  Satellite Synthetic Aperture Radar (SAR) observations can provide a global
view of ocean swell fields when using a specific "wave mode" sampling. A
methodology is presented to routinely derive integral properties of the longer
wavelength (swell) portion of the wave spectrum from SAR Level 2 products, and
both monitor and predict their evolution across ocean basins. SAR-derived
estimates of swell height, and energy-weighted peak period and direction, are
validated against buoy observations, and the peak directions are used to
project the peak periods in one dimension along the corresponding great circle
route, both forward and back in time, using the peak period group velocity. The
resulting real time dataset of great circle-projected peak periods produces
two-dimensional maps that can be used to monitor and predict the spatial
extent, and temporal evolution, of individual ocean swell fields as they
propagate from their source region to distant coastlines. The methodology is
found to be consistent with the dispersive arrival of peak swell periods at a
mid-ocean buoy. The simple great circle propagation method cannot project the
swell heights in space like the peak periods, because energy evolution along a
great circle is a function of the source storm characteristics and the unknown
swell dissipation rate. A more general geometric optics model is thus proposed
for the far field of the storms. This model is applied here to determine the
attenuation over long distances. For one of the largest recorded storms,
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. In this
case, swell dissipation is a significant term in the wave energy balance at
global scales.
