On internal waves near the high‐frequency limit in an enclosed basin

Abstract

Observations of the internal wave field using thermistor chains in Lake Kinneret (Israel) showed a substantial energy peak at frequencies just below the maximum buoyancy frequency. The waves were of the first vertical mode, occurred in groups, and resulted in isotherm displacements of up to 4 m in 22 m of water. The appearance of this high‐frequency internal wave energy was closely related to the strength of the wind field. A weaker relationship was found with the phase of the basin‐scale internal waves, with more energy observed under the same wind conditions when the metalimnion was closer to the surface. The relationship of the high‐frequency waves to the wind and the basin‐scale internal waves implies a generation mechanism associated with shear in the surface layer (observed to be approximately 0.08 s−1), similar to that responsible for generating high‐frequency internal waves in the equatorial Pacific. A linear stability analysis of the mean state during periods of high‐frequency wave activity revealed the existence of unstable modes, whose period and frequency bandwidth closely matched field observations.