Alongshore Variability of Shoaling Internal Bores on the Inner Shelf

Abstract

Temperature and velocity measurements from 42 moorings were used to investigate the alongshore variability of nonlinear internal bores as they propagated across the central California inner shelf. Moorings were deployed September – October 2017 offshore of the Pt. Sal headland. Regional coverage was ~30 km alongshore and ~15 km across shore, spanning 9 – 100 m water depths. In addition to subtidal processes modulating regional stratification, internal bores generated complex spatiotemporal patterns of stratification variability. Internal bores were alongshore-continuous O(10s km) at the 50 m isobath, but the length scales of frontal continuity decreased to O(1 km) at the 25 m isobath. The depth-averaged, bandpass filtered (3-min – 16-hr) internal bore kinetic energy (KEIB¯) was found to be nonuniform along a bore front, even in the case of an alongshore-continuous bore. The pattern of along-bore KEIB¯ variability varied for each bore, but a 2-week average indicated that KEIB¯ was generally strongest around Pt. Sal. The stratification ahead of a bore influenced both the bore’s amplitude and cross-shore evolution. The data suggest that alongshore stratification gradients can cause a bore to evolve differently at various alongshore locations. Three potential bore fates were observed: 1) bores transiting intact to the 9 m isobath, 2) bores being overrun by faster, subsequent bores, leading to bore merging events, and 3) bores disappearing when the upstream pycnocline was near or below mid-depth. Maps of hourly stratification at each mooring and the estimated position of sequential bores demonstrated that an individual internal bore can significantly impact the waveguide of the subsequent bore.

The Inner-Shelf Dynamics Experiment

“…As internal waves propagated into shallower water, their evolution depended on the shelf stratification and background shear. The pycnocline depth ahead of a bore was found to influence the evolution of frontal steepness (McSweeney et al 2020a). Cross-shore mooring transects illustrate that an internal bore can maintain a sharp front from the 150 m to 9 m isobath, while higher-frequency internal waves evolve over shorter distances (Fig.…”

“…Spatial heterogeneity of the semidiurnal internal tide signals from the outer to the inner shelf have been investigated using both observations and numerical model simulations (Kumar et al 2019;Fertitta 2019;McSweeney et al 2020a). Mean (Sep., 2017) semidiurnal internal tidal energy fluxes at 50 and 30 m isobath are strongest adjacent to Point Sal, decreasing in magnitude both towards the north and south (Fig.…”

“…2020a). Furthermore, internal tide properties, such as energy, amplitude, and frontal structure, are influenced by the spatial heterogeneity of stratification(McSweeney et al 2020a). For example, nonlinear internal bore fronts are found to be along-shore continuous O (10 km) at the 50 m isobath and only O (1 km) at the 25 m isobath(McSweeney et al 2020a).…”

“…15d). The enhanced dissipation rates in the shallower reaches of the smoother inner shelf may be associated with a variety of processes including surface layer turbulence by wind and non-linear internal wave dynamics (Colosi et al 2018;McSweeney et al 2020a). Enhanced dissipation occurs at the headlands and complicated bathymetry of Points Sal, Purisima and Arguello with a more randomly distributed pattern with offshore distance.…”

The Evolution of a Northward‐Propagating Buoyant Coastal Plume After a Wind Relaxation Event

“…Poleward propagating thermal plumes contribute to temperature and stratification variability on the inner continental shelf, but inner‐shelf conditions are also influenced by other processes with similar spatial and temporal scales. For example, remotely and locally generated internal tides influence the SMB region and propagate shoreward across the shelf roughly every 6 hr (Colosi et al., 2018; McSweeney, Lerczak, Barth, Becherer, Colosi, et al., 2020; McSweeney, Lerczak, Barth, Becherer, MacKinnon, et al., 2020). The coastline also features several headlands, which can introduce semidiurnal temperature variability nearshore (Feddersen et al., 2020).…”