Abstract: Dissipation Mechanisms of Internal Solitary-like Waves in the Ocean
Authors: Kevin Lamb
Internal solitary-like waves (ISWs) are ubiquitous, highly energetic features in the coastal ocean where they are predominately generated by tide-topography interaction. There are many unanswered questions about the generation and fate of these waves and a better understanding of these processes is necessary for developing parameterizations of their effects for use in large scale models. Several sets of observations have suggested that the mixing associated with ISW trains is important for setting the stratification in some regions of the coastal ocean (e.g, the Scotian Shelf and the Portuguese Shelf). This talk will begin with a discussion of the energetics of large amplitude internal waves. I will then discuss three dissipation mechanisms for ISWs and consequences for mixing: instabilities in the bottom boundary layer, the breaking of shoaling waves, and shear instabilities in the pycnocline. Results from 2D numerical simulations will be presented for all three mechanisms, with a focus on shear instabilities.Instabilities in the bottom boundary layer can occur beneath ISWs of elevation or depression where they are triggered by the adverse pressure gradient associated with the wave-induced current pulse beneath the wave. In idealized numerical experiments over a flat bottom they appear to require a background current however they can occur in the absence of a background current if topographic features are present, for example, under shoaling waves.Strong mixing occurs when ISWs shoal and break, a process which has been the subject of many experimental, observational and numerical investigations. Accurate estimates of reflectance (the ratio of reflected to incident energy flux) requires a proper evaluation of kinetic (KE) and available potential energy (APE) fluxes. It has been common practice in the past to assume equipartition of KE and APE or of their associated fluxes. The accuracy of these assumptions will be discussed and reflectances for a range of toographic slopes obtained from 2D numerical simulations will be presented.Several field observations of ISWs exhibiting shear instabilities have been made in the coastal ocean. In the final part of this talk I will present some results from a numerical investigation of shear instabilities in an ISW. This work was motivated by, and is based on, an ISW observed on the Oregon Shelf. The simulations are done in a reference frame moving with the ISW. Small perturbations of a prescribed frequency are introduced ahead of the wave. The ISW is unstable in a band of forcing frequencies provided that the minimum Richardson number in the pynocline is sufficiently small (less than about 0.1). The rate at which energy is extracted from the ISW is frequency dependent and is consistent with estimates of the decay of ISWs in field observations. The simulations on which the above work is based are two-dimensional, however 3D simulations of shear instabilities have recently been initiated. It is hoped that results from these simulations will also be presented.