The Unified Response of the Atlantic Multidecadal Oscillation and Quasi-Biennial Oscillation on Indian Summer Monsoon Rainfall

This study investigates the relationship between Indian summer monsoon (ISM) rainfall and the Atlantic Multidecadal Oscillation (AMO) on a monthly and seasonal basis with respect to the quasi-biennial oscillation (QBO) during the period 1953–2016. The analysis is performed for full time series of monthly and seasonal ISM rainfall and AMO index data as well by regrouping according to the westerly and easterly phase of the QBO (at 50 hPa). A direct positive association is observed between ISM rainfall and the AMO for the full time series during the pre-monsoon (March–April–May) and winter (January–February) seasons via the Rossby wave train from the North Atlantic across South Asia, thus resulting in an increase in the temperature gradient between the Indian Ocean (IO) and Eurasia which strengthened the ISM. The strongest association was found during the pre-monsoon season and especially during April. The variability in ISM rainfall was more prominently modulated by the pre-monsoon warm phase of the AMO along with the westerly phase of the QBO as compared with the easterly phase. The elevated ISM rainfall during the warm AMO phase and westerly phase of the QBO ultimately triggered low IO sea surface temperature and salinity during September–October.