Driven by a need to understand the propagation and stability of abyssal ocean currents, there have been numerous idealised studies examining the dynamics in a rotating frame of reference of dense fluid on a slope underlying a less dense ambient fluid. This circumstance is characteristic, for example, of the Denmark Strait Overflow and the Western Boundary Undercurrent. A starting point of many theoretical and numerical studies is to assume the ambient is stationary and the current moves initially at a constant speed set by geostrophic balance. However, recent laboratory experiments (e.g. Lane-Serff and Baines (1998)) have shown that the continuous injection of a dense current from a localised source can significantly accelerate the ambient fluid and the consequent interaction between the two moving fluids cannot be neglected.

We have performed a series of laboratory experiments designed to examine the temporal as well as spatial stability characteristics of the current. In these experiments a 90 centimetre diameter cylindrical tank on a rotating table is filled with fresh water. Dyed salt-water is injected uniformly through an annular slit on the conical-shaped bottom of the tank thus creating a uniform circular current. When instability occurs we observe a sinusoidal mode with phase speed approximately equal to that of the induced surface flow. The results are shown to be consistent with barotropic instability of the ambient rather than baroclinic instability of the dense current.