Observations of Nonlinear Internal Wave Run-Up to the Surfzone

The cross-shore evolution of nonlinear internal waves (NLIWs) from 8-m depth to shore was observed by a dense thermistor array and ADCP. Isotherm oscillations spanned much of the water column at a variety of periods. At times, NLIWs propagated into the surfzone, decreasing temperature by approximate to 1 degrees C in 5 min. When stratification was strong, temperature variability was strong and coherent from 18- to 6-m depth at semi-diurnal and harmonic periods. When stratification weakened, temperature variability decreased and was incoherent between 18- and 6-m depth at all frequencies. At 8-m depth, onshore coherently propagating NLIW events had associated rapid temperature drops (Delta T) up to 1.7 degrees C, front velocity between 1.4 and 7.4 cm s(-1), and incidence angles between -5 degrees and 23 degrees. Front position, Delta T, and two-layer equivalent height z(IW) of four events were tracked upslope until propagation terminated. Front position was quadratic in time, and normalized Delta T and z(IW) both decreased, collapsing as a linearly decaying function of normalized cross-shore distance. Front speed and deceleration are consistent with two-layer upslope gravity current scalings. During NLIW rundown, near-surface cooling and near-bottom warming at 8-m depth coincide with a critical gradient Richardson number, indicating shear-driven mixing.