INTERDECADAL VARIABILITY IN AN IDEALIZED MODEL OF THE NORTH-ATLANTIC

A coarse resolution model is developed to study the thermohaline circu lation of the North Atlantic, This model is driven by the annual mean Hellerman and Rosenstein wind stress field, Levitus sea surface restor ing temperatures, and Schmitt, Bogden, and Dorman freshwater flux fiel ds (mixed boundary conditions) together with various parameterizations of Arctic freshwater export into the North Atlantic. The model simula tions indicate the existence of self-sustained, internal variability o f the thermohaline circulation with a period of about 20 years. Associ ated with the variability is a large variation in the deep-water forma tion rate in the Labrador Sea and hence the poleward heat transport in the North Atlantic. It is shown that the variability is insensitive t o the freshwater flux and wind forcing used and that the timescale for this thermally driven convective/advective oscillation is set by the cooling time of the Labrador Sea. The variability is robust to various parameterizations of Arctic freshwater export but may be suppressed i f there is a strong freshwater flux through the Canadian Archipelago ( or equivalently, large precipitation) into the Labrador Sea. The impor tance of topography, although poorly resolved in this coarse resolutio n study, is addressed and the results are compared with a coupled atmo sphere-ocean simulation and observations taken over the North Atlantic .