1961-2018年西南地区夏季干旱变化特征及其与环流异常的联系

利用1961—2018年中国西南地区312站降水观测资料、NCEP/NCAR再分析资料及海表温度资料,采用夏季SPI(Standardized Precipitation Index)指数作为干旱指数,研究了西南地区夏季干旱变化特征及其与环流异常的联系。结果表明:西南地区夏季总体呈现变干趋势,尤其在云南、四川东南部干旱化趋势显著。当西南地区夏季显著干旱时,该地区对流层低层辐散、上层辐合,且向该地区的水汽输送偏少。造成西南地区干旱维持的原因可归结为大气波动活动异常和海温异常强迫。前者通过西风带扰动向下游的能量频散,为西南地区低层辐散、上层辐合的环流异常的形成和维持提供了必要的扰动能量积聚;后者通过热带西北太平洋异常热源对大气的强迫,使得该地区对流层低层(上层)形成异常辐合(辐散),在西南地区和热带西北太平洋形成了斜向垂直环流,使西南地区受下沉气流控制,从而形成了利于降水显著偏少和干旱发生并维持的条件。

Variation characteristics of summer drought in Southwest China and its relationship with circulation anomalies from 1961 and 2018

Based on the observed precipitation data from 312 stations in Southwest China, NCEP/NCAR reanalysis data and sea surface temperature data from 1961 to 2018, the summer SPI (Standardized Precipitation Index) index was used to study the variation characteristics of summer drought in Southwest China and its relationship with circulation anomalies.Results indicate that Southwest China has a drying trend in summer, especially in Yunnan and southeast Sichuan.In the typical dry summers, Southwest China is characterized by the lower troposphere divergence and upper troposphere convergence, and the water vapor transport to the area is less.The reason of drought maintenance in Southwest China can be attributed to the abnormal atmospheric wave activity and the abnormal sea surface temperature forcing.The former disperses energy downstream through the disturbance of the westerly zone, which provides the necessary disturbance energy accumulation for the formation and maintenance of the circulation anomalies of lower troposphere divergence and upper troposphere convergence in Southwest China.The latter forces the atmosphere through anomalous heat sources in the tropical Northwest Pacific, causing the lower (upper) troposphere in the region to form an abnormal convergence (divergence), and an oblique vertical circulation to be formed in Southwest China and the tropical Northwest Pacific, which makes Southwest China controlled by the sinking airflow, thus forming conditions conducive to the occurrence and maintenance of significantly less precipitation and drought.These results are helpful to understand the formation mechanism of drought in Southwest China and provide useful clues for prediction.