The Meridional Land-Sea Thermal Contrast And Its Relation To The Asian Summer Monsoon Onset And Precipitation Change

The Asian monsoon is a product of the thermal contrast between the Eurasia and its surrounding oceans(including the southern Indian Ocean and eastern Pacific) and its seasonal changes. Therefore, the heat anomalies of the Qinghai-Tibet Plateau and the variations of the thermal contrast between it and the southern tropical ocean plays an important role in the establishment and development of the Asian monsoon.Using 1979-2014 daily tropospheric reanalysis data provided by the European Centre for Medium-Range Forecasting and gridded daily precipitation data prepared by the US Climate Prediction Center, we analyzed the influence of TP thermal forcing on the troposphere temperature gradient and 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 principal conclusions of this analysis are as follows:(1) There are different reversal times of meridional temperature gradient at different levels in the middle-upper troposphere(600hPa-200hPa) 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.(2) Composite analyses of 36 years data and we found that the Asian atmospheric circulation and precipitation undergo abrupt changes with the reversal of 400 hPa temperature gradient. The strong south westerly flows greater than 6 m/s at 850 hPa expand from the south of 10 oN to the north of 20 oN and enters the 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 200 mm in central India and 120 mm in the middle, corresponding to a sudden outbreak of the Indian monsoon and rapid occurrence of the plum rain season over Jianghuai Basin.(3) There are correlations between the reversal of tropospheric meridional temperature gradient south of the Tibetan Plateau and the plum rain season in the Yangtze River valley. So the reversal of temperature gradient can be used as a reference indicator to forecast the change of Meiyu precipitation. When the 400 hPa TGI index in March is lower(higher) than the annual average, the 500 hPa TGI index in May is also lower(higher) than the annual average, then the 600 hPa reversal date is later(earlier) than the annual average,corresponding to it, the plum rain season date of type I is later(earlier) than the annual average.