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Observations of shoaling nonlinear internal bores across the central California inner shelf

TitleObservations of shoaling nonlinear internal bores across the central California inner shelf
Publication TypeJournal Article
Year of Publication2020
AuthorsMcSweeney J.M, Lerczak J.A, Barth J.A, Becherer J., Colosi J.A, MacKinnon JA, Macmahan J.H, Moum J.N, Pierce S.D, Waterhouse A.F
Date Published2020/01
Type of ArticleArticle
ISBN Number0022-3670
Accession NumberWOS:000505499600001
KeywordsContinental shelf; evolution; Fronts; generation; internal waves; model; nonlinear dynamics; numerical experiments; oceanography; rotation; slope; solitary waves; solitons; tide; topography; transformation; Wave properties

We present observations of shoaling nonlinear internal bores off the coast of central California. The dataset includes 15 moorings deployed during September-October 2017 and cross-shore shipboard surveys. We describe the cross-shore structure and evolution of large-amplitude internal bores as they transit from 9 km (100-m depth) to 1 km offshore (10 m). We observe that two bores arrive each semidiurnal period, both propagating from the southwest; of the total, 72% are tracked to the 10-m isobath. The bore speeds are subtidally modulated, but there is additional bore-to-bore speed variability that is unexplained by the upstream stratification. We quantify temporal and cross-shore variability of the waveguide (the background conditions through which bores propagate) by calculating the linear longwave nonrotating phase speed c(o) and using the nonlinearity coefficient of the Korteweg-de Vries equation alpha as a metric for stratification. Bore fronts are generally steeper when alpha is positive and are more rarefied when alpha is negative, and we observe the bore's leading edge to rarefy from a steep front when alpha is positive offshore and negative inshore. High-frequency alpha fluctuations, such as those nearshore driven by wind relaxations, contribute to bore-to-bore variability of the cross-shore evolution during similar subtidal waveguide conditions. We compare observed bore speeds with c(o) and the rotating group velocities c(g), concluding that observed speeds are always faster than c(g) and are slower than c(o) at depths greater than 32 m and faster than c(o) at depths of less than 32 m. The bores maintain a steady speed while transiting into shallower water, contrary to linear estimates that predict bores to slow.

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