The Strengthening of the Atlantic Ocean Meridional Overturning Circulation Caused by Enhanced Indian Ocean Warming

It is expected that the Atlantic meridional overturning circulation (AMOC) will weaken with the future climate change, but the extent and timing of this weakening varies significantly across climate models as many different factors affect the AMOC. Here, we describe how a salient feature of anthropogenic climate change – enhanced warming of the tropical Indian ocean (TIO) – can strengthen the Atlantic meridional overturning circulation (AMOC) by modulating salinity distribution in the Atlantic (Hu and Fedorov 2019). The mechanism of this AMOC strengthening is related to the remote effect of TIO warming on rainfall over the tropical Atlantic – by strengthening the Atlantic Walker cell and increasing atmospheric vertical stability TIO warming reduces precipitation in the tropical Atlantic. On multi-decadal timescales (30-50 years) the resultant positive salinity anomalies are advected to northern high latitudes, which strengthens the AMOC. This effect is further amplified by the salt-advection feedback. In coupled model simulations, a TIO warming of 0.1°C above the mean warming of all tropical oceans increases the AMOC intensity by ~1 Sv. Thus, the Indian ocean 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 of the past 50 years further confirm this TIO-AMOC link, suggesting that the Indian ocean warming might be already playing a role in sustaining the AMOC.

Reference: Hu, S., and Fedorov, A.V.: Indian Ocean warming can strengthen the Atlantic meridional overturning circulation. Nature Climate Change, in press (September 2019).