On July 20, 1990, the north edge of the Gulf Stream (36.7-degrees-N, 7
2.0-degrees-W) was sampled by the R/V Cape Henlopen and simultaneously
imaged by the Jet Propulsion Laboratory's airborne synthetic aperture
radar (SAR). Hydrographic measurements show an abrupt surface front s
eparating warm, salty Gulf Stream water in the south from a filament o
f cool, fresh (<33 practical salinity unit (psu)) water to the north.
The filament lies within the stream and is likely water entrained from
the continental shelf. The southern boundary of the filament is marke
d by increased surface wave breaking in a 100- to 200-m-wide zone, acc
umulations of Sargassum, and an orthogonal velocity change of 20 cm/s.
The front is manifested in a sequence of SAR images as a narrow line
having returns 1-2 dB higher than background. (A second, transient SAR
line occurs near the northern filament boundary.) The observations ar
e compared with model calculations of the surface wave hydrodynamics a
nd radar scattering. The ocean waves are driven by southwesterly 8-m/s
winds and interact with the front to produce primarily an enhancement
of 2- to 3-m waves over a less-than-or-equal-to200-m-wide region cent
ered downwind of the front. Using a composite scattering radar model a
long with measured breaking-wave statistics, we show that the observed
modulations in the radar backscatter can be accounted for through bre
aking-wave and tilted Bragg wave scattering effects. These results fur
ther show that SAR images of the ocean surface can be exploited for de
tailed study of particular ocean processes.