Mechanism of interannual to decadal variability of the North Atlantic circulation

A model of the Atlantic Ocean was forced with decadal-scale time series of surface fluxes taken from the National Centers for Environmental Prediction -National Center for Atmospheric Research reanalysis. The bulk of the varia bility of the oceanic circulation is found to be related to the North Atlan tic oscillation (NAO). Both realistic experiments and idealized sensitivity studies with the model show a fast (intraseasonal timescale) barotropic re sponse and a delayed (timescale about 6-8 yr) baroclinic oceanic response t o the NAO. The fast response to a high NAO constitutes a barotropic anticyc lonic circulation anomaly near the subpolar front with a substantial decrea se of the northward heat transport and an increase of northward heat transp ort in the subtropics due to changes in Ekman transport. The delayed respon se is an increase in subpolar heat transport due to enhanced meridional ove rturning and due to a spinup of the subpolar gyre. The corresponding subpol ar and subtropical heat content changes could in principle act as an immedi ate positive feedback and a delayed negative feedback to the NAO.