Source regions and reflection of infragravity waves offshore of U.S.'s Pacific Northwest

Infragravity waves are oceanic surface gravity waves but with wavelengths (10's km) and periods (>30s) 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 USA were studied using Differential Pressure Gauges which were deployed as part of the Cascadia Initiative array from September 2012-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 100-200s. 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.