The effect of stratification on tidal current profiles

Tidal current profile measurements by acoustic doppler current profilers and recording current meters at a continental shelf sea site showed significant changes between winter (homogeneous) and summer (stratified) conditions. In winter the vertical variation of phase was small, whereas in summer there was a 30° phase and a 25° orientation difference between the near surface and near bed regions. This has been modelled using a continuous piece-wise linear eddy viscosity profile. For winter conditions the profile consisted of a linear region in the bottom 4 m, capped by a region of constant eddy viscosity. In summer, when the thermocline was at 30 m depth, the bottom linear layer was thinner (∼1 m) and the thermocline corresponded to very low eddy viscosities, decoupling the surface and sea-bed layers. The inferred bed roughness in summer (z0=0.001 m) was 400 times larger than its winter value. Consequently, the bottom stress was twice as large in summer compared with winter. The cause of the increase is postulated to be the effect of benthic biological activity.