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Observations of Inflow of Philippine Sea Surface Water Into the South

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Observations of Inflow of Philippine Sea Surface Water Into the South JANUARY 2004 CENTURIONI ET AL. 113 Observations of In¯ow of Philippine Sea Surface Water into the South China Sea through the Luzon Strait LUCA R. CENTURIONI AND PEARN P. N IILER Scripps Institution of Oceanography, La Jolla, California DONG-KYU LEE Busan National University, Busan, South Korea (Manuscript received 19 February 2003, in ®nal form 12 June 2003) ABSTRACT Velocity observations near the surface made with Argos satellite-tracked drifters between 1989 and 2002 provide evidence of seasonal currents entering the South China Sea from the Philippine Sea through the Luzon Strait. The drifters cross the strait and reach the interior of the South China Sea only between October and January, with ensemble mean speeds of 0.7 6 0.4ms21 and daily mean westward speeds that can exceed 1.65 ms21. The majority of the drifters that continued to reside in the South China Sea made the entry within a westward current system located at ;208N that crossed the prevailing northward Kuroshio path. In other seasons, the drifters looped across the strait within the Kuroshio and exited along the south coast of Taiwan. During one intrusion event, satellite altimeters indicated that, directly west of the strait, anticyclonic and cyclonic eddies resided, respectively, north and south of the entering drifter track. The surface currents measured by the crossing drifters were much larger than the Ekman currents that would be produced by an 8±10 m s 21 northeast monsoon, suggesting that a deeper westward current system, as seen in historical watermass analyses, was present. 1. Introduction latitude can be successfully computed from the wind- This work describes observations of velocity made at driven vorticity dynamics of linear and nonlinear re- a nominal depth of 15 m with satellite-tracked drifters duced-gravity circulation models. and provides new direct evidence of seasonal near-sur- At 188N the Kuroshio is a well-formed, northward- face ¯ow from the Philippine Sea into the South China ¯owing western boundary current concentrated entirely Sea through the Luzon Strait (Fig. 1). Such ¯ow is fre- west of 1248E; its high-speed core is positioned at quently described as a westward branch of the Kuroshio. 1238E, and its baroclinic structure is evident in the upper Mainly hydrographic methods have provided evidence 600 m (Toole et al. 1990; Qu et al. 1998). Before reach- of the seasonal penetration of the Philippine Sea water ing Taiwan, the Kuroshio encounters the Luzon Strait, into the South China Sea (e.g., Fang et al. 1998). which is the deepest passage from the Paci®c Ocean to The surface circulation south and east of the Luzon the South China Sea. At the southern portion of the Strait is dominated by strong and persistent subtropical strait, the Kuroshio takes a westward set and makes a current systems. At the surface, the yearly mean Paci®c detour into the South China Sea through the deepest North Equatorial Current bifurcates at ;138N near the channels of the Luzon Strait: the Balintany Channel and east coast of Luzon to form the northward-¯owing Ku- south of Babuyan Island. West and north of Batan Is- roshio and the southward-¯owing Mindanao Current land, the Kuroshio ¯ows within the Bashi Channel until (Nitani 1972; Toole et al. 1990; Qu and Lukas 2003). it reaches the southeast coast of Taiwan (Gilson and The near-surface bifurcation latitude moves between Roemmich 2002). 118N in May and 14.58N in November, and at depth it Between the north coast of Luzon and the southeast is even farther north of its surface expression (Nitani coast of Taiwan, the Kuroshio is occasionally referred to 1972; Qu and Lukas 2003). Qiu and Lukas (1996) have as a ``loop current'' because it makes a loop, or excursion, noted that the interannual variations of the bifurcation into the South China Sea. The largest loop occurs be- tween October and January when the winds are domi- nated by the northeast monsoon. A description of the Corresponding author address: Luca R. Centurioni, Scripps In- stitution of Oceanography, 9500 Gilman Dr., La Jolla, CA 92093- loop current can be found, for example, in Nitani's (1972) 0213. maps of the geomagnetic electrokinetograph data and dy- E-mail: [email protected] namic heights compiled from several hydrographic sur- q 2004 American Meteorological Society Unauthenticated | Downloaded 09/30/21 01:18 AM UTC 114 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 34 FIG. 1. Bathymetry