The Role of Temperature Feedback in Stabilizing the Thermohaline Circulation

Abstract

Ocean climate models traditionally compute the surface heat flux with a restoring boundary condition of the form Q = ?(T* ? To). This implies an atmosphere of fixed temperature and breaks down when large-scale changes in the ocean circulation are considered, which have a feedback effect on atmospheric temperatures. To include this important feedback, a new thermal boundary condition of the form Q = ?(T* ? To) ? ??2(T* ? To) is proposed. This is derived from an atmospheric energy balance model with diffusive lateral heat transport. The effects of this new parameterization are examined in experiments with the GFDL modular ocean model for two model basins. ?Conveyor belt? circulation states are compared using traditional mixed boundary conditions and our new coupling. With the new coupling, a realistic temperature contrast is obtained between the North Atlantic and the Pacific, caused by free adjustment of surface temperature to the oceanic heat transport. The results show that a temperature feedback involving horizontal heat transport regulates the overturning rate of the conveyor. A second feedback involving vertical convection of heat stabilizes the conveyor belt when freshwater anomalies are added to the North Atlantic, making it harder to interrupt convection and trigger a halocline catastrophe.