Strong dissipation of steep swells observed across oceans

Global observations of ocean swell propagation is presented and analyzed, using on four years of satellite Synthetic Aperture Radar data. Tracking swells along their propagation paths yields an estimation of the dissipation of their energy. Swells can be very persistent with energy e-folding scales exceeding 30,000 km. For increasing swell steepness, this scale shrinks down to 2700 km, revealing a significant loss of swell energy. This pattern is consistent with a laminar to turbulent transition of the boundary layer, induced by the opposite wave-induced motions of air and water, with a threshold Reynolds number of the order of 100,000. This finding opens the way for more accurate wave forecasting models, and provides a constraint on swellinduced air-sea fluxes of momentum and energy.