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A damped oscillatory mode of the thermohaline circulation (THC), which may play a role in interdecadal climate variability, is identified in a global primitive equation model. This analysis is done under mixed boundary conditions using an adjoint of the primitive equation model. The linearized versus nonlinear stability behavior of the model is studied by comparing the adjoint analysis to runs of the fully nonlinear model. It is shown that a steady-state solution obtained under larger amplitude freshwater surface forcing (and hence with a weaker North Atlantic overturning) is unstable, while a steady- state solution with stronger THC is stable. In a certain intermediate parameter regime it is found that the full nonlinear model state may be unstable, while the linearized analysis indicates that the model state is stable. It is proposed that this may be because either the instability mechanism at this intermediate regime is nonlinear or, while the model is linearly stable at this regime, it allows for temporary growth of small perturbations due to the non-normal nature of the problem. A clear signal of variations is not found in the amplitude of the horizontal gyre circulation, possibly indicating that the gyre effect that was found in THC oscillations in some previous studies may not be essential for the existence of the THC oscillation. The long timescale of the oscillation in the present model also seems to indicate that the gyre effect may not be a main active participant in the thermohaline oscillation mechanism.

Identifying a Damped Oscillatory Thermohaline Mode in a General Circulation Model Using an Adjoint Model