01-11-2010: The stability of the MOC as diagnosed from model projections for pre-industrial, present and future climates

Paleoclimatic proxy data of the last glacial cycle show evidence of abrupt climate changes that are associated with major reorganizations of the Atlantic Meridional Overturning Circulation (MOC). The stability of the MOC was investigated for various climate scenario runs, using data from the CMIP3 archive of coupled atmosphere-ocean models. Apart from atmospheric feedbacks, the sign of the salt flux into the Atlantic basin that is carried by the MOC determines whether the MOC is in a stable regime with only one equilibrium, or in a regime where abrupt transitions between two stable states are possible. The amplitude of the salt advection feedback was analyzed by diagnosing the freshwater and salt budgets for the combined Atlantic and Arctic basins. Consistent with the finding that almost all coupled climate models recover from hosing experiments, it was found that most models feature a negative salt advection feedback in their pre-industrial climate, excluding the existence of a stable off-state for the MOC. There is strong evidence that the "real" MOC resides in the multiple equilibrium regime, indicating that those models are biased towards too stable conditions. All models feature enhanced evaporation over the Atlantic basin in future, warmer climates, but for a moderate increase in radiative forcing (B1 and 2 CO2 scenarios), there is a decrease of the fresh water flux carried by the MOC into the Atlantic, making the MOC less stable in the future. (Drijfhout, et al)