Variations of the thermohaline circulation (THC) in the North Atlantic on multidecadal time scales were simulated in a series of integrations using the GFDL coupled ocean-atmosphere model. The temperature changes associated with these THC variations resemble recent observational analyses using both instrumental and proxy data. Such multidecadal variations can interact with anthropogenic climate change signals. In particular, the oceanic fresh water budget plays a key role in both the natural (multidecadal) and forced (anthropogenic) THC fluctuations.
Experiments were conducted to assess the role of the atmosphere in these multidecadal THC variations. It is shown that the THC fluctuations in this coupled model may be at least partially viewed as the excitation of an oceanic mode of variability by stochastic atmospheric forcing. Further, additional experiments were conducted to assess the relative contributions of surface heat, water, and momentum fluxes in exciting this THC variability. It is shown that variations in surface heat flux forcing were the most important term driving this multidecadal variability. The heat flux forcing had a large scale spatial structure but little temporal coherence.