of the study region (Smith and Sandwell 1997, bathymetry version 8.2). veys. Li et al. (1998) used hydrographic data to show and intrusions in autumn and winter with sea surface the looping of the Kuroshio in the Luzon Strait in summer temperature (SST) maps made from Advanced Very High months in the upper 200 m. Between October and March Resolution Radiometer data. As de®ned by the SST, the the warm surface water of Philippine Sea origin contrasts loop intrudes most severely into the South China Sea with the surrounding colder South China Sea water. This during the October±January period, when signi®cant condition was exploited by Farris and Wimbush (1996), amounts of Philippine Sea water are also found below who observed the occurrence of Kuroshio surface loops the surface. For example, Shaw (1989) used conductiv- TABLE 1. Summary of the Lagrangian statistics in the region bounded, in the zonal direction, by 1208 and 1358E and, in the meridional direction, by 108 and 258N. Here T is the Lagrangian timescale, L is the Lagrangian length scale, and s denotes the standard deviation. The ®rst ®gure in the columns labeled with NT is the number of 6-h-interval time series longer than 20 days for which the velocity autocovariance converges, and the second ®gure is the total number of time series examined. Here E and N in the subscripts refer to the zonal and meridional direction, respectively, and NP is the total number of velocity observations in the region. JFM is for Jan±Mar, AMJ is for Apr±Jun, JAS is for Jul±Sep, OND is for Oct±Dec, and All is for all months. TE s(TE ) LE s(LE ) TN s(TN ) LN s(LN ) 4 4 4 4 Months (days) (days) (10 m) (10 m) NTE (days) (days) (10 m) (10 m) NTN NP JFM 2.7 2.0 4.4 3.7 90/95 2.6 1.6 4.5 3.6 93/95 21 442 AMJ 3.3 1.9 6.7 5.4 93/96 2.9 1.4 5.4 3.5 93/96 24 856 JAS 2.5 1.4 4.5 3.0 98/102 2.5 1.4 4.6 3.5 96/102 25 530 OND 2.8 1.8 4.9 3.9 90/90 2.8 1.7 5.1 4.0 88/90 21 075 All 3.2 2.1 6.0 4.8 259/269 3.0 1.6 5.5 4.0 255/269 92 903 Unauthenticated | Downloaded 09/30/21 01:18 AM UTC JANUARY 2004 CENTURIONI ET AL. 115 FIG. 2. Six-hour-interval positions of the drifters, color-coded in accordance with the local instantaneous speed. The 24 626 data shown here were collected between Sep 1987 and May 2002. The black lines represent the 500-m depth contours. ity±temperature±depth (CTD) pro®les to demonstrate that are the breadth of the gap relative to the width of the Philippine Sea water extends down to 500 m on the con- Munk viscous boundary current and the Reynolds num- tinental slope west of the southern tip of Taiwan in the ber of the ¯ow, de®ned as the ratio between the transport March±August period. Shaw (1991), by analysis of a rate of mass per unit depth and the lateral eddy diffusion larger hydrographic dataset, tracked water of Kuroshio coef®cient. The general theoretical results are that a origin as far as west of 1158E in the upper 250 m and strong current ``leaps the gap'' and a weak current loop along the continental slope of the northern South China propagates westward into the gap, forming eddies in the Sea. Qu (2002) reached similar conclusions about intru- intrusion. sions upon inspection of the distribution of oxygen con- Even in the absence of direct evidence of the circu- centration. Although the in¯ow is apparent from water- lation patterns (but see Farris and Wimbush 1996), ob- mass properties, diapycnal mixing in the upper South servations and theory are supporting the view that water China Sea and the out¯ow needed to achieve a mass of Philippine Sea origin can reach the interior of the balance of the basin are not known well. South China Sea. In numerical studies of the circulation Multilayer ocean general circulation model simula- of the South China Sea, the processes that cause the tions of the transport of upper-layer water through the in¯ow appear to be associated with, or produce, me- Luzon Strait are discussed in Metzger and Hurlburt soscale eddies. Observational evidence for this meso- (1996, 2001). In these simulations, the in¯ow is a result scale variability has recently become available from hy- of the seasonally varying basin-scale circulation of the drography (Li et al. 1998). entire western subtropical and tropical Paci®c. These In this work, we present and discuss observations of works associate the seasonal in¯ow with the lowering the surface circulation in the western tropical Paci®c and of sea level caused by the increased cyclonic circulation in the South China Sea and we report on the observed of the South China Sea that results from the positive ¯ow from the Philippine Sea to the South China Sea.
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