On the spontaneous transition to asymmetric thermohaline circulation

Abstract

The stability of symmetric thermally dominated thermohaline circulation in a two-hemisphere basin is explored with the aid of a conceptual as well as a numerical model. The conceptual model has two layers, representing the water masses of the thermocline and the deep ocean, respectively. The stability of the symmetric solutions of the model to small symmetric or antisymmetric perturbations are considered. It is found that the symmetric equilibria are considerably more sensitive to antisymmetric than to symmetric perturbations. The reason is that a symmetric flow anomaly by necessity causes a perturbation of the thermocline depth; a state of affairs which provides a negative feedback. The strength of this stabilizing feedback depends on the properties of the vertical mixing in the interior ocean. In contrast, the antisymmetric flow perturbations assume a pole-to-pole overturning pattern with negligible vertical motion at low latitudes; this essentially removes the stabilizing feedback due to thermocline’depth adjustment. As a consequence, the dynamics of antisymmetric perturbations are basically independent of the vertical mixing. It is concluded that the symmetric circulation is always unstable to antisymmetric perturbations if Δρs/ΔρT > 1/2, where Δρs and ΔρT are the equator-to-pole density contrasts created by salinity and temperature, respectively. However, if the interhemispheric density gradients in the deep ocean are assumed to strengthen the antisymmetric flow anomaly, the instability should occur for a considerably smaller value of Δρs/ΔρT. Numerical simulations are presented that support the theoretical results and furthermore suggest that the density gradients in the deep ocean indeed augment the antisymmetric flow perturbations, thereby acting to destabilize the symmetric branch of thermally dominated circulation.