FIGURE 3.4 Response to the thermohaline circulation (THC) to a slow increase and subsequent slow decrease in freshwater forcing for the two cases in Figure 3.3.

concerns the response to a sudden external perturbation, such as a disintegrating ice sheet. Manabe and Stouffer (1995) released a short, strong pulse of freshwater (1 Sv = 106 m3/s for 10 years) into the northern North Atlantic in their coupled model. The THC responded instantaneously with a reduction of almost 70 percent within a decade but recovered to the original strength within less than 200 years. Although the THC shut down almost completely, a threshold was not crossed, and no transition to a new equilibrium was simulated. The model does, however, have a second equilibrium that is achieved if the perturbation is much stronger (Manabe and Stouffer, 1999). It appears, therefore, that the behavior of the model is qualitatively similar to the standard case of the Stommel two-box model (see Plate 6, orange line). Depending on the amplitude of the perturbation, the coupled GCM thus exhibits either a transition to a new stable state or a reversible change (which might still be abrupt and of large amplitude). Manabe and Stouffer (1999) also showed that simply increasing the vertical diffusivity in their model led to a situation with a single equilibrium; this implies that processes other than those discussed above could qualitatively change the hysteresis structure of the model.

Behavior similar to that in Manabe and Stouffer’s (1995) model was observed by Schiller et al. (1997), albeit because of a different forcing. They

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