Two different two-dimensional models of the thermohaline circulation o
f the ocean have been used to study the loss of stability of a thermal
ly dominated symmetrical two-cell circulation. Although the models dif
fer in their momentum budget, their behavior was qualitatively similar
: the symmetrical solution was found to lose its stability at a critic
al strength of the salinity forcing with respect to two asymmetrical s
olutions that are mirror images of each other. The supercritical pitch
fork bifurcation that describes this process was calculated with a num
erical continuation technique. An analysis of the linear stability of
the system yields an eigenfunction structure that allows the identific
ation of the processes causing the instability. For the current surfac
e forcing and parameter values that put the system in a thermal regime
, this physical mechanism can be understood from meridional advection
of salt and heat anomalies alone. This explains why the phenomenon of
symmetry breaking is observed in such a wide range of studies and that
model properties such as the choice of the convection scheme are only
of quantitative importance.