Detection and classification of ocean eddies using ERS-1 and aircraft SAR images

The interaction of ocean waves with surface currents having a significant horizontal shear causes either an amplification or a reduction of the surface wave spectral density, depending on the wave propagation direction. The radar backscatter modulations that result from this interaction are therefore also a function of the radar look direction relative to the direction of the current shear. This effect causes distinctive patterns in radar images of the ocean surface which can be used to detect the presence of ocean eddies and to determine the direction of their rotation. This approach has been applied to a set of ERS-1 and aircraft SAR images collected off the coast of Norway on October 9, 1993. The ERS-1 image contains a semicircular feature that changes in contrast from bright to dark, consistent with model predictions for an anticyclonic eddy. Aircraft SAR images collected over the same feature about 11 hours later show similar patterns. Furthermore, images collected with orthogonal look directions show a.contrast reversal as predicted for this type of current field. These images also reveal an additional fine structure which suggests the presence of internal waves near the boundary of the eddy.