Journal of Oceanography Vol. 49, pp. 581 to 592. 1993

Observations of the Countercurrent on the Inshore Side of the Kuroshio Northeast of Taiwan WEN-SSN CHUANGl, HSIEN-WEN LI 2, T. Y. TANGI and CHONG-KUNG Wu 3

l lnstitute of Oceanography, National Taiwan University, Taipei, Taiwan, ROC 2Department of Oceanography, National Taiwan Ocean University, Keelung, Taiwan, ROC JDepartment of Marine Resources, National Sun Yat-Sen University, Kaoshiung, Taiwan, ROC (Received 24 December 1992; in revised form 2 April 1993; accepted 20 April 1993)

Intensive current measurements in the area northeast of Taiwan indicate subsurface, southwestward flow existed between the inshore edge of the Kuroshio and the East China Sea continental slope. At 70 km away from Taiwan, this countercurrent has a mean speed about 30 cm s-~ at mid-depth. Closer to Taiwan, the flow turns along with the topography, and subjects to sidewall and bottom friction. Both the magnitude and the vertical shear of this countercurrent are comparable with that inferred from hydrographic survey. The wind field features short-period (a few days) fluctuations associated with the cold front passages, however, this is not reflected on the current records. It appears that the countercurrent is fairly steady. Together with similar reversing flow found at places much further to the north, the overall pattern seems to be a general quasi-steady feature along most part of the shelf edge of the East China Sea. 1. Introduction The Kuroshio current, originating from east of Philippine Islands, flows northward along the east coast of Taiwan into the East China Sea (ECS), and thence to the Pacific through the Tokara Strait. The mean western edge of the surface Kuroshio pattern, as inferred from 1953-1984 GEK observations (Fig. 1), approximately follows the 200-m isobath in the ECS (Qiu et al., 1990). The only exception is at the northeastern area of Taiwan where a part of the surface Kuroshio water branches onto the shelf (Hsueh et al., 1992). This feature was further revealed in the top 100 meters of the moored ADCP measurements over the continental slope (Tang and Yang, 1993), and studied extensively using a barotropic inflow-outflow model (Qiu and Imasato, 1990) as well as a three-dimensional, primitive-equation model (Chao, 1990). Not only surface intrusion, hydrographic surveys in the same area also found subsurface Kuroshio water on the shelf(e.g., Chern et al., 1990). Analysis of the chemical properties (mainly nutrients and oxygen) suggested that this water is originated from about 300 m and raised to the shelf through the shelf edge upwelling (Won~ et al., 1991). The upwelling appears to be a yearround phenomenon, but reaches a maximum m mid-September, shortly after the onset of winter northeasterly monsoon wind (Liu etal., 1992). Another noteworthy feature of the Kuroshio in the ECS is first reported by Miyaji and lnoue (1983). From geostrophic computation, they found that near the latitude of 26~ the core of the Kuroshio was at the slope area but a countercurrent was over the shelf break in the winter of 1982. Later, farther to the north at latitude of 29 ~ and 30~ Miyaji and Inoue (1986) monitored the 100 and 290 m current at two sites over the slope (Station MI-A and MI-B in Fig. l, water depth: 300 m) during the winter of 1983 and found opposing mean flow between the upper and lower layers

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Longitude (*E) Fig. 1. Map of the East China Sea region. The long dashed line indicates the mean western edge of the surface Kuroshio current (from Qiu et al., 1990). The rectangular box outlined by the dashed line is the area enlarged in Fig. 2. The 3 current meter stations marked (MI-A, MI-B, E) are observations reported previously (see text).

at both sites. They concluded that even when the Kuroshio moved closer to the shelf, the countercurrent still prevailed in the waters adjacent to the shelf edge. Tang and Yang (1993) identified similar reversing flow pattern above and below 120 m in their winter time ADCP mooring records (Station E in Fig. 1, water depth: 386 m). Huseh et al. (1993), in a theoretical model, predicted that when a bottom current (upper layer motionless) collides head-on with a step topography, deflection will give rise to a countercurrent along the step. In this study, we tried to assemble the recent direct current measurements from the Kuroshio Edge Exchange Processes (KEEP) study conducted in the area northeast of Taiwan, and draw evidence to support the notion that subsurface eountercurrent westward of Kuroshio is a general feature in the ECS with little temporal variations. 2. Data Description From October 1990 to February 1992, in three separate occasions (named period I, II and Ill), we deployed several current meter moorings simultaneously in the slope region. The study area features a sharply curved shelf(roughly in the NE-SW direction) and a canyon cuts perpendicularly onto the shelf(Fig. 2). A total of six stations around the canyon had been occupied, and they were designated with letters A to F in Fig. 2. Among them, Station A is placed right over a steep slope, B is very close to the shelf, C is a long-term station (occupied in every period) near the axis of

