Properties of the flow generated by a continuous source of dense
fluid on a slope in a
rotating system are investigated with a variety of laboratory experiments.
The dense
fluid may initially flow down the slope but it turns (under the influence
of rotation) to
flow along the slope, and initial geostrophic adjustment gives it an anticyclonic
velocity
profile. Some of the dense fluid drains downslope in a viscous Ekman layer,
which may
become unstable to growing waves. Provided that the viscous draining is
not too
strong, cyclonic vortices form periodically in the upper layer and the
dense flow breaks
up into a series of domes. Three processes may contribute to the formation
of these
eddies. First, initial downslope flow of the dense current may stretch
columns of
ambient fluid by the ‘Taylor column’ process (which we
term ‘capture’). Secondly, the
initial geostrophic adjustment implies lower-layer collapse which may stretch
the fluid
column, and thirdly, viscous drainage will progressively stretch and spin
up a captured
water column. Overall this last process may be the most significant, but
viscous
drainage has contradictory effects, in that it progressively removes dense
lower-layer
fluid which terminates the process when the layer thickness approaches
that of the
Ekman layer. The eddies produced propagate along the slope owing to the
combined
effects of buoyancy–Coriolis balance and ‘beta-gyres’.
This removes fluid from the
vicinity of the source and causes the cycle to repeat. The vorticity of
the upper-layer
cyclones increases linearly with Γ=Lα/D
(where L is the Rossby deformation radius,
α the bottom slope and D the total depth), reaching
approximately 2f in the
experiments presented here. The frequency at which the eddy/dome structures
are
produced also increases with Γ, while the speed at
which the structures propagate along
the slope is reduced by viscous effects. The flow of dense fluid on slopes
is a very
important part of the global ocean circulation system and the implications
of the
laboratory experiments for oceanographic flows are discussed.
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