An observational study on the onset and break of the Asian summer monsoon over south China
Student thesis: Master's Thesis
A study of the physical processes responsible for the onset and break (inactive period) of summer monsoon over South China (SC) is presented here. They are defined using the rainfall data over Hong Kong. Meteorological parameters provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) and blackbody temperature (Tbb) from the Geostationary Meteorological Satellite-4 (GMS-4) for the period 1985 - 1990 are examined. Daily values of each parameter for the six years are then composited each day for the period ten days before to ten days after the onset and, 5 days before to 1 day after the break respectively. Results show that the possible causes of the onset of summer monsoon over SC include: 1) an establishment of a north-south temperature gradient between SC and northern Australia; 2) a development of a surface low over Beibu Wan and; 3) an enhancement of the southerly flow by the cross-equatorial flows. These lead to the increase in low-level convergence over SC. A monsoon trough is then established along the SC coast and convection activity increases, is indicated by the Tbb data. This represents the onset of the summer monsoon over South China. It is found that several days before the break, changes opposite to those occurred during the onset and active periods. This suggests that a feedback mechanism is present which tends to restore the atmosphere to a more stable state. The convective clouds formed during the onset and maintenance periods reduces the solar radiation reaching the ground, together with the cooling of the atmosphere due to large amounts of rainfall, lead to a gradual drop in the temperature. When this happens, the pressure over the SC region becomes higher which in turn induces the intrusion of the subtropical ridge. Also, the decrease in temperature over SC may shift the average position of the heat sources to the west, which leads to a westward shift of the convergence of the southerlies and results in less moisture-laden air reaching the SC region. The atmosphere then becomes less favourable for heavy convection. Thus, an inactive period which lasts for several days to weeks occurs. The physical processes which lead to the onset and break of the summer monsoon over SC are also compared. It is found that most of the onset characteristics are "reversed during the period of break. However, the existence of some atmospheric features of the onset during the break period suggested that the atmosphere is just under an unstable state that it tends to return to the state which favours the onset / maintenance of the summer monsoon.
- Monsoons, China