Thermohaline Oscillations Induced by Strong Steady Salinity Forcing of Ocean General Circulation Models

A series of numerical experiments is conducted with a three-dimensional ocean general circulation model and a two-dimensional counterpart both designed for efficient integration over diffusive (millennial) time scales. With strong steady salinity fluxes (salting at low latitudes and freshening at high), basin mean temperature and several other diagnostics show a series of self-sustaining oscillations. The oscillations termed deep decoupling oscillations, exhibit halocline catastrophes at regular intervals, followed by warming deep decoupled phases (when the deep overturning is weak), cooling flushes, and in the lower range of salinity forcing, a coupled phase when the deep ocean advective/diffusive heat balance is almost, but not quite, met. It is suggested that oscillations arise when a steady overturning circulation encounters a contradiction: the poleward salt and heat transport needed to maintain convection in the polar ocean requires more overturning than is consistent with the reduced thermocline depth that results. This hypothesis is supported by the sensitivity to variations in the vertical diffusivity: increased vertical diffusivity stabilizes oscillating solutions into steady, thermally direct circulations. Although deep decoupling oscillations appear in both two- and three-dimensional models, they occur over a much broader range of forcing in the three-dimensional model. This is shown to be due to heat and salt transports by the horizontal plane (gyre) motions in the three-dimensional model that intensify in the upper polar ocean in response to the formation of a halocline and eventually destabilize it. Increasing the wind stress in the three-dimensional model and the horizontal diffusivity in the two-dimensional model stabilizes oscillating solutions. The amplitude, shape, and period of the oscillations are also sensitive to the strength of the salinity forcing. Another kind of oscillation, termed a loop oscillation, with a smaller amplitude and an overturning time scale, is found in some of the more weakly forced experiments with both models. These oscillations are shown to be a result of the advection of salinity anomalies by the deep overturning, affecting its strength in a manner that leads to their further amplification by feedback from the salinity flux boundary condition. A simple thermohaline loop model demonstrates the essential advective mechanism for this kind of oscillation.