1997–1998, causing remarkable changes in the monsoon system over the SCS basin. The response of the SCS upper layer circulation to the monsoon wind anomalies during 97/98 El Niño will be investigated by a 3-D ocean numerical model. This paper is arranged as follows. In the next section, the model and setup of climatological and interannual runs are described. Section 3 checks monsoon wind anomalies over the SCS during 97/98 El Niño. The SCS upper layer circulation during the 97/98 El Niño event is examined through numerical modelling in the Discussion.

Model and setup description

The Princeton Ocean Model (POM) (Blumberg and Mellor, 1987) implemented for the SCS with surface wind forcing is used to simulate the SCS upper layer circulation which occurred during 97/98 El Niño. The POM is a time-dependent, primitive equation circulation model on a three-dimensional grid that includes realistic topography and a free surface.

The model domain is from equator to 25o N and from 98.75o E to 121o E, with the horizontal resolution of 0.25o latitude and longitude and 21 vertical sigma coordinate levels. The bottom topography is obtained from the smoothed Naval Oceanographic Office Digital Bathymetry Data Base 5 minute resolution. The model is forced by the monthly mean wind stress derived from the Florida State University (FSU) pseudo-stress product for the period 1970–2003 (Luther and O’Brien, 1985; Legler et al., 1989).

Chu et al. (1999) pointed out that flows through SCS straits are quite uncertain. It is not an easy job to choose one among various estimations. Like Chu et al. (1999), Wyrtki's (1961) data are used for a balanced estimation of volume transports for the Luzon Strait, Taiwan Strait, and Gasper-Karimata Strait with seasonal variations (Table 1). Since there are no reliable estimations at the Balabac Channel, Mindoro Strait, and Strait of Malacca, transport there is assumed to be zero.

The model is spun up for 10 yr from rest with temperature and salinity fields in January derived from the World Ocean Atlas 2001 (WOA01) as initial fields, and is forced by the climatological monthly mean wind stress derived from the FSU pseudo-wind stress product. The interannual run is from January 1995 to December 2000, the 97/98 El Niño event included.

Wind anomalies during 1997/1998 El Niño event

The SCS is subjected to a seasonal monsoon system, winter monsoon and summer monsoon (Wyrtki, 1961). The monsoon cycle of the wind-stress fields is shown in Figure 2, for January and August from the FSU monthly-mean climatology. Due to the Siberian high pressure system, the winter monsoon winds are directed away from the Asian continent, causing northeasterly wind stresses over the SCS basin. The winter monsoon season persists from November to March. During the summer monsoon season, stresses are southwesterly over the SCS basin, which lasts from April to August. The transitional periods are marked by highly variable winds (Chu et al., 1999). In contrast to summer monsoon, not only the persistence, but also the intensity of the winter monsoon is higher.

This El Niño caused remarkable changes in the direction and magnitude of monsoon system over the SCS basin (Figures 3 and 4). In January 1998, the mature phase of 97/98 El Niño, the wind stress vector anomalies were marked by strong southerly wind component in the northern and central SCS (Figure 3a). This means the anomalies had the reverse direction of the climatologies which resulted in a weakening of the winter monsoon over the SCS. The scope of significant negative wind stress anomalies occupied the deep basin of the SCS and orients southwest-northeast roughly with the typical magnitude of above 20 m2 s−2 (Figure 3b). Over Sunda Shelf and Gulf of Thailand, southern SCS, the wind stress vector anomalies blew westward (Figure 3a) and increased slightly the magnitude of wind stress with the scale of positive 5–10 m2 s−2 (Figure 3b).

In August 1998, demise phase of 97/98 El Niño, the wind stress vector anomalies had the following features: the anomalous vector reflected a strong northeasterly flow in the central SCS (Figure 4a), reversal of the climatological vector. This corresponds to the reduction of wind stress with the scale of negative 20–40 m2 s−2 (Figure 4b). The isogram of wind stress oriented southwest-northeast slightly. The wind stress vector anomalies were westward over Gulf of Thailand, southward over Sunda Shelf. Remarkably, the wind stress vector anomalies over the northern SCS were much weaker (Figure 4a).

Upper Layer Circulation of the SCS

Climatology

During the winter monsoon period (November to March) the winter Asian high pressure system brings strong winds from the northeast (Chu et al., 1999). The major feature of the SCS surface circulation pattern is generally cyclonic in the winter. As the western intensification of the cyclonic circulation increases, the western boundary current flows southward along the Vietnam coast (Figure 5a). The Natuna eddy is another major feature during winter monsoon periods. The western boundary current flows away from the Vietnam coast. One of its branches, narrow and fast, continues to move southward and eastward along the Sunda Shelf, and then leaves the coast of Borneo and turns northward and westward to merge into the western boundary current again. Thus, the cyclonic Natuna eddy is formed, which locates at (7°N, 111°E). Another branch, broad and slow, flows southwestward along the southern coast of Vietnam, and turns southward near the mouth of Gulf of Thailand toward Gaspar-Karimata Straits.

