Internal wave‐driven transport of fluid away from the boundary of a lake

Abstract

A field experiment was conducted to study transport of fluid from the boundary to the interior of a lake. Tracking of a tracer injected into the metalimnion was combined with measurements of meteorological forcing, internal waves, and temperature microstructure. Seiches of vertical mode 2 and horizontal modes 1 and 2 were initiated after a wind event, and the tracer moved 950 m into the interior after 29.2 h. Four potential mechanisms for spreading of the tracer from the boundary to the interior were considered: intrusions from boundary mixing, horizontal dispersion, advection by seiches, and advection and dispersion driven by internal waves. Some evidence of boundary mixing was observed 0.5 h before the dye injection, when the speed of seiche‐driven currents was large, but a model of an intrusion driven by steady input overpredicted the propagation distance by a factor of about two. A one‐dimensional model with only dispersion yielded a dispersion coefficient of 0.8 m2 s−1, and a one‐dimensional model with only advection caused by internal waves predicted the position of the peak concentration and a change in longitudinal variance that was 60% of the measured change. Estimates of dispersion caused by the interaction of vertical diffusion with velocity gradients in the internal wave field are large enough to explain the rest of the spreading and suggest that the transport can be modeled as wave‐driven advection and dispersion.