Interhemispheric thermohaline circulation in a coupled box model

interhemispheric thermohaline circulation is examined using Rooth's three-b ox ocean model, whereby overturning strength is parameterized from density differences between high-latitude boxes. Recent results with general circul ation models indicate that this is a better analog of the Atlantic thermoha line circulation than a single-hemisphere box model. The results are compar ed with those of hemispheric box model studies, where possible, and the rol e of asymmetrical freshwater forcing is explored. Using both analytical and numerical methods, the linear and nonlinear stabi lity of the model is examined. Although freshwater forcing in the Southern Hemisphere alone governs overturning strength, increasing fresh water forci ng in the Northern Hemisphere leads to a heretofore unrecognized instabilit y in the northern sinking branch due to an increasingly positive ocean sali nity feedback. If the northern forcing is instead made weaker than the sout hern forcing, this feedback becomes negative.;In contrast, the ocean salini ty feedback is always positive in single-hemisphere models. Nonlinear stabi lity, as measured by the size of the perturbation necessary to induce a per manent transition to the southern sinking equilibrium, is also observed to depend similarly on the north-south forcing ratio. The model is augmented with explicit atmospheric eddy transport parameteriz ations, allowing examination of the eddy moisture transport (EMT) and eddy heal transport (EHT) feedbacks. As in the hemispheric model, the EMT feedba ck is always destabilizing, whereas;the EHT may stabilize or destabilize. H owever, in this model whether the EHT stabilizes or destabilizes depends la rgely on the sign of the ocean salinity feedback and the size of the pertur bation. Since oceanic heat transport in the Southern Hemisphere is weak, th e Northern Hemisphere EMT and EHT feedbacks dominate.