Near-inertial waves in Lake Erie

Abstract

Near-inertial (Poincaré) waves with a period Tp ∼ 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 Poincaré 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 Poincaré wave cells form in both the Central and Eastern Basins when wind events have a duration of 0.25 Tp to 0.5 Tp, are impulsive and periodic at Tp, or have anticyclonic rotation with a duration of Tp. Momentum transfer from longer wind events (> 0.5 Tp) will oppose the Coriolis-force rotated currents and damp Poincaré wave generation. In agreement with theory, the most efficient wind events are observed and computationally modeled to have a duration of 0.25 Tp; causing an excitation peak at ∼ 0.4 Tp and converting ∼ 0.8% of the wind energy input to Poincaré waves. The efficiency of wind work in generating Poincaré wave kinetic energy is given by (1-cos (2πf t)) t-1, where f is the inertial frequency and t is the wind duration. Therefore, the efficiency peaks during each nTp period, where n is a non-negative integer, and decreases significantly for longer wind events.