Seasonal variation of deep vertical velocity in the South China Sea

Abstract

On the basis of the Simple Ocean Data Assimilation (SODA) and the Ocean General Circulation Model for the Earth Simulator (OFES), it is surprised that a distinct seasonal variation is found in the deep vertical velocity fields of the South China Sea (SCS). In summer, an upwelling band extending from the northeast to the southwest across the SCS deep basin is mainly located in the interior region of the basin, surrounded by the downwelling areas situated in the marginal region of the basin. In winter, the vertical motion is almost completely reversed, which is also enhanced in its strength and transport. A more extended downwelling band in winter replaces the upwelling band in summer, and the upwelling occurs in the marginal region. The upwelling or downwelling in the interior region of the basin is compensated by the reverse flow near the marginal basin in summer or winter, maintaining the balance of the whole deep basin without water exchange. The seasonal structures of potential density and potential temperature are consistent with the seasonal variation of the SCS deep vertical velocity. In the interior region, their isolines uplift in summer and drop in winter. The seasonal variation of the SCS deep vertical velocity in the interior region also has a good correspondence with that of basin-scale horizontal circulation, the seasonal pattern of which is cyclonic in summer and anticyclonic in winter. Just as the vertical transports always maintain a balanced, the vorticity averaged over the entire basin always remains positive, and the values in winter are one order of magnitude smaller than that in summer. This correspondence can be explained by the Sverdrup relation. The mechanism of the seasonal variation of the SCS deep vertical velocity may be related to the seasonal deepwater overflow through the Luzon Strait.