Multidecadal thermohaline circulation variability driven by atmospheric surface flux forcing

Previous analyses of an extended integration of the Geophysical Fluid Dynam ics Laboratory coupled climate model have revealed pronounced multidecadal Variations of the thermohaline circulation (THC) in the North Atlantic. The purpose of the current work is to assess whether those fluctuations can be viewed as a coupled air-sea mode tin the sense of ENSO), or as an oceanic response to forcing from the atmosphere model, in which large-scale feedbac ks from the ocean to the atmospheric circulation are not critical. A series of integrations using the ocean component of the coupled model are performed to address the above question. The ocean model is forced by suit ably chosen time series of surface fluxes from either the coupled model or a companion integration of an atmosphere-only model run with a prescribed s easonal cycle of SSTs and sea-ice thickness. These experiments reveal that 1) the previously identified multidecadal THC variations can be largely vie wed as an oceanic response to surface flux forcing from the atmosphere mode l, although air-sea coupling through the thermodynamics appears to modify t he amplitude of the variability, and 2) variations in heat flux are the dom inant term (relative to the freshwater and momentum fluxes) in driving the THC variability. Experiments driving the ocean model using either high- or low-pass-filtered heat fluxes, with a cutoff period of 20 yr, show that the multidecadal THC variability is driven by the low-frequency portion of the spectrum of atmospheric flux forcing. Analyses have also revealed that the multidecadal THC fluctuations are driven by a spatial pattern of surface h eat flux variations that bears a strong resemblance to the North Atlantic o scillation. No conclusive evidence is found that the THC variability is par t of a dynamically coupled mode of the atmosphere and ocean models.