Mechanisms of the ENSO-Monsoon Interaction During Mature-Decay Phase of ENSO Cycle

One of the puzzling aspects of ENSO-summer monsoon interaction lies in the fact that the mature phase of ENSO occurs most frequently in boreal winter, which is half year out of phase with the season of peak monsoon rainfall. This is perhaps why the linkage between the Asian summer monsoon rainfall and the Pacific SST anomalies is so tantalizing. Observational evidence is presented to show a delayed impact of mature phase ENSO on East Asian monsoon during the decay-transition phases of ENSO. During the late fall of the ENSO development year, an anomalous low-level subtropical anticyclone develops rapidly over the Philippine Sea that persists to the following spring or early summer, causing anomalous wet (dry) conditions along the East Asian front stretching from southern China northeastward to the Kuroshio extension. The delayed impact of ENSO on the East Asian summer monsoon is thus due to the persistence of the subtropical anomalies excited by ENSO during its mature phase. How can the subtropical anomalies persist against damping? We found that the development and maintenance of the anomalous subtropical high is concurrent with the enhancement and maintenance of the in situ sea surface cooling. The persistence of the western North Pacific subtropical anticyclonic anomalies is attributed to a positive feedback between the atmospheric Rossby wave response and the associated sea surface cooling through wind-evaporation-SST feedback. This interaction depends on the presence of the mean northeasterly trades.