Dept. of Appl. Ocean Phys. and Eng., Woods Hole Oceanogr. Inst., Woods Hole, MA 02543
James F. Lynch
Woods Hole Oceanogr. Inst., Woods Hole, MA
Scripps Inst. of Oceanogr., La Jolla, CA
Six near-surface acoustic transceivers were moored in the Greenland Sea in an array approximately 200 km in diameter from October 1988 to August 1989. Transmissions with a center frequency of 250 Hz and a bandwidth of 100 Hz were made at 4-h intervals over most of the array, resulting in a multipath structure of early ray arrivals and later near-surface modal arrivals. During February and March the almost isothermal temperature profile resulted in an arrival pattern with the later arriving lower-order modes have greater amplitude. However, from mid-April onwards the pattern changed dramatically as amplitudes of the lowest-order mode became far smaller, and slightly earlier arriving modes became somewhat louder. At the same time, net surface fluxes changed sign, so one expects the formation of a shallow warm surface mixed layer (although the somewhat sparse historical data suggests that warm surface mixed layers are not seen for at least another month). One explanation for the amplitude changes is that modal coupling is induced by horizontal variations in the depth of this warm layer. [sup a)]On leave from Shanghai Acoustics Laboratory.