Instabilities in nonlinear internal waves on the Washington continental shelf

TitleInstabilities in nonlinear internal waves on the Washington continental shelf
Publication TypeJournal Article
Year of Publication2015
AuthorsZhang S., Alford MH
JournalJournal of Geophysical Research-Oceans
Date Published2015/07
Type of ArticleArticle
ISBN Number2169-9275
Accession NumberWOS:000359776000036
Keywordsbreaking; dissipation; new-england shelf; ocean; overturns; slope; solitary waves; solitons; topography; turbulence

Previous studies have identified two primary mechanisms (shear instability and convective instability) by which nonlinear internal waves (NLIWs) induce mixing on continental shelves. To determine the relative importance of these and their dependence on background flow conditions, we examine a much longer (6 month) data set from a moored ADCP/thermistor chain with 2 m vertical spacing in which over 600 NLIWs are detected. Turbulent properties of the 318 waves with detectable overturning instabilities are documented using Thorpe scales. The 130 waves detected while an ADCP was functioning are classified based on a Froude number criterion (Fr = u/c, where u is velocity in the wave propagation direction, c is the wave phase speed). Of these, 108 waves are identified as shear-instability (Type I; Fr < 1) waves and 22 as convective instability (Type II; Fr > 1). Composites are constructed by averaging in a wave coordinate frame over all waves in each category, showing the mean spatial structure of dissipation and other wave quantities. Turbulence is highest at the sheared interface for Type I waves and throughout the wave core for Type II waves. No relationship between wave instability mechanisms and wave/background parameters such as wave steepness, stratification, or mean flow is found, except that unstable waves tend to be more energetic, demonstrating a need to better understand wave propagation and breaking in complex and variable coastal oceanographic background flows.

Short TitleJ Geophys Res-Oceans
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