Observation of swell dissipation across oceans
  Global observations of ocean swell, from satellite Synthetic Aperture Radar data, are used to estimate the dissipation of swell energy for a number of storms. Swells can be very persistent with energy e-folding scales exceeding 20,000 km. For increasing swell steepness this scale shrinks systematically, down to 2800 km for the steepest observed swells, revealing a significant loss of swell energy. This value corresponds to a normalized energy decay in time {\ss} = 4.2 x 10-6 s -1 . Many processes may be responsible for this dissipation. Because no particular trend is found with wind magnitude and direction, the increase of dissipation rate in dissipation with swell steepness is interpreted as a laminar to turbulent transition of the boundary layer, with a threshold Reynolds number of the order of 100,000. These observations of swell evolution open the way for more accurate wave forecasting models, and provides a constraint on swell-induced air-sea fluxes of momentum and energy. 
Fabrice Ardhuin (SHOM), Bertrand Chapron (LOS), Fabrice Collard