Environmental forcing of ambient noise in the Nansen and Amundsen Basins of the Arctic Ocean
Bourke, Robert H.
Wilson, James H.
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The AREA 1992 experiment inserted three ANmmT buoys on separate ice floes about 600 ka north of Franz Josof Land. The buoys drifted in unison for most of the experimnt and provided 12-19 months of hourly ambient noise data between 5 and 4000 Hz while obtaining limited weather data. The drift pattern was neatly divided into five legs of nearly uniform ice velocities in response to major changes in the wind field. The annual median spectra of each buoy were nearly identical at or above 200 Nz but diverged below 200 Hz. The largest differences were recorded between the two closest buoys. The annual spectra wer 10 d4 greater than the long term ]urasian Basin median spectra at all frequencies. The annual median spectra was 6-7 dB greater than th,, CZAR= 1988/89 median spectra below 100 HN but was quieter than CZARIX above 100 Hz. Persistent extreme noise levels above the 95" or below the 5 eS percentiles were rare. Sustained 95• percentile noise levels were caused by the ice field convergence resulting from storms passing near the buoy cluster. Sustained noise levels near the 5"t percentile occurred during periods of slow, steady winds. Temporal coherency of the year-long record ranged from 12-23 hours at all frequencies, comparable to other reported data. Significant energy was found at synoptic periods of 16-148 hours and near the tidal/inertial 12 hour period at all three buoys, implying the smem forcing mechanisms were important in spite of buoy separations up to 300 ka. Spatial coherency between the buoys showed the highest correlation between the closest buoy pair. Differences in correlation coefficients mere mller at higher frequencies due to the increased iportance of local effects at higher frequencies. ce speed was the beat environmental correlate with ambient noise from 5-10 Hz, wind speed was best from 32-100 Hz, and wind stress was best above 100 Hz. Three periods of extreme noise levels (two loud, one quiet), each lasting for several days, were investigated in detail to establish the role of wind forcing on ambient noise generation. Periods of loud noise were associated with periods of high wind/ice speed coupled with rapid changes in direction, i.e., loud noise levels are the result of large ice convergence and shearing moment. Quiet periods occur when the buoy drift speed is slow. One of the loud noise events showed that periods of ice convergence on nearby land will increase the noise level, even during times of moderate wind speeds.
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