Amplified internal pulsations on a stratified exchange flow excitedby interaction between a thin sill and external seiche

Exchange flow over a thin sill is studied in a laboratory tank. Characteristic pulsation in the flow intensity is visualized and measured in the vicinity of the sill. The frequency of pulsation is identified as the first eigenmode of external seiche (sloshing) of the whole water body. Control experiments in a simplified setup unambiguously confirmed that initial configurations with horizontal pressure gradient always generate such oscillations. The almost invisible, small-amplitude vertical vibration is magnified by the sill generating internal waves in the pycnocline. These results can serve to increase awareness, also among researchers analyzing field data, of the potential for external seiching to drive internal flow characteristics.