Internal Bore Evolution across the Shelf near Pt. Sal, California, Interpreted as a Gravity Current

Abstract

Off the central California coast near Pt. Sal, a large-amplitude internal bore was observed for 20 h over 10 km cross shore, or 100–10-m water depth (D), and 30 km along coast by remote sensing, 39 in situ moorings, ship surveys, and drifters. The bore is associated with steep isotherm displacements representing a significant fraction of D. Observations were used to estimate bore arrival time tB, thickness h, and bore and nonbore (ambient) temperature difference ΔT, leading to reduced gravity g′. Bore speeds c, estimated from mapped tB, varied from 0.25 to 0.1 m s−1 from D = 50 to 10 m. The h varied from 5 to 35 m, generally decreased with D, and varied regionally along isobath. The bore ΔT varied from 0.75° to 2.15°C. Bore evolution was interpreted from the perspective of a two-layer gravity current. Gravity current speeds U, estimated from the local bore h and g′, compared well to observed bore speeds throughout its cross-shore propagation. Linear internal wave speeds based on various stratification estimates result in larger errors. On average bore thickness h = D/2, with regional variation, suggesting energy saturation. From 50- to 10-m depths, observed bore speeds compared well to saturated gravity current speeds and energetics that depend only on water depth and shelf-wide mean g′. This suggests that this internal bore is the internal wave analog to a saturated surfzone surface gravity bore. Along-coast variations in prebore stratification explain variations in bore properties. Near Pt. Sal, bore Doppler shifting by barotropic currents is observed.

Interactions Between Internal Tidal Bores and Submesoscale Currents on the Continental Shelf

“…For this study, we take N, u to mean a depth-average . Many theoretical frameworks offer a more formal representation of c, for example, a two-layer gravity current framework (Benjamin, 1968;Spydell et al, 2021) or KdV theory (Benney, 1966;McSweeney, Lerczak, Barth, Becherer, Colosi, et al, 2020). However, (Equation 6) captures the generic stratification dependence and Doppler shifting that is universal to any diagnosis of bore propagation.…”

Kinematics of an Ebb Plume Front in a Tidal Crossflow

“…Through the use of remote sensing technologies, spatially and temporally dense observations of surface fronts and internal bores (or jumps) (D. A. Honegger, 2015; D. A. Honegger et al., 2017; Rijnsburger et al., 2018; Spydell et al., 2021; X. Wu et al., 2021) are becoming more common. The water surface manifestations of the front, such as enhanced surface roughness and wave breaking (Lyzenga, 1991; Orton & Jay, 2005; O’Donnell et al., 1998), enable the time‐resolved tracking of plume fronts via X‐band marine radar (Kilcher & Nash, 2010; Pritchard, 2000; Rijnsburger et al., 2018).…”