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 Ni˜no–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 Ni˜no), 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 Niña summers, however, the seasonal march of the monsoon and Asian jet does not exhibit clear variations, suggesting asymmetrical impacts between El Ni˜no and La Ni˜na. Furthermore, we explain these variations in the seasonal evolution of rainfall and jet and the asymmetrical impacts between El Ni˜no and La Ni˜na through the variations in tropical tropospheric temperatures and the resultant changes in meridional temperature gradient.