Northwest Pacific Anticyclonic Anomalies during Post–El Niño Summers Determined by the Pace of El Niño Decay

Abstract

Abstract This study investigates the characteristics and maintaining mechanisms of the anomalous northwest Pacific anticyclone (NWPAC) following different El Niño decaying paces. In fast decaying El Niño summers, the positive SST anomalies in the tropical central-eastern Pacific (TCEP) have transformed to negative, and positive SST anomalies appear around the Maritime Continent (MC), whereas in slow decaying El Niño summers, positive SST anomalies are present in the TCEP and in the tropical Indian Ocean (TIO). During fast decaying El Niño summers, the cold Rossby wave in response to the negative TCEP SST anomalies has a primary contribution to maintaining the NWPAC anomalies. The warm Kelvin wave response and enhanced Hadley circulation anomalies forced by the positive MC SST anomalies also facilitate developing the NWPAC anomalies. During slow decaying El Niño summers, the warm Kelvin wave anchored over the TIO plays a crucial role in sustaining the NWPAC anomalies, while the warm Rossby wave triggered by the positive TCEP SST anomalies weakens the western part of the NWPAC anomalies. The southwesterly anomalies of the NWPAC anomalies during fast decaying El Niño summers can reach to higher latitudes than those during slow decaying El Niño summers. Correspondingly, positive rainfall anomalies appear in northern China and the Yangtze River basin in fast decaying El Niño summers but are only distributed in the Yangtze River basin in slow decaying El Niño summers. This study implies that the El Niño decaying pace is a key factor in East Asian summer climate.