The impact of changes in atmospheric and land surface physics on the thermohaline circulation response to anthropogenic forcing in HadCM2 and HadCM3

We have undertaken a comparative study of the mechanisms which drive the response of the Atlantic thermohaline circulation (THC) to a fourfold increase in CO₂ over 70 years with stabilisation thereafter in HadCM2 and HadCM3. In both models, the THC changes are driven by surface flux forcing, with advection (and diffusion in HadCM2) acting in the opposite sense to limit the circulation change. In both cases, heat fluxes are more important than those of freshwater. We find that different patterns of heat flux forcing in HadCM2 and HadCM3 are the prime determinants of the differing response in the two models. The increased northerly component to the near surface winds (associated with an increase in reflective low level cloud), leads to enhanced heat loss in the west-central North Atlantic, which in turn tends to steepen the steric gradient and strengthen the THC. By contrast, in HadCM3 the winds become more westerly rather than northerly, there is no dynamically-forced enhancement of surface heat loss, and the heat flux in the North Atlantic continues to be strongly positive, relative to the control, leading to a reduction in the meridional steric gradient, and a weaker overturning circulation. Differences in atmospheric response patterns appear to be caused by improvements to atmospheric and land surface physics, and suggest that the THC response in HadCM2 is less credible than in HadCM3.