Swell Scintillation
Abstract
"Ocean surface waves represent the dynamic boundary between the atmosphere and ocean, which is important for air-sea interaction, and the global exchange of matter and momentum in the ocean-atmosphere system. Ocean waves are generated during energetic storms and, after decoupling from the wind, radiate across ocean basins, developing into long-period and narrowbanded wave fields typically referred to as ocean swell. While radiating across open oceans, ocean swell fields drive upper ocean circulation and affect air-sea interaction. The breaking of these waves on exposed coastlines transfers the ordered wave energy and momentum into coastal currents, infragravity waves, heat, sound, and turbulence. Open ocean swell propagation is assumed to occur along the rays of geometrical optics defined by weak refraction on meso- and synoptic-scale flows. In this case, the evolution of swell is predominantly governed by geometric spreading and frequency dispersion. However, large differences between observed and predicted directional apertures, consistent errors in swell arrival times, and unexplained swell decay rates, indicate that our theoretical framework of swell propagation is likely incomplete. To improve this, we have to consider that the upper ocean is not a homogeneous medium on the submesoscale. Instead, the upper ocean is filled with energetic submesoscale turbulent flow features, which results in random scattering of waves. This is analogous to star scintillation due to scattering of light by atmospheric turbulence.In this project we consider the hypothesis that interaction with submesoscale surface flows is responsible for the noted discrepancies between theories and observations of swell. We develop a new theoretical framework to quantify and model the effects of oceanic submesoscale flows on swell evolution and make detailed and falsifiable predictions of swell propagation characteristics. We will deploy an oceanic sensor array of a 100 wave resolving drifters in the South Pacific to observe swell evolution from the Southern Ocean to the U.S. West Coast to test our hypotheses.Numerical simulations with the third-generation wave model WaveWatch III, equipped with a new diffusion term that follows from the proposed theory, will enable direct comparison of theory predictions to worldwide coastal stations, and to data acquired from our dedicated sensor array. The combination of a new theoretical framework, a dedicated oceanic sensor array, and a numerical implementation of the theory will provide an unambiguous validation of the hypothesis that swell evolution can be completely explained if scattering of waves on the submesoscale nearsurface flow field is accounted for. If proven correct, these results will lead to a paradigm shift in our understanding and operational modeling capabilities of long-distance swell propagation.Further, the observations will represent a unique, continuous and long-term dataset of oceanwaves, surface currents, sea-surface temperature and surface winds in the Southern Pacific basin.The dataset thus has broad use beyond the current project in verification of Navy (operational)models and understanding of marine boundary layer dynamics in general"
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- May 08, 2020
- Source ID
- N000142012439
Entities
People
- Pieter Smit
Organizations
- Office of Naval Research
- United States Navy