We reconsider the problem of the stability of the thermohaline circulation as described by a two-dimensional Boussinesq model with mixed boundary conditions. We determine how the stability properties of the system depend on the intensity of the hydrological cycle. We define a two-dimensional parameters' space descriptive of the hydrology of the system and determine, by considering suitable quasi-static perturbations, a bounded region where multiple equilibria of the system are realized. We then focus on how the response of the system to finite-amplitude changes in the surface freshwater forcings depends on their rate of increase. We show that it is possible to define a robust separation between slow and fast regimes of forcing. Such separation between slow and fast regimes is obtained by singling out an estimate of the critical rate of increase for the anomalous forcing. The critical rate of increase is of the same order of magnitude of the ratio between the typical intensity of the hydrological cycle and the advective time scale of the system. Basically, if the change of the forcing is faster than the estimated critical rate, the system responds similarly to the case of instantaneous changes of the same amplitude. Specularly, if the change of the forcing is slower than the critical rate, the behavior of the system resembles the response to quasi-static changes of the same amplitude. Furthermore, since the advective time scale is proportional to the square root of the vertical diffusivity, our analysis supports the conjecture that the efficiency of the vertical mixing might also be one of the key factors determining the response of the THC system to transient changes in the surface forcings. These results should be taken into account when engineering global warming scenario and paleoclimatic experiments with GCMs. © Springer-Verlag 2005.

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