Climatological aspects and mechanism of spring persistent rains over Central China

In this study we examine the large-scale atmospheric circulation associated with the spring persistent rains (SPR) over Central China in March and April based on the climatological means and we propose a physical explanation of this rainy season. Low-level southwesterlies to the south of the middle and lower reaches of the Yangtze River (southern China) are responsible for SPR. Low-level southwesterlies are identified over southern China on the climatological mean wind field in SPR, and the appearance of the southwesterlies at the end of February is consistent with the onset of SPR. The southerlies, which are limited to southern China, the Indochina Peninsula and the South China Sea, are important for moisture transport to Central China and the moisture convergence there. Seasonal evolutions of low-level temperature, geopotential height and wind fields suggest that the low-level southerlies over southern China, the Indochina Peninsula and the South China Sea in SPR are caused by the westward pressure gradient associated with the eastward temperature gradient around the region from the Indochina Peninsula to the western North Pacific to east of the Philippines. The southerlies are the geostrophic winds associated with the westward pressure gradient. The eastward temperature/westward pressure gradients are most evident in March and April, and they are a result of the time-lag in the seasonal warming between the Indochina Peninsula and the western North Pacific to east of the Philippines. In addition, the coincidences of spatial distributions and seasonal evolutions from February through May between the low-level temperature and the surface sensible heat flux (SHF) suggest that the differential heating due to SHF between the two regions is likely responsible for the east-west thermal contrast. Much higher correlations than the 99 % significance level among the year to year fluctuations of SPR, the eastward temperature/westward pressure gradients over the region from the Indochina Peninsula to the western North Pacific to east of the Philippines are identified. The close relationship between SPR and the eastward temperature/westward pressure gradients on both the seasonal and interannual bases strongly suggests that the east-west thermal contrast in spring between the Indochina Peninsula and the western North Pacific to the east of the Philippines plays the primary role in SPR formation.