Dependence of multiple climate states on ocean mixing parameters

Multiple equilibria of the climate system, inferred from paleo reconstructi ons, have also been observed in both ocean-only and coupled ocean-atmospher e general circulation models. These multiple states are thought to be assoc iated with different modes of operation of the meridional overturning circu lation in the North Atlantic. It has recently been suggested that the stabi lity of these states depends on the amount of vertical mixing in ocean mode ls. Here we investigate the dependence of the hysteresis behaviour of the t hermohaline circulation to sub-gridscale mixing processes in the ocean. Usi ng a simplified coupled ocean-atmosphere model we find that both vertical a nd horizontal diffusivities have considerable influence on the stability of the different circulation modes. They also change the transition points fr om one circulation pattern to another. Larger vertical diffusivities lead t o higher values of additional precipitation into the Worth Atlantic being n ecessary to stop the formation of deep water. However, for slightly increas ed evaporation, the state without deep water formation becomes increasingly unstable for stronger vertical diffusion. Larger values of horizontal mixi ng lead to a narrowing of the phase space for which two equilibria are stab le. These results suggest that it is currently not possible, given the larg e uncertainty in ocean mixing, to quantitatively determine possible thresho lds for the transition of the North Atlantic thermohaline circulation betwe en on and off modes. This presents a policy predicament as it makes it extr emely difficult to assign a probability for the future occurence of such no nlinear climate transition.