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 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 9km (100m depth) to 1km offshore (10m). We observe that two bores arrive each semidiurnal period, both propagating from the southwest and 72% of which are tracked to the 10m 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 bores propagate through) 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 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 to co and the rotating group velocities (cg), concluding that observed speeds are always faster than cg and are slower than co at depths >32m and faster than co at depths <32m. The bores maintain a steady speed while transiting into shallower water, contrary to linear estimates which predict bores to slow.