Reversal of tropospheric meridional temperature gradient south of the Tibetan Plateau during spring-summer transition and its relation to Asian summer monsoon onset and precipitation change

Using 1979—2014 daily tropospheric reanalysis data provided by the European Centre for Medium-Range Weather Forecasts(ECMWF) and gridded daily precipitation data prepared by the US Climate Prediction Center(CPC),we analyzed the reversal of tropospheric meridional temperature gradient south of the Tibetan Plateau during spring-summer transition and its relation to Asian summer monsoon onset and precipitation change.The results show that there are different reversal times of meridional temperature gradient at different levels in the middle-upper troposphere(600~200 hPa) south of the plateau from spring to summer.As the latest one in all levels,the reversal of 400 hPa temperature gradient marks the warming of the whole column over the Tibetan Plateau and establishment of Asian summer monsoon.Composite analyses of 36 years have found that the Asian atmospheric circulation and precipitation undergo abrupt changes with the reversal of 400 hPa temperature gradient.The strong southwesterly flows greater than 6 m·s~(-1) expand at 850 hPa from the south of 10°N to the north of20°N and enter central India.The southerly flows also increase significantly and move northward over East Asia,forming a stable convergence region in the middle and lower reaches of the Yangtze River.This circulation pattern results in dramatic increase in rainfall in the core areas of South and East Asia monsoons.The rainfall for 25 days after the reversal of 400 hPa temperature gradient is more than 220 mm in central India and 140 mm in the middle and lower reaches of the Yangtze River compared with that for 25 days before the reversal,corresponding to a sudden outbreak of the Indian monsoon and rapid occurrence of the plum rain season over Jianghuai Basin.