Mechanisms and Impacts of a Partial AMOC Recovery Under Enhanced Climate Forcing

A weakening of the AMOC has been found in models and in paleo proxies to be accompanied by a gradual warming and reduction in density in the subsurface Atlantic. Here we perform freshwater perturbation experiments with a 1° fully coupled climate model and then with a zonally-averaged ocean model to demonstrate that slow subsurface property changes (i) erode ocean stratification and thereby introduce a negative feedback that permits a partial re-invigoration of convection and the AMOC, and (ii) ensure that the AMOC Meridional Heat Transport (MHT) weakens less than the volume transport. In the coupled model, with a 0.1 Sv net freshwater flux introduced around Greenland, an initial 22% AMOC reduction over the first 40 years is then followed by a recovery over the following 350 years, such that the final reduction is only 12% relative to the unpurturbed state. An ensuing recovery of the MHT results in a final reduction of only 7% compared to its initial state. A similar response to the imposed forcing in the idealized ocean model demonstrates that the changes are controlled by simple 2D dynamics that involve vertical stratification. Close agreement between the modeled patterns of change with depth and observations taken in the Atlantic raises important questions about how we interpret and predict observed AMOC trends and impacts.