A Reexamination of the polar Halocline Catastrophe and Implications for Coupled Ocean-Atmosphere Modeling

In this paper, the physical mechanism of the polar halocline catastrophe (PHC) is reexamined with emphasis on the role played by the surface heat flux. It is argued that, in a coupled ocean–atmosphere system, thermal changes in the atmospheric state in response to changes in heat flux from the ocean weaken the feedback responsible for the PHC. So far, the PHC has been observed in models that use mixed boundary conditions; that is, the freshwater flux is specified, but the surface temperature is relaxed to a specified value. Previous explanations of the PHC have focused on the role of the freshwater flux in establishing a freshwater cap and shutting off the deep convection. However, the establishment of a freshwater cap reduces the depth of the water column that is cooled by surface heat loss. As a consequence, the surface temperature is reduced. Since the difference between this and atmospheric restoring temperature is now less, there is a corresponding reduction in the surface heat loss to the a...