Asymmetry in the dominant co-variation mode of boreal summer monsoon rainfall regulated by the ENSO evolution

Abstract

Interannual variability of boreal summer monsoon (BSM) rainfall exhibits a prominent co-variation mode, which affects the regions with the highest population density in the world. The mode intensity depends on the rainfall variation over the western North Pacific (WNP) and the North America (NAM) monsoon regions. This study suggests that relative importance of the WNP and NAM rainfall anomalies is asymmetric between the strong positive (SP) and strong negative (SN) phases of this mode, which can be attributed to the distinct seasonal evolution of ENSO events. In the SP-phase, the suppressed summer rainfall over the WNP determines the mode following a decaying El Nino event. When the tropical Indian Ocean (TIO) warms and the eastern Equatorial Pacific (EEP) cools, the emanating Kelvin wave- and zonal SST difference-induced low-level WNP anticyclone thus gets enhanced to damp the local rainfall, which further affects the other members of the BSM. However, the SN-phase of this mode, showing closer association with anomalous NAM rainfall, appears with a developing El Nino event. The tropical North Atlantic (TNA) becomes colder from spring to summer, when the EEP warms due to the zonal circulation anomalies. A stronger zonal SST difference between the warm EEP and cold TNA suppresses the NAM rainfall by strengthening the low-level anticyclone in situ. Such asymmetry in different phases of the BSM co-variation mode is validated by the pre-industrial control runs in a state-of-the-art CGCM, indicating the roles of ENSO evolution in regulating the interaction between the BSM members.