The variability of the East Asian summer monsoon (EASM) on the interannual timescale is jointly affected by the thermal forcing from the ocean and land.
A new study published in Journal of Geophysical Research: Atmospheres and led by the scientists from the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences and National University of Singapore (NUS) revealed how the response of the EASM to the global sea surface temperature anomaly (SSTA) is regulated by the Tibetan Plateau (TP) thermal feedback.
"The EASM is known to have very different responses to the SSTA in developing and decaying years. Yet, there's almost no convincing explanation for such an asymmetric response," said Prof. DUAN Anmin from the IAP, the corresponding author of the study.
To investigate the underlying mechanism, the scientists used an atmospheric general circulation model called Finite-volume Atmospheric Model, developed by the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG) of the IAP, to design and conduct the numerical experiments.
The results showed that the global SSTA during the four-year cycle has significant impacts on the anomaly of diabatic heating over the TP in summer.
In the developing year summer, the diabatic heating weakens over the whole TP, leading to less rainfall over East Asia, an anomalous cyclone over the northwestern Pacific and an anticyclone in north China. In the decaying year summer, the SSTA induces an anomalous anticyclone in south China and an anomalous cyclone in north China. The TP heating shows an anomalous negative-positive pattern.
"Without the anomalous TP thermal forcing, in both developing and decaying years the EASM shows similar response patterns with an anomalous anticyclone over the northwestern Pacific, an anomalous cyclone in north China, and an enhanced main rainfall band," Prof. DUAN added.
Prof. DUAN along with the collaborators from National University of Singapore concluded that the TP thermal feedback plays a key role in the asymmetric responses of the EASM to the global SSTA during the different stages, which causes less precipitation in developing years and an enhanced main rainfall band in decaying years.
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