During the summer monsoon period (mid-May to August) winds blow from the southwest. The upper SCS circulation is generally anticyclonic in the southern basin. Although along the Vietnam coast, the western boundary current flows northward, reversing direction during the winter monsoon (Figure 5b). A current, originating from Gaspar–Karimata Straits, flows northward, and near the Gulf of Thailand turns northwestward along the southern coast of Vietnam, then turns northward along the eastern coast of Vietnam. Thus the western boundary current is formed which separates near 6–15°N into three branches: two offshore branches and one alongshore branch. The alongshore branch is weak and continues northward, then east at Hainan Island. The two offshore branches split at about 10°N The zonal current, north of 10°N, crosses the SCS basin to the western coast of Philippines and follows the coast northward, while the other current, south of 10°N, turns to the south.

Circulation during the 97/98 El Niño Event

Figure 6a shows the SCS upper layer circulation for January 1998 (representing winter), which is the mature phase of 97/98 El Niño. The upper layer circulation pattern shows less difference in structure from the climatology results (Figure 5a), which indicates that the winter monsoon over SCS during 97/98 El Niño does not change the general circulation pattern. However, our computation shows that it causes a prominent change in the strength of the western boundary current.

Figure 6b is a plot of the difference in upper layer circulation for the 1998 results minus climatology results, and represents the effects of winter monsoon over SCS during the mature phase of 97/98 El Niño. The anomalous circulation shows an anti-cyclonic gyre in the whole SCS basin and an anti-cyclonic eddy in the southern SCS. The anti-cyclonic gyre structure indicates that although the western boundary current is still along the Vietnam coast, the strength of the basin-wide cyclonic pattern and western boundary current decreases (Figure 6a) due to anomalous winter monsoon during 97/98 El Niño. The anti-cyclonic eddy illustrates that the Natuna eddy is weaker than in a normal year.

Figure 7b is a plot of the difference in upper layer circulation in August for the 1998 results minus climatology results and represents the effects of summer monsoon over SCS during the demise phase of 97/98 El Niño. The anomalous circulation shows that a broad cross-basin zonal current, spanning from about 8°N to 11°N in meridional direction, moves westward from the west coast of Palawan. The anomalous current bifurcates into two branches near the eastern coast of Vietnam at about 11°N. The strong branch moves northward along the eastern coast of Vietnam, and further splits into two sub-branches: One sub-branch turns eastward and southward, and forms an anti-cyclonic eddy. Another sub-branch continues northward, then east at Hainan Island. The weak branch moves southwestward along the southeastern coast of Vietnam, turns southward and eastward along the Sunda Shelf, and forms a cyclonic eddy which orients south-north with dual cores.

The above circulation anomalies correspond to the following features of upper layer circulation for August 1998 (representing summer). A current, originating from Gaspar–Karimata Straits, flows northward, and near the Gulf of Thailand turns northeastward along the southern coast of Vietnam. Then the current merges into the western boundary current along the eastern coast of Vietnam. From 8°N to 12°N, the western boundary current turns southward. From 12°N to 16°N, the second branch crosses the SCS basin to the western coast of Philippines. The third branch is weak and continues northward, then east at Hainan Island.

Discussion and Conclusions

The influence of El Niño on the SCS is believed to be through the atmospheric bridge of circulation changes. During El Niño, the regional Hadley circulation in the Indo-western Pacific sector was weakened, with anomalous decent over the tropical Indo-western Pacific (Klein et al., 1999; Wang, 2002, 2005; Wang C. et al., 2006). The anomalous anticyclone in the north western Pacific, associated with the anomalous decent motion, altered the local SCS monsoon wind and in turn induced oceanic circulation variations over the SCS (Wang, C. et al., 2006; Wang, D. et al., 2006).

In January 1998, when 97/98 El Niño matured in boreal winter, the SCS showed strong southerly wind anomalies in the northern and central SCS (Figure 3a), resulting in weakening of the winter northeast monsoon (Figure 3b), with maximum weakening occuring in the northern and central SCS and with the typical negative magnitude of above 20 m2 s−2. Corresponding to the weakening of the winter monsoon, the strength of the general circulation showed a prominent change. The mean velocity of upper ocean current over the SCS decreased from 75 cm s−1 for climatological January to 37 cm s−1 for January 1998. The values were from our ocean numerical modeling results. However, the anomalies of the SCS winter monsoon did not change the general circulation pattern. The SCS upper layer circulation for January 1998 (Figure 6a) showed less difference in structure from the climatology results (Figure 5a).

In August 1998, demise phase of 97/98 El Niño, the wind anomalies over the SCS were northeasterly (Figure 4a), a reversal of the climatology, resulting in the reduction of wind stress with a scale of negative 20–40 m2 s−2 (Figure 4b). Correspondingly, the upper ocean circulation weakened, with the mean velocity of upper ocean current over the SCS decreasing from 50 cm s−1 for climatological August to 27 cm s−1 for August 1998. Different from winter, the upper layer circulation for August 1998 (Fig- ure 7a) showed changes not only in magnitude, but also in structure from the climatology results (Figure 5b). The changes of the structure lie in the bifurcation of the western boundary current. The zonal cross-basin current, which is one of the offshore branches, crossed the SCS basin to the western coast of Philippines from 12°N to 16°N in the climatology summer pattern, but from 10°N to 14°N in the 1998 summer pattern. The 1998 cross-basin current moved farther north than the climatologcial one.

Acknowledgements

This study is supported by programs under contract No. 2011CB403502, NCET-08-0510 and 201105002-11.

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