Open Ocean Swell Dispersion from Moving Wind Fetches
Momme Claus Hell1, Alex Ayet2, Bertrand Chapron2, Prof. Sarah T Gille, PhD3 and Laure Baratgin4, (1)Scripps Institution of Oceanography, La Jolla, CA, United States, (2)IFREMER, Univ. Brest, CNRS, IRD, Laboratoire d'Océanographie Physique et Spatiale, Brest, France, (3)Scripps Institution of Oceanography, UCSD, La Jolla, United States, (4)Ecole Polytechnique, Palaiseau Cedex, France
Abstract:
Swell waves are generated by areas of high winds under extra-tropical cyclones and are co-located with processes such as wave breaking, white capping of shorter waves and enhanced upper ocean mixing. While cyclones generate waves, they translate at about 10 m/s for about week, thus displacing the region of relative constant-direction winds known as the fetch. In sharp contrast to this moving fetch area under storms, swell events, observed at distant locations, show dispersed swell event that suggest that the event's energy comes from a point source in the open ocean (Munk 1947, Barber and Ursell, 1948, Snodgrass et al 1966). In this study we unify both ideas by outlining the life cycle of open-ocean generated swell in three stages: the generation of a non-linear sea state under a moving storm, the transformation region where the non-linear sea state transforms into linear swell waves, and finally propagation as ordinary linear waves.
We calculate the peak period of the non-linear sea state (Kudryavtsev et. al 2015) from high-frequency scatterometer winds and estimations of non-linear spectral energy fluxes (Longuet-Higgins, 1976) to show how the continuous transformation of wind-generated high wave number energy to longer scales will eventually lead to wave energy that out runs the moving fetch at its leading edge. Once the wave energy exits the fetch area, we can estimate the time and distance over which the non-linear sea state transforms to a linear, less step-like wave spectrum by triangulating the dispersion point from buoy observations. Waves start to disperse in frequency at the point when the spectrum is linear.
Using theoretical and semi-empirical laws, we show how the large-scale space-time evolution of surface winds drives swell generation. The analysis reveals how the structure of surface winds driven by cyclone dynamics determines the timing and amplitude of swell wave arrivals. This analysis framework lays the groundwork for improving long standing biases in numerical swell forecast models as well as improving our understanding of the scales at which surface winds drive the upper ocean.
