春夏转换期青藏高原南侧对流层大气经向温度梯度逆转与亚洲夏季风建立及降水变化的联系

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

利用欧洲中期数值预报中心(ECMWF)提供的1979—2014年逐日对流层大气再分析资料和美国气候预测中心(CPC)整理的同期逐日格点降水资料,分析了春夏季节转换期间青藏高原南侧对流层大气经向温度梯度逆转时间与亚洲季风建立及降水变化的关系。结果表明,从春到夏高原南侧对流层中上层(600~200 hPa)不同层次经向温度梯度逆转的时间不同,逆转时间最晚的400 hPa温度梯度逆转可作为青藏高原上空整层气柱变暖和亚洲夏季风全面建立的标志。36年的合成分析发现,随着400 hPa温度梯度的逆转,850 hPa上大于6 m·s^(-1)的强西南季风从10°N以南扩大到20°N以北并进入印度中部,东亚地区偏南气流也明显增强北进且在长江中下游形成稳定的辐合区,从而导致南亚和东亚季风区的核心地区降水急剧增加,温度梯度逆转后25天较前25天在印度中部和长江中下游地区的降水量分别增加220 mm和140 mm以上,对应印度季风突然爆发和江淮流域迅速进入梅雨期。

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 of 20°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.