Distinct Impacts of Two Types of Summer ENSO with Different Temporal Evolutions on the Asian Summer Monsoon: Role of the Tropical Indo-Western Pacific

Abstract

Abstract ENSO shows great diversity in temporal evolution, exerting different climatic impacts on the global scale. Previous studies suggest that summer ENSOs can be classified into two types with different evolutions: “continuing ENSOs” persist from the previous winter and “emerging ENSOs” newly develop from late spring. In this study, we define two indices for continuing and emerging ENSOs via the linear combination of the two leading modes of the evolutionary anomalies of tropical sea surface temperature (SST) from the prior September to August, which reflects the distinct characteristics of the two types of ENSO well. The two types of ENSO dominate the two leading modes of the SST variability in the tropical Indo–western Pacific (TIWP) and the related Asian summer monsoon anomalies. During continuing ENSOs, the related TIWP SST anomalies show positive anomalies in the tropical Indian Ocean and northwest Pacific and negative ones in the New Guinea region. The warm Kelvin wave forced by the tropical Indian Ocean warming propagates into the tropical western Pacific, exciting the meridional East Asia–Pacific/Pacific–Japan pattern and causing a dipole precipitation pattern in East Asia. During emerging ENSOs, the related TIWP SST anomalies show negative anomalies in the warm pool and positive ones in the western Indian Ocean. The negative SST anomalies in the warm pool induce a westward equatorial Rossby wave conducive to the suppression of the Indian summer monsoon rainfall, whereas the Matsuno–Gill response to the warm pool cooling leads to negative anomalies in East Asian summer rainfall. This mechanism can be simulated well in the CMIP6 and Pacific Ocean–Global Atmosphere experiments. Significance Statement ENSO displays diversity in its amplitude, spatial pattern, temporal evolution, and impact. Although previous studies have systematically investigated the diversity of ENSO spatial locations, there are few studies on the diversity of ENSO evolution and the impacts on the Asian summer monsoon. Here, we define two indices for so-called continuing and emerging ENSOs via the linear combination of the two leading modes of the evolutionary anomalies of tropical sea surface temperature (SST) from the prior September to August. Then, we reveal that the two leading modes of the tropical Indo–western Pacific SST variability are dominated by the two types of ENSO, respectively, and act as a bridge via which the two types of ENSO can exert different impacts on the Asian summer monsoon.