Salinity-dominated thermohaline circulation in sill basins: can two stable equilibria exist?

The dynamics of a salinity-dominated thermohaline circulation in a sill basin is examined using a two-layer model. A prescribed freshwater supply acts to establish a stable stratification, working against a prescribed destabilizing temperature difference. The upper-layer outflow is in geostrophic balance and the upwelling is driven by a fixed energy supply to small-scale vertical mixing. The salinity-dominated flow may have two qualitatively different modes of operation. First, a mixing-limited regime, where the upper layer is shallower than the sill and the flow strength decreases with increasing density difference. Second, an overmixed regime, where the upper layer extends below the sill and the flow strength increases with density difference. Possibly, mixing-limited and overmixed equilibria, with widely different upper-layer depths, can exist for the same external parameters. In such cases, transitions between the two regimes are associated with abrupt changes of the salinity, depth and flow strength. The present results may be of relevance for ocean circulation in glacial climates and for interpretations of marine palaeo data, issues that are briefly discussed in the context of the Arctic Ocean.