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the canyon, D and E are at the northern flank of the canyon, and F is farther away to the north (about 70 km from Taiwan). Table 1 lists the basic information of all the moorings. The records are denoted by station name and instrument depth and grouped by periods. Most of these moorings were equipped with two Aanderaa RCM-7 current meters at about 100 m apart, and the lower one was about 50 or 100 m above the bottom. Station E in period ][IIwas an exception; it was an ADCP mooring and only the data at 150 and 250 m depth were used in this study. Generally speaking, period I covers October and November of 1990, period II March and April of 1991, and period III October and November of 1991. Wind data in the study area is acquired from the surface meteorological observations at Peng-Chia-Yu, an island near the mooring sites (Fig. 2). The progressive vector diagram of the wind starting from October 20, 1990 and lasting for eighteen months is shown in Fig. 3. Apparently, northeasterly wind dominates over most of the year. For only a short period about three months (from mid-May to mid-August), southerly wind prevails. This is considered to be

584

W.-S. Chuang et al.

Table 1. Mean flow for KEEP current meter stations. Period

III

Station-

Water depth

Starting date

C.M. depth (m)

[Duration] (days)

Speed

Direction

(m)

(cm/s)

(~

Mean flow

A-202 A-307

452

10/20/90 [41 ] 10/20/90 [41 ]

8.8 0.9

-166 -162

C-286

339

10/20/90 [46]

5.1

140

D-314 D-370

398,

10/20/90[47] 10/20/90 [46]

6.7 4.0

-130 -132

B-190 B-245

270

3/8/91 [55] 3/4/91 [49]

9.2 2.5

-99 -104

C-210 C-299

352

3/4/91 [48] 3/4/91 [58]

15.3 6.4

-172 97

D-241 D-344

396

3/4/91 [48] 3/4/91 [62]

17.2 9.9

-I 15 -172

E~267 E-362

507

3/5/91 [40] 3/5/91 [52]

21.5 4.5

- 130 -98

F-250 F-351

403

3/6/91 [60] 3/5/91 [58]

33.4 20.7

-103 -129

B- 105 B-200

300

10/5/91 [45] 10/5/91 [45]

21.5 4.3

- I0 I -! 02

C-328

378

8/29/91 [91]

8.2

78

E-150 E-250

386

9/28/91 [61] 9/28/91 [61]

19.7 15.8

-110 -143

F-275

328

10/5/91 [116]

22.9

-138

very typical in the southern part o f the ECS. Our measurements seem to be all falling into the northeasterly monsoon season, though the wind in part of the period II might be in the transition between seasons (see section on wind forcing). 3. Mean Flow We computed the vector averages for the entire length o f the individual current meter record and listed in Table 1. As all the records are relatively long (>40 days) compared with the dominant short-period fluctuations at 3-5 days in the study area (Chuang and Wu, 1991), the averages may

Observations of the Countercurrent on the Inshore Side

585 Oct.20 1990

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be regarded as the monthly or even seasonal means. Most of the mean flow was in the direction between -99 ~ and -172~ or pointing toward between west and south. This is just opposite to the nearby Kuroshio current which is known to be flowing in the northward or northeastward direction (Nitani, 1972). For the 21 measurements altogether, there were only three exceptions to this general trend of counter flow, and they all occurred at the deep measurements (50 m above the bottom) of Station C in the canyon (C-286, C-299, and C-328 in periods I, II, and III, respectively). They were directing between 78 ~ and 140 ~ T, i.e., pointing offshore along the canyon. Upper flow at C-210 of period II (about 150 m above the bottom), however, seems to be not affected by the canyon topography underneath and remained in the southward direction. The pattern of countercurrent can be simply displayed by plotting the averaged velocity vector on the map for each period (Fig. 4). For clarity, only the 400 m isobath is superimposed on the map. It is evident that the flows roughly follow the orientation of the local isobath. Some general trends can also be identified from these plots. At Stations D, E, and F (off the shelf break), the deeper currents were subjected to more "topographic steering", i.e., align with the bottom contours, and currents at intermediate depth shifted west and toward the shelf. Veering, however, was not found at Station A (located over the slope) where nearly southward flow was registered in period I, and Station B (near the shelf) with nearly westward flow in period I1 and III. The magnitudes of mean flow had considerable variations, ranging from almost nil to over 30 cm s-L It was also apparent that the mean speed consistently decreased with depth at all stations when data from two levels were available. Figure 5 shows a plot of mean flow magnitudes as function o f the distance from the bottom. Regardless of the observation period, the scattered data can be divided into three groups: Station A, B to E, and F. A rough estimation indicates vertical shear nearly 10 cm s-t for each 100 m separation. Taking mid-depth (150 m from the bottom) as a reference level, mean flow was vanishing at Station A, about 15 cm s ' at B to E, and 30 cm s-~ at F, a slow increase toward the north. It seems both sidewall and bottom

