An Experimental Study of Baroclinic Flows with and without Two-Wave Bottom Topography

Abstract

A series of laboratory experiments was performed in a thermally-driven rotating annulus of fluid with and without two-wave bottom topography. Velocity measurements were made by illuminating a thin layer of fluid at mid-depth and photographing successive positions of tracer particles suspended in the fluid. Streamfunctions were determined from the calculated vorticity. Comparison of the rotational velocity field with the measured velocity field revealed negligible differences, indicating that the flow was horizontally quasi-nondivergent. A detailed analysis was made of one experiment with and one without topography at the same point in dimensionless parameter space. The results indicate that the effect of topography is 1) to modulate the synoptic-scale waves in both space and time, 2) to suppress the odd modes and 3) to force a “planetary” scale mode which oscillates about a climatological mean position (with high pressure centers located in this experiment approximately 22° upstream of the mountain ridges). Synoptic wavenumbers 4 and 6 have a common frequency of wave passage, which is the same as the frequency of oscillation of the planetary scale wavenumber 2. The wave amplitudes, as well as the zonal mean velocity profile, also vacillate with time at this same frequency. The combined energy of the synoptic-scale waves is concentrated about 45° downstream from the mountain ridges. A hierarchy of experiments is recommended for the future in which sloping upper and/or lower boundaries are used to simulate the β effect at other points in parameter space and additional Fourier components are added to the bottom topography with relative amplitudes and phases equal to those found on the earth.