Intraseasonal Variability of Northeast India Monsoon Rainfall: Modulation by the Madden–Julian Oscillation under El Niño–Southern Oscillation and Indian Ocean Dipole Conditions

This study examines the intraseasonal modulation of June–September (JJAS) rainfall over Northeast India (NEI) by the Madden–Julian Oscillation (MJO) and evaluates its possible conditioning by the El Niño–Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) during the common complete analysis period 1979–2023. Phase-dependent variations in mean rainfall and extreme-rainfall probability, defined using the pooled JJAS 95th-percentile threshold of 29.33 mm day-1, are quantified for dynamically active MJO days with RMM amplitude ≥1. Mean rainfall and extreme-rainfall probability are highest during MJO Phases 1–2 and lowest primarily during Phases 4–5, with suppression extending into Phase 6 in some diagnostics. Lagged composites indicate that extreme-rainfall probability reaches its maximum during Phase 2 at a lag of approximately three days, suggesting delayed moisture accumulation and convective organization. Continuous wavelet power, cross-wavelet power and wavelet coherence reveal intermittent rather than persistent variability within the canonical 30–60-day MJO band. ENSO- and IOD-conditioned composites show descriptive differences in the magnitude and phase-wise expression of rainfall; however, no phase-specific climate-state contrast remains statistically significant after false-discovery-rate correction. Annual MJO-band coherence is descriptively higher during La Niña than during Neutral and El Niño conditions, although the overall ENSO comparison is not statistically significant. Combined ENSO–IOD distributions are interpreted descriptively because the categories are highly unequal and some contain only one or two years, preventing reliable statistical inference. The results demonstrate a statistically supported phase- and lag-dependent association between the MJO and NEI rainfall while indicating that the apparent conditioning by ENSO and IOD remains descriptive. These findings may support subseasonal identification of rainfall-risk windows over Northeast India.