Response of the thermohaline circulation to cold climates

A coupled atmosphere-ocean-sea ice-land surface-ice sheet model of intermediate complexity, the so-called McGill Paleoclimate Model, is employed to study the response of the thermohaline circulation (THC) to various global climate coolings, which are realized by increasing the present-day planetary emissivity to various values. Generally, it is found that the response of the THC to global cooling is nonlinear: For a slightly cold climate the THC in the North Atlantic and the Pacific upwelling become intensified. For a very cold climate the THC in the North Atlantic may be weakened or even collapsed. The associated Pacific upwelling for a very cold climate also becomes weak when the THC is weakened, and intermediate deep water may form in the Pacific when the THC is collapsed. Some support for this nonlinear response is found in recent paleoceanographic data. The reduced atmospheric poleward moisture transport due to the global cooling is mainly responsible for the intensification of the THC in the North Atlantic for a slightly cold climate. For a very cold climate the global cooling may lead to a decrease of the meridional surface density gradient and an increase of the vertical density difference (lower layer density minus upper layer density) in the deep water formation region, which can weaken or shut down the THC. It is the temperature-dependent part of the density differences that is mainly responsible for the weakening or shutting down of the THC. The potential influence of surface temperature changes must be taken into account for a full understanding of the role of the THC in the climate system