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Observations of In¯ow of Philippine Surface Water into the through the Strait

LUCA R. CENTURIONI AND PEARN P. N IILER Scripps Institution of Oceanography, La Jolla, California

DONG-KYU LEE Busan National University, Busan,

(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 through the . 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 Ϯ 0.4msϪ1 and daily mean westward speeds that can exceed 1.65 msϪ1. The majority of the drifters that continued to reside in the South China Sea made the entry within a westward current system located at ϳ20ЊN 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 . 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 Ϫ1 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 18ЊN 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 124ЊE; its high-speed core is positioned at quently described as a westward branch of the Kuroshio. 123ЊE, 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 ϳ13ЊN 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 Current land, the Kuroshio ¯ows within the 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). 11ЊN in May and 14.5ЊN 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-

᭧ 2004 American Meteorological Society

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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 120Њ and 135ЊE and, in the meridional direction, by 10Њ and 25ЊN. Here T is the Lagrangian timescale, L is the Lagrangian length scale, and ␴ 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 ␴(TE ) LE ␴(LE ) TN ␴(TN ) LN ␴(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

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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 115ЊE 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. wind stress curl of the northeast monsoon. Sheremet (2001) used a barotropic model to address the local 2. The data parameter dependence of a western boundary current Direct velocity measurements in the surface mixed that encounters a gap along the coast. The parameters layer were obtained with Argos satellite-tracked drifters

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FIG. 3. Seasonal displacement diagrams of the drifters: (a) Jan±Mar, (b) Apr±Jun, (c) Jul±Sep, and (d) Oct±Dec. Contours of bottom depth are 200 m. drogued at a nominal depth of 15 m. Between November The dimensions of each averaging cell were chosen of 1986 and May of 2002, 380 instruments were de- to be the eastward and northward Lagrangian space ployed or drifted into the region delimited by 105Њ± scales, respectively (and up to 2 times these values for 135ЊE and 10Њ±25ЊN. Only 14 deployments were made the sparser seasonal datasets). Within each cell, the in- in the South China Sea. For a description of the drifters, dependent estimates of the zonal and meridional veloc- wind slip correction, and data-processing methods see ities were computed by time averaging the 6-h velocity Niiler (2001). In this analysis, the positions of each time series in bins with amplitude given by the upper drifter, interpolated to 6-h intervals, were used to con- bound of the largest Lagrangian timescale [T ϩ ␴(T) struct the velocity vectors, which are called instanta- in Table 1]. The ensemble mean velocity and its standard neous velocities. The estimated accuracy of the velocity error ellipse referred to the principal axes (Freeland et measurements in a 10 m sϪ1 wind is 10Ϫ2 msϪ1 (Niiler al. 1975) were calculated for each cell. The North Equa- et al. 1995). The Lagrangian statistics and the ensemble torial Current bifurcation latitudes in each season were mean velocity ®eld were calculated in the region delim- chosen where the zonal average (between 127Њ and ited by 120Њ and 135ЊE longitude and by 10Њ and 25ЊN 130ЊE) of the seasonal ensemble mean northward ve- latitude. For each drifter trajectory that was longer than locity components was zero. Unless stated otherwise, 20 days, we computed the Lagrangian decorrelation all uncertainties are assumed to be 1 standard deviation. timescale and length scale in the zonal and in the me- ridional directions (e.g., Freeland et al. 1975; Poulain 3. Flow of the Philippine Sea water through the and Niiler 1989). Nonconverging autocovariance func- Luzon Strait tions (the ones without zero crossings) were discarded. Seasonal Lagrangian timescale and length scale were The ensemble of the individual drifter tracks (Fig. 2) also computed. The results are summarized in Table 1. shows how the North Equatorial Current intensi®es to

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FIG. 4. Displacement diagram of the drifters that crossed the line 120.8ЊE (the dashed line labeled ``D'') at least once; 29 drifters satis®ed this condition. The trajectories of the drifters that moved back to the Paci®c Ocean are plotted in red. The others reached the interior of the South China Sea and are plotted in blue. See main text for the meaning of A, B, and C. The 200-m contours of bottom depth are shown with a solid black line. form the Kuroshio, made visible by instantaneous average speeds in excess of 0.8 m sϪ1 were observed speeds in excess of 0.7 m sϪ1, east of Luzon between when the drifters traveled southward over the narrow 16.5Њ and 18.5ЊN. To the north, the drifters in this ¯ow shelf area located along the southeast Vietnamese coast move northwest and enter the Luzon Strait principally (marked ``A'' in Fig. 4). The drifters whose positions through the channels north (Balitany Channel) and south are plotted in red on Fig. 4 recrossed the strait eastward of Babuyan Island. Two drifters moved very rapidly and moved north, some of them following a small loop westward on the continental shelf north of Luzon. Some pattern southwest of the south coast of Taiwan (marked drifters within the South China Sea continued to exhibit ``B'' in Fig. 4). Five of these drifters had lost the drogue, instantaneous speeds in excess of 0.6 m sϪ1 within the implying that the position error accumulates at a rate of current system that appears to surround its entire perim- at most 10 km dayϪ1 downwind because of the slip of eter. the ¯oat relative to a drogued drifter (Niiler 2001), an The seasonal displacement diagram of the drifters error that tends to push the tracks southwestward during (Fig. 3) shows that a westward ¯ow through the Luzon the northeast monsoon. Some of the red drifters were Strait occurs only between October and March. Of the trapped in a low-velocity region (marked ``C'' in Fig. 29 drifters that crossed the meridional line at 120.8ЊE 4) near the southeast tip of Taiwan before rejoining the between Luzon and Taiwan at least once, 15, as shown northward Kuroshio ¯ow. In the proximity of the Luzon in blue in Fig. 4, (5 of which had lost the drogue) moved Strait there were data in 13 of the 17 yr analyzed. In to the interior of the South China Sea and 6 reached 10 of the 13 yr, the drifters crossed the strait between the broad continental-shelf area south of Vietnam. Daily October and January and remained in the South China

