川渝地区夏季干旱气候特征及成因分析

利用1979—2011年英国东英吉利大学气候研究中心（CRU）整理的地面月降水资料和日本气象厅（JMA）加工的全球高空等压面月平均气象场再分析资料，采用标准化降水指数（SPI），确定了川渝地区典型干旱年，讨论了川渝地区夏季典型干旱的气候特征及其大气环流特征。结果表明：川渝地区夏季典型干旱年具有气温高、降水少的气候特征；东亚中纬度盛行纬向气流，西风锋区偏北，冷空气多偏北东移，川渝地区及其我国北方大部分地区受大陆带状高压控制。500 hPa带状高压与100 hPa南亚高压中心位置基本一致，且500 hPa高压中心与温度场的暖中心相对应，是持续性川渝夏季干旱具有近似正压结构的典型环流特征；同时川渝地区夏季干旱年对流层低层从云贵高原到四川盆地的西南气流明显偏弱，风场距平合成表明，云贵高原到川渝地区为东北距平风，距平风场上华南地区为明显的气旋式环流；川渝地区夏季典型干旱年整层西南气流水汽输送与常年比较明显偏小。

Analysis of climatological characteristics and causes of summer droughts in Sichuan-Chongqing region

Using the monthly precipitation data from Climate Research Unit (CRU), University of East Anglia in England and the global monthly mean meteorological reanalysis data at standard isobaric levels from Japan Meteorological Agency (JMA) from 1979 to 2011, the criterion of typical drought years in Sichuan-Chongqing region was determined with the Standardized Precipitation Index (SPI), and the climatological characteristics and possible causes of summer droughts in Sichuan-Chongqing region were analyzed from the aspect of atmospheric circulation. The results show that, during years with typical summer droughts, the surface air temperature is higher and the precipitation is less than n ormal. The zonal circulation develops in mid-latitude regions of East Asia, and the westerly frontal zone and the cold air mass shift northward during summer droughts. Meanwhile, Sichuan-Chongqing region and most parts of northern China are under the control of mainland belt high pressure. The location of high pressure at 500 hPa field is nearly the same with the center of South Asian high pressure at 100 hPa field. Moreover, the high pressure center at 500 hPa corresponds with the warm center in the temperature filed. This quasi-barotropic structure is the typical circulation feature of a persistent summer drought in Sichuan-Chongqing region. At the same time, the southwesterly flow is obviously weakened from Yunnan-Guizhou Plateau to Sichuan Basin in the low troposphere. In the anomaly wind field, there exist northeasterly flows from Yunnan-Guizhou Plateau to Sichuan Basin, and there is a cyclonic circulation over southern China. The entire-layer southwesterly water vapor transport in typical summer drought years is much weaker than normal in Sichuan-Chongqing region.