Shelfbreak circulation and thermohaline structure in the northern south China Sea-contrasting spring conditions in 2000 and 2001

Two intensive, high-resolution hydrographic surveys during April 2000 and May 2001 are used to characterize the thermohaline and current structure at the shelfbreak in the South China Sea. In 2000, a strong anticyclonic circulation was present in the northern portion of the South China Sea with strong onshore currents east of Dongsha Island. The flow became polarized along isobaths as it encountered shallow water, with northeastward flows of over 0.9 m/s along steep topography. The flow was driven by strong density contrasts between waters of the outer shelf and upper slope. Shelf water was both cooler and more fresh than the water offshore, which had salinities close to that of Kuroshio water. In contrast, the mean flow in the northern South China Sea was predominantly cyclonic in 2001. Flow over the slope was to the southwest at up to 0.2 m/s. The water mass properties of the outer shelf and upper slope were similar, so that there were not the strong cross-shelf density gradients present as in 2000. A potential difference between the water mass structure of the two years was the difference in cooling during the preceding winters. In December, 1999, unusually strong cooling may have resulted in cooler shelf waters relative to the following year. The ASIAEX study area may be a particularly sensitive region to both seasonal and interannual variability, as it is near a bifurcation point associated with the Kuroshio Intrusion into the South China Sea.