Box 3.2 A Minimal Model for the Thermohaline Circulation

For many years, the textbook example for explaining multiple states of the THC has been an idealized two-box model confined to a single hemisphere (Stommel, 1961), although a two-hemisphere version (Rooth, 1982) would be more appropriate to describe the Atlantic THC (Marotzke, 2000). Nevertheless, Stommel’s configuration (Figure 3.2) is useful to classify existing simulations of abrupt climate change involving the THC and to identify some crucial open questions concerning the future stability of the Atlantic’s THC.

FIGURE 3.2

decrease as well but then responds with a sudden transition to the reverse mode. This is an abrupt climate change according to the definition used in this document; the response is considerably faster than the change in forcing.

As for the real Atlantic and climate system, the crucial question is whether the THC is best characterized by the “lower” or by the “upper” case. Is there a possibility that a temporary perturbation in, say, the freshwater forcing can induce a permanent change in the THC? This would



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