Orbital Velocities Induced by Surface Waves

Abstract

During the third intensive observational period of the Surface Wave Dynamics Experiment (SWADE), an aircraft-based experiment was conducted on 5 March 1991 by deploying slow-fall airborne expendable current profilers (AXCPs) and airborne expendable bathythermographs (AXBTs) during a scanning radar altimeter (SRA) flight on the NASA NP-3A research aircraft. As the Gulf Stream moved into the SWADE domain in late February, maximum upper-layer currents of 1.98 m s?1 were observed in the core of the baroclinic jet where the vertical current shears were O(10?2 s?1). The SRA concurrently measured the sea surface topography, which was transformed into two-dimensional directional wave spectra at 5?6-km intervals along the flight tracks. The wave spectra indicated a local wave field with wavelengths of 40?60 m propagating southward between 120° and 180°, and a northward-moving swell field from 300° to 70° associated with significant wave heights of 2?4 m. As the AXCP descended through the upper ocean, the profiler sensed orbital velocity amplitudes of 0.2?0.5 m s?1 due to low-frequency surface waves. These orbital velocities were isolated by fitting the observed current profiles to the three-layer model based on a monochromatic surface wave, including the steady and current shear terms within each layer. The depth-integrated differences between the observed and modeled velocity profiles were typically less than 3 cm s?1. For 17 of the 21 AXCP drop sites, the rms orbital velocity amplitudes, estimated by integrating the wave spectra over direction and frequency, were correlated at a level of 0.61 with those derived from the current profiles. The direction of wave propagation inferred from the AXCP-derived orbital velocities was in the same direction observed by the SRA. These mean wave directions were highly correlated (0.87) and differed only by about 5°.