The El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) significantly regulate sea surface height (SSH) variability in northern IO through equatorial and coastal waveguides. However, the phase-dependent relationship between concurrent ENSO-IOD events and equatorial-coastal waveguides remains unclear. Long-term SSH observations reveal two distinct waveguide modes during autumn-winter: an eastern mode propagating from eastern IO to Bay of Bengal (BOB), and a western mode extending from central IO to Arabian Sea (AS). These modes exhibit significant coherence in central IO exclusively during concurrent ENSO-IOD events, with stronger SSH anomalies occurring during positive phase. We identify a complete wave-mediated pathway in which equatorial Kelvin-coastal Kelvin waveguide interacts with equatorial Rossby-coastal Kelvin waveguide in central IO, transmitting SSH signal from eastern IO through BOB to AS coast. These results establish a dynamical framework linking ENSO-IOD phases to waveguide connectivity, improving predictability of climate-driven air–sea variability in northern IO.

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