Multiple Equilibrium States in Combined Thermal and Saline Circulation

Abstract

Structure and stability of the multiple equilibria of the thermohaline circulation am studied using 2 ? 2 and 3 ? 2 box models. Thermohaline catastrophe is a shallow phenomenon and its time evolution consists of three stages: the search stage, the catastrophic stage, and the adjustment stage. A 3 ? 2 box model is introduced to permit fitting a somewhat less truncated form of freshwater forcing. Although the total number of possible modes of circulation increases quickly as the number of boxes is increased, the grand thermal mode and grand saline mode remain. Some multiple solutions are found to be statistically improbable, and they belong to the so-called minor modes. With the more realistic freshwater forcing appropriate for the present-day North Atlantic, our simple 3 ? 2 model predicts that the thermohaline cimulation is near a critical mate, because a small increase in freshwater flux can cause the system to collapse from the present thermally dominated circulation to an intermediate mode of circulation with strong sinking at midlatitude and a rather sluggish circulation in the polar basin.