Changing spatial structure of the thermohaline circulation in response to atmospheric CO2 forcing in a climate model

The heat transported northwards by the North Atlantic thermohaline circulat ion warms the climate of western Europe(1-3) previous model studies(4-6) ha ve suggested that the circulation is sensitive to increases in atmospheric greenhouse-gas concentrations, but such models have been criticised for the use of unphysical 'flux adjustments' (7-9) (artificial corrections that ke ep the model from drifting to unrealistic states), and for their inability to simulate deep-water formation both north and south of the Greenland-Icel and-Scotland ridge, as seen in observations(10,11), Here we present simulat ions of today's thermohaline circulation using a coupled ocean-atmosphere g eneral circulation model without flux adjustments, These simulations compar e well with the observed thermohaline circulation, including the formation of deep water on each side of the Greenland-Iceland-Scotland ridge. The mod el responds to forcing with increasing atmospheric greenhouse-gas concentra tions by a collapse of the circulation and convection in the Labrador Sea, while the deep-water formation north of the ridge remains stable. These cha nges are similar in two simulations with different rates of increase of CO2 concentrations. The effects of increasing atmospheric greenhouse-gas conce ntrations that we simulate are potentially observable, suggesting that it i s possible to set up an oceanic monitoring system for the detection of anth ropogenic influence on ocean circulation.