春夏季青藏高原积雪对中国夏末秋初降水的影响及其可能机制

Impacts of Late Spring Tibetan Plateau Snow Cover on Early Autumn Precipitation

本文首先利用最大协方差分析方法，探讨青藏高原积雪与中国降水之间的联系，发现中国夏末秋初（8-10月，简称ASO）降水与前期及同期高原积雪有着显著联系，当春夏季青藏高原西部多雪时，其后ASO中国长江及其以南地区多雨，而东部沿海的狭长区域少雨。进一步引入最大响应估计等方法，研究中国区域降水对高原积雪异常的响应及其可能的物理机制，结果表明，冬春季高原多雪异常可持续到夏季，并通过改变地表热力状况，导致ASO南亚高压减弱，同时在高、低空激发出两支波列：高层200hPa波列沿中高纬西风急流传播，自高原经蒙古到达日本呈现明显的“负-正-负”位势高度异常传播，日本上空为气旋性异常环流；低层850hPa波列起于高原，经孟加拉湾至中国南海，沿着西南气流传播，导致台湾附近的反气旋性异常环流，其西侧的偏南气流，将南海丰富的水汽输送至中国南部湖南、广西；而高层中心位于日本的气旋性异常环流西侧的偏北气流利于北方天气尺度扰动向南移动，它们为长江中下游及其以南地区多雨提供了有利条件。进一步计算定常波波数也表明，高层西风急流与低层西南季风气流作为波导，有利于高原上空的扰动沿着高、低空2支通道向东传播。由于东部沿海浙江、福建为正位势高度异常区，低层反气旋性异常环流则抑制了该区域的降水。

Maximum covariance analysis is utilized to study the impacts of Tibetan Plateau （TP） snow cover on precipitation in China. The results show a significant correlation between early autumn （ASO） Chinese precipitation andthe preceding TP snow cover. A strong pattern with lead positive mode snow cover anomalies over the western TP signifies enhanced ASO rainfall over Yangtze River basin and southern China and reduced rainfall over the southeast coastline of the country. Maximum response estimation and diagnostic analyses are also utilized to investigate the impacts of TP snow cover on Chinese precipitation and its possible mechanism. The results indicate that positive early spring snow cover anomalies in TP can persist until summer and can modify the surface thermal conditions, which results in a weak South Asian high with the disturbance propagating downstream along the westerly jet through Mongolia to Japan. An additional wave train propagates along the lower southwesterly airflow through the Bay of Bengal to the South China Sea, which leads to low-level anti-cyclonic circulation over Taiwan. The southerly flow to the west of the anti-cyclonic anomalous circulation strengthens abundant moisture transport from the South China Sea to southern China. Moreover, the northerly flow to the northwest of the upper-level cyclonic anomalous circulation over Japan contributes to southward movement of synoptic disturbances from the north. Both conditions lead to increased rainfall in central and southern China. The calculated stationary wave number reveals that as waveguides, the high-level westerly jet and the low-level southwesterly monsoon flow can contribute to the eastward propagation of disturbances over the TP. The positive anomalies of geopotential height over the eastern coastline accompanied by the low-level anti-cyclonic anomalous circulation suppresses the rainfall along southeast coast.