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 fresh-
water 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 fresh-water 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
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