Instability of two-dimensional thermohaline circulation

Abstract

Two different two-dimensional models of the thermohaline circulation of the ocean have been used to study the loss of stability of a thermally dominated symmetrical two-cell circulation. Although the models differ in their momentum budget, their behavior was qualitatively similar: the symmetrical solution was found to lose its stability at a critical strength of the salinity forcing with respect to two asymmetrical solutions that are mirror images of each other. The supercritical pitchfork bifurcation that describes this process was calculated with a numerical continuation technique. An analysis of the linear stability of the system yields an eigenfunction structure that allows the identification of the processes causing the instability. For the current surface forcing and parameter values that put the system in a thermal regime, this physical mechanism can be understood from meridional advection of salt and heat anomalies alone. This explains why the phenomenon of symmetry breaking is observed in such a wide range of studies and that model properties such as the choice of the convection scheme are only of quantitative importance.