Freshwater forcing as a booster of thermohaline circulation

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Abstract
Making use of a simple two-layer model, we analyze the impact of freshwater forcing on thethermohaline circulation. We consider the forward-type circulation dominated by thermal forcing,implying that the freshwater forcing acts to reduce the density contrast associated withthe equator-to-pole temperature contrast (prescribed in the model). The system is described bytwo variables: the depth of the upper layer (H) and the density contrast between the upper and lower layer (∆p), which decreases with salinity contrast. The rate of poleward flow of lightsurface water and the diapycnal flow (i.e., upwelling) driven by widespread small-scale mixingare both modeled in terms of H and Dr. Steady states of thermohaline circulation are foundwhen these two flows are equal. The representation of the diapycnal flow (MD) is instrumentalfor the dynamics of the system. We present equally plausible examples of a physically basedrepresentation of MD) for which the thermohaline circulation either decreases or increases withdensity contrast. In the latter case, contrary to the traditional wisdom, the freshwater forcingamplifies the circulation and there exists a thermally dominated equilibrium for arbitrary intensityof freshwater forcing. Here, Stommel’s famous feedback between circulation and salinitycontrast is changed from a positive to a negative feedback. The interaction of such a freshwaterboosted thermohaline circulation with the climate system is fundamentally different from whatis commonly assumed, an issue which is briefly addressed. DOI: 10.1034/j.1600-0870.2001.00263.x

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