Effects of upstream surface heat fluxes on the evolution of the South China Sea summer monsoon

This study uses a regional climate model to investigate the effects of upstream surface heat fluxes on the evolution of the East Asian summer monsoon, especially the South China Sea (SCS) summer monsoon onset and its subsequent evolution. Upstream refers to the regions west of the SCS: Bay of Bengal (BOB), Indochina Peninsula (ICP) and the Indian subcontinent (IND). Five sensitivity experiments have been conducted by cutting off the different surface heat fluxes over one or more of these regions. Results show that by cutting off the heat fluxes in one or more of these upstream regions, the precipitation decreases and low-level geopotential heights increase not only locally, but also in remote regions, including South China, East China Sea, southern Japan, and western North Pacific. The strength of the south Asia high in the mid-upper troposphere also decreases. The westerly (easterly) winds at 850 hPa (200 hPa) are generally weaken in May and June, especially before and during the onset of the SCS summer monsoon in late May and onset of the Indian summer monsoon in mid June. The impact of cutting off sensible heat flux over ICP is comparable to that of cutting off the latent heat flux over BOB. The least impact occurs when cutting off the sensible heat flux over IND. Removing the heat fluxes over two or three upstream regions gives a larger effect in the precipitation decrease and circulation change, and the magnitude of the effect increases almost quasi-linearly with the number of regions in which the heat flux is cut off, but the combined effect is not simply equal to (mostly less than) the sum of the individual effects. The effects of the heat flux removals are more significant in May than in June especially in the mid-lower troposphere. This result implies that the heat fluxes in the upstream regions are more important in May during the period of the SCS monsoon onset, but less at the lower levels in June after the onset of the Indian summer monsoon, although the most prominent effects occur at the mid-higher levels in June. © Springer-Verlag 2008.