Interhemispheric thermohaline circulation in a coupled box model

Abstract

The interhemispheric thermohaline circulation is examined using Rooth's three-box ocean model, whereby overturning strength is parameterized from density differences between high-latitude boxes. Recent results with general circulation models indicate that this is a better analog of the Atlantic thermohaline circulation than a single-hemisphere box model. The results are compared with those of hemispheric box model studies, where possible, and the role of asymmetrical freshwater forcing is explored. Using both analytical and numerical methods, the linear and nonlinear stability of the model is examined. Although freshwater forcing in the Southern Hemisphere alone governs overturning strength, increasing freshwater forcing in the Northern Hemisphere leads to a heretofore unrecognized instability in the northern sinking branch due to an increasingly positive ocean salinity feedback. If the northern forcing is instead made weaker than the southern forcing, this feedback becomes negative. In contrast, the ocean salinity feedback is always positive in single-hemisphere models. Nonlinear stability, as measured by the size of the perturbation necessary to induce a permanent 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 parameterizations, allowing examination of the eddy moisture transport (EMT) and eddy heat transport (EHT) feedbacks. As in the hemispheric model, the EMT feedback is always destabilizing, whereas the EHT may stabilize or destabilize. However, in this model whether the EHT stabilizes or destabilizes depends largely on the sign of the ocean salinity feedback and the size of the perturbation. Since oceanic heat transport in the Southern Hemisphere is weak, the Northern Hemisphere EMT and EHT feedbacks dominate.