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east and southeast of the Luzon Strait (Fig. 6), shows that the Kuroshio system is relatively well mapped. The northward boundary current forms near the coast where the North Equatorial Current separates in two branches. The latitude of this bifurcation point, computed from the annual mean surface (15 m) velocity ®eld, is located at 12.4ЊϮ0.3ЊN (dashed line in Fig. 6), and it varies seasonally by 2.6 Ϯ 1.5Њ, reaching its northernmost position of 15.0ЊϮ0.8ЊN in the October± December period (not shown), in general agreement with the results of Qu and Lukas (2003). The Kuroshio accelerates as mass is added from the east, and, by 14.8ЊN, the average speed increases above 0.5 Ϯ 0.2 m sϪ1 (computed in box 1, Fig. 6). A second concentrated stream appears to join the Kuroshio from the east at about 20ЊN. The mean westward speed of this current is 0.20 Ϯ 0.05 m sϪ1 (computed in box 2, Fig. 6) and increases to 0.34 Ϯ 0.06 m sϪ1 if only autumn data are considered (average computed in a slightly larger box, not shown). This stream is mainly sampled by the drift- ers that crossed the Luzon Strait and moved into the South China Sea in the October±January period, as de- picted and discussed in Figs. 3 and 4. This relatively strong current that crosses the Kuroshio path appears in both the global and autumn ensemble means. However, the interannual variability of this westward current structure is not well determined by the drifter data, im- plying that the estimate of the standard error is based on potentially time-aliased data and should be consid- ered as a lower bound. The mesoscale activity between 15 December 1997 and 18 January 1998 can be seen in the sea level anom- aly of Centre National d'eÂtudes Spatiales Archivage, FIG. 5. Daily average velocity for drifters crossing the line 120.8ЊE. Validation et InterpreÂtation des DonneÂes des Satellites The thick vectors correspond to the drifters that reached the interior of the South China Sea (with a mean speed of 0.7 Ϯ 0.4 m sϪ1). The OceÂanographiques (CNES/AVISO) Ocean Topography thinner vectors correspond to the drifters that crossed the line back Experiment (TOPEX)/Poseidon±European Remote toward the Philippine Sea. The thin dashed lines are the bottom depth Sensing Satellite (ERS) merged altimeter data of sea (200 m) contours. surface height anomaly (SLA) and the Tropical Rainfall Measuring Mission Microwave Imager (TMI) 3-day- averaged SST data (Fig. 7). During this period, one Sea (the blue drifters in Fig. 4). In 8 of the 13 yr, the drifter crossed the Luzon Strait. The water warmer than drifters made the small loop across the Luzon Strait 26ЊC from the Philippine Sea intruded north of 20ЊN between October and March (the red drifters in Fig. 4). and is visible in Fig. 7 because the surrounding South Five drifters (all with their drogues attached) reached China Sea surface water was colder than 24ЊC. The most the strait by traveling westward and crossing directly obvious intrusion of warm water along the northern over the mean Kuroshio path. Most of the trajectories boundary of the South China Sea extended westward north of 15ЊN (Fig. 4) indicate that besides the seasonal to ϳ117.5ЊE, with a weaker continuation east of westward current into the Luzon Strait there is, to the ϳ116ЊE. An anticyclonic eddy (A) was centered at ap- east of the strait, a vigorous eddy ®eld. proximately (21.6ЊN, 117.8ЊE) near the 200-m isobath. The velocities at the crossing line are plotted in Fig. In the Luzon Strait and in the South China Sea, south 5. The average speed computed only with data from the of approximately 20.5ЊN, the sea level was lower than drifters that reached the interior of the South China Sea the average and a cyclonic eddy was located directly is 0.7 Ϯ 0.4msϪ1. The maximum, almost westward, north of Luzon (B). The drifter track in Fig. 7b crosses speed of 1.65 Ϯ 0.01 m sϪ1 (the instrumental error is the Luzon Strait westward along the border between the used here) was observed at 20.7ЊN, 120.8ЊE in Decem- cold and the warm water that lies between the anticy- ber of 1997. clonic and cyclonic eddies at approximately 20.5ЊN. The The ensemble mean velocity ®eld, computed with a trajectory in Fig. 7b is shown 5 days before and after resolution of approximately 0.5Њ from all of the data the date of the SLA and SST observations. The average

