Two-Layer Models of Abyssal Equator-Crossing Flow

Abstract

The role of baroclinicity in the dynamics of abyssal equator-crossing flows is examined by studying two-layer models of the flow valid in the equatorial region. Three new analytical models are derived from two-layer shallow-water theory. One of these models (Equatorial Model I, or EMI) reduces to the Swaters and Flierl coupled model in the midlatitude limit. In the equatorial limit, the lower-layer dynamics of EMI are that of the complete shallow-water equations, and the upper-layer dynamics are built upon quasigeostrophic potential vorticity conservation with a balance equation to relate the streamfunction and pressure. Simple numerical simulations are performed using this model to investigate its behavior in certain idealized situations, including equator-crossing lenses and currents. In the midlatitudes, the dynamics of EMI are characterized by strong baroclinic interactions between the layers, while near the equator all three models exhibit a partial decoupling of the layers. This motivates the use of a one-layer reduced-gravity model to simulate abyssal dynamics in the immediate vicinity of the equator. Such simulations are reported elsewhere. A uniformly valid metamodel is derived that contains all of the necessary terms so that it may reduce, in the appropriate parameter limit, to any of the three models derived here.