Volume 8, Number 4, Winter 2000
MODELING EQUATOR-CROSSING CURRENTS
ON THE OCEAN BOTTOM
PAUL F. CHOBOTER AND GORDON E. SWATERS
Abstract. Observations unambiguously show that deep
ocean currents carry a significant amount of fluid across the
equator. Away from the equator in either hemisphere, these
currents are relatively quiescent so that planetary vorticity
dominates relative vorticity within the fluid. Thus, the potential vorticity
of cross-equatorial flow changes sign en route.
The breakdown of geostrophic balance at the equator because
of the vanishing horizontal component of the Coriolis force
and the fact that potential vorticity is not conserved in these
flows constitute formidable challenges to modeling these cross equatorial currents.
Recent research points to friction as being crucial to the
crossing process since it provides the mechanism by which
potential vorticity can be altered. As well, since these flows
are bottom-dwelling currents, the geometry of the bottom
topography is an important factor in determining the portion
of the current which successfully crosses the equator.
We examine the dynamical balances within equator-crossing
flows by studying a simplified model of the flow in the equatorial region.
This model retains the effects of friction and bottom
topography. We compare the predictions of this model
with the predictions of more sophisticated numerical models
and with observations. It is shown that, despite the simplicity
of the model, it captures certain aspects of the flow quite well.