Diagnosing Factors Influencing Amoc Decline in Climate Models

Climate models project a decline in Atlantic Meridional Overturning Circulation (AMOC) as a result of CO2 forcing, but the magnitude and timescales of these projections vary greatly. We use a modified Stommel box model to study the fundamental relationship between overturning decline and fluxes of heat and freshwater. The box model—calibrated with GCM outputs—is able to reproduce a positive connection between control AMOC and the change in AMOC under the conditions of heat flux forcing. In addition, previous studies have noted that GCMs with larger AMOC often reach quasi-equilibrium overturning values faster than those that do not. This is true in the box model representation as well, with the sensitivity being most pronounced in the cases of large surface temperature coupling. We discuss the box model results in the context of prior work supporting the dominance of heat flux relative to the freshwater flux forcing of the projected AMOC decline.