利用NCEP/NCAR再分析资料，统计了1954-2014年间夏半年（5-10月）西南低涡发生次数的年际变化，并着重分析了西南低涡异常发生年份的气候特征。结果表明，西南低涡多发年，低层流场在西南低涡关键区表现为西南风旺盛且辐合异常强，气旋性切变加大，低纬季风环流增强，导致大量正角动量输送至关键区，有利于西南低涡生成；同时印度洋输送至关键区的水汽通量增加，也有利于降水发生。而西南低涡少发年，低纬季风减弱，关键区为异常北风控制，南支绕流偏弱，水平散度场表现为辐散异常强，造成角动量输送减弱，不利于西南低涡生成；且来自于印度洋的季风水汽输送减弱，亦不利于降水发生。因此，除地形和加热作用外，西风带以及季风环流带来的水汽和角动量输送也是影响西南低涡发生的重要因子。
Based on the NCEP/NCAR reanalysis data, we analysis the interannual variability of southwest vortex frequency in the summer half years (May to October) from 1954 to 2014, and discuss the climate characteristics in southwest vortex anomaly years emphatically. The result shows that the number of the southwest vortex annual average time is 43 in the summer half year from 1954 to 2014, June is the most powerful month of the southwest vortex, and the weakest month is August. The years with more southwest vortex are 1954, 1955, 1956, 1963, 1964, 1997, 1998, 2005, 2007 and 2009. The years with less southwest vortex are 1960, 1961, 1962, 1972, 1974, 1975, 1981, 1982, 1988 and 2014. This statistical results have a good corresponding wih the history extreme weather. The southwest wind and the anomaly of convergence flow are strong in the key area of southwest vortex, and the cyclonic shear and the monsoon circulation with low latitude increase in the multiple years of southwest vortex, these characteristics are advantaged to the development of southwest vortex, with a large number of positive angular momentum transporting into the key area of southwest vortex with a fluctuant change. Meanwhile, the water vapor flux of the key area from the Indian Ocean increases, which is advantageous to precipitation. During the southwest vortex less years, the monsoon circulation of low latitude weakened, and the meridional wind present a north wind anomaly and the anomaly of divergence flow are strong in the key area, these characteristics are disadvantaged to the development of southwest vortex with a weakened transporting of positive angular momentum in the key area. The key area water vapor flux which comes from the Indian Ocean decreases, these are disadvantageous to precipitation. Thus, in addition to the terrain and the heating effect, the water vapor and the angular momentum transporting which are brought by westerlies and monsoon circulation are also the important factors for the development of southwest vortex. Finally, to be sure, as be a preliminary statistics of southwest vortex, we used the reanalysis data with 2.5°×2.5° resolution. The data with different resolution has an influence of the statistics of southwest vortex. So using the proper resolution reanalysis data is an important point for the statistics of southwest vortex in the future. And this study also shows that: in the year with more southwest vortex, the angular momentum and the water vapor flux of the key area increase and in downstream region of the key area, the angular momentum and the water vapor flux decrease. In the year with less southwest vortex, the angular momentum and the water vapor flux of the key area decrease and the angular momentum and the water vapor flux increase in downstream region of the key area. This kind of reverse phase change needs to be further study.
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