Multidecadal fluctuations of the North Atlantic Ocean and feedback on the winter climate in CMIP5 control simulations

This study examines the relationship between the Atlantic Multidecadal Variability (AMV) and the wintertime atmospheric circulation of the North Atlantic in simulations of the fifth Coupled Model Intercomparison Project (CMIP5). Comparisons of internal (using preindustrial control simulations) and externally forced (using historical and Representative Concentration Pathways 8.5 simulations) simulated AMV with observations suggest that the CMIP5 models lack internally generated AMV, except for two models (GFDL-ESM2G and HadGEM2-ES). A long-term influence of the winter North Atlantic Oscillation (NAO) on the AMV is identified, but no consistent feedback of the AMV onto the atmospheric circulation is found among the models. However, GFDL-ESM2G and HadGEM2-ES show a small lagged NAO signal that suggests a driving role of the ocean on decadal fluctuations of the atmosphere, with two different potential mechanisms. HadGEM2-ES exhibits a latitudinal shift of the Atlantic Intertropical Convergence Zone that can modulate the NAO through a Rossby wave train emanating from the tropics. In GFDL-ESM2G, the AMV is associated with a decrease in storm track activity and a shift of the intraseasonal weather regimes toward the negative NAO regime. These results raise hope that some long-term predictability of the winter climate over the North Atlantic and surrounding continents could be extracted from long-term oceanic fluctuations of the North Atlantic Ocean, provided that the AMV is correctly represented in coupled ocean-atmosphere models.