123 The Western Boundary Current in the Far-Western Pacific Ocean Dunxin HU and Maochang CUI Institute ofOceanology, Academia Sinica 7 Nanhai Road, Qingdao, People's Republic ofChina 1. Introduction The Western Boundary Current (WBC) is well known, not only because of its being one of the strongest currents in the world ocean, but also its extremely important role in the global transport and redistribution of heat. The WBC in the North Atlantic (the Gulf Stream) is quite well understood by long period of various investigations. However, the WBC in the North Pacific (PWBC), especially east of the Philippines is relatively poorly understood, except the Kuroshio south of Japan, although much work has been done during CSK (Cooperative Study of the Kuroshio, 1965-1977). The PWBC is different from the WBC in the North Atlantic, where only exists a northward current in the Gulf Stream. Here, PWBC consists of two currents east of the Philippines : the Kuroshio a northward current, and the Mindanao Current towards the south. The latter is poorly understood. From 1986 through 1988 about 70 ern stations (Fig. 1) were occupied by RN Science 1 of Academia Sinica Institute of Oceanology in each October. In the present paper some features of the PWBC are extracted from the ern data by inversion manipulation. 2. Method Since all the ern casts did not reach the bottom, instead, just 1500 m, Wunsch's inverse method (1978) should be modified for calculation. In consideration of the fact that the velocity field should be of potential with f-plane approximation, we add one more equation IVXds=O to A b=-r where V is the velocity vector, ds the tangential element vector, c the closed cruise track (loop). A is an m x n matrix, b a column vector consisting of n components, velocities at reference level, I'a column vector of residual term, as described by Wunsch (1978). Then we have: E B· pr, !J. 'V . b, 0 JJ "'J J = for reference level, say, 1500 m, where Bj is +1 or -1 depending on whether the water flows into or out of the box, p r j the density at the reference level at the j-th station pair, !J. Xj the separation of the j-th station pair, and bj the reference velocity at the j-th station pair at reference level. As 1500 m is quite shallow and geostrophy is not quite valid in the upper layer, we divided the water column into two layers by O't =27 or 27.5, which has been tested being the optimal, for every box, in which mass is conserved. Then some features are realized as follows. 125" 130" [ 125" 130" 120" 30" 125" 130" 135" 30C i i I N ~ I~ t\ =-':"1 ~ i i K I , 25" 25" 25" .1 I i.e-. ~ • 11. • 72 • 10• .'0 11. • 20" • 20" 20" 20" • .71 • 13. • • • .51 I~ .23 • ' ••13 •••••• •• ...., ..... .10 • •H·n ·.56 2~. • •• •• • -~ • • IS" • .u IS" • IS" • • • •• D • • • ~~. .~~{; ~~·24 • . • \}~ 0 ~3' •••• ·35 \J, •)r'S1••••• 31 \J~ll~~ • .'1 • D<fJ~' 51 • 10" 10" .. 10· 10" • 10" 10" ~O~ • ~ • • 6'0' • . .41 1: ..... ..... .43 e •• ~ @ 1 ' V~ 5. 5"' , 50.e.. 5" 120" . , S" - 125" 130" 125" 130" [ 120" . 125" 130" 135" 140" [ FIG.I. Cruise tracks in October 1986 (left), October 1987 (middle), and October 1988 (right). 125 3. Some Features of the PWBC 3.1. The Kuroshio at its origin The Kuroshio is defined differently by different oceanographers (Kishindo, 1931; Sverdrup et al., 1942; Wyrtki, 1961; Nitani, 1972). Nitani (1972) stated that the current from just north of the place where the North Equatorial Current is separated into two branches offshore east of the Philippines to the east of Japan where the current veers away from land can be called the Kuroshio in a broad sense. At least, the beginning of the Kuroshio is in the region offthe east coast ofLuzon. In order to detect the Kuroshio three sections east of northern Luzon and Taiwan were designed (Fig. 1). However, the latitude of the section east of the north end of Luzon for 1986 is half degree different (more) than 1987 and 1988. A summary can be made about the Kuroshio as follows from the inversion-calculated results. a) Volume transport ofthe Kuroshio in its originarea October 1986. The strength of the Kuroshio can be examined