Role of surface films in ERS SAR signatures of internal waves on the shelf3. Mode transitions

It was shown recently that three different types of short-period internal w ave (IW) synthetic aperture radar (SAR) signatures occur on the Iberian She lf depending on parameters of surface active films. Large-scale modulation of film characteristics by currents associated with internal tidal waves ca n 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. H ere onshore propagating short-period IWs can exhibit different SAR signatur es within the same wave packet according to their position relative to the phase of the internal tidal wave. These transitions of signature mode can b e explained by large-scale variations of film parameters produced by the in ternal tidal waves. At low-to-moderate winds, leading waves in the IW packe ts usually exhibit double sign signatures, which evolve to negative sign si gnatures for the subsequent waves in the packets. At very low, near-thresho ld winds the IW signatures can be in the form of the double sign type under going transition to positive sign signatures when compared to the backgroun d behind the IW packet. This is a consequence of different threshold wind s peeds for excitation of Bragg waves at different film pressures. An increas e of unperturbed film pressure in the direction of the ranking order of sol itons in a packet of short-period IWs, which is predicted by a theoretical model and in agreement with measurements of film elasticity, is demonstrate d. Observations of SAR signature mode transitions are concluded to be consi stent with previous model results, and a diagram describing mode transition s of the IW signatures in relation to film pressure and wind speed is prese nted.