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RESEARCH INTO THE WESTERN BOUNDARY

CURRENTS OF THE PACIFIC OCEAN IN

CHINA (1998-2002)

WANG Fan

Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China

The western boundary currents in the Pacific Ocean were one of topics studied by Chinese oceanographers during the period of 1998 – 2002, with emphasis on the Kuroshio in the East China Sea and to the east of Taiwan and Ryukyu Islands, the Kuroshio origin, the Mindanao Current, the undercurrents beneath the western boundary currents, etc.

The western boundary currents in the tropical Pacific and their seasonal variations were examined with assimilation productions (Wang et al., 2001, 2002; Yu, 2000, 2002). The transports of the Mindanao Current (MC), the Kuroshio origin, the North Equatorial Current (NEC) and the North Equatorial Countercurrent (NECC), which estimated by using the assimilation data, display the similar seasonal cycles ,roughly speaking, they are stronger in spring and weaker in autumn with a little phase difference (Wang et al., 2002). The results also reveal that the Indonesian throughflow (ITF) is stronger in summer during the southeast monsoon period, which is different from those of its upstreams, the MC and the New Guinea Coastal Current (NGCC). It is noteworthy that the estimations by another assimilation output show different seasonal cycles of the MC in the opposite phase to that of the Kuroshio and NEC (Yu et al., 2000, 2002). The interannual variations of the Kuroshio, NEC, NECC and MC related to the ENSO events were also discussed, which revealed larger transports of the NEC, the Kuroshio and NECC in the E1 Niño years (Wang et al., 2001; Yu, 2000). It is found that the MC was stronger during the E1 Niño years in the 1980s (Yu, 2000) while weaker during the E1 Niño years in the 1990s (Wang et al., 2001). There were also some quasi-biennial signals superimposing the ENSO-like oscillations in the flows (Wang et al., 2001; Qu et al., 1998).

Based on the repeated hydrographic section data near the Philippine coast observed through 14 cruises from 1986 through 1991, a striking subsurface undercurrent system including the Mindanao Undercurrent (MUC), the Luzon Undercurrent (LUC) and the North Equatorial Undercurrent (NEUC) was found below and opposite the MC, the Kuroshio and the NEC, respectively (Wang et al., 1998; Qu et al., 1998). These undercurrents , which have the multiyear mean transports of about 5.9´106 m3/s, 4´106m3/s and 8´106m3/s (relative to 1500 dbar), respectively, are found to be of a feature of both the synoptic and the multiyear mean velocity fields. The MUC with maximum velocity >10cm/s and more than one core appears in and below the westward deepening thermocline off the Mindanao coast (Wang and Hu, 1998a). The synoptic volume transports of the MUC during 11 cruises varied from 6.2 to 28.4´106m3s-1 (average of 14.4´106m3s-1), while the multiyear mean MUC transport is 5.9´106m3s-1. The difference between the multiyear mean and synoptic transports comes from the MUC spatial and temporal variations. A part of the water carried by the MUC and NEUC belongs to two water masses with their salinity extremes of 34.6 psu at 26.9 st and 34.52 psu at 27.2 st, which are associated with the Antarctic Intermediate Water (AAIW) and the lower part of the Southern Pacific Subtropical Water (SPSW), respectively (Wang and Hu, 1998b; Wang et al., 1998). The water masses cross the equator via the New Guinea Coastal Undercurrent, indicate that the existence of the MUC is not a local transient phenomenon. The AAIW and the lower part of the SPSW spread to the east crossing 130°E via the NEUC at latitudes south of 10°N.

To explore the dynamics of the western boundary undercurrents such as the MUC, the criterion of the geostrophic velocity reversal in and below the thermocline was derived by a simple conceptual model of the stratified ocean (Wang and Hu, 1999). The criterion,  and , suggests that the velocity below the thermocline can be opposite to the surface velocity only if the slopes of the thermocline ( ) and the sea surface ( ) are opposite to each other and  is strong enough to satisfy the latter inequality. Attributed to both the wind-driven basin-scale circulation and the local geostrophic balance, the criterion is satisfied off the Mindanao coast so that the MUC forms. An improved barotropic model of wind-driven circulation was developed to examine the circulation in the North Pacific Ocean, including the Kuroshio (Zhang and Qu, 1999). A modified solution of the linear Munk boundary-layer theory proposed that spatially varying eddy viscosity affects the construction of the western boundary current (Lu and Wu, 1999).

The Kuroshio east of Taiwan was examined by the improved inverse and diagnostic calculations based on the survey data (Yuan et al., 1998a, b, 2000a, b). The calculations showed that transport of the Kuroshio across 21°30'N was 57.8 ´ 106 m3s-1 in Oct., 1995 and 44.6 ´ 106 m3s-1 in May, 1996, while 37.6 ´ 106 m3s-1 and 27.6 ´ 106 m3s-1 in July and Dec., 1997, respectively, suggesting that the Kuroshio be weaker in the 1997 E1 Niño year. A part of the Kuroshio enters the East China Sea and the rest flows to the east of Ryukyu Islands as its branch (Yuan et al., 1998a). The branch is not found from the observed data in July and Dec., 1997 (Yuan et al., 1998b, 2000b). A southward flowing countercurrent induced by topography was to the east of the Kuroshio and reported below (Yuan et al., 2000a, b).

The Kuroshio in the East China Sea displays significant seasonal variation, that is, its multi-year mean transport between 1985 and 1998 is the biggest in summer and the smallest in autumn with an annual mean transport of 27 ´ 106 m3s-1 across the PN section (Yuan et al., 1998a, 2000a, b; Liu and Yuan, 1998, 1999a, b). The results also revealed abnormal seasonal variation signal of the Kuroshio in the East China Sea in 1995 and 1998, which showed it's the biggest in spring and the smallest in summer (Liu et al., 1999b; Yuan et al., 2001). The Kuroshio in the East China Sea is reported to have multi-core structure (Yuan et al., 1998a, 2000a, b; Liu and Yuan, 1998, 1999a, b). A numerical experiment by the POM model suggested that the growth of frontal meander of the Kuroshio in the East China Sea be restrained by topography (Luo et al., 2001).

The seasonal variation of Ryukyu Current, a steady northeastward flowing western boundary current east of Ryukyu Islands, was not so obvious in 1987-1997, with larger transport in autumn and summer (Liu et al., 1998, 2000a, b). The Ryukyu Current was found to have 3 sources: the anti-cyclonic recirculation southeast of Okinawa Island, the westward flowing current across 130°E at mid-latitude and the branch of the Kuroshio east of Taiwan (Yuan et al., 1998a; Bu et al., 2000). A southwestward flowing countercurrent below and to the east of the Ryukyu Current with interannual variation has been reported (Liu et al., 1998, 2000a, b). The Kuroshio and its countercurrent across 137°E and their seasonal variability were discussed on the basis of geostrophic calculation by using the hydrographic data between 1967 and 1995 (Sun, 1999).

Based on monthly precipitation data at 18 stations in the northern China during 1951-1996,a statistical analysis shows that there is a close relationship between precipitation in the rainy season in the northern China and the wintertime heat transport of the Kuroshio in the same year (Zhang and Weng, 1999).

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