The source of uncertainty in projecting the anomalous western North Pacific anticyclone during El Niño–decaying summers

Abstract

AbstractThe western North Pacific anomalous anticyclone (WNPAC) is a key bridge that links El Niño and East Asian climate variability. Future projections of ENSO-related WNPAC changes under global warming are highly uncertain across climate models. Based on a 40-member ensemble from the Community Earth System Model Large Ensemble (CESM-LE) project, we investigate the effects of internal variability on the El Niño-related WNPAC projection. Here, we first develop a decomposition method to separate the contributions of El Niño amplitude change and non-amplitude change from the leading uncertainty in the El Niño-related WNPAC projection. Based on the decomposition, approximately 23% of the uncertainty in the El Niño-related WNPAC projection is attributed to the El Niño amplitude change, while the remaining 77% is from the non-amplitude change, which is mainly related to the change in the El Niño decaying pace. A larger (smaller) El Niño amplitude can enhance (weaken) the WNPAC through a stronger (weaker) tropical Indian Ocean (TIO) capacitor effect. For non-amplitude change, a faster (slower) El Niño decaying pace can also enhance (weaken) the WNPAC through descending Rossby waves in response to cold sea surface temperature anomalies (SSTA) over the tropical central-eastern Pacific. The decomposition method can be generalized to investigate the sources of uncertainty related to El Niño properties in climate projections and to help improve the understanding of changes in the interannual variability of East Asian-western Pacific climate under global warming.