Several numerical experiments are conducted with a zonally-averaged latitude-depth model developed by Wright and Stocker (1992) to study the thermohaline circulation in an isolated two-hemisphere ocean basin, with asymmetric basin geometry and surface forcings.
It is shown that the steady southern-sinking circulation obtained under restoring boundary conditions is unstable after a switch to mixed boundary conditions and eventually under-goes a transition to a stable northern-sinking circulation. The instability and transition of the thermohaline circulation consists of four characteristic phases: unstable oscillation with century time-scale, collapse during which a halocline catastrophe occurs and the overturning circulation breaks down, quiescence in which the thermohaline circulation is in a collapsed state except for the Ekman overturning circulation in the upper ocean, and establishment of a reverse overturning circulation during which convection at high northern latitudes is triggered by diffusion and deep water formation gradually develops. The unstable oscillation is shown to be due to interaction among advection, convection, and the salinity flux boundary condition, in a way that leads to the growth of negative salinity anomalies and hence the halocline catastrophe. This instability mechanism is a combination of Bryans (1986) advective feedback mechanism and convective feedback mechanism.
It was demonstrated that a similar unstable oscillation may also occur in the Howard-Malkus thermohaline loop model, which is analogous to the Wright-Stocker model in internal dynamics. This unstable oscillation is caused by an advective mechanism, part of the instability mechanism above.
The advective-convective instability mechanism was investigated further in a series of sensitivity tests. In each case, the experimental results are consistent with the proposed instability mechanism.