Interdecadal Variability of the Afro-Asian Summer Monsoon System

摘要

The Afro-Asian summer monsoon is a zonally planetary-scale system,with a large-scale rainbelt covering Africa,South Asia and East Asia on interdecadal timescales both in the past century (1901-2014) and during the last three decades (1979-2014).A recent abrupt change of precipitation occurred in the late 1990s.Since then,the entire rainbelt of the Afro-Asia monsoon system has advanced northwards in a coordinated way.Consistent increases in precipitation over the HuangheHuaihe River valley and the Sahel are associated with the teleconnection pattern excited by the warm phase of the Atlantic Multidecadal Oscillation (AMO).A teleconnection wave train,with alternating cyclones/anticyclones,is detected in the upper troposphere.Along the teleconnection path,the configuration of circulation anomalies in North Africa is characterized by coupling of the upper-level anticyclone (divergence) with low-level thermal low pressure (convergence),facilitating the initiation and development of ascending motions in the Sahel.Similarly,in East Asia,a coupled circulation pattern also excites ascending motion in the Huanghe-Huaihe River valley.The synchronous increase in precipitation over the Sahel and Huanghe-Huaihe River valley can be attributed to the co-occurrences and in-phase changes of ascending motion.On the other hand,the warm phase of the AMO results in significant warming in the upper troposphere in North Africa and the northern part of East Asia.Such warming contributes to intensification of the tropical easterly jet through increasing the meridional pressure gradient both at the entrance region (East Asia) and the exit region (Africa).Accordingly,precipitation over the Sahel and Huanghe-Huaihe River valley intensifies,owing to ageostrophic secondary cells.The results of this study provide evidence for a consistent and holistic interdecadal change in the Afro-Asian summer monsoon.