Nonlinear features of internal waves off Baja California as observed from the SEASAT imaging radar

Abstract

The synthetic aperture radar (SAR) on SEASAT has yielded well‐defined images of quasi‐periodic internal waves in the waters on the west side of Baja California. These waves occur in groupings separated by 15–23 km, each group having 2–20 striations with wavelengths of the order of 300 m or longer. They appear in water approximately 200 m deep, shoreward of several banks at the continental shelf edge whose depths are as shallow as 15 m. The wave surface signatures exhibit clear nonlinear features: higher‐than‐linear group velocities, and decreases in wavelength, crest length, and amplitude toward the rear of the packet. Environmental data for the area have been examined, and these show a well‐developed mixed layer, low winds, and vigorous tidal action. Thus, stratification, wind speed, tidal current, and bottom topography apparently combine to establish proper conditions for tidal generation of internal waves. The images have been analyzed from the standpoint of nonlinear wave theory, and it is shown that the vertical displacement of an isopycnal surface can be estimated from a combination of SAR imagery, vertical density profiles, and simple theoretical expressions. Numerical examples are given.