J. Aucan

Frequency and Duration of Coinciding High Surf and Tides along the North Shore of Oahu, Hawaii, 1981–2007

Abstract Wave run-up along the north shore of Oahu has an annual cycle with a maximum centered on boreal winter. An understanding of the variability of high wave wash is important for coastal planning, transportation, safety, and property protection. Wave run-up increases with increasing surf size and tidal height. This study analyzed hourly historic wave data and predicted tides from 1981 to 2007 to better understand wave run-up potential based on the frequency and duration (in hours) that high surf and tides coincided as categorized by thresholds of surf and tidal height. The Waimea buoy, located just 5 km offshore, was the primary source for deep-water wave height and period. However, this series only began in 2001. The National Data Buoy Center platform 51001 extends back to 1981. Its location is sufficiently remote from Oahu to warrant a correction in significant wave height. The correction was made based on a regression analysis between daily mean wave heights from these two buoys. From the final, deep-water, nearshore Oahu, hourly series of swell height and period, surf heights were calculated using an empirical transformation scheme. Thresholds were defined for four surf heights and three tidal elevations. Recurrence was calculated for each of the 12 categories. Duration was computed by summing consecutive hours for each event above each threshold. Historical evidence of sand wash onto select portions of the coastal highway was used to qualitatively rank the 12 categories as marginal, significant, or extreme. One important application of these results would be improvement to surf-related coastal flood forecasts by the National Weather Service in pursuit of protection of life and property. For future design considerations, a joint probability model was constructed to better understand the annual average number of hours exceeding any given paired surf and tidal height.

Boundary Mixing Associated with Tidal and Near-Inertial Internal Waves

Abstract Moorings deployed on the south (August–November 2002) and north (November 2002–June 2003) flanks of the Kaena Ridge, Hawaii, are used to document the flow variability associated with mixing within 200 m from the boundary, deep along the ridge, as part of the Hawaii Ocean Mixing Experiment (HOME). At both sites, strong temperature inversions are detected with vertical scales of ∼100 m. A Thorpe-scale analysis of the overturns yields a time-averaged dissipation near the bottom at the south site (1.2 × 10−8 W kg−1) that is 10 times higher than the north site (1.9 × 10−9 W kg−1), with both higher than the dissipation at similar depths 30 km from the ridge. On the south flank, observed horizontal currents and vertical displacements are dominated by the semidiurnal internal tide. On the north flank, the semidiurnal tide is less energetic than on the south, with a different vertical structure as tidal amplitudes decrease toward the boundary. These differences are attributed to greater separation from th...

The deep sound of one wave plunging

The ability to provide infrasound estimates of breaking ocean wave height and period in shallow reefs, steep rocky coastlines, and sand beaches has been demonstrated in previous work. Yet defining the source process and isolating the source pressure function remained elusive because of ambiguity introduced by complex coastlines and multiple breaker zones. Due to the steep bathymetry and the its proximity to land, the Temae reef in the northeast coast of Moorea island, French Polynesia, provided a well constrained experimental environment where individual breaking waves could be identified and recorded. Synchronous wave height, infrasonic, seismic, and visual recordings of individual waves breaking against the shallow reef ledge were made and correlated. We characterize a possible source mechanism for surf infrasound and demonstrate the capability to acoustically track alongshore traveling (peeling) plunging waves. We also introduce preliminary results on remote infrasonic monitoring of the surf zone in th...

Multiparameter studies of surf infrasound

Infrasound stations on islands or near coastlines routinely detect signals associated with breaking ocean waves. Although the source mechanisms of these infrasonic surf signals are not well understood, they might provide useful insight into coastal processes and permit an assessment of wave energy distribution in the littoral zone. Near‐shore infrasonic arrays, ocean bottom sensors, an infrared imager, and a video camera were deployed on rocky and sandy coastlines in Hawaii during the 2004–2005 Winter swell season, with the aim of establishing a relationship between the directional ocean swell height and the infrasonic source distribution, intensity, and spectral content. The cameras targeted acoustically active regions for selected time periods to associate the timing of the infrasonic signal arrivals with the breaking and dissipation of ocean wave sets. A wide range of sea states and weather conditions were captured during two separate deployments. The results of our multiparameter analyses and prelimin...

The Aloha Cabled Observatory: New Insights into hurricane generation of the seismo-acoustic noise spectrum

The close passage of Hurricane Lester near the Hawaiian Islands in September 2016 afforded an in-depth, close-up study of storm generation of the largest background vibrations observed planet wide. The observations at the ALOHA Cabled Observatory on the seafloor below the Hurricane, coupled with seismic sensors on Oahu, and ocean wave buoys off shore, present a detailed picture connecting the storm to the ocean and Earth. Wave interactions from a distant typhoon near Japan play an important role. Vibration energy levels observed on Oahu closely match those on the sea floor 100 km north of Oahu, where ALOHA Cabled Observatory is the world’s deepest seafloor observatory at 4,728 m depth. Characteristic vibrations generated radially from the Hurricane were observed, along with unexpected transverse motions perpendicular to the radial waves. This latter observation is consistent with a broad source region extending from Hurricane Lester and generating the vibrations. Evidence for substantial scattering of the...