Abstract
East China has experienced positive precipitation anomalies in post-El Niño summers, mainly in the Yangtze-Huaihe River Valley. This kind of monsoonal rainfall change induced by El Niño, however, is not always the same due to El Niño diversity and mean state change. Here, we use cluster analysis on the post-El Niño (PE) East China summer precipitation anomalies to identify the diversity of this El Niño-induced monsoon change. The result shows that PE East China summer rainfall anomalies mainly display three different modes for all selected 20 El Niño events from 1957 to 2016. Cluster 1 shows the middle and lower reaches of the Yangtze River demonstrate strong wet anomalies, while South and North China are dominated by dry anomalies, similar to a sandwich mode. Cluster 2 is distinguished by dry anomalies over South China and wet anomalies over North China, exhibiting a dipole mode. Compared with Cluster 1, the change caused by Cluster 3 is different, showing negative anomalies over the Yangtze-Huaihe River Valley. The three clusters are correlated with successive events of El Niño, a quick transfer to a strong La Niña and a quick transfer to a weak La Niña respectively. The associated anomalous anticyclone (AAC) focuses on (120°E, 20°N) in Cluster 1, which expands southward for Cluster 2 and moves eastward for Cluster 3. The feedback of AAC-sea surface temperature (SST) mainly works for supporting the AAC in Cluster 1, but it is weak for Cluster 2; the strong easterly anomalies related to La Niña contribute to the AAC location change for Cluster 2. Both AAC-SST feedback and easterly anomalies support the AAC of Cluster 3. The CMIP5 output can capture these diverse responses in circulation except that their simulated AAC for Cluster 1 is significant to the east of the observed.
Highlights
East Asian summer monsoon (EASM) plays a significant role in Asian monsoon system
The first mechanism reveals the importance of the anomalous anticyclone (AAC) interacting with the western North Pacific (WNP) sea surface temperature (SST) cooling under the control of background average flow (Wang et al, 2000), which has been confirmed by coupled general circulation model (CGCM) experimental test (Wang et al, 2013)
The WNP AAC associated with subtropical, cold SST anomalies in the models, moves too far to the east compared with the observation, which locates at 150°E and 25°N
Summary
East Asian summer monsoon (EASM) plays a significant role in Asian monsoon system. The weather and climate in a large part of the East Asian region are affected by the variability and evolution of EASM (Chen et al, 1992; Wang et al, 2003). In cluster 1, in the middle and lower reaches of the Yangtze River (MLRYR), there are positive anomalies of heavy rainfall and negative anomalies in South and North China, showing a tripolar pattern (Figure 1B). In cluster 2, the WNP AAC expands southward, which is associated with equatorial easterly wind anomaly, and Southeast China is controlled by the AAC (Figure 2B).
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