Abstract
Hybrid coastal and interior modes
for two-dimensional homogeneous flow in a cylindrical ocean
by Onno Bokhove and E.R. Johnson
Flows on coastal shelves and in the deep interior ocean are often considered separately
but transport of fluid between these two regions can have important biologial or environmenta
l consequences.
This paper considers a linear coupled coastal and deep interior-ocean model
in the idealized context of a homogeneous two-dimensional cylindrical ocean with a rigid lid
and axisymmetric step shelf topography. Both a semi-analytical mode-matching approach
and brute-force finite-element numerics have been used to analyze the linear dynamics.
It is shown that hybrid planetary beta-plane Rossby and topographic shelf modes emerge.
The structure of these inviscid modes is clarified by considering their frequency dependence
on shelf break radius, by contrasting the evolution of hybrid modes to the evolution of pure
shelf and pure
beta-plane Rossby modes (considering streamfunction fields and particle paths, and
by showing solutions of the initial-value problem.
Both ``ocean'' and ``laboratory'' parameter values are considered.
Hybrid modes exchange information between the deep ocean and coastal shelves, especially at t
he intermediate
frequencies where the separate planetary Rossby mode and topographic shelf mode dispersion cu
rves overlap.
The role of these modes
is particularly clear in an initial-value problem wherein a localized initial condition
on the southern shelf later leads to large-scale interior-ocean circulation.
Forced-dissipative calculations reveal the sensitivity of resonantly generated hybrid Rossby-
shelf
modes to the strength of Ekman damping. For typical ``oceanic'' and ``laboratory''
parameter values hybrid modes are altered by increasing Ekman damping but do not disappear.