Is a shutdown of the thermohaline circulation irreversible?

The Atlantic thermohaline circulation (THC) plays a vital role in explaining past abrupt climate changes and in maintaining the current climate. Its remarkable nonlinear dynamics, first demonstrated by H. M. Stommel, have 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 an uncoupled ocean general circulation model (OGCM) and a coupled atmosphere‐ocean general circulation model (AOGCM) to investigate the nonlinear response of the THC to freshwater perturbations in the northern North Atlantic. We find that the THC shuts down irreversibly in the uncoupled OGCM simulations but reversibly in the coupled AOGCM simulations. This occurs because of different feedback processes operating in the uncoupled OGCM and AOGCM. The reversal of the THC in the uncoupled OGCM tends to stabilize the “off” mode of the THC by decreasing the mean salinity of the Atlantic, whereas a crucial negative feedback in the AOGCM helps the THC recover. This negative feedback results from complex air‐sea interactions, and its operation needs the full participation of the atmosphere. Thus given the more realistic simulation by the AOGCM, the irreversible shutdown of the THC caused by freshwater addition appears to be an artifact of the uncoupled OGCM rather than a likely outcome of global warming.