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Observations of the Countercurrent on the Inshore Side 121

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588

W.-S. Chuang et al.

friction effects are acting upon the mean flow field. However, note that at the present region Kuroshio intrusion often occurs in the upper 100 m layer (Tang and Yang, 1993), and it is not possible to extrapolate the empirical relation much above the mid-depth.

4. Wind Forcing Since the observed trend of water movements coincides with the northeasterly monsoon wind, it is of interest to examine its relationship with wind forcing. In order to minimize the canyon effects, we chose the two upstream records at Station F (F-351 and F-275 obtained in period II and III, respectively, both at the level about 50 m above the bottom) for further analysis. The wind and current data were low-pass filtered using Inverse Fourier Transform method with cut-off period set at 34 hours to eliminate tidal and higher frequency oscillations. During period II when the monsoon transition occurred, prevail wind was toward the southwest in March then reversed to northward or northeastward in the first half of April (Fig. 6). The current, however, was continually southwestward without significant changes in this transition season. During the period III, over a 4-month span, the wind was rather steady in October and November, then became a little more variable as successive winter cold fronts passed the ECS (Fig. 7). The current was always southwestward, only its magnitude reduced in November. A cross-spectral analysis of the NE-SW wind and the components of the current along the mean flow direction (which was very close to the NE-SW direction, see Table l) was also performed (figures not shown). The current autospectra were red, i.e., most of the energy were contained in the very low frequency band. The wind autospectra, on the other hand, showed a distinct peak around 0.3 ~ 0.5 cpd, which was associated with the successive winter cold front passages. The current and wind fluctuations were not coherent at those energetic wind frequency band. The short period wind pulse forcing thus may not be able to alter the flow. The current

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Fig. 6. Stick diagram of low-passed time series of wind and current (F-351) in period l I,

Observations of the Countereurrent on the Inshore Side

589

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Fig. 7. Stick diagram of low-passed time series of wind and current (F-275) in period 1II.

records, howev.er, were not long enough to fully exploit the relationship for the seasonal fluctuations.

5. Density Field A rather persistent, distinctive feature like the countercurrent should be identifiable from the density field. For comparison, a recent hydrographic (CTD) survey conducted in September 1315, 1992 is presented in Fig. 8. The transect covers the entire stretch from shelf, slope to the deep ocean, including the Station F (Fig. 2). The sharp uplifting of the > 17~ isotherms over the outer slope is clear as associated with the Kuroshio. The salinity maximum (>34.7 psu) away from the shelf break represents the core of the Kuroshio main stream (Nitani, 1972). For the water below 150 m with temperature <17~ and salinity >34.7 psu over the slope (within a range about 30 km from the shelf break), the isotherms and isohalines bend downward on the inshore side of the Kuroshio. Vertical stratification below 150 m is roughly 0.5 in tyr for depth change of 100 m. Between Station F and nearby offshore station, the degree of sloping isopycnals (descending about 30 m over 25 km distance) translates to a geostrophic velocity shear at about 10 cm s-t per 100 m vertical separation, similar to the estimation from the mooring records. Assuming the velocity vanishes at the bottom (water depth: 400 m), computed geostrophic current at 150 m is about 25 ctn s-~ toward Taiwan, which is very close to the measured current at Station F. The agreement between the mean current field and the density distribution from a single survey may not be coincidental, and a quasi-steady state seems to be the only plausible explanation. 6. Discussion Direct current measurements in the winter time confirm that between the ECS shelf water and the offshore Kuroshio stands a subsurface, southwestward, countercurrent over the slope

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Observations of the Countercurrent on the Inshore Side

