The stability and internal variability of the ocean's thermohaline circulation is investigated using a coarse-resolution general circulation model of an idealized ocean basin, in one hemisphere. The model circulation is driven, in addition to wind forcing, by restoring the surface temperature to prescribed values, and by specifying freshwater fluxes in the surface salinity budget (mixed boundary conditions). All forcing functions are constant in time.
The surface freshwater forcing is the dominant factor in determining the model's stability and internal variability. Increasing the relative importance of freshwater flux versus thermal forcing, in turn, one stable steady state of the model, two stable ones, one stable, and one unstable equilibrium, or no stable steady states at all are found. If the freshwater forcing is sufficiently strong, self-sustained oscillations exist in the deep-water formation rate, which last thousands of years. One type of oscillation occurs on the time scale of decades and is associated with the advection of high-latitude salinity anomalies. The other type has a diffusive time scale of centuries or longer and marks periods of complete absence of deep-water formation followed by violent overturning events (flushes).
When a stochastic component is added to the steady freshwater flux forcing, internal decadal variability persists if the background steady freshwater flux is sufficiently strong. Periodic flushes also exist under stochastic forcing; with increasing magnitude of the stochastic term the frequency of the flush events increases while their intensity decreases.