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Distribution of peaks of 70 kHz acoustic scattering in relation to depth and temperature during day and night at the edge of the Gulf Stream—EchoFront 83

1989; Pergamon Press; Volume: 36; Issue: 4 Linguagem: Inglês

10.1016/0198-0149(89)90008-3

1878-2477

Richard D.M. Nash, John J. Magnuson, T. K. Stanton, C. S. Clay,

Oceanographic and Atmospheric Processes

The depth and temperature distribution of acoustical scattering peaks in the near-surface frontal zone of the Gulf Stream was examined with a 70 kHz downward-looking sonar to 130 m. A peak was defined as a layer with relatively more scattering than layers just above or below it. Data were collected during the day and night in July 1983, approximately 115 km ENE of Cape Hatteras in a front with sharp thermal gradients above 130 m, and in the Gulf Stream with rather homogeneous temperatures above 130 m. "Peaks" in the front occurred more frequently both day and night above 60 m and at temperatures warmer than 15°C; in the Gulf Stream peaks occurred more frequently only above 20 m and at temperatures warmer than 24.5°C and then only during the day. The distribution of peaks was more homogeneous with depth and temperature in the Gulf Stream than in the front. Day and night distribution also differed with temperature in the front but with depth in the Gulf Stream. Acoustical scattering intensity in the peaks generally increased at night in the Gulf Stream at all depths above 130 m; in the front scattering levels appeared to increase both above and below the gradient but decrease in the gradient. Differences in the nocturnal increase in scattering between the front and Gulf Stream supports the concept of thermoclines as barriers to vertical migrants.