Freshwater forcing as a booster of thermohaline circulation

Making use of a simple two‐layer model, we analyze the impact of freshwater forcing on the thermohaline circulation. We consider the forward‐type circulation dominated by thermal forcing, implying that the freshwater forcing acts to reduce the density contrast associated with the equator‐to‐pole temperature contrast (prescribed in the model). The system is described by two variables: the depth of the upper layer (H) and the density contrast between the upper and lower layer (Δρ), which decreases with salinity contrast. The rate of poleward flow of light surface water and the diapycnal flow (i.e., upwelling) driven by widespread small‐scale mixing are both modeled in terms of H and Δρ. Steady states of thermohaline circulation are found when these two flows are equal. The representation of the diapycnal flow (MD) is instrumental for the dynamics of the system. We present equally plausible examples of a physically based representation of MD for which the thermohaline circulation either decreases or increases with density contrast. In the latter case, contrary to the traditional wisdom, the freshwater forcing amplifies the circulation and there exists a thermally dominated equilibrium for arbitrary intensity of freshwater forcing. Here, Stommel's famous feedback between circulation and salinity contrast is changed from a positive to a negative feedback. The interaction of such a freshwater boosted thermohaline circulation with the climate system is fundamentally different from what is commonly assumed, an issue which is briefly addressed.