In:
Journal of Fluid Mechanics, Cambridge University Press (CUP), Vol. 399 ( 1999-11-25), p. 205-236
Abstract:
We report new experiments with the ‘sliced-cylinder’
β-plane model of Pedlosky & Greenspan (1967) and Beardsley (1969), but with a much wider basin such that the western boundary current and its eddies occupy a small fraction of the basin width.
These experiments provide new insights into nonlinear aspects of the flow: th e critical
conditions for boundary current separation and the transition from stable to unstable flow are redefined, and a further transition from periodic to chaotic eddy shedding
under strong anticyclonic forcing is also found. In the nonlinear regimes the western boundary current separates from the western wall and shoots into the interior as a
narrow jet that undergoes a rapid adjustment to join with the broad slow interior flow. In the unstable regimes this adjustment involves eddy shedding. Each transition
occurs at a fixed critical value of a Reynolds number Re γ based on the velocity and width scales for a purely viscous boundary current:
the flow is unstable for Re γ 〉 123±4 and
aperiodic for Re γ 〉 231±5.
The results provide evidence that the mechanism causing instability is shear in the separated jet rather than the breaking of
a large-amplitude Rossby wave. A quasi-geostrophic numerical model applied to the laboratory conditions yields a stability boundary and detailed characteristics of the
flow largely consistent with those determined from the experiments. It also reveals a strong dependence of the circulation pattern on basin aspect ratio, and shows that an
adverse higher-order pressure gradient is responsible for western boundary current separation in this model. Eddy–eddy interactions and feedback of fluctuations from
the eddy formation region to upstream parts of the boundary current contribute to aperiodic behaviour. As a result of eddy shedding, passive tracer from each streamline
in the boundary current can be stirred across much of the width of the basin.
Type of Medium:
Online Resource
ISSN:
0022-1120
,
1469-7645
DOI:
10.1017/S0022112099006370
Language:
English
Publisher:
Cambridge University Press (CUP)
Publication Date:
1999
detail.hit.zdb_id:
1472346-3
detail.hit.zdb_id:
218334-1
