Indian Ocean Warming Can Strengthen the Atlantic Meridional Overturning Circulation

The slowdown of the Atlantic meridional overturning circulation (AMOC) and the accelerated warming of the tropical Indian Ocean (TIO) are two robust features projected for anthropogenic greenhouse warming, affecting both regional and global climates. Here we use coupled climate simulations to investigate a previously overlooked link between the two phenomena. We demonstrate that TIO warming reduces rainfall over the tropical Atlantic by strengthening the Walker circulation and increasing atmospheric vertical stability. The resultant ocean salinity increase intensifies the AMOC as salinity anomalies are advected to northern high latitudes. Additionally, TIO warming enhances westerly winds over the subpolar North Atlantic, which helps maintain the stronger AMOC. A TIO warming of 0.1°C above the mean warming of tropical oceans intensifies the AMOC by ~1 Sv, leading to a stronger interhemispheric asymmetry and a northward shifted Intertropical convergence zone (ITCZ). Thus, TIO warming could delay the AMOC weakening under greenhouse warming. Indeed, we find that the AMOC weakens more strongly or completely collapses if we suppress TIO warming under doubled and quadrupled CO₂ scenarios. Simulations replicating the observed tropical ocean warming further confirm this TIO-AMOC link, suggesting that the observed TIO warming might be already playing a role in sustaining the AMOC.