Imperfections of the thermohaline circulation: Latitudinal asymmetry and preferred northern sinking

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