The Diversity of ENSO Evolution during the Typical Decaying Periods Determined by an ENSO Developing Mode

Abstract

Abstract The evolution of El Niño–Southern Oscillation (ENSO) is an essential predictor for global climate anomalies, but ENSO’s effect is regulated by the considerable diversity of post-ENSO evolution. This study identified that there coexist a decaying mode and a developing mode of ENSO in the ENSO decaying periods (from September to August) through empirical orthogonal function (EOF) and extended EOF analyses. The diversity of ENSO evolution during the decaying periods is actually determined by the ENSO developing mode, which is unsteady and susceptible to other signals, whereas the ENSO decaying mode is quite steady. Using multiple datasets, segmented datasets, and sea surface temperature anomalies (SSTAs)-residual EOF, the key signals related to the diversity of ENSO evolution are identified and verified in the South Atlantic and equatorial South Pacific. Based on the decaying and developing modes, a prediction model was constructed to predict the boreal summer Niño-3 index. The ENSO developing mode, explaining very small variance, plays a decisive role in determining the prediction of the summer SSTAs in the Pacific. The forecasting skill of summer ENSO can also be greatly improved when continuous SSTAs and interbasin interactions in the whole tropics are considered. This prediction result indicates that identified precursory disturbance signals and interbasin interactions for ENSO development are crucial for boreal summer ENSO prediction. Significance Statement ENSO often shows great diversity in the temporal evolution during post-ENSO periods, thus limiting the predictability of tropical SSTAs in the following spring and summer. Identifying the signal impacting the diversity of ENSO is crucial for predicting ENSO. Here, we offer a new understanding of the essence of ENSO’s temporal diversity from the perspective of interbasin interactions. We identified that the ENSO typical decaying period contains a steady decaying mode and an unsteady developing mode simultaneously, and the developing mode induces the great diversity in ENSO’s evolution during post-ENSO periods. Two signals located in the South Atlantic and equatorial South Pacific are recognized as the main sources influencing the diversity of ENSO evolution.