Modeling equator-crossing currents on the ocean bottom

Abstract

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 crossequatorial 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.

Shallow water modeling of Antarctic Bottom Water crossing the equator

“…It is shown that the lack of fluid inertia in the FG model prevents the fluid from flowing against a dynamic pressure gradient, significantly restricting the motion. This is consistent with Choboter and Swaters [2000], who found that the FG model did not allow currents to flow as high on idealized topography as the shallow water model.…”

Two-Layer Models of Abyssal Equator-Crossing Flow

“…Notable differences between the shallow-water simulation and the FG simulation include the fact that the shallow-water lens penetrates the equator as it turns eastward where the FG eddy does not and that fluid in the shallow-water simulation flows to a higher point on the eastern slope than fluid in the FG simulation. These two differences hold over the whole range of parameters tested, and both are a result of the lack of fluid inertia in the FG model (Choboter and Swaters 2000).…”

“…This confirms that the EMI model derived is, indeed, a generalization of the Swaters and Flierl (1991) model to the case of varying Coriolis parameter, including allowing for f ϭ 0 in the domain. Choboter and Swaters (2000) investigated the dynamics of equator-crossing currents using simplified onelayer models and idealized topography. To explore the effects of baroclinicity on near-equatorial motions in the EMI model, we reproduce the simulations of Choboter and Swaters (2000), with and without a dynamically active upper layer, and quantitatively compare the results (Figs.…”

“…3-7). Choboter and Swaters (2000) compared the dynamics of a ''frictional geostrophic'' (henceforth FG) model with the dynamics of the shallow-water equations. The FG model may be written…”

“…This was done to observe the breakdown of geostrophic balance in the fluid as it approached the equator, and for comparison with the similar shallow-water simulations of Borisov and Nof (1998). The channel used by Choboter and Swaters (2000) was specified to be h B ϭ , and the latitudinal variation of the Coriolis 2 ͙x ϩ 1 parameter was f ϭ tanh(y). The same functional form of the topography and Coriolis parameter are used here.…”