Abstract
AbstractInfragravity waves are oceanic surface gravity waves but with wavelengths (tens of km) and periods (>30 s) much longer than wind waves and swell. Mostly studied in shallow water, knowledge of infragravity waves in deep water has remained limited. Recent interest in deep water infragravity waves has been motivated by the error they may contribute to future high‐resolution satellite radar altimetry measurements of sea level. Here deep water infragravity waves offshore of the Pacific Northwest of the U.S. were studied using differential pressure gauges which were deployed as part of the Cascadia Initiative array from September 2012 to May 2013. Cross correlation of the records revealed direction of infragravity wave propagation across the array, from which source regions were inferred. The dominant source was found to be the coastline to the east, associated with large wind waves and swell incident on the eastern side of the basin. The source shifted southward during northern‐hemisphere summer, and on several days in the record infragravity waves arrived from the western side of the Pacific. Asymmetry of cross‐correlation functions for five of these westerly arrivals was used to calculate the ratio of seaward to shoreward propagating energy, and hence estimate the strength of infragravity wave reflection at periods of 100–200 s. Reflection of these remote arrivals from the west appeared to be strong, with a lower bound estimate of r = 0.49 ± 0.29 (reflection coefficient ± standard error) and an upper bound estimate of r = 0.74 ± 0.06. These results suggest that reflection at ocean boundaries may be an important consideration for infragravity waves in the deep ocean.
Highlights
Low-frequency infragravity waves are associated with wave groups of the higher-frequency sea waves and swell in the coastal zone
A recent study by Crawford et al [2015] on deep ocean infragravity waves in the Atlantic has shown that other factors such as short-wave incidence angle or spread and coastal morphology may be more important for infragravity wave generation than wave heights and periods
The red points mark the mean value of r from the seven east-west station pairs at each period, with error bars of two standard deviations (1 SD positive and 1 SD negative)
Summary
Low-frequency infragravity waves are associated with wave groups of the higher-frequency sea waves and swell in the coastal zone. Two mechanisms have been proposed for the generation of infragravity waves from the short-wave groups. One is that the interaction of shoreward propagating swell creates ‘‘bound’’ or ‘‘forced’’ infragravity waves [Longuet-Higgins and Stewart, 1962; Herbers et al, 1995a]. As the swell waves break, the forced infragravity waves are released as free infragravity waves and are reflected from the beach. Free infragravity waves satisfy the dispersion relation for surface gravity waves and have longer wavelengths than forced waves of the same frequency [Webb et al, 1991]. The second mechanism is that infragravity waves are generated by a time-varying breakpoint, with standing waves shoreward of the breakpoint and progressive infragravity waves radiating seaward [Symonds et al, 1982]
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