591

northeast of Taiwan. Hydrographic data not only comply with the mooring measurements, but also suggest that the offshore extension of the countercurrent is bounded within about 30 km from the slope wall. Similar flow pattern has also been found in the area northwest of Okinawa Island (Miyaji and Inoue, 1986), or more than 400 km north of the study area. The mechanism for generating this countercurrent may be due to the defection of the Kuroshio blocked by the ECS shelf (Hsueh et al., 1993). Since the curvature of the current path are peculiar to the Kuroshio, a continuous drainage of Kuroshio. water along most part of the ECS shelf edge is thus envisioned. The direction of the countercurrent near the bottom is dictated by a local canyon topography and the speed is retarded through frictional effects. Above the shelf depth, an onshore component exists and may contribute to the mixture of the coast-originated water and the Kuroshio water (Chern et al., 1990). North of the canyon, the strength of countercurrent is not affected by the dominant short-period fluctuations of northeasterly monsoon wind. South of the canyon, the flow is blocked by the island of Taiwan and turning offshore. Owing to the existence of a lateral boundary, the wind takes full effect and strongly modulates the flow regime (Chuang and Wu, 1991). In conclusion, our results suggest that northeast of Taiwan, the countercurrent is a continuation of remote origin, rubs the slope, spills over the shelf edge, and then feeds back to the ocean. Its dynamical significance may be comparable with other known phenomena, for example, branching, upwelling, and frontal eddies found in this area.

Acknowledgements The authors would like to express their gratitude to the anonymous reviewers for their helpful suggestions, and the crew and technicians of R/V Ocean Researcher I for their help in the deployment and retrieval of current meter moorings during many cruises. The mooring program was part of the KEEP project supported by the National Science Council of ROC.

References Chao, S.-Y. (1990): Circulation of the East China Sea, a numerical study. J. Oceanogr. Soc. Japan, 46, 273-295. Chem, C.-S., J. Wang and D.-P. Wang (1990): The exchange of Kuroshio and East China Sea shelf water. Z Geophys. Res., 95, 16017-16023. Chuang, W.-S. and C.-K. Wu (199 l): Slope-current fluctuations northeast of Taiwan, winter 1990. J. Oceanogr. Soc. Japan, 47, 185-193. Hsueh, Y., J. Wang and C.-S. Chern (1992): The intrusion of the Kuroshio across the continental shelf northeast of Taiwan. J. Geophys. Res., 97, 14323-14330. Hsueh, Y., C.-S. Chern and J. Wang (1993): The blocking of the Kuroshio by the continental shelf northeast of Taiwan. J. Geophys. Res. (in press). Liu, K.-K., G.-C. Gong, C.-Z. Shyu, S.-C. Pai, C.-L. Wei and S.-Y. Chao (1992): Response of Kuroshio upwelling to the onset of northeast monsoon in the sea north of Taiwan: Observations and a numerical simulation. J. Geophys. Res., 97, 12511-12526. Miyaji, K. and N. Inoue (1983): Characteristics of the ~low of the Kuroshio in the vicinity of Senkaku Islands. Bull. Seikai Reg. Fish. Res. Lab., 60, 57-70 (in Japanese with English abstract). Miyaji, K. and N. Inoue (1986): Characteristics of the flow of the Kuroshio in Northwest waters offAmami Oshima in the East China Sea. Bull. Seikai Reg. Fish. Res. Lab., 63, 1-14 (in Japanese with English abstract). Nitani, H. (I 972): Beginning of the Kuroshio. p. 129-163. In Kuroshio, Its PhysicalAspects, ed. by H. Stommel and K. Yoshida, Univ. Tokyo Press, Tokyo. Qiu, B. and N. Imasato ( 1990): A numerical study on the formation of the Kuroshio countercurrent and the Kuroshio branch current in the East China Sea. Cont. ShelfRes., 10, 165-184. Qiu, B., T. Toda and N. Imasato (1990): On Kuroshio front fluctuations in the East China Sea using satellite and in situ observational data. J. Geophys. Res., 95, 18191-18204.

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Tang, T. Y. and Y. J. Yang (1993): Low frequency current variability on the shelf break northeast of Taiwan..]. Oceanogr., 49, 193-210. Wong, G. T. F., S.-C. Pai, K.-K. Liu, C.-T. Liu and C. T. A. Chen (1991): Variability of the chemical hydrography at the frontal region between the East China Sea and the Kuroshio northeast ofTa!wan. Estuarine Coastal Shelf Sci., 33, 105-120.