Effects of a wind-driven gyre on thermohaline circulation variability

A simplified model is used to study the possible effects of the horizontal upper-ocean wind-driven circulation (WDC) on the variability of the overturning meridional circulation driven by buoyancy fluxes. It is found that the added interaction with the WDC adds interesting new classes of variability. First, self-sustained variability of the thermohaline circulation (THC) becomes possible, on time scales of interdecades to a few centuries. Furthermore, these oscillations may be either small amplitude or large amplitude and either periodic or chaotic, depending on the amplitude of the freshwater forcing and on the strength of the WDC. Even a relatively weak WDC changes the well-known stability properties of the THC that are seen in numerous models of the THC alone. The variability modes found here may account for similar modes of variability observed in GCM studies.