Upslope Internal-Wave Stokes Drift, and Compensating Downslope Eulerian Mean Currents, Observed above a Lakebed

Abstract

AbstractIn a small lake, where flows were dominated by internal waves with 10–32-h period, slow but persistent mean transport of water over many wave periods was examined. Acoustic Doppler profilers (ADPs) and a vertical string of temperature loggers were deployed where the lower thermocline intersected the sloping lakebed. Near (<1 m above) the bed, internal waves, coherent with a lakewide seiche, propagated upslope at ~0.023 m s−1. Near-bed wave-induced water velocity fluctuations had a standard deviation of <0.02 m s−1. Near the surface, velocity fluctuations had similar magnitude, but lateral wave propagation was unclear. Averaged over many wave periods, the near-bed Eulerian velocity flowed downslope at ~0.01 m s−1, and was roughly cancelled by an upslope internal-wave Stokes drift (estimated by assuming that weakly nonlinear waves propagated without change of form). To examine net transport, while relaxing approximations used to estimate the Stokes drift, the observed temperature range (9°–25°C) was divided into 0.5°C increments, and the depth-integrated, wave-averaged flux of water in each temperature class was calculated. The coldest (near-bed) water was slowly transported onshore, opposite the Eulerian mean velocity. Onshore flux of warm near-surface water was comparable to an Eulerian-mean flux, indicating minimal near-surface Stokes drift. Intermediate water, from the middle of the water column and the outer boundary layer, was transported offshore by an offshore Stokes drift. The downslope near-bed Eulerian mean velocity, together with intensification of mean stratification within 0.4 m of the bed, may enhance boundary layer mixing.