Warmer SSTs in the Equatorial Eastern Pacific Delay the Seasonal March of the Asian-Pacific Summer Monsoon Onset

This study investigates the impact of El Nino-Southern Oscillation (ENSO) on the seasonal evolution of the Asian-Pacific summer monsoon. The results show that during the summers when the SSTs in the equatorial eastern Pacific are above normal (i.e., during developing El Nino), the seasonal march of the Asian-Pacific summer monsoon is delayed. Specifically, the stepwise northeastward extension of the western North Pacific monsoon and the northward migration of East Asian and Indian monsoon are all delayed, resulting in rainfall deficiency in the western North Pacific and India, and more rainfall in the Yangtze River valley but less rainfall in North China due to the delayed northward migration and resultant prolonged rainy season in the Yangtze River valley. In addition, the seasonal march of the Asian upper-tropospheric westerly jet, which is mainly characterized by the northward migration during the monsoon development, is also delayed, concurrent with the delayed march of monsoonal rainfall. During La Nina summers, however, the seasonal march of the monsoon and Asian jet does not exhibit clear variations, suggesting asymmetrical impacts between El Nino and La Nina. Furthermore, we explain these variations in the seasonal evolution of rainfall and jet and the asymmetrical impacts between El Nino and La Nina through the variations in tropical tropospheric temperatures and the resultant changes in meridional temperature gradient. SIGNIFICANCE STATEMENT: The seasonal march of the Asian-Pacific summer monsoon exhibits large spatial and temporal variations, but shows some regular features such as stepwise northward migrations. The impacts of ENSO on the seasonal evolution of the components of Asian-Pacific summer monsoon have been shown in previous studies. However, there have been remarkable uncertainties in the previous results, and more emphasis has been put on the impacts of preceding ENSO. This study starts from El Nino and La Nina summers to reduce uncertainty caused by defining the seasonal march of the monsoon, and includes the variations in circulations that correspond well to monsoonal rainfall, making the conclusions more reliable.

Automated summary

This study investigates the impact of El Nino-Southern Oscillation (ENSO) on the seasonal evolution of the Asian-Pacific summer monsoon. The results show that El Nino delays the seasonal march of the monsoon and Asian jet, while La Nina does not exhibit clear variations. These variations can be explained by the changes in tropical tropospheric temperatures and meridional temperature gradient.