Changes in the Atlantic Meridional Overturning Circulation

Introduction: the concept of the MOC Climate models project a slow down of the Atlantic Meridional Overturning Circulation (MOC) in the twenty-first century. This slow down is expected to affect climate over Europe. In particular, it damps the temperature rise due to the emission of greenhouse gases. The MOC is associated with the thermohaline circulation (ThC), which is the large-scale ocean circulation driven by fluxes of heat and freshwater at the surface. It is difficult to define the ThC exactly. In the real world we cannot disentangle the various driving forces of the ocean circulation, and exclude, for instance, the wind forcing. Even in idealized models in which the wind stress is neglected, a ThC which only responds to the ‘push’ of surface waters, becoming convectively unstable and sinking, cannot be sustained. Also the ‘pull’ by small-scale mixing, that gradually lightens the deep waters, is necessary. And the main energy source for the small-scale mixing is mechanical (winds and tides). Clearly, the ThC is only a theoretical concept. For practical purposes, oceanographers prefer to speak of the MOC: The zonally-integrated mass transport in the Changes in the Atlantic Meridional Overturning Circulation