Why was the western Pacific subtropical anticyclone weaker in late summer after the 2015/2016 super El Niño?

Abstract

The western Pacific subtropical anticyclone (WPSA), which bridges the El Niño events and the East Asian summer monsoon, is generally enhanced in boreal summer after El Niño events. However, it becomes weaker in boreal late summer (July–August) of 2016, following the 2015/2016 super El Niño event, indicating failure of the previous experience. Our results suggest that the weaker WPSA in late summer 2016 is associated with the extinction of the Indian Ocean capacitor effect, presenting the cold sea surface temperature anomalies (SSTAs) in the western tropical Indian Ocean (WTIO) after the fast decaying of the 2015/2016 super El Niño in spring. Compared with the case in 1982/1983 and 1997/1998, both data analysis and numerical experiments indicate that the cold WTIO SSTAs could induce an anomalous zonal circulation, with its descending branch over the WTIO and the ascending branch over the western North Pacific (WNP). Afterwards, the thermal structure in the upper troposphere of the WNP is altered to enhance the local wave guide, which facilitate the equatorward propagation of the mid‐latitudinal wave train into the WNP. Consequently, the abnormal ascending over the WNP weaken the WPSA in conjunction with the anomaly of the upper‐tropospheric cyclone due to the southward intrusion of the wave train during late summer 2016. This implicates the possible influence of El Niño evolution in spring on the WPSA intensity in the subsequent summer.