从中期天气过程看近几年长江中下游梅雨偏少的原因

长江中下游地区自2000年到2007年连续八年梅雨期降水偏少。本文从中期天气过程的角度分析了这八年不利于长江中下游梅雨的主要因子, 有东亚高空急流中准定常波动、 西太平洋暖池强对流活动和西北太平洋热带气旋活动。西太平洋副热带高压受这些因子的直接影响, 在中期时间尺度上副高环流形势发生变异, 从而造成长江中下游梅雨期的降水异常减少。在这八年的梅雨期中, 这些因子的特别异常, 更主要的由于它们的组合作用是导致近年来长江中下游梅雨偏少的直接原因。并且, 同样是长江中下游梅雨偏少, 不同因子的组合方式也影响着长江中下游梅雨偏少的降水异常分布背景。本文还初浅地讨论了在季节内预测长江中下游梅雨时对中期天气过程的参考。

Causes of Deficit Meiyu over the Middle and Lower Reaches of the Yangtze River in the Last Several Years Viewed from Medium Range Weather Process

Deficit Meiyu occurred over the middle and lower reaches of the Yangtze River during 2000－2007. In this study, the causes of deficit Meiyu are analyzed from view of the medium range weather process, which includes the propagation of stationary Rossby waves along the Asian jet, convective activities in the tropical western Pacific warm pool and tropical cyclones (TC) activities over the western North Pacific. The western Pacific subtropical high was affected directly by these medium range factors, and induced the anomalous Meiyu over the middle and lower reaches of the Yangtze River. The unusual anomalies of the medium range factors, especially their combinations were the immediate causes of deficit Meiyu during the eight years. Different combinations of these factors afforded the different distribution of precipitation anomalies, although deficit Meiyu occurred in each year during 2000－2007. A discussion on how to refer the medium range factors in predicting the Meiyu in the middle and lower reaches of the Yangtze River is also been held.