2010年以来华西秋季降水年代际增多原因初探

Preliminary analysis on the interdecadal increase of autumn rainfall in western China since 2010

基于1961—2016年降水资料和NCEP/NCAR再分析数据,分析了华西秋季降水的年代际变化特征。结果表明,华西秋季降水在20世纪80年代中期前偏多,之后减少,21世纪10年代又开始增多,而且该时段的降水量多于20世纪80年代中期前。进一步从大气环流角度,初步探讨了这种年代际变化的成因。研究发现,与华西降水偏少阶段相比,在两个降水偏多期,巴尔喀什湖低压槽加深、亚洲区域海平面气压(Sea Level Pressure,SLP)负异常,利于高纬冷空气南下入侵华西区域;东亚低空盛行异常偏南风,利于低纬水汽向华西区域输送;东亚西风急流位置偏北,提供华西降水发生的动力抬升条件,共同造成华西秋季降水增加。相比于20世纪80年代前,21世纪10年代后欧洲高压脊和巴尔喀什湖槽更强,更利于高纬冷空气进入华西区域。同时,有来自南半球印度洋的水汽向华西区域输送。更强的冷空气活动和更多的水汽输送导致后一阶段降水更多。

In the present study,based on the CN05.1 gridded rainfall data and the NCEP/NCAR reanalysis for the period of 1961—2016,the interdecadal change of autumn rainfall in western China (WCAR) is investigated.The results reveal that the WCAR shifted from a positive phase to a negative phase in the mid-1980s.After entering the 2010s,the WCAR tends to increase again,with the increasing amplitude becoming even greater than that before the mid-1980s.Next,the physical mechanism responsible for the interdecadal change in the WCAR is further explored from the perspective of large-scale atmospheric circulations,in particular focusing on the similarities and differences in the atmospheric circulations for the increase of the WCAR after the 2010s and before the mid-1980s.During these two periods,in comparison with the negative phase of the WCAR,both the Lake Balkhash trough and Asian sea level pressure are strengthened,favoring the southward outbreak of cold air from the high latitudes into western China.Meanwhile,the southerly anomalies prevail in the lower troposphere of East Asia,thereby benefiting from the transport of water vapor from the low latitudes toward western China.In addition,the East Asian jet stream shifts northward,providing favorable dynamical conditions for the occurrence of the WCAR.Under such a configuration,the WCAR is strengthened.With respect to the counterparts before the mid-1980s,the blocking over Europe and trough over Lake Balkhash are both much stronger,which may cause more cold air from the high latitudes to invade western China.At the same time,more warm-moist airflows from the Indian Ocean in the Southern Hemisphere are transported toward western China.Finally,it is shown that more cold air and warm-moist airflows which are encounter in the targeted region contribute to the greater rainfall occurring after the 2010s than before the mid-1980s.