Coastal trapped waves in high-scattering regions: Observations of scattering modes and comparison to theory near the Outer Banks, North Carolina

Coastal trapped waves (CTWs) become scattered when they encounter irregular coastlines and bathymetry during propagation. Classical analytical models using a first-order wave equation in the long-wave limit require assumptions of a straight coastline with similar shelf bathymetry and, therefore, cannot capture CTW scattering. More complicated analytical and modeling studies have provided some information about the different types of shelf geometries that can induce scattering, but much of the CTW scattering process remains a large knowledge gap. The continental shelf off the coast of North Carolina provides a unique region for studying CTWs as the coastline is relatively straight locally but is bounded by an estuary to the north and shelf narrowing to the south, both of which induce scattering. Observations of CTW velocity and pressure from a mooring array in this region show that seasonality of the CTW signal and the importance of the role of friction follow theory. However, poor agreement between cross-shelf CTW spatial modes from the observations and two different classical analytical solutions is due to scattering and indicates the basic assumptions are too rigid to allow the theory to be globally applicable. High-frequency radar surface velocity data from this same study region provides additional CTW spatial and propagation information, while the application of real vector empirical orthogonal function analysis allows the identification of CTW scattering modes. This analysis indicates that these modes are highly complex; scattering induces a change in the particle velocity magnitude and rotates vectors away from a theoretical alongshore propagation to cross-shelf direction. Furthermore, increased stratification not only reduces the magnitude of CTWs but also increases scattering into progressively higher order modes through a cascading process.