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The Atlantic THC is a self-sustained nonlinear phenomenon, prone to instability and multiple equilibria. At present formation of deepwater in the Atlantic THC seems to be thermally driven, with a source of cold deepwater in the high-latitude Atlantic Ocean. However, variations of salinity in the deepwater source region may modify the THC. If, for example, the northward transport of saline subtropical waters decreases by ice berg melt, increased precipitation, and river runoff, the density of high-latitude waters may be reduced so that these waters are no longer able to sink according to Eq. (10.13). Then the THC with a near-polar deepwater source will terminate and the now more stagnant subtropical surface water may increase in salinity and density due to ongoing evaporation, until it reaches a density that allows deep convective mixing and subtropical downwelling. Then the circulation direction of the THC is reversed, and a salinity-driven THC will emerge. In order to study the occurrence of these different equilibria for the THC and the transition from one equilibrium to another, simple models for the THC are used with simplified boundary conditions that are assumed to present the essentials of the feedback processes in the THC.

The simple models discussed in the previous chapter suggest that the THC and climate may change due to feedback processes. The available oceanographic observations only cover the last 100 years, a period too short to display large changes of the THC. Geologists are able to study past changes in the climate and ocean circulation and numerical modelers are able to simulate such changes with coupled ocean-atmosphere models.