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The impacts of explosive volcanism on the densely populated Indian Ocean (IO) region remain elusive. Dedicated sensitivity experiments indicate that tropical volcanic eruptions induce a stronger surface cooling over Africa than of ocean, promoting westerlies in the equatorial IO. These westerlies drive a subsurface response reminiscent to that of a negative IO Dipole (IOD) during autumn in the year of eruption. The eruption also drives an enhanced cooling over the northwestern IO as a direct response to climatological cloud cover distribution. The resulting anomalous zonal sea surface temperature gradient contributes to enhance equatorial westerly anomalies in summer. The response is sensitive to the IO preconditioning, being larger when the system is favorable to a positive IOD development. Volcanic eruptions also induce a subsurface IOD-like response in the multimodel database from the Coupled Model Intercomparison Project Phase 5 as well as a primary productivity decrease in the eastern IO.

Plain Language Summary Explosive volcanism induces a global surface cooling, but its regional impact on the densely populated Indian Ocean sector is poorly documented. Here we show that the volcanic eruptions induce a subsurface ocean response in autumn of the eruption's year, reminiscent to that of a negative Indian Ocean Dipole, an intrinsic climate mode in the Indian Ocean equivalent to El Nino in the Pacific. This subsurface response is driven by equatorial westerly anomalies resulting from the stronger surface cooling over the African landmass than over the ocean. The response is sensitive to the Indian Ocean preconditioning, being larger when the system is preconditioned toward a positive Indian Ocean Dipole development.