An observational study on the onset of the summer monsoon over South China around Hong Kong

Abstract

This paper presents a study of the physical processes responsible for the summer monsoon onset over South China, which is defined using the rainfall data over Hong Kong. Four sets of meteorological parameters are examined: 1) mean-sea-level pressure; 2) surface and upper-level winds; 3) surface and the 850 hPa temperatures; and 4) blackbody temperature (Tbb). The first three sets are from the gridded analyses of the European Centre for Medium-Range Weather Forecasts (ECMWF) for the period 1985-1990 while the Tbb data are from the Geostationary Meteorological Satellite-4 (GMS-4). Daily values of each parameter for the six years are then composited each day for the period ten days before to after the onset day. The results of the composite study suggest the following sequence of events. Ten days before the onset, a north-south temperature gradient between South China (SC) and northern Australia is first established as the temperature over the latter decreases. This temperature gradient is further enhanced as time goes on when the temperatures over SC and South China Sea (SCS) increase. Development of a surface low over Beibu Wan (around 107°E 18°N) is also observed four days before onset. This surface low deepens throughout the onset period. The enhanced temperature gradient and the deepening of the low apparently lead to an eastward shift of the cross-equatorial flow originally present around 60°-80°E. Together with another branch of this flow within 90° to 110°E, the low-level southerly winds over the SCS are therefore enhanced. This results in an increase in low-level convergence in the region of 100°-120°E, 15°-25°N which leads to enhanced rising motion. This is indicated by the enlargement of the 200-hPa anticyclone on Day(0). The northerlies on the eastern flank of this anticyclone then form a prominent return flow towards the equator. A monsoon trough is thus established along the SC coast and convection activity increases, as indicated by the Tbb data. This represents the onset of the summer monsoon over South China.