Kuroshio in the Luzon Strait

Three acoustic Doppler current profilers (ADCPs) were deployed in the central Luzon Strait to monitor current velocity. The profilers were deployed from 1997 to 1999, with the duration of deployment varying by location. The observed current velocities indicated that the Kuroshio flowed consistently into the South China Sea. Further information provided by composite shipboard ADCP data showed that the Kuroshio intruded into the Luzon Strait through the deepest channels (similar to 20.5 degrees N). Most of the intruded component made a loop and flowed out of the northern Luzon Strait, but a branch of the Kuroshio intruded into the South China Sea. While the current velocity obtained from mooring data did not show seasonal variation, significant intraseasonal variation ranging from several to 100 days was found. The seasonal reversal of monsoonal winds (i.e., northeasterly in winter and southwesterly in summer) did not cause noticeable variation in current velocity. The Miami Isopycnic Coordinate Ocean Model, together with monthly wind data, provided by the European Centre for Medium-Range Weather Forecasts was used to interpret the observed current velocity. Validation of the model results by the ADCP data showed that the model can explain the seasonal and interannual variations in the observed velocity fields. For example, the model reproduced the branch of the Kuroshio that consistently intruded into the South China Sea. The modeled velocity showed slight seasonal variation in the central Luzon Strait, although the large interannual signal could have hindered the seasonal timescale fluctuation. The interaction between the Kuroshio and South China Sea cyclonic flow caused variation in current velocities in both the Luzon Strait and the northern sector of the South China Sea. The model results also indicated that the Kuroshio axis bent clockwise within the Luzon Strait and flowed into and out of the South China Sea through the central and northern Luzon Strait, respectively. This pattern is similar with that shown by composite shipboard ADCP data. In summer, the Kuroshio looped west of the Luzon Strait, with a small intruding branch confined to the northwestern South China Sea. In winter, the intruding branch of the Kuroshio extended west and into the interior of the South China Sea. The net zonal transport of inflow and outflow across the Luzon Strait calculated from the model results was primarily westward, with the maximum varying from 4.8 to 6.5 Sv. Eastward transport was found occasionally in summer in some years, with a maximum from 0.3 to 2.2 Sv. While the Kuroshio intruded consistently into the South China Sea, transport out of the South China Sea was also observed. In summer, the current on the northern South China Sea shelf break contributed to the outflow. Variation in zonal transport was caused by variation in the sea surface height (SSH) west of northern Luzon caused by wind stress curl. Ekman transport, driven by monsoonal winds, and the strength of the Kuroshio off the Luzon Strait had little impact on the intrusion, which varied with fluctuation of the SSH west of northern Luzon.