Transport and Mixing by Shoaling Waves of Elevation

Non-linear internal waves are important to processes such as nutrient replenishment and sediment transport on continental shelves. Here we present unique field observations of shoaling nonlinear internal waves (NLIWs) of elevation, formed by the fission of nonlinear waves of depression as they propagated into water with a depressed pycnocline. Many of the waves possessed a trapped core, leading to the in-shore transport of mass over 10s of km. Large density overturns accompanied the trains of waves, which were driven by a variety of instability mechanisms. We observed both shear and kinematic instabilities, but the kinematic instabilities led to the largest overturns and diapycnal mixing. The first two waves in each packet had the largest diapyncal diffusivity. The high diffusivity was local to the nonlinear waves of elevation, indicating the waves transported the turbulence inshore rather than the dissipation occurring locally. The time- and vertically-averaged diapycnal diffusivity was up to three orders of magnitude larger during the NLIW of elevation events. These observations demonstrate the importance of non-linear internal waves to cross-shelf transport and vertical mixing.