North Atlantic Interannual Variability in a Coupled Ocean–Atmosphere Model

Abstract

The primary mode of sea surface temperature variability in the North Atlantic on interannual timescales during winter is examined in a coupled ocean?atmosphere model. The model, developed at die Geophysical Fluid Dynamics Laboratory, is global in domain with realistic geography and a seasonal cycle of insulation. Analyses performed on a 1000-year integration of this model show that this mode is characterized by zonal bands of SST anomalies in the North Atlantic and bears a distinct resemblance to observational results. The largest anomalies in the model are to the southeast of Newfoundland. The model SST variations appear to be related to a north?south dipole in the atmospheric 500-mb geopotential height field, which resembles the North Atlantic oscillation and the Western Atlantic pattern. Analyses are presented that show that this mode of SST variability is primarily driven by perturbations to the surface heat fluxes, which are largely governed by atmospheric variability. Changes in model ocean circulation also contribute to this mode of variability but appear to be of secondary importance. Additional integrations are analyzed to examine the above conclusion. The same atmospheric model used in the above integration was coupled to a 50-m slab ocean and integrated for 500 years. The primary mode of SST variability in this model, in which there were no effects of ocean dynamics, resembles the primary mode from the coupled model, strengthening the conclusion that the surface fluxes are the primary mechanism generating this oceanic variability. One notable difference between the two models is related to the presence of deep vertical mixing at high latitudes in the model with a fully dynamic ocean. An additional 500-year integration of the atmospheric model with a prescribed seasonal cycle of SSTs lends further support to this conclusion, as do additional diagnostic calculations in which a 50-m slab ocean was forced by the time series of surface fluxes from both the prescribed SST and fully coupled model.