Is a Shutdown of the Thermohaline Circulation Irreversible?

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 various types of climate models. This has led to severe concerns that global warming may shut down the THC irreversibly, with consequent catastrophic climate changes, particularly for Europe. Here we use a suite of models to investigate the nonlinear response of the THC to freshwater perturbation in the northern North Atlantic. We find that 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. This negative feedback results from complex air-sea interactions and its operation needs detailed and comprehensive simulation about the climate system. Analysis of Stommel's 2-box ocean model supports this finding. The simulations from the uncoupled OGCM and AOGCM correspond to different parameter regimes of the 2-box model. Thus, the irreversible shutdown of the THC caused by freshwater addition appears to be a model artifact rather than a likely outcome of global warming. 2 1. Introduction The importance of the Atlantic thermohaline circulation (THC) stems from its two unique properties: its huge northward heat transport and remarkable nonlinear dynamical behavior. Most previous research on the THC focused on issues related to these two properties. For example, it has been shown that the reduction of the northward heat transport induced by the shutdown of the THC can result in considerable cooling over the northern If such a cooling actually occurred, it could be detrimental to the world's environment and economy. Based on contemporary climate model simulations, the Third Assessment Report of the Intergovernmental Panel on Climate Change found that this scenario cannot be completely excluded if the concentrations of greenhouse gases (GHG) continues to increase [Cubasch et al., 2001]. Consideration of the possible future slowdown/shutdown of the THC requires knowledge of the dynamical behavior of the THC. the THC is a highly nonlinear dynamical system. In response to a sequential increase and then decrease of the freshwater input to the northern North Atlantic, the THC in these simplified models exhibits bistability, bifurcation, hysteresis and irreversibility. Such an input of freshwater into the North Atlantic Ocean is what might occur …