An application of ocean wave-current refraction to the Gulf Stream using SEASAT SAR data

Abstract

When ocean waves in deep water interact with a current, the direction of propagation and characteristics of the waves such as height and length are affected. Swell in the open ocean can undergo significant refraction as it passes through major current systems like the Gulf Stream or Antarctic Circumpolar Current. Remote sensing techniques such as synthetic aperture radars (SAR) have the potential to detect wave systems over a wide geographical area. Combining a model for wave refraction in the presence of currents with SAR measurements, the inverse problem of using the measured wave data can be solved to determine the direction and magnitude of the intervening currents. In this study the behavior of swell measured by SAR on a satellite pass over the Gulf Stream is examined. The refraction predicted by a numerical model under conditions of varying current profiles and velocities is compared to SAR generated wave spectra. By matching the current profile which results in the best correlation of wave refraction to the SAR data, the tomographic problem of measuring the Gulf Stream current is solved. The best correlation between the model and SAR data is obtained when a current is modeled by a top hat velocity profile with a direction of 75° and a current speed of 2 m/s. The direction agrees with that visually observed from the SAR images, and the direction and speeds are close to the Coast Guard estimates for the Gulf Stream at the time of the SEASAT,pass. The current profiles used did not take into account a possible widening of the Gulf Stream at the position of the satellite overpass. There is a great deal of scatter in the SAR data, both before and in the Gulf Stream, so it is difficult to correlate every point with specific current behavior, but the increase in wave length and change in wave angle in the center of the Gulf Stream seem to indicate that there may be a non-uniform feature such as the formation of an eddy or other lateral variability near the current's edge.