CLIMATOLOGICAL ASPECTS AND MECHANISM OF SPRING PERSISTENT RAINS OVER CENTRAL CHINA

In this study we examine the large-scale atmospheric circulation assoc iated with the spring persistent rains (SPR) over Central China in Mar ch and April based on the climatological means and we propose a physic al explanation of this rainy season. Low-level southwesterlies to the south of the middle and lower reaches of the Yangtze River (southern C hina) are responsible for SPR. Low-level southwesterlies are identifie d over southern China on the climatological mean wind field in SPR, an d the appearance of the southwesterlies at the end of February is cons istent with the onset of SPR. The southerlies, which are limited to so uthern China, the Indochina Peninsula and the South China Sea, are imp ortant for moisture transport to Central China and the moisture conver gence there. Seasonal evolutions of low-level temperature, geopotentia l height and wind fields suggest that the low-level southerlies over s outhern China, the Indochina Peninsula and the South China Sea in SPR are caused by the westward pressure gradient associated with the eastw ard temperature gradient around the region from the Indochina Peninsul a to the western North Pacific to east of the Philippines. The souther lies are the geostrophic winds associated with the westward pressure g radient. 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 N orth Pacific to east of the Philippines. In addition, the coincidences of spatial distributions and seasonal evolutions from February throug h May between the low-level temperature and the surface sensible heat flux (SHF) suggest that the differential heating due to SHF between th e 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 pr essure gradients over the region from the Indochina Peninsula to the w estern North Pacific to east of the Philippines are identified. The cl ose relationship between SPR and the eastward temperature/westward pre ssure gradients on both the seasonal and interannual bases strongly su ggests that the east-west thermal contrast in spring between the Indoc hina Peninsula and the western North Pacific to the east of the Philip pines plays the primary role in SPR formation.