Mechanisms determining the Atlantic thermohaline circulation response to greenhouse gas forcing in a non-flux-adjusted coupled climate model

Models of the North Atlantic thermohaline circulation (THC) show a range of responses to the high-latitude warming and freshening characteristic of gl obal warming scenarios. Most simulate a weakening of the THC, with some sug gesting possible interruption of the circulation, but others exhibit little change. The mechanisms of the THC response to climate change using the Had CM3 coupled ocean-atmosphere general circulation model, which gives a good simulation of the present-day THC and does not require flux adjustment, wer e studied. In a range of climate change simulations, the strength of the TH C in HadCM3 is proportional to the meridional gradient of steric height (eq uivalent to column-integrated density) between 30 degreesS and 60 degreesN. During an integration in which CO, increases at 2% per year for 70 yr, the THC weakens by about 20%, and it stabilizes at this level if the CO, is su bsequently held constant. Changes in surface heat and water fluxes are the cause of the reduction in the steric height gradient that derives the THC w eakening, 60% being due to temperature change (greater warming at high lati tudes) and 40% to salinity change (decreasing at high latitude, increasing at low latitude). The level at which the THC stabilizes is determined by ad vective feedbacks. As the circulation slows down, less heat is advected nor thward, which counteracts the in situ warming. At the same time, northward salinity advection increases because of a strong increase in salinity in th e subtropical Atlantic, due to a greater atmospheric export of freshwater f rom the Atlantic to the Pacific. This change in interbasin transport means that salinity effects stabilize the circulation, in contrast to a single ba sin model of the THC, where salinity effects are destabilizing. These resul ts suggest that the response of the Atlantic THC to anthropogenic forcing m ay be partly determined by events occurring outside the Atlantic basin.