Persistency of ocean swell fields observed from space Fabrice

Using global satellite Synthetic Aperture Radar (SAR) observations, swell systems are confirmed to travel over very large distances across ocean basins. Satellite observations can provide a global view of swell systems when using a specific ”wave mode” sampling. The data are gathered globally and systematically, but the SAR-derived peak wavelength and directional parameters are restricted to the longer swell detected systems for which the SAR-mapping distortion is minimal. Owing to the quality of the retrieved swell parameters, the methodology is shown to provide accurate estimates of the swell arrival times up to 6 days ahead. When reversed, propagation yields well defined swell sources associated with storms. Far away from these sources, and in the absence of dissipation, the swell height should decrease asymptotically like 1/ √ (α sinα) with α the spherical distance on the Earth surface. In practice, estimated values of Hss can deviate significantly from that asymptote, providing a measure of swell energy dissipation. One of the largest recorded storms provides a striking example in which observations of 15 s period swells are consistent with a constant dissipation rate that corresponds to a 3300 km e-folding scale for the energy. This relatively high dissipation is a significant term in the energy balance of ocean waves at global scales.