Mechanism of Interannual to Decadal Variability of the North Atlantic Circulation

Abstract

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 variability of the oceanic circulation is found to be related to the North Atlantic oscillation (NAO). Both realistic experiments and idealized sensitivity studies with the model show a fast (intraseasonal timescale) barotropic response and a delayed (timescale about 6?8 yr) baroclinic oceanic response to the NAO. The fast response to a high NAO constitutes a barotropic anticyclonic circulation anomaly near the subpolar front with a substantial decrease of the northward heat transport and an increase of northward heat transport in the subtropics due to changes in Ekman transport. The delayed response is an increase in subpolar heat transport due to enhanced meridional overturning and due to a spinup of the subpolar gyre. The corresponding subpolar and subtropical heat content changes could in principle act as an immediate positive feedback and a delayed negative feedback to the NAO.