近30余年来盛夏东亚东南季风和西南季风频率的年代际变化及其与青藏高原积雪的关系

利用地面观测资料和NCEP/NCAR再分析资料集,使用相关分析、合成分析等方法,在将地面风分为东南季风和西南季风的基础上,分析了近30余年来盛夏东亚季风频率的年代际变化特征。结果表明:盛夏东南季风、西南季风频率和前期春季青藏高原积雪均在21世纪初期发生了显著的年代际变化;东南季风、西南季风频率由较少改变为较多,春季青藏高原积雪则由深变浅。由于青藏高原积雪厚度发生了年代际变浅,说明青藏高原发生了年代际变暖和南亚高压变强,南亚高压的年代际变强,使得其下游对流层低层(18°~28°N,108°~118°E)的反气旋性环流异常增强,有利于东亚西南季风频率的增加;同时,由于高原发生湿反馈作用,使得淮河地区降水发生年代际变多,由Sverdrup涡度平衡关系,降水的异常增多通过潜热释放,使得东亚副热带高压异常加强,而副热带高压异常变强则有利于盛夏东亚东南季风频率发生年代际增加。

Interdecadal Variations of Summer Southeast and Southwest Monsoon Frequency over East Asia and Its Relationship with Snow Cover over the Tibetan Plateau for Recent 30 Years

On the basis of surface data and National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis, the characteristics of long-term variability of summer southeast and southwest monsoon frequency over East Asia for the recent 30 years are examined by correlation and composite analysis by dividing the surface wind into southeast and southwest monsoons. The results show that the frequency of southeast and southwest monsoons and the snow depth over the Tibetan Plateau (TP) experienced interdecadal transitions after 2000 such that the frequencies of southwest and southeast monsoons have enhanced, while the snow depth over the TP has decreased. The interdecadal decrease of the snow depth over the TP indicates warming of the TP and strengthening of the South Asia high. The latter induces a strong anticyclonic anomaly at the lower levels of the troposphere (18°-28°N, 108°-118°E) and increased southwest monsoon frequency in East Asia. Moreover, in the presence of moisture feedback, the TP warming induced an increase in precipitation in the Huaihe River basin, which would favor the strengthening of the Western Pacific subtropical high couplet through the Sverdrup vorticity balance, and the northward shift of the subtropical high is beneficial to the interdecadal increase in southeast monsoon frequency over East Asia.