Internal seiche modes and bottom boundary‐layer dissipation in a temperate lake from acoustic measurements

Abstract

Measurements with Acoustic Doppler Current Profilers (ADCPs) and a thermistor string were used to investigate the seasonal variation of internal seiches in a 5‐km‐long temperate lake and the dissipation induced by the seiches in the bottom boundary layer. Velocity data, from a bottom‐mounted 1.2‐MHz ADCP were analyzed by cross‐spectral analysis to determine the seiche modes. The first vertical mode was generally dominant but the second mode made a significant contribution at times and was present throughout the summer season. The modal periods, which were consistent with normal mode analysis, exhibited a marked seasonal change that correlated closely with the evolution of stratification. Measurements in the bottom boundary layer, by another ADCP, in pulse‐pulse coherent‐mode sampling at 4 Hz, allowed determination of the rate of turbulent kinetic energy dissipation via the Structure Function Method. Episodes of energy dissipation ~ 10‐7 W kg‐1 occurred during peak flow (~ 0.05 m s‐1) in the seiche motion (period ~ 10 h) and coincided with times of significant Reynolds stress. Between these episodes, dissipation fell below the noise level (~ 5 × 10‐9 W kg‐1). The ratio of internal wave energy to the boundary‐layer dissipation rate implies a decay time of ~ 75 h, which is similar to that for energy decay in strong seiches, indicating that boundary‐layer dissipation makes a substantial contribution to energy loss from seiche motions. A major mixing event in August caused by near‐resonant wind forcing brought metalimnion water to the surface and induced substantial mixing.