Shallow water modeling of Antarctic Bottom Water crossing the equator

Abstract

[1] The dynamics of abyssal equator-crossing flows are examined by studying simplified models of the flow in the equatorial region in the context of reduced-gravity shallow water theory. A simple ''frictional geostrophic'' model for one-layer crossequatorial flow is described, in which geostrophy is replaced at the equator by frictional flow down the pressure gradient. This model is compared via numerical simulations to the one-layer reduced-gravity shallow water model for flow over realistic equatorial Atlantic Ocean bottom topography. It is argued that nonlinear advection is important at key locations where it permits the current to flow against a pressure gradient, a mechanism absent in the frictional geostrophic model and one of the reasons this model predicts less cross-equatorial flow than the shallow water model under similar conditions. Simulations of the shallow water model with an annually varying mass source reproduce the correct amplitude of observed time variability of cross-equatorial flow. The time evolution of volume transport across specific locations suggests that mass is stored in an equatorial basin, which can reduce the amplitude of time dependence of fluid actually proceeding into the Northern Hemisphere as compared to the amount entering the equatorial basin. Observed time series of temperature data at the equator are shown to be consistent with this hypothesis.

Deep variability in the Atlantic Western Boundary Current based on altimetric and expendable bathythermograph data

“…Our results can aid in the partitioning between density‐driven and wind‐driven flows from a given deep current measurement or estimate. For example, how much of the annual signal studied by Choboter and Swaters [2004] is, as assumed by the authors, related to seasonal changes in the rate of Antarctic Bottom Water (AABW) production and how much of it can, in fact, be attributed to wind‐forcing (which they neglected)?…”

“…Sea surface slopes generated by the divergence of Ekman transport at the coast have been proposed as a mechanism that relates alongshore wind stress to WBC transport [ Lee and Williams , 1988; Greatbatch and Goulding , 1990; Kubota et al , 1995; Lee et al , 2001]. A third mode of WBC variability has been suggested on the basis of numerical experiments that show a correlation between the rate of Antarctic Bottom Water formation and Deep Western Boundary Current (DWBC) transport at the equatorial region [ Choboter and Swaters , 2004].…”

Hamiltonian Structure and a Variational Principle for Grounded Abyssal Flow on a Sloping Bottom in a Mid‐Latitude β‐Plane

“…because h(x, y) + h B (x, y) is the geostrophic pressure. In practice one determines τ (x, y) from (9) and substitutes into (8) to determine h(x, y).…”

Flow of grounded abyssal ocean currents along zonally-varying topography on a rotating sphere

“…In addition, the model we examine here cannot describe the cross-equatorial flow of grounded abyssal currents where the assumptions of a geostrophically balanced flow must break down (see, e.g. Edwards and Pedlosky 1998a,b, Nof and Borisov 1998, Choboter and Swaters 2003, 2004, or the super-inertial instability associated with frictional super-critical abyssal overflows (e.g. Swaters 2009).…”