THE SEARCH FOR TOPOGRAPHIC ROSSBY WAVES IN THE GAPPY CURRENT RECORDS AT SITE D.

Abstract : Evidence for the existence of equivalent-barotropic topographic (Rossby) waves is found in current records from instruments moored near depths of 500, 1000, and 2000 m at Site D (39 degrees 20 minutes N, 70 degrees W). Spectra are found from the very gappy records by a new form of complex demodulation, allowing examination of periods up to 100 days. The autospectra level off at the lowest frequencies, no longer increasing as a power law. Significant coherencies of 0.6 and up between the levels and between the U and v components at each level are common. The different levels are nearly in phase, giving significant negative correlation and momentum flux. The records taken near 100 m show few of these results, suggesting a different regime. The interpretation given to the observations is of quasi-barotropic topographic waves generated in the decay region of the Gulf Stream, being filtered by the shelf so the group velocity is nearly straight up the slope for the frequencies examined (periods of 15 to 100 days), and being absorbed by the continental shelf and upper slope. This interpretation is supported by a numeric model. (Author)