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

The Atlantic Meridional Overturning Circulation (AMOC) has undergone large changes in strength during past climatic changes, and it is expected to weaken throughout the 21st century due to increased surface buoyancy. A weakening of the AMOC in climate models, as well as in paleo-proxy observations, is reported to be accompanied by a reduction in density in the deep North Atlantic ocean. This potential for a reduction in the stratification therefore raises the possibility for a re-invigoration of high latitude convective mixing. Here we perform freshwater perturbation experiments with both a 1° coupled model and an idealized zonally averaged ocean-only model to demonstrate that multi-decadal to centennial-timescale subsurface property changes (1) introduce a negative feedback that eventually erodes the stratification and partially reinvigorates North Atlantic convection and the AMOC, and (2) ensure the meridional heat transport weakens less than the AMOC. For example, in the coupled model with a 0.1-Sv net freshwater flux introduced around Greenland, an initial 22% AMOC reduction over 40 years is followed by a recovery of almost half the lost strength after 400 years. The final heat transport, however, is weakened by only 7%. Similar responses in the idealized model demonstrate that simple 2-D ocean-only dynamics control the changes.