Contrasting influences of ENSO on the East Asian monsoon during different PDO phases

Abstract:

El Nino-Southern Oscillation (ENSO) exerts significant impacts on the East Asian climate including the East Asian winter monsoon (EASM) and the East Asian summer monsoon (EASM). Based on reanalysis and observational datasets, it reveals that the impacts of ENSO on the EAWM and the EASM depend on the phase of the Pacific Decadal Oscillation (PDO). During the negative phase of PDO, EAWM is significantly weakened in El Nino winters with robust warming over East Asia. In the following summer, EASM rainfall anomalies feature a dipole pattern. The sub-seasonal evolution of the EASM rainfall anomalies is characterized by clear two-step northward shift from June to August. During the positive phase of PDO, in contrast, the weakening of the EAWM is insignificant during El Nino winters with slightly warming over East Asia. In the following summer, EASM rainfall anomalies feature a tripole pattern with a gradual northward march. Such contrasting influences of ENSO on the East Asian monsoon can be attributed to the different anomalous lower-tropospheric anti-cyclone over Northwestern Pacific in both winter and the following summer, and the different decaying speed of El Nino during different phases of PDO. The reproducibility of these phenomena in CMIP5 models is also evaluated.