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Nonlinear Rossby adjustment in a channel: beyond Kelvin waves

Published online by Cambridge University Press:  26 April 2006

Albert J. Hermann ,
Peter B. Rhines  and
E. R. Johnson
Albert J. Hermann
Affiliation:
School of Oceanography, WB-10, University of Washington, Seattle, WA 98195, USA Present address: Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA.
Peter B. Rhines
Affiliation:
School of Oceanography, WB-10, University of Washington, Seattle, WA 98195, USA
E. R. Johnson
Affiliation:
Department of Mathematics, University College London, WC1E 6BT, UK
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Abstract

Nonlinear advective adjustment of a discontinuity in free-surface height under gravity and rotation is considered, using the method of contour dynamics. After linear wave-adjustment has set up an interior jet and boundary currents in a wide ([Gt ] one Rossby radius) channel, fluid surges down-channel on both walls, rather than only that wall supporting a down-channel Kelvin wave. A wedgelike intrusion of low potential vorticity fluid on this wall, and a noselike intrusion of such fluid on the opposite wall, serve to reverse the sign of relative vorticity in the pre-existing currents. For narrower channels, a coherent boundary-trapped structure of low potential vorticity fluid is ejected at one wall, and shoots ahead of its parent fluid. The initial tendency for the current to concentrate on the ‘right-hand’ wall (the one supporting a down-channel Kelvin wave in the northern hemisphere) is defeated as vorticity advection shifts the maximum to the left-hand side. Ultimately fluid washes downstream everywhere across even wide channels, leaving the linearly adjusted upstream condition as the final state. The time necessary for this to occur grows exponentially with channel width. The width of small-amplitude boundary currents in linear theory is equal to Rossby's deformation radius, yet here we find that the width of the variation in velocity and potential vorticity fields deviates from this scale across a large region of space and time. Comparisons of the contour dynamics solutions, valid for small amplitude, and integration of the shallow-water equations at large amplitude, show great similarity. Boundary friction strongly modifies these results, producing fields more closely resembling the linear wave-adjusted state. Observed features include those suggestive of coastally trapped gravity currents. Analytical results for the evolution of vorticity fronts near boundaries are given in support of the numerical experiments.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 205 , August 1989 , pp. 469 - 502
Copyright
© 1989 Cambridge University Press

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