Multiscale ENSO–IOD–MJO Interactions Modulating Indian Summer Monsoon Rainfall Variability

Abstract

The Indian Summer Monsoon Rainfall (ISMR) is strongly influenced by large-scale climate variability associated with the El Niño–Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), and the Madden–Julian Oscillation (MJO). While the individual influences of these climate modes on the monsoon are well established, their combined impact across interannual and intraseasonal timescales remains insufficiently understood. This study investigates the joint influences of ENSO, IOD, and MJO on ISMR during 1975–2025. Monsoon years were classified into nine ENSO–IOD categories using the Niño3.4 and Dipole Mode indices, followed by analyses of tropical sea surface temperature (SST) evolution, seasonal rainfall, atmospheric circulation, MJO characteristics, and phase-dependent intraseasonal rainfall anomalies. The results show that seasonal monsoon rainfall is governed by the combined state of the Pacific and Indian Oceans. Positive IOD is predominantly associated with an Eastern Pacific El Niño, whereas neutral IOD conditions exhibit a Central Pacific El Niño pattern. These distinct oceanic backgrounds produce contrasting monsoon circulation and rainfall responses, with La Niña accompanied by a negative IOD generating the strongest positive rainfall anomalies over India. Seasonal rainfall anomalies vary considerably among the ENSO–IOD categories and are dynamically consistent with changes in low-level monsoon circulation and mid-tropospheric vertical motion. Analysis of MJO characteristics demonstrates that both the amplitude and phase occupancy of active MJO events are strongly modulated by the background ENSO–IOD state. La Niña with a negative IOD favors prolonged MJO residence over Phases 1–2, whereas El Niño with a positive IOD exhibits enhanced activity during the Maritime Continent and western Pacific (Phases 5-6). These findings highlight the importance of multiscale interactions among Pacific and Indian Ocean SST conditions and MJO evolution in governing ISMR variability and suggest that incorporating ENSO flavor, IOD state, and MJO phase characteristics into operational prediction frameworks could improve seasonal monsoon forecasts over India.