Near-inertial waves in Lake Erie

Near-inertial (Poincare) waves with a period T-p similar to 17 h are the dominant wind-induced internal wave motions in central Lake Erie and consequently have a substantial influence on lake circulation, mixing and biogeochemistry. However, due to the complex three-basin bathymetry in Lake Erie, the vertical and horizontal modal structure of these waves remain poorly understood. In this study, we analyze field data to show wind events energize frequent vertical mode-one Poincare waves. The horizontal modal structure was also investigated, in a sensitivity analysis, using a calibrated three-dimensional hydrodynamic transport model forced with observed and idealized spatially uniform wind events. Strong horizontal mode-one Poincare wave cells form in both the Central and Eastern Basins when wind events have a duration of 0.25 T-p to 0.5 T-p, are impulsive and periodic at T-p, or have anticyclonic rotation with a duration of T-p. Momentum transfer from longer wind events (> 0.5 Tp) will oppose the Coriolis-force rotated currents and damp Poincare wave generation. In agreement with theory, the most efficient wind events are observed and computationally modeled to have a duration of 0.25 T-p; causing an excitation peak at similar to 0.4 T-p and converting similar to 0.8% of the wind energy input to Poincare waves. The efficiency of wind work in generating Poincare wave kinetic energy is given by (1-cos (2 pi f t)) t(-1), where f is the inertial frequency and t is the wind duration. Therefore, the efficiency peaks during each nT(p) period, where n is a non-negative integer, and decreases significantly for longer wind events.