| The thermohaline circulation (THC) in the North Atlantic plays a vital role in explaining past abrupt climate changes and in maintaining the current climate. Its remarkable nonlinear dynamics, first demonstrated by Stommel, has been supported by different types of models. This has led to concern that global warming may shut down the THC irreversibly, with consequent catastrophic climate changes, particularly for Europe. However, recent simulations by complex atmosphere/ocean general circulation models show a great suppression of the nonlinear response of the THC to external freshwater forcing. In this study a suite of models are used to investigate the nonlinear response of the THC to freshwater addition. It is found that the THC simulated by an ocean general circulation model responds very differently depending on whether it is uncoupled or coupled to an atmosphere general circulation model. The THC shuts down irreversibly in the uncoupled ocean general circulation model (OGCM) simulations, but reversibly in the coupled atmosphere/ocean general circulation model (AOGCM) simulation. This occurs because of a crucial negative feedback in the AOGCM simulation that cannot occur in the OGCM simulations. Analysis of Stommel's 2-box ocean model within different parameter regimes supports this finding. Thus, the irreversible shutdown of the THC caused by freshwater addition appears to be a model artefact rather than a likely outcome of global warming. |
