Persistency of ocean swell fields observed from space

Using global satellite Synthetic Aperture Radar (SAR) observations, swell systems are confirmed to travel over very large distances across ocean basins. To yield global and almost continuous coverage of these very well organized wave systems emanating from the different intense storms, a geometrical optics approach is applied. 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. Satellite Synthetic Aperture Radar (SAR) observations can provide a global view of ocean swell fields when using a specific ”wave mode” sampling. In this global and systematic dataset, we restricting the SAR-derived peak wavelength and directional parameters to the longer swell systems for which the SAR mapping distortion is minimal. The persistency of swells, that travel over very large distances across ocean basins, is used to fill gaps in the observation record with a geometrical optics approach. This provides a continuous coverage of very well organized wave systems that emanate from intense storms. 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.