On the Variety of Internal Wave Radar Observations in the Ocean. 2. Rip-like Signatures

In the accompanying paper (part 1. Slick-like signatures) we describe and present results of a recent classification scheme for synthetic aperture radar (SAR) signatures of internal waves (IWs) that has been proposed based on the analysis of ERS SAR images of the Iberian shelf zone. According to this classification IWs display three different signatures depending on the parameters of surface-active films and wind speed. In particular, it was 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. Here, we present results of observations of predominant single positive signatures even when the background backscatter is well above the noise floor. A variety of observations from different oceanic regions, such as the Strait of Gibraltar, Andaman Sea, Sea of Bali, Gulf of California, etc., are presented as evidence of the phenomena. 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 (Cp>1 m/s). However, analysis of a data set of the West Iberian shelf also shows that predominant positive signatures can occur in this region, where typical values of IW phase speed is Cp 0.5 m/s. This has been made clear from analysis of the mode of signature, which is defined by a parameter that evaluates the IW intensity profiles in relation to the mean backscatter of the image background. The signature mode parameter is found to be correlated with the angle between the IW propagation direction and the wind velocity. A simple model which takes into account modulation of short-scale surface waves by the relative wind velocity to the surface current generated by the IWs is considered and was found to be in good qualitative agreement with the experimental results. While in the accompanying paper (part 1) we found the signature mode parameter to be sensitive to the local concentration of films, in this paper we demonstrate that it can also be sensitive to the relative wind direction to the IW propagation.