Instability of the thermohaline ocean circulation on interdecadal time scales

Abstract

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.