Relationship between the Tibetan Plateau surface thermal anomalies and the summer circulation over East Asia and rainfall in the Yangtze and Huaihe River areas.

The relationship between the surface thermal anomalies (STA) over the Tibetan Plateau (TP) and the summer rainfall over the Yangtze and Huaihe River (Jianghuai) areas, has been studied by many researchers. However, most previous studies focused on the temporal variation of STA,and less attention was paid to the correlation between the spatial variation of STA over TP and the Jianghuai summer rainfall.The present paper aims to explore this correlation.By use of the NCEP/NCAR Reanalysis monthly mean data, the precipitation data at 160 stations in China, and the index data of the Western Pacific subtropical high ridgeline (WPSHR) provided by the National Climate Center of China, the spatial features of the mean surface temperature over the TP and its circumjacent areas during the seasonal transition (March to June) are analyzed.By mean square deviation analyses,three areas are selected:1) the TP main area(28°-38°N，75°-100°E), 2) the area to the northeast of the TP(38°-48°N，90°-105°E), and 3) the area to the northwest of the TP(38°-45°N，75°-90°E). The relationship between the spatial variation of STA over the three areas in May and the summer circulation over East Asia and rainfall over the Jianghuai areas from 1951—2002 is further investigated. The results show that the STA over the three areas are evidently related to the 850 hPa summer circulation.A large-scale thermal contrast exists between the TP and the area to its north in May, and an index is defined to reflect the intensity of the large-scale thermal contrast.When the thermal contrast enhances (weakens), the south-north gradient of the mean air temperature in the mid-upper troposphere (500—200 hPa) enhances (weakens), too.This results in the speeding up (slowing down) of the westerly winds in the mid-upper troposphere and their shifting to further south (north).The WPSHR also shifts to further south (north) than normal. So the East Asian summer monsoon weakens (strengthens), and the rainfall increases (decreases) over Jianghuai areas.