青藏高原地区5月热力差异和后期夏季北疆降水的联系

基于美国国家环境预测中心/美国国家大气研究中心(NCEP/NCAR)再分析月平均资料和新疆70站降水资料,分析了1961~2010年5月青藏高原地区地表热力异常差异和新疆夏季降水的联系。奇异值分解(SVD)分析发现,当5月青藏高原视热源偏弱,高原西北部地区偏强时,北疆夏季降水偏多。定义了一个热力差异指数来表征两个区域热力异常的对比程度,发现考虑这种大尺度热力差异对比要比单一地区与区域气候有更为密切的联系。当热力差异指数为负时,即5月青藏高原视热源偏弱,其西北部视热源偏强时:(1)西亚副热带西风急流位置偏南;(2)500 h Pa中亚上空和贝加尔湖上空分别对应异常气旋和反气旋环流,在二者共同作用下,北疆上空盛行异常的偏南气流,有利于低纬度的暖湿气流北上,形成有利于降水的环流形势;(3)印度半岛上空为异常反气旋环流,中亚上空为异常气旋环流,形成北疆夏季降水水汽的两步型输送,阿拉伯海水汽被输送至中亚和新疆地区。偏相关分析发现,青藏高原热力异常主要影响对流层中高层大气环流和水汽输送的第二步环流条件,高原西北部热力异常则影响水汽输送的第一步环流条件。

The Relation between Thermal Anomaly Contrast over the Tibetan Plateau and Its Surrounding Areas in May and Summer Rainfall in Northern Xinjiang

Based on the NCEP/NCAR reanalysis monthly mean data and the precipitation data at 70 stations in Xinjiang, the relationship between the thermal anomaly contrast over the Tibetan Plateau and its surrounding areas in May and summer rainfall in Xinjiang were analyzed. By using the singular value decomposition (SVD) method, it was found that when the thermal anomalies were negative over the Tibetan Plateau and positive over the northwestern portion of the Tibetan Plateau, more summer rainfall occurred in northern Xinjiang. An index was defined to reflect the intensity of the large-scale thermal contrast between the two regions, and it was found that the thermal contrast between the two regions had a close relation with summer rainfall in northern Xinjiang. This relation is closer than that between thermal anomalies in any single region and summer rainfall in northern Xinjiang. When the index is negative, it indicates that the apparent heat source is weaker over the Tibetan Plateau and stronger over northwestern portion of the Tibetan Plateau. Under such a thermal pattern, (1) the subtropical westerly jet shifted further south; (2) an anomalous cyclone was located over central Asia and an abnormal anticyclone developed over Baikal Lake at 500 hPa, respectively. As a result, anomalous southerly winds prevailed over northern Xinjiang, which were favorable for the transport of warm and moist air from low latitude regions; (3) combined with the anomalous anticyclone over the Indian Peninsula and the anomalous cyclone over central Asia, which formed the two-step transport of water vapor, the water vapor from the Arabian Sea could be transported to central Asia and northern Xinjiang. Partial correlation analysis indicated that thermal anomalies over the Tibetan Plateau largely affect circulations in the middle and high troposphere and the second step of water vapor transport, while the thermal anomalies over northwestern Plateau mainly influence the first step of water vapor transport.