The stability of three-dimensional thermally driven ocean flows in a single
hemispheric sector basin is investigated using techniques of numerical bifur-
cation theory.Under restoring conditions for the temperature,the flow is
stable.However,when forced with the associated heat flux,an interdecadal
oscillatory time scale instability appears.This occurs as a Hopf bifurcation
when the horizontal mixing coeffcient of heat is decreased.The physical
mechanism of the oscillation is described by analyzing the potential energy
changes of the perturbation flow near the Hopf bifurcation.In the relatively
slow phase of the oscillation,a temperature anomaly propagates westwards
near the northern boundary on a background temperature gradient,thereby
changing the perturbation zonal temperature gradient,with corresponding
changes in meridional overturning.This is followed by a relatively fast phase
in which the zonal overturning reacts to a change in sign of the perturbation
meridional temperature gradient.The different responses of zonal and merid-
ional overturning cause a phase difference between the effect of temperature
and vertical velocity anomalies on the buoyancy work anomaly,the latter
dominating the changes in potential energy.This phase difference eventually
controls the time scale of the oscillation