Energetics of Shoaling Internal Waves and Turbulence in the St. Lawrence Estuary

Abstract

The shoaling of horizontally propagating internal waves may provide an important source of mixing and transport in estuaries and coastal seas. Parameterizing such effects in numerical models demands better understanding of several aspects of wave energetics, especially relating to horizontal energy flux and turbulence generation. Observations are needed to build this understanding. To address some of these issues in the estuarine context, an intensive field program was undertaken in Summer 2008 in the St. Lawrence Estuary, involving shore-based photogrammetry, ship-based surveys, and moorings that held conventional and turbulence-resolving sensors. The measurements reveal that waves generally arrived during the rising phase of the M2 tide. Shoreward of the 40m isobath, waves traversed the field site perpendicular to bathymetry, a pattern that continued as the waves transformed nonlinearly. A tight temperature-salinity relationship permits the estimation of the time-varying density field from a moored chain of temperature-depth recorders. A new method for inferring the heaved internal wave density field is developed, using a relaxation solver to determine the wave streamfunction. The method is applied to discrete events measured with acoustic Doppler profilers to estimate the kinetic and available potential energy, as well as the nonlinear horizontal energy flux. Acoustic Doppler velocimeters were used to infer near-bottom turbulent energetics, revealing two main features. First, a period of wave incidence had turbulence dissipation rates that exceeded values associated with tidal shear by an order of magnitude. Second, the evolving spectral signatures associated with a particular wave-shoaling event indicate that the turbulence is at least partly locally generated. A simple model for wave-induced turbulence is proposed based on the energy flux measurements. Generally, the results suggest that during the rising phase of the tide, energy input from shoaling waves is required to explain the observed levels of dissipation. Estimates of vertical diffusivity during times of wave shoaling are on average 3 times larger than values predicted by tidal shear alone.