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FIG. 6. Mean velocity and standard-error ellipses plotted on the same scale: Nov 1986±May 2002. Averaging grid (lon±lat) is 0.56Њϫ0.49Њ, approximately corresponding to the Lagrangian length scales. Here 12 734 out of 92 634 measurements are independent (based on a decorrelation timescale of 5.3 days). Gray cells contain less than 15 (at least 3) independent measurements. The 500-m contours of bottom depth are shown with a thin dashed black line. The latitude of the North Equatorial Current bifurcation point is (12.4 Ϯ 0.3)ЊN and is marked with the thick dashed black line. The mean velocities were computed in boxes 1 and 2 (see main text). speed of the drifter for the ®rst 5 days, west of ϳ118ЊE, does not appear in the drifter data between April and was 1.4 Ϯ 0.2 m sϪ1. For the last 5 days, east of ϳ118ЊE, September. The loop is con®ned to the north of the when the drifter began to leave the warm water, the Luzon Strait in the remaining months, when its west- average speed was 0.5 Ϯ 0.2 m sϪ1. Figures 7c and 7d ward extension reaches to 119.2ЊE. A net westward Ek- show that the warm-water intrusion separated from the man transport through the Strait of Luzon, computed warm water along the southern coast of Taiwan by being from Special Sensor Microwave Imager (SSM/I)-de- entrapped in the westward-moving anticyclonic eddy. rived winds (Fig. 8) exists only during the northeast The drifter moved to the south and west in a cyclonic monsoon, reaching a maximum of 0.6 Sv (1 Sv ϵ 106 circulation around a cyclonic feature (C) that propagated m3 sϪ1). Figure 8 also shows that the drifters enter the westward in the northern part of the South China Sea South China Sea preferentially during the early part of basin. the onset of the northeast monsoon. The drifters that crossed the strait were entrained by the Kuroshio east of Luzon or crossed the Kuroshio path inside the Luzon 4. Discussion and summary Strait within a strong westward jet. The ensemble mean Velocity observations made with drifters drogued to velocity ®eld computed between October and December 15-m depth describe a seasonally recurring in¯ow of (not shown) indicates that a nearly zonal westward ¯ow surface water from the Philippine Sea into the South of 0.34 Ϯ 0.06 m sϪ1 occurred east of the Luzon Strait China Sea through the Luzon Strait between October at ϳ20ЊN. The sampling of the drifters was not uniform and January. The permanent looplike intrusion of the in time; however, this jet appears between ϳ18.5Њ and Kuroshio described, for example, by Nitani (1972) and, ϳ20.5ЊN in all of the years (1989, 1992, 1994, and with numerical models, Metzger and Hurlburt (2001) 1995) during which there were data in the region. The

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FIG. 7. Maps of SLA [10-day average, contoured with thick dashed (continuous) lines for negative (positive) values, in meters] and SST (3-day average): (a) 15 Dec 1997, (b) 25 Dec 1997, (c) 4 Jan 1998, and (d) 14 Jan 1998. The purple marks show the trajectory of a drifter (5 days before and after the nominal date). Contours of bottom depth (200 m) are shown with a thin black dashed line. near-surface Ekman currents produced by 10 m sϪ1 northeast monsoon [as hypothesized, e.g., by Shaw northeast monsoon winds would be at most 0.1 m sϪ1 (1989, 1991)] and that, in the Luzon Strait region, en- (Ralph and Niiler 1999). The much stronger surface trains Kuroshio water. The study of Sheremet (2001) currents observed with the drifters indicate that a deeper predicts that the Kuroshio can penetrate the South China current system to the west must be present during the Sea when its strength is reduced. This weakening occurs in the autumn months (see, e.g., Qu et al. 1998), when the North Equatorial Current system shifts northward. The depth of the penetration and the ¯ow regime must also depend on the topography of the Luzon Strait, a problem not dealt with well by Sheremet's (2001) theory or the low-vertical-resolution models of Metzger and Hurlburt (1996, 2001).

Acknowledgments. The help of Sharon Lukas, Mayra Pazos, and Jessica Redman for processing and recov- ering the most recent drifter data is gratefully acknowl- edged. TMI data and images are produced by Remote Sensing Systems and are sponsored by NASA's Earth Science Information Partnerships (ESIP; a federation of information sites for earth science) and by NASA's TRMM Science Team. SSM/I data and images are pro- duced by Remote Sensing Systems and are sponsored FIG. 8. Ekman transport across the Luzon Strait into the South by the NASA Path®nder Program for early Earth Ob- China Sea computed between 1992 and 2001 from monthly average serving System (EOS) products. This work was sup- SSM/I satellite winds (solid line, right scale), and number of drifters per month entering the South China Sea from Kuroshio (histogram, ported at the Scripps Institution of Oceanography by left scale). Two yearly cycles are repeated. NOAA Grant NOAA-NA-17R1231.

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