The setup and decadal variability of thermocline depth in the South China Sea

The thermocline depth (TD) in the South China Sea (SCS) is much shallower than that in the adjacent western North Pacific. The relative shallow TD in the SCS has a great impact on ocean dynamical processes, biogeochemical processes as well as tropical cyclone intensification. The reasons accounting for the difference in the TD, however, has not been fully addressed. Using a reduced gravity model, we calculated the upper layer thickness in the North Pacific and the SCS. In general, the pattern estimated from this simple reduced gravity model agrees well with the TD estimated from the WOA13 data. It is found that the large-scale wind pattern over the North Pacific as well as the strong Kuroshio currents set up the TD along the eastern boundary of the SCS, which can explain about 80 m shoaling of the TD. The wind over the SCS can further reduce the TD about 20 m from the eastern SCS to the western SCS. Compared to the meridional wind, the zonal wind is more important in the setup of the distribution of TD inside the SCS. Furthermore, the wind stress curl over the Pacific can affect the decadal variability of TD along the eastern boundary of SCS from oceanic processes, and subsequently affect the TD in the SCS. Since the decadal variability of WSC is related to PDO, the study also gave a new perspective on the impact of PDO on the SCS.