| Mesoscale numerical models have difficulty in representing oceanic flows past steep topography. Here we describe laboratory experiments for comparison with numerical models and to explore the character of instabilities that occur. The experiments were conducted for geometrically identical systems, on both a large turntable (13 m in diameter) and a smaller one (1.8 m in diameter) with the flow in the former being transitional or turbulent and laminar in the latter. The topography, which is placed in the center of the test cell, includes a vertical coastline, a horizontal shelf, a continental slope and a deep ocean. Experiments were conducted for both homogeneous and linearly stratified fluids that were initially in a state of solid body rotation. The motion relative to the topography was established by impulsively increasing (or decreasing) the rotation rate, to provide along-isobath, downwelling (upwelling) favorable currents. The flow properties depend on the Rossby, the Burger and to a lesser extent, the Ekman number. Spin-up or spin-down first develops on the spin-up timescale, and then possible instabilities develop. This baroclinic flow was either stable as evidenced by circular, along-isobath streamlines, or unstable, in the sense that the azimuthal motions began to develop small scale disturbances which eventually developed into basin-scale eddies which encopassed the entire test facility. These observations have been compared with theoretical model of the extended Eady type, and quantitative agreement between the two is highly satisfactory. These results show that baroclinic eddy formation occurs naturally as part of stratified spin-up/down, and may be a significant factor in producing eddies in coastal currents. |
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