First page Back Continue Last page Overview Graphics

Overview of physical processes in large lakes: Internal waves : Internal Seiches

During the stratified period the most common effect of the wind forcing is the generation of basin-scale, internal seiches [7]. The largest horizontal flow is at the node (maximum kinetic energy) and at a minimum at the points of maximum vertical deflection (maximum potential energy). If Earth's rotation is ignored the points of maximum vertical deflection are at the upwind and downwind ends of the lake and the node would then be situated in the center of the lake for the main harmonic. In Lake Ontario, multinodal seiches form the dominant type of resonance in the lake because of wind forcing, dampening and other short period disturbances [9]. During sustained winds both momentum and turbulent kinetic energy (TKE) is transferred to the surface water layer. The TKE distributes momentum in the water column, pushing down the surface layer (downwind), which results in a thermocline depression at the downwind end and upwelling at the upwind end of the basin [6]. On wind cessation the gravity balances the forces and initiates an oscillatory movement of the thermocline in the entire basin. This movement is creating basin-wide isotherms in a fan-shape, which varies the density profile across the lake [9]. These long waves have wavelengths of the same order of magnitude as the basin dimensions. Incident waves are reflected at the lake boundaries and combine into standing wave patterns on the thermocline forming the internal seiches [9].