Observations of Shoaling Nonlinear Internal Bores across the Central California Inner Shelf

Abstract

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 co and using the nonlinearity coefficient of the Korteweg–de Vries equation α as a metric for stratification. Bore fronts are generally steeper when α is positive and are more rarefied when α is negative, and we observe the bore’s leading edge to rarefy from a steep front when α is positive offshore and negative inshore. High-frequency α 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 co and the rotating group velocities cg, concluding that observed speeds are always faster than cg and are slower than co at depths greater than 32 m and faster than co 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.

The Inner-Shelf Dynamics Experiment

“…The onshore propagating warm bore was at least 10 m thick with 0.2-3 m s −1 onshore velocities. These warm bores were similar to those seen around Oceano (McSweeney et al 2020b) and they propagate deep into the bay in the lee of Point Sal (not shown).…”

“…Internal waves are important to the inner shelf over a range of time and spatial scales. Highly nonlinear internal tides and high-frequency internal waves occur in the inner shelf and are responsible for large changes in stratification and strong cross-shore currents over short time scales (Lee 1961;Cairns 1967;Winant 1974;Scotti and Pineda 2004;Sinnett et al 2018;McSweeney et al 2020b;Feddersen et al 2020), and also transport heat from the inner shelf to the surf zone (e.g., Sinnett and Feddersen 2019). These waves also transport plankton and nutrients across the inner shelf (Shanks and Wright 1987;Pineda 1991;Leichter et al 1998;Lennert-Cody and Franks 1999;Shroyer et al 2010); and can resuspend and transport sediment (Butman et al 2006).…”

“…Internal Wave DynamicsObserved nonlinear internal waves (NLIWs) included steep bores, undular bores, and highfrequency waves of elevation and depression. Internal bores propagated into the region every 6 hours(McSweeney et al 2020b) and were detectable in both in situ and remote observations. Data from satellite SAR and land-and ship-based radar stations demonstrate that internal bores were…”

Inner‐Shelf Vertical and Alongshore temperature Variability in the Subtidal, Diurnal, and Semidiurnal Bands Along the central California coastline with headlands

“…Temperature observations versus 2017 yearday and height above the bed at M15 for the (a) total, (b) subtidal, (c) diurnal, and (d) semidiurnal frequency bands (seeFigure 1b). (e) Tidal elevations versus 2017 yearday (McSweeney et al, 2019)…”

Vorticity Recirculation and Asymmetric Generation at a Small Headland With Broadband Currents

“…Both EOF modes 1 and 2 for ζ W are similar to those for ζ S in both vertical structure (not shown) and variance fraction (65% and 27%, respectively). The vertically smooth ζ / f means and low mode dominance of ζ / f variability indicates that the vorticity estimation method using 2 h filtered and cEOF reconstructed velocities (Section 2.2.1) is not noise contaminated due to aliasing of short scale variability associated with internal warm bores or solitons (Colosi et al., 2018; McSweeney et al., 2020) that likely have contributions at cEOF mode 2. If noise were significant, one would expect small‐scale vertical variation in the statistics due to estimation error.…”