Collapse and slow recovery of the Atlantic Meridional Overturning Circulation (AMOC) under abrupt greenhouse gas forcing.

Modeling studies which abruptly increase atmospheric CO2 concentration evidence a rapid reduction or collapse of the Atlantic Meridional Overturning Circulation (AMOC) from its present state of strong overturning circulation into one characterized by a nearly absent deep ocean overturning in the Atlantic and a reduced northward heat transport. Similar transitions are frequently discussed in the context of both present and past global climate changes. Far less discussed, however, is the eventual recovery of the circulation and the establishment of a new equilibrium state, which may occur, or not, many millennia following the initial collapse. Here, we use the Community Earth System Model (CESM1) and a range of abrupt CO2 forcing scenarios (0.5xCO2, and 2xCO2 up to 16xCO2) to evaluate the timescales, and the factors influencing these timescales, in the AMOC's slow evolution to equilibrium. This takes ~ 2000 years for 2xCO2, but over 10,000 years for 8xCO2 in the model. We focus on the interplay between upper- and deep-ocean temperature and salinity, and Arctic sea ice, to diagnose the mechanisms of AMOC recovery and its transition towards equilibrium. We show that the freshening of the Arctic and Subpolar region can delay or possibly halt AMOC recovery. Critically, even after radiative balance is reached at the top of the atmosphere, ocean temperature and salinity keep evolving for thousands of years, affecting the AMOC. These results highlight the long timescales needed for ocean adjustment to radiative forcing, and also shed light on AMOC characteristics in past climates with atmospheric CO2 concentrations markedly different from the present day. [ABSTRACT FROM AUTHOR]