Behavior of the ocean radar cross-section at low incidence, observed in the vicinity of the Gulf Stream

Observational results derived from the airborne C-band scatterometer RESSAC looking at small incidence angles (⩽21°) were acquired in the vicinity of the Gulf Stream during the Surface Wave Dynamics Experiment in February and March 1991. Although the instrument was primarily designed to study the directional spectra of long waves, it is also possible to process the data to examine the behaviour of the radar cross-section σ⁰ versus incidence θ and azimuth φ. Although the classical physical or empirical models of σ⁰(θ,φ) predict that the anisotropy of σ⁰ versus azimuth increases with wind speed for a given incidence angle θ, it is found that under some conditions, this behaviour does not occur. By contrast, the anisotropy shows strong minima on certain occasions. Given the high-resolution upper ocean current measurements from Airborne expendable Current Profilers deployed on 5 March 1991, the regions of anisotropic minima could be identified from the three sets of observations (March 4, 5, and 7) downwind of the core of the Gulf Stream with a maximum current of 2 m s^-1, and downwind of the current speed maxima of an anticyclonic-rotating warm core ring juxtaposed along the North Wall. On three other days (February 27, March 2 and 6), no clear anisotropic minima of was found, because the upper ocean current effects were obscured by variability in surface wind field