Resonance Effect in Interaction Between South Asian Summer Monsoon and ENSO During 1958–2018

Abstract

The study has shown that the shear component of the vertical integrated kinetic energy (Ks) over the box (40oE-100oE, 0-20oN) can be used to measure the intensity of the South Asian summer monsoon (SASM). Based on its value averaged between June and August, the SASM can be divided into strong and weak monsoon episodes. Between 1958 and 2018, there existed 16 (16) strong (weak) monsoon episodes. Based on the calendar year, the relationship between the SASM and ENSO episodes can be grouped into six patterns: weak monsoon - El Nino (WM-EN), normal monsoon - El Nino (NM-EN), weak monsoon - non ENSO (WM-NE), strong monsoon - La Nina (SM-LN), normal monsoon - La Nina (NM-LN) and strong monsoon - non ENSO (SM-NE). Previous studies suggest that the WM-EN and SM-LN patterns reflect the correlated relationship between the SASM and El Nino/Southern Oscillation (ENSO) events. Therefore, we name these two strongly coupled categories WM-EN and SM-LN as the resonance effect. Two important circulations, i.e., Walker circulation (WC) and zonal Asian monsoon circulation (MC), in the vertical plane are found to be not always correlated. The MC is controlled by thermal gradients between the Asian landmass and the tropical Indian Ocean, while the WC associated with ENSO events is primarily the east-west thermal gradient between the tropical South Pacific and the tropical Indian Ocean. Furthermore, the gradient directions caused by different surface thermal conditions are different. The main factor for the resonance effect is the phenomenon that the symbols of SSTA in the tropical Indian Ocean and the equatorial eastern Pacific are the same, but are opposite to that of the SSTA near the maritime continent.