Abstract
As part of the Surface Wave Dynamics Experiment on 5 March 1991, upper ocean current profiles and directional surface-gravity wave spectra were acquired to examine the interactions of low-frequency surface waves (swell) with strong mean currents on a combined Airborne expendable Current Profiler (AXCP) and Scanning Radar Altimeter (SRA) research flight on the NASA NP-3A. These data indicated a spatially-evolving surface wave field encountering a highly-sheared baroclinic current regime associated with the Gulf Stream (GS) and Warm Core Ring (WCR). A maximum near-surface current of 2 m s -1 was observed in the core of the GS, and a WCR was juxtaposed along the North Wall of the GS. Near-surface currents from the AXCPs (2 m) and those observed from an Acoustic Doppler Current Profiler (ADCP) (6 m) were used to shift the grid of surface currents derived from the AVHRR images 20 km northward. Surface current vorticity ranged from - 0.3f c to 0.7f c (f c is the local Coriolis parameter) with the largest values located on the boundary between the GS and the WCR. A ray tracing model was used to simulate the behavior of a 10 s swell crossing the adjusted surface velocity field at various angles from 310 to 10 ° N. The wave group and the individual wave direction were refracted by these baroclinic features forming caustic regimes at distances of 80-100 km to the east of the anticyclonically-rotating WCR and weaker caustics located on the west side of the feature. The directional wave spectra were traced from 310° to 50° N in direction, and from 0.06 Hz to 0.14 Hz in frequency based upon the conservation of wave action along the ray paths. The highest correlation (0.57) between the observed and simulated directional wave spectra was found when the GS and WCR were displaced 20 km northward as suggested by the measurements. Realistic surface current structure has to be included in models to forecast accurately the wave field in regions of strong baroclinic currents.