The influence of island-generated eddies on chlorophyll distribution : a study of mesoscale variation around Gran Canaria

Island wakes and headland eddies in coastal waters are identified from spaceborne and airborne remotely sensed imagery in the visible wavelengths. The imagery were obtained for the Bristol and English channels, United Kingdom, which are characterized by high tidal currents and levels of turbidity. Suspended matter in the surface waters is used as a passive tracer for the flow features. Scaling parameters (e.g., the Reynolds, Ekman, and Rossby numbers), obtained from the depth averaged equation of motion, are presented based upon information on oceanographic conditions at the times of the overpasses. The parameters are compared then with data obtained from laboratory model investigations, presented by other experimenters in the published literature. The remotely sensed data demonstrate that in coastal waters with nonuniform obstacles and bathymetry, for a given Ekman number the flow regimes occur at a lower Rossby number than might be expected from laboratory experiments. In the absence of a method for the accurate determination of the horizontal eddy viscosity, hence the Reynolds number, it is concluded that an “island wake parameter” (Wolanski et al., 1984a) should be adopted to describe such features in coastal waters.

Island wakes and headland eddies: A comparison between remotely sensed data and laboratory experiments

The flow of a linearly stratified fluid past a sphere is considered experimentally in the Froude number Fi, Reynolds number Re, ranges 0.005 ≤ Fi ≤ 20 and 5 ≤ Re ≤ 10000. Flow visualization techniques and density measurements are used to describe the rich range of characteristic flow phenomena observed. These different flow patterns are mapped on a detailed Fi against Re flow regime diagram. In most instances the flow patterns were found to be very different from those observed in homogeneous fluids. Vortex shedding characteristics, for example, were found to be dramatically affected by the presence of stratification. Where possible, the results are compared with available analytical and numerical models.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112092000119

Stratified flow past a sphere

The islands of the Lesser Antilles present a porous meridional barrier to North Brazil Current (NBC) rings. To better understand if, when, and how a NBC ring could be squeezed into the Caribbean Sea through such a gappy barrier, the encounter of a westward drifting eddy with a porous meridional wall is considered. First an eddy encountering a single aperture is modeled. To do so, both an analytical model and a reduced-gravity primitive equations numerical model are used. This was followed by numerical experiments that explored eddy collisions with barriers containing many gaps. In all of these models, the collisions were forced by either β or a steady advection through the gap(s). Using integrated constraints, an analytical solution was constructed for a zero potential vorticity lens passing through a single gap on a β plane. The solution involves a small parameter e, the ratio of the short timescale associated with the f-plane adjustment, and the long encounter timescale due to β. It is found th...

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The Squeezing of Eddies through Gaps

▪ Abstract This article reviews some aspects of the roles that laboratory experiments have played in the study of orographic effects in the Earth's atmosphere and oceans. The review focuses on, but is not restricted to, physical systems for which the effects of both background stratification and rotation are important. In the past, such laboratory studies have been largely decoupled from attempts to make quantitative comparisons with the results of numerical-model studies or observations from field programs. Rather, they have been used mostly in the important task of better understanding the physics of rotating and stratified flows. Furthermore, most laboratory experiments concerned with the effects of orography on either homogeneous or stratified rotating fluids have considered laminar flows, whereas their counterpart flows in the atmosphere and ocean are turbulent. We argue that laboratory investigations are likely to be more useful in addressing critical environmental problems if the studies are more c...

D. L. Boyer

Papers

Flow Past a Right Circular Cylinder in a Rotating Frame