Recent warming of the tropical Indian Ocean may sustain the AMOC from slowing down

A recent study published in Nature Climate Change used a series of coupled ocean-atmosphere model simulations to demonstrate that the tropical Indian Ocean (TIO) warming reduces rainfall over the tropical Atlantic by strengthening the Walker circulation (i.e., increased ascending motion over TIO and increased subsidence over tropical Atlantic). This increases salinity of the upper tropical Atlantic Ocean. The increased salinity anomalies are in turn carried to the deep convection sites in the high-latitude North Atlantic to increase the AMOC. Sensitivity experiments showed that a TIO warming of 0.1 °C above the mean warming of tropical oceans intensifies the AMOC by ~1 Sv. Therefore, TIO warming could potentially delay the AMOC weakening, both projected in response to the anthropogenic greenhouse warming. Based on these findings, the study also suggests a possibility that the recent TIO warming is already playing a role in sustaining the AMOC from slowing down.

Figure 2 from Hu and Fedorov (2019): The effect of changing TIO SST on the AMOC. The evolution of AMOC intensity in the pre-industrial control and several perturbation experiments: 1 °C TIO warming (TIO+1C), 2 °C TIO warming (TIO+2C), 1 °C TIO cooling (TIO–1C), quadrupling of CO₂ (4×CO₂) and quadrupling of CO₂ but with superimposed climatological TIO SST (4×CO₂, TIOclim). Perturbations are imposed at time zero and maintained for the duration of the experiments. The AMOC intensity is estimated as the maximum streamfunction within 500–5,500 m, 28° N to 90° N. An 11-year running mean is applied to all the curves. The dots and bars indicate average values and interannual variability over the past 50 years, respectively; the latter is estimated as standard deviation relative to an 11-year running mean.

Hu, S. and Fedorov, A. V. (2019). Indian Ocean warming can strengthen the Atlantic meridional overturning circulation. Nature Climate Change, 1-5, https://doi.org/10.1038/s41558-019-0566-x