Variable and Robust East Asian Monsoon Rainfall Response to El Ni(n)o over the Past 60 Years (1957-2016)

摘要

Severe flooding occurred in southern and northern China during the summer of 2016 when the 2015 super El Ni(n)o decayed to a normal condition.However,the mean precipitation during summer (June-July-August) 2016 does not show significant anomalies,suggesting that-over East Asia (EA)-seasonal mean anomalies have limited value in representing hydrological hazards.Scrutinizing season-evolving precipitation anomalies associated with 16 El Ni(n)o episodes during 1957-2016 reveals that,over EA,the spatiotemporal patterns among the four categories of El Ni(n)o events are quite variable,due to a large range of variability in the intensity and evolution of El Ni(n)o events and remarkable subseasonal migration of the rainfall anomalies.The only robust seasonal signal is the dry anomalies over central North China during the El Ni(n)o developing summer.Distinguishing strong and weak El Ni(n)o impacts is important.Only strong El Ni(n)o events can persistently enhance EA subtropical frontal precipitation from the peak season of El Ni(n)o to the ensuing summer,by stimulating intense interaction between the anomalous western Pacific anticyclone (WPAC) and underlying dipolar sea surface temperature anomalies in the Indo-Pacific warm pool,thereby maintaining the WPAC and leading to a prolonged El Ni(n)o impact on EA.A weak El Ni(n)o may also enhance the post-El Ni(n)o summer rainfall over EA,but through a different physical process:the WPAC re-emerges as a forced response to the rapid cooling in the eastern Pacific.The results suggest that the skillful prediction of rainfall over continental EA requires the accurate prediction of not only the strength and evolution of El Ni(n)o,but also the subseasonal migration of EA rainfall anomalies.