Destabilization of the Thermohaline Circulation by Atmospheric Eddy Transports

Simple process models have been developed to investigate the role of atmosphere-ocean feedbacks in the stability of the current mode of the thermohaline circulation in the North Atlantic. A positive feedback between the meridional atmospheric transport of moisture and the high-latitude sinking thermohaline circulation (EMT feedback) has been found to help destabilize the latter. The minimum perturbation required to shut off the high-latitude sinking is considerably smaller when this feedback is included. Also, the high-latitude sinking is shut off much faster with this feedback than without it, given a perturbation of the same magnitude. There is also a strong positive feedback between atmospheric heat transport and the thermohaline circulation, but this can be modeled accurately on the global scale by using a properly tuned Newtonian cooling law for the surface heat flux. Idealized flux adjustment experiments suggest that the sensitivity of the real climate is not represented well in coupled atmosphere-ocean general circulation models that require O(1)adjustments in the surface fluxes of heat and freshwater to simulate the current climate.