On the basic structure of oceanic gravity currents |
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Authors: | Achim Wirth |
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Institution: | (1) LEGI / MEOM, CNRS, BP 53, 38041 Grenoble Cedex 9, France |
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Abstract: | Results from numerical simulations of idealised, 2.5-dimensional Boussinesq, gravity currents on an inclined plane in a rotating
frame are used to determine the qualitative and quantitative characteristics of such currents. The current is initially geostrophically
adjusted. The Richardson number is varied between different experiments. The results demonstrate that the gravity current
has a two-part structure consisting of: (1) the vein, the thick part that is governed by geostrophic dynamics with an Ekman
layer at its bottom, and (2) a thin friction layer at the downslope side of the vein, the thin part of the gravity current.
Water from the vein detrains into the friction layer via the bottom Ekman layer. A self consistent picture of the dynamics
of a gravity current is obtained and some of the large-scale characteristics of a gravity current can be analytically calculated,
for small Reynolds number flow, using linear Ekman layer theory. The evolution of the gravity current is shown to be governed
by bottom friction. A minimal model for the vein dynamics, based on the heat equation, is derived and compares very well to
the solutions of the 2.5-dimensional Boussinesq simulations. The heat equation is linear for a linear (Rayleigh) friction
law and non-linear for a quadratic drag law. I demonstrate that the thickness of a gravity current cannot be modelled by a
local parameterisation when bottom friction is relevant. The difference between the vein and the gravity current is of paramount
importance as simplified (streamtube) models should model the dynamics of the vein rather than the dynamics of the total gravity
current. In basin-wide numerical models of the ocean dynamics the friction layer has to be resolved to correctly represent
gravity currents and, thus, the ocean dynamics. |
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Keywords: | Ocean dynamics Gravity current |
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