A close link between the formation of global-scale water masses, such as North Atlantic Deep Water (NADW) and Antarctic Intermediate Water (AAIW), and two stable modes of the thermohaline circulation (THC) is investigated in a coupled model. In the upper 2-3 km of the Atlantic, the THC modes are characterized by meridional overturning circulations of opposite sign, with either a dominance of the AAIW cell over the NADW cell ("off" THC model or vice versa ("on" THC mode). A transition between these THC modes is controlled by the relationship between the densities in the source regions of formation of AAIW and NADW water masses. This is shown by applying a freshwater perturbation in the region of enhanced AAIW formation in the Southern Ocean to obtain a hysteresis loop of the NADW circulation. Transitions between the two modes of the THC occur when the densities in the source regions of AAIW and NADW become comparable to each other.
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