Abstract
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 Huanghe–Huaihe 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.
摘要
亚非季风系统是一个横跨非洲, 南亚和东亚的纬向行星尺度系统, 在过去百年(1901-2014)和近三十年(1979-2014)均体现出完整的大尺度雨带. 最近, 亚非夏季风降水在20世纪90年代中后期出现了年代际转折, 季风雨带出现了一致北移的变化. 从20世纪90年代中后期, 萨赫勒地区和我国黄淮地区降水增多, 呈同相变化. 造成这种变化的原因在于大气环流从温, 压, 风三个方面对AMO暖位相的耦合响应(coupled response of). AMO暖位相通过在对流层上层激发出伴随气旋反气旋的东传的Rossby波列. 沿遥相关传播路径, 在北非地区, 高层为反气旋和低层为热低压, 这种高层辐散, 低层辐合的环流异常配置, 有利于激发和加强萨赫勒地区的上升运动. 同样地, 在东亚地区, 高低层耦合的环流配置也激发了黄淮地区的上升运动. 萨赫勒地区和黄淮地区共同发生和同相变化的上升运动, 导致了这两个地区降水同步增加. 另一方面, AMO暖位相使得对流层上层温度在北非和东亚北部地区显著增暖, 进而加强了热带东风急流入口区(东亚)和出口区(北非)的经向气压梯度, 这对热带东风急流的加强和北移起到了关键的作用. 同时, 由于热带东风急流入口区和出口区非地转次级环流的作用, 萨赫勒地区和中国黄淮地区降水增加.
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Acknowledgements
This study was jointly supported by the National Basic Research Program of China (Grant Nos. 2013CB430203 and 2012CB417205), the National Key Research and Development Program of China (during the 13th Five-year Plan) (Grant No. 2016YFA0601501), and the China Meteorological Special Programs (Grant No. GYHY201306033).
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Li, Y., Ding, Y. & Li, W. Interdecadal variability of the Afro-Asian summer monsoon system. Adv. Atmos. Sci. 34, 833–846 (2017). https://doi.org/10.1007/s00376-017-6247-7
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DOI: https://doi.org/10.1007/s00376-017-6247-7
Key words
- Afro-Asian summer monsoon
- precipitation
- Atlantic Multidecadal Oscillation (AMO)
- teleconnection
- tropical easterly jet