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 Point 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 on the order of tens of kilometers 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 (from 3min to 16 h) internal bore kinetic energy (K̅E̅I̅B̅) was found to be nonuniform along a bore front, even in the case of an alongshorecontinuous bore. The pattern of along-bore K̅E̅I̅B̅ variability varied for each bore, but a 2-week average indicated that K̅E̅I̅B̅ was generally strongest around Point 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 middepth. 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.