西藏夏季降水的季节内变化特征及其影响系统

Intraseasonal Variation Characteristics of Summer Precipitation over Tibet and the Corresponding Large-Scale Circulation Anomalies

利用西藏地区1980-2013年夏季降水量资料、NCEP再分析资料等,分析了西藏地区夏季降水主模态季节内变化特征,尤其是盛夏7和8月降水异常对应的大尺度环流特征和影响系统。结果表明:西藏夏季降水存在明显的季节内变化,6和7月降水主模态的时间系数变化具有较好的持续性,而7和8月降水主模态的时间系数的相关关系明显减弱。西藏地区7和8月降水偏多年,西藏地区上游低层纬向风场均呈西风异常,但是水汽来源有差异;同时欧亚中高纬地区对流层中高层环流存在显著差异。西藏7月降水与南亚高压强度存在显著负相关关系,南亚高压偏强/弱时,降水偏少/多。西藏8月降水与南亚高压的位置关系更密切,南亚高压偏南/北,降水偏多/少。

Based on the summer precipitation data over Tibet and monthly NCEP/NCAR reanalysis data during the period from 1980 to 2013, we analyzed the intraseasonal variation of summer precipitation principal mode, especially the large scale circulation features of corresponding precipitation anomalies in July and August. The results show that obvious intraseasonal variation exists in summer rainfall over Tibet. The correlation coefficient of PC1 between June and July principal mode is significant while weak between July and August. In more rain years, the upstream zonal wind of the Tibetan Plateau is westerly anomaly in July and August but the moisture water vapours come from different regions. And the upper troposphere circulation in the mid high latitude over Eurasia in July and August is different significantly. Further research shows a strong negative correlation between precipitation and the intensity of South Asian high （SAH） in July, which means more rain/less rain in July, SAH is weaker/stronger. In August, there is a strong correlation between precipitation and the position of SAH. That is, SAH southward/northward to its normal position may lead to flood/drought over Tibet in August.