Observations collected as part of the Cascadia Amphibious Array (Oregon-Washington coast) show that ocean surface infragravity (IG) waves (periods from 30 to 250 s) in the Pacific Ocean originate from the coastal zone. With the deepest observations at 1909 m, we identify tidal modulation and a dramatic reduction in energy as IG waves cross the shelf break. Observations are consistent with radiation from an isotropic, nearshore source, and provide quantitative verification linking oceanic IG fluctuations with nearshore motions. Our analysis demonstrates that trapping of IG waves on the shelf break strongly reduces energy radiation into ocean basins. Further, for narrow continental shelves (U.S. West Coast), energy levels past the shelf break depend only weakly on shelf geometry. Consequently, oceanic energy levels can be related to coastal sources by scaling relations predicted by geometric optics, simplifying the effort to estimate the impact of radiated IG-waves on remote sites (e.g., to estimate seismic noise levels or impact on ice shelves). |
