Internal waves of the second vertical mode in a stratified lake

Abstract

A time series of measurements of thermal structure and near‐bottom currents in a stratified lake revealed both first and second vertical mode internal seiching. The transition into second‐mode seiching occurred after first‐mode waves, usually generated by pulses of strong winds, died down. We describe the major features of the second‐mode seiche and discuss its effects on mixing in the lake below the thermocline. On the basis of calculations of eddy diffusion coefficients, there is only weak mixing in the interior of the hypolimnion. Scaling arguments suggest that mixing at the lake’s edges due to motion of the isotherms is not significant despite the larger amplitudes of the second‐mode seiche. The most important mixing region appears to be a turbulent bottom boundary layer driven by seiche currents. However, resuspension of bottom sediments did not occur, probably due to a combination of sediment nature and low current speeds.