ERS SAR observations of long internal waves with positive backscatter signatures

A recent classification scheme for synthetic aperture radar (SAR) signatures of internal waves (IWs) has been proposed based on the analysis of ERS-1 SAR images of the Iberian shelf zone. According to this classification IWs which propagate with relatively low phase velocities (C_p~0.5 m/s) display three different signatures depending on the parameters of surface-active films and wind speed. In particular, it is found that at very low wind speeds IWs display single positive backscatter signatures as a result of the image background falling to the noise floor at such low wind conditions. However, in different oceanic regions, the authors have observed predominant single positive signatures even when the background backscatter is well above the noise level. In this paper they report observations of IW signatures which mainly display positive backscatter variations on a background above the noise level. They seek for a common factor in the variety of observations from different oceanic regions, (such as the Strait of Gibraltar, Andaman Sea, Sulu Sea, Sea of Ball, Gulf of California, etc.) that may lead to an explanation of the phenomenon. One common factor associated with these SAR observations seems to be that they correspond to comparatively long IWs (their wavelength can be up to several kilometres) which are known to propagate with high phase velocities (C _p>1 m/s)