  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.
