Density staircases are observed in an idealised model of a deep western
boundary current upon crossing the equator. We propose that the
staircases are generated by the excitement of symmetric instability as
the current crosses the equator. The latitude at which symmetric
instability is excited can be predicted using simple scaling arguments.
Symmetric instability generates overturning cells which, in turn, cause
the inhomogenous mixing of waters with different densities. The mixing
barriers and well mixed regions in density profiles coincide,
respectively, with the boundaries and centres of the overturning cells
generated by the symmetric instability. This new mechanism for producing
density staircases may require us to re-evaluate the origins of some of
the density staircases observed in the Tropical Atlantic.