Role of surface films in ERS SAR signatures of internal waves on the shelf: 3. Mode transitions

Abstract

It was shown recently that three different types of short‐period internal wave (IW) synthetic aperture radar (SAR) signatures occur on the Iberian Shelf depending on parameters of surface active films. Large‐scale modulation of film characteristics by currents associated with internal tidal waves can also produce a signature in ERS SAR images. On the western Iberian Shelf short‐period IWs typically propagate in groups of packets that are believed to be generated by the evolution of internal tidal waves over the shelf. Here onshore propagating short‐period IWs can exhibit different SAR signatures within the same wave packet according to their position relative to the phase of the internal tidal wave. These transitions of signature mode can be explained by large‐scale variations of film parameters produced by the internal tidal waves. At low‐to‐moderate winds, leading waves in the IW packets usually exhibit double sign signatures, which evolve to negative sign signatures for the subsequent waves in the packets. At very low, near‐threshold winds the IW signatures can be in the form of the double sign type undergoing transition to positive sign signatures when compared to the background behind the IW packet. This is a consequence of different threshold wind speeds for excitation of Bragg waves at different film pressures. An increase of unperturbed film pressure in the direction of the ranking order of solitons in a packet of short‐period IWs, which is predicted by a theoretical model and in agreement with measurements of film elasticity, is demonstrated. Observations of SAR signature mode transitions are concluded to be consistent with previous model results, and a diagram describing mode transitions of the IW signatures in relation to film pressure and wind speed is presented.