JET INSTABILITY AND THE STABILIZING EFFECT OF TOPOGRAPHY ON JETS IN 2-LAYER ROTATING SYSTEMS

Laboratory experiments concerning azimuthal jets in two-layer rotating systems in the absence and presence of bottom topography aligned along the jets have been conducted. The jets were forced by the selective withdrawal of fluid from the upper layer of a two-fluid system contained in a circular dishpan geometry. The principal parameters measured in the experiments were the jet Rossby number, Ro, and a stratification parameter [formula omitted] where r1 is the radius of the circular disc used for the selective withdrawal (i.e., r1 is the approximate radius of curvature of the jet) and λ1,λ2 are the internal Rossby radii of deformation in the upper and lower fluids, respectively. The no-topography experiments show that for a sufficiently small F, the particular value depending on Ro, the jet is stable for the duration of the experiment. For sufficiently large F, again as a function of Ro, the jet becomes unstable, exhibiting horizontal wave disturbances from modes three to seven. An Ro against F flow regime diagram is presented. Experiments are then conducted in the presence of a bottom topography having constant cross-section and extending around a mid-radius of the dishpan. The axis of the topography is in the vicinity of the jet axis forced in the no-topography experiments and the crest of the topography is in the vicinity of the interface between the two fluids (i.e., the front associated with the jet). The experiments show that in all cases investigated the jet tends to be stabilized by the bottom topography. Experiments with the topography in place, but with the interface between the fluids being above the topography crest, are shown to be unstable but more irregular than their no-topography counterparts. Various quantitative measurements of the jet are presented. It is shown, for example, that the jet Rossby number defined in terms of the fluid withdrawal rate from the tank, Q, can be well correlated with a dimensionless vorticity gradient, VG, across the upper layer jet. This allows for an assessment of the stability characteristics of a jet based on a knowledge of VG (which can be estimated given a jet profile) and F. © 1990 Gordon and Breach, Sciena Publishers. Inc.