Imperfections of the Thermohaline Circulation: Latitudinal Asymmetry and Preferred Northern Sinking*

The present Atlantic thermohaline circulation is dominated by deep water formation in the north despite the fact that surface buoyancy forcing has relatively modest latitudinal asymmetry. Many studies have shown that even with buoyancy forcing that is symmetric about the equator, spontaneous symmetry breaking can produce a single overturning cell with intense sinking in the north. This occurs by salt advection at sufficiently large freshwater forcing. In this symmetry-breaking case, a southern-sinking solution and a symmetric solution are also possible. A simple coupled ocean–atmosphere model of the zonally averaged thermohaline circulation is used to examine the effect of latitudinal asymmetries in the boundary conditions. The greater continental area in the Northern Hemisphere, combined with the slight asymmetry in the observed freshwater flux, induce a strong preference for the northern-sinking solution. Examining the relation to the solution under symmetric conditions, the salt-advection mechanism still acts to enhance the overturning circulation of the northern-sinking branch, but multiple equilibria are much less likely to occur within the realistic parameter range. The most plausible shift between equilibria for paleoclimate applications would be between a strong northern-sinking branch and a weak northern-sinking branch that is an asymmetric version of the thermally driven solution. However, this is possible only in a very limited range of parameters. There is a substantial parameter range where the northern-sinking branch is unique. The role of the fractional region of air–sea interaction at each latitude is substantial in producing north–south asymmetry.