The South China Sea, a cul-de-sac of North Pacific Intermediate Water

This study discusses branching of the Kuroshio Current including North Pacific Intermediate Water (NPIW) into the South China Sea (SCS). The spreading path of the subtropical salinity minimum of NPIW is southwestward pointing to the Luzon Strait between Taiwan and Luzon islands. Using a large collection of updated hydrography, results show that the SCS is a cul-de-sac for the subtropical NPIW because even the NPIW’s upper boundary neutral density surface σ N = 26.5 is completely blocked by the Palawan sill and partly blocked by the southern Mindoro Strait. In autumn, NPIW is driven out of the Luzon Strait by the preceding anticyclonic summer monsoon due to an intraseasonal variation and seasonal phase lag response to the weaker summer monsoon. Stronger inflow under winter monsoon than outflow under summer monsoon results in a net annual transport of NPIW of about 1.1 ± 0.2 Sv (1 Sv = 106 m3s−1) into the SCS. This net transport accounts for the anomaly in NPIW transport across the World Ocean Circulation Experiment section P8 (130° E). An earlier study estimated a large westward NPIW transport of about 3.9 ± 0.2 Sv, resulting in a difference of 1.2 ± 0.2 Sv from the basin-wide mean of 2.7 ± 0.2 Sv. Observations are generally in agreement with numerical results although the intraseasonal signal seems to cause a slight bias and remains to be simulated by future model experiments.