Characterizing spatial and temporal distributions of turbulent mixing and dissipation in Lake Erie

Abstract

The knowledge about the distribution of small-scale turbulence in Lake Erie is limited. The present study analyzed >600 temperature microstructure casts, from multiple stations collected during the spring and summer of 1997 and 2008–09, to map the characteristic distributions of the dissipation rate of turbulent kinetic energy (ε) and the vertical turbulent diffusivity (Kz) in Lake Erie. The observations were classified into three groups based on depth (Group West: <12 m; Group Central: 12–25 m; Group East: >25 m), with the Group Central divided into seasonally stratified and unstratified period. Group West had the highest mean ε=3.8×10-8m2s-3, over all depths and lowest mean Kz10-5m2s-1 through the bottom layer. Due to the shear instability, generated by internal waves, elevated ε ~5×10-8m2s-3 was observed in the metalimnion of the Group Central, during the seasonally stratified period, when Kz was limited. The hypolimnion was calm in Group Central (ε10-8-10-9m2s-3), except when entrainment or basin-scale seiches accelerated the flow, leading to enhanced ε10-7m2s-3 and Kz10-3m2s-1. We extended the turbulent kinetic energy budget, in Lake Erie, to include the surface layer. Around 58% of the energy flux entering the lake dissipated in the surface layer, and the remaining 42% in the lake interior. Both lake surface area and Wedderburn numbers provided predictions on the magnitude of Kz, which decreased with increasing Wedderburn numbers at a rate slower than with increasing lake number.