ROLE OF SURFACE-FILMS IN ERS SAR SIGNATURES OF INTERNAL WAVES ON THE SHELF - 1 - SHORT-PERIOD INTERNAL WAVES

Analysis of synthetic aperture radar (SAR) images of internal waves (I Ws) on the Iberian shelf and classification of the IW surface signatur es is presented. There are three types of IW signatures in the form of bright/dark, dark, and bright bands that correspond to positive/negat ive, negative, or positive variations of radar backscatter, respective ly. Strong positive sign IW signatures occur at very low wind velociti es (less than 2m/s). At winds higher than 2 m/s both positive/negative and negative signatures are observed for range-propagating IWs. For a zimuth-propagating IWs, negative signatures prevail. Two groups of exp eriments carried out in the frame of the Multidisciplinary Oceanograph ic Research in the Eastern North Atlantic (MORENA) project on the Iber ian shelf in August 1994 are described: (1) IW measurements simultaneo us with ERS-1 SAR overpasses and (2) IW measurements, visual observati ons of the surface, and sampling of films from the sea surface. IW man ifestations at low to moderate winds took the form of slicks locating over IW troughs; at the near-threshold wind velocity they were of the form of intensified decimeter-scale waves (antislicks) located over IW crests. Measurements of wave damping cue to films collected from the slick and nonslick areas showed the concentration of surfactants and r etrieved film elasticity to be higher in the slicks than in the nonsli ck areas. A theoretical model of the surface wave modulation by IWs is developed to include surface wave straining by the IW current and sur face wave damping due to surfactant films. Pressure-area curves for re al marine films are used in the model. The relation between the film a nd the straining effects on the surface wave spectrum in the IW field is shown to depend on film parameters, surface wavelength, and wind ve locity. The model predicts strong damping of centimeter-scale waves ov er IW troughs mainly due to the film effect and intensification and de pression of decimeter-scale waves due to both straining and film effec ts, Dependencies of the model spectrum variations as a function of win d velocity for range and azimuth propagating IWs are obtained, the neg ative sign contrast being shown to predominate for azimuth propagating IWs. Model variations of the spectrum of decimeter-scale waves are sh own to be significantly sensitive to surfactants (film pressure), the negative sign contrasts due to films being superimposed on the straini ng effects, and these are eventually capable of suppressing the positi ve sign contrasts. It is concluded that the occurrence of the differen t classes of IW signatures in the SAR images can be explained by the a ction of surface-active films.