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江淮入梅前后大气环流的演变特征和西太平洋副高北跳西伸的可能机制
THE FEATURES OF ATMOSPHERIC CIRCULATION DURING MEIYU ONSET AND POSSIBLE MEHANISMS FOR WESTWARD EXTENSION (NORTHWARD SHIFT) OF PACIFIC SUBTROPICAL HIGH
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摘要: 该文首先采用合成分析的方法研究了江淮入梅前后大尺度大气环流的演变特征和西太平洋副热带高压西伸北跳的可能机制。研究结果表明, 江淮入梅前期的最显著的特征是:副热带高压首先在太平洋中部增强北跳, 而后向西扩展导致太平洋副高西部脊 (120°E) 的增强北跳。进一步分析表明, 在太平洋中部副热带高压的增强北跳和西伸之前, 副热带高压南侧ITCZ中对流和孟加拉湾北部的对流活动明显并且都经历了一次增强活跃过程, 这意味着热带ITCZ和孟加拉湾北部对流的异常活跃可能对副热带高压的增强北跳西伸产生影响。全球大气环流模式模拟结果表明, 赤道中太平洋ITCZ中对流异常活跃不仅可导致副热带高压的增强北移, 而且还可导致副热带高压西伸, 与诊断分析结果相一致。Abstract: Through composite analysis, the features of the atmospheric circulation during Meiyu onset and possible mechanisms responsible for the westward extension and northward shift of the western Pacific Subtropical High are investigated. Results indicate that the most pronounced characteristics of the circulation evolution prior to the Meiyu onset are: the subtropical High intensifies first, jumps northward in the middle Pacific, and then extends westward, thus resulting in the reinforcement (northward migration) of the western part of the Pacific Subtropical High ridge at 120°E. The further study reveals that the enhancement (northward movement) and westward shift of the Subtropical High over the middle Pacific are closely related to the abnormally active convection in the ITCZ to the south and over the northern Bay of Bengal. A GCM simulation is performed to study the effects of abnormal convection over the middle equatorial Pacific on the Subtropical High and results show that the abnormal convection can result in both the enhancement (northward migration) of the Subtropical High over the middle Pacific and its westward extension as well, which are in agreement with diagnostic results.
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图 2 15年 (1979~1993年) 500 hPa高度场以江淮入梅日为0天的逐日合成场沿120°E的时间-剖面图
(等值线间隔为: 10 gpm阴影区为高度场值大于588 dagpm)
图 3 15年 (1979~1993年) 500 hPa高度场以江淮入梅日为0天的逐日合成场沿150°E~180°的时间-剖面图
(等值线间隔为: 10 gpm; 阴影区为高度场值大于590 dagpm)
图 4 1979~1993年15年的OLR以江淮入梅日作为0天的前后逐日合成场沿150°E~180°的时间-纬度剖面图
(等值线间隔为10 W/m2; 阴影区为OLR小于240 W/m2)
图 5 1979~1993年15年的OLR以江淮入梅日作为0天的逐日合成场沿17.5°~22.5°N纬度带平均的时间-经度剖面图
(等值线间隔为10 W/m2; 阴影区为OLR小于240 W/m2)
图 6 热源异常试验和对比试验500 hPa高度场之差 (ANT-CTL) 沿150°E~180的时间-纬度剖面图
(等值线间隔: 20 gpm, 阴影区为高度场差值大于0)
表 1 南京气象学院气象台1979~1993年入梅日期统计表
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