by westward volume transport through the meridional section east of Luzon Strait (approximately 122°20'E) and northward through the zonal sections (18°30'N and 22°1O'N). It is seen from Fig. 2 that the main flows are concentrated in the upper 600m layer. A very narrow northward flow between st. 7 and 9 is more than 19.106 m3.s·1 in transport (Fig.2a). There are two westward flows between st. 9 and 11 (east of Taiwan; with 11.106 m3.s·1 in transport and between st. 12 and 14 (north of Luzon) with 19.10 m3.s·1 (Fig. 2b). A narrow eastward flow of 3.106m3.s·1 in transport gets through the meridional section (Fig. 2b). So if we use the northwestward flow from st. 7 and 14 to indicate the Kuroshio, the transport is about 46.106 m3.s·1 without the leakage in between st. 14 and Luzon coast (they are about 46 km apart. The velocity near st, 14 is more than 30 cm.s'. Suppose the average velocity in the upper 600 m is 18 cm.s! southeast of st. 14, then the transport leaked is about 6.106m3.s·l. Therefore, the volume transport of the Kuroshio east of Luzon and Taiwan is about 52.106 m3.s·1 which is about 1.5 times of the normal transport (Hu, 1989). This can be circumstantially verified by the O't =26 topography (Fig.5a). October 1987. Through 122°40'E section (Fig. 3a) gets a westward flow between st. 18 and 21 with a volume transport of 18.1()6 m3.s·l , and eastward flow with 18.7.106m3.s·1 in transport including a narrow eastward flow between st. 17 and 18 with 1.5 X 106m3.s'l • Through 18°N section (Fij!;. 3b) take place a northward flow between st. 17 and 14 (transport is about 15.106 m3.s·r), and two bands of southward flow offshore with transport of 19.106 m3.s· between st. 14 and 13 and 5.106 m3.s·1 between st. 9 and 8 in the upper layer. In consideration of the fact that st. 17 is about 50 km away from land, northwestward leakage between st.17 and Luzon might be about 5.106 m3.s'l • And then the transport of the Kuroshio is only about 20.106 m3.s·l , which is only half of the normal (Hu, 1989). Obviously, the Kuroshio is weaker during this time. A great deal of water getting through the section of 132°E (Fig. 3d-st. 5 to 1) turns right to the north in the place east ofst. 1. This also can be verified by the O't = 26 topography (Fig. 5b). October 1988. An obvious westward flow through the 122°40'E section (Fig. 4a) is between st. 10 and 15 mainly in the upper 600 m layer with a volume transport of about 30.106m3.s·1 and a return (eastward) flow in the upper 300 m layer exists between st. 8 and 10 with 7.106 m3.s·1 in transport. The section along 18°N (Fig. 4b) shows a strong 28.106m3.s·1 in transport and wide (about 300 km in width) northward flow, in the upper 600 m layer between st. 15 and 18 and a return flow, which is 300 km wide and with volume transport of 10.106 m3.s·1 in the upper 500 m layer in between st. 18 and 21. This year the volume transport of the Kuroshio here is about 33.106 m3.s·1 with the same estimate as above since the distance between st. 15 and Luzon is about 33 km. This tendency can be verified from figure 5c. 122 'er 123 'IT 12C' IT 125 ' 57 I' . ~ • I 131 ITE 122; 1/"123' 28"124 " 13'124" er 125" 46' IZ8' 31' 127:.. 129"18' ,123''!IT' 124 ~ !IT 125 "!IT ' 127:5TE IS'IT 19'25'" l!I'3!' 21 '!IT 22'llrw 14 15 16 17 18 19 20 21 23 987654 ·3 1 14 13 12 11 9 i 4 i i 100 ~II \~~\ 100 100 \JII 2( 'r, 'l' 300 300 500 500 5 ::It"" o .., 700 ;: 700 ::It..,"" 700 2:..,"" ~ - o ~ e :5 900 5 900 [ -0\N ~ 1100 1100 1100 o 1300 1300 1300 1500 .0I, !, " ',,, I , , ! I 1500' .• , , 800 <KM) r 200 400 600 o 200 400 <KM) 200 400<KM) FIG,2, Velocity distribution in October 1986along 22·10'N (left), In·20'E (middle), and IS·30'N (right), Positive (negative) flow into (out ot) the paper, 127 13I'W 132'we , 131'·W • 1 ~ 51)0 .'":z: " 100 1/00 liDO 1311U 1300 200 4011 lUll (KM> a. alollg 122. f1 . E b. along,,· N 19'42' 21'31' 'W'W m:~ ~.~. 44 J I IIIU JOU SOO ~ 700 ~ 700 ...t '00 Cl 990 1I0U IIUU . IJUO . IJUO 15UO L 0 ZUU 4UU 'UU IUU IUUU UUU ,I·\UU 1500 .1. ~1 ..nD 132' E 0 ZUO 4UO (KM) e • n.ln"!J , . JO'N FIG.3. Velocity disbibution in October 1987.