Abstract
A comprehensive observational data set was used to examine shoreward propagating semidiurnal internal tides as they shoal, break and run-up as turbulent boluses across the edge of the Australian North West Shelf (NWS), offshore Dampier, during late winter 2013. The measured waveforms and wavefields supported the grouping of events into two distinct categories: (1) pre-; and, (2) post- wave breaking. It was found that the transition from (1) to (2) was marked by the rise of nonlinear steepening (α) and reduction in dispersion (β), both coefficients that parameterize nonlinear wave effects on the Korteweg-de Vries (KdV) equation. We introduced a criterion for wave breaking from the dimensionless parameter (δ) that relates these two terms: wave breaking occurs when δ < 1. In the first group, dispersive effects were dominant to spread energy out of the semidiurnal wave to a dispersive wave packet of short-period internal solitary waves (ISWs). In the second, dispersion was considered small compared to the cumulative effect of nonlinear steepening. Here, the semidiurnal wave built sufficient energy at its rear face to generate wave breaking, which has been known to produce multiple turbulent boluses. Similar observations have not been described for this region during winter months and highlight that the nonlinear internal wave field is an important feature on the NWS throughout the year. Additionally, measurements obtained through autonomous ocean glider profiles revealed some of the post-breaking characteristics that included intensive vertical mixing and transport of dense water and suspended material onshore of the shelf break.
Highlights
Internal tides are internal waves at the tidal frequency, commonly observed wherever strong barotropic tidal currents and stratification occur simultaneously near steep topography (Gerkema and Zimmerman, 2008)
In specific cases where the tidal bore disturbs a pycnocline closer to the bottom than to the surface and dispersion is considered small compared to the cumulative effect of nonlinear steepening, waveforms different from internal solitary waves (ISWs) are generated (Vlasenko et al, 2005). i.e., the tidal bore could propagate across the shelf edge without necessarily transfer most of its energy into short-period ISWs (e.g., Lamb, 1994; Holloway et al, 1997; Noble et al, 2009)
The knowledge of nonlinear internal tide dynamics on the Australian North West Shelf (NWS) is well established during the austral summer, comparatively little is known about the consequences of the setup of a near-bottom thermocline under austral wintertime conditions
Summary
Internal tides (or baroclinic tides) are internal waves at the tidal frequency, commonly observed wherever strong barotropic tidal currents and stratification occur simultaneously near steep topography (Gerkema and Zimmerman, 2008). Internal wave breaking is an important topic in coastal oceanography for many practical reasons, as most of the dissipation exists where the wave breaks and overturns the vertical stratification (Gregg et al, 2003) This process greatly influences the resuspension of bottom material (e.g., Boczar-Karakiewicz et al, 1991; Bogucki et al, 1997; Klymak and Moum, 2003; Hosegood and van Haren, 2004; Quaresma et al, 2007; Bourgault et al, 2014; Zulberti et al, 2020), nutrients and contaminants input to the water column (e.g., Sandstrom and Elliott, 1984; Holloway et al, 1985; Leichter et al, 1996), and cross-shelf exchange of water and material (e.g., Cacchione and Southard, 1974; Davis and Monismith, 2011; Nam and Send, 2011). It has implications on the management strategy plans for fishery (e.g., Holloway et al, 1985; Pineda, 1991; Miller and Shanks, 2004), wastewater disposal (e.g., Boehm et al, 2002; Petrenko et al, 2000), and for oil/gas operations (e.g., Holloway, 1983a; Chen, 2011; Jones and Ivey, 2017)
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