High frequency acoustic observations of episodic mixing events in Lunenburg Bay

Abstract

High frequency (0.6-5 MHz) acoustic backscatter data has been collected during the past four years (2002-2006) at Dalhousie's coastal ocean observing system at Lunenburg Bay (www.cmep.ca). Severe storm events, including hurricanes, produce high wave (Hs 2m) and wind (U10 10 m/s) conditions in the area for sustained periods (T 1 day) during late summer and autumn. Coincident with periods of high waves or high winds or both, enhanced acoustic backscatter is observed to occur throughout the water column. The acoustical backscatter observations recorded via acoustic current profilers and velocimeters at three locations within the bay are shown. The subplots labeled BN1-3 correspond to locations around the bay (buoy node 1 is at the head of the bay while BN2 and 3 are at the south and north shores of the mouth of the bay). Six periods of enhanced surface-intensified backscatter (at days 267, 272, 277, 280, 287, 289) and 3 periods of enhanced scattering near the bottom (days 267, 272 and 289) occur between yearday 260 to 290 (September 17 to October 17). To varying degrees, local sheltering of wind and waves resulted in distinct data sets from each location for any given period of enhanced scattering