A mechanistic model of ocean interdecadal thermohaline oscillations

Oceanic interdecadal thermohaline oscillations are studied in the context of a conceptual two box model. In this mechanistic model, two essential processes associated with convection, subsurface advective heating and surface freshening, are modeled in terms of horizontal relaxation processes, with the surface relaxation timescale shorter than the subsurface one. The model can oscillate with interdecadal periods within a certain parameter regime, and captures the fundamental characteristics of interdecadal oscillations: a clockwise trajectory in the T-S plane. The dependence of the amplitude and period of the oscillation on different model parameters is explored. It is shown that there are two important parameters in the model: 1) p, the ratio of the timescale of the surface freshening process to that of the subsurface advective heating process, and 2) q, the ratio of the saline forcing to the thermal forcing in maintaining the vertical halocline and inverted thermocline structure. Oscillatory solutions exist when p < q < 1. The period of the oscillation increases with q and decreases with p. When p is small enough, the period of the oscillation is mostly determined by q. The amplitude of the oscillation increases with the period.