Half‐year‐long measurements with a buoy‐mounted acoustic Doppler current profiler in the Somali Current

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Abstract
A self‐contained, upward‐looking acoustic Doppler current profiler (ADCP), mounted in the top float of a subsurface mooring was deployed in the Somali Current at 2°14′N, 45°55′E from September 17, 1985, to April 25, 1986. The instrument operated at a frequency of 150 kHz, with a vertical beam angle of 20°. Vector‐averaged profiles of horizontal and vertical velocity were recorded every 4 hours, using 200 pings per ensemble at a vertical bin length of 8.7 m. The mooring was deployed in very rough topography, settling in a trough at 337 m depth with the ADCP located at 267 m depth. Data retrieval over the entire recording period was complete, with Doppler biasing from side lobe reception of vertically traveling rays affecting only the top 20 m below the surface. Over the 7‐month deployment the instrument recorded current profiles encompassing the end of the 1985 summer monsoon and entire winter monsoon and also through the spring transition into the early onset phase of the 1986 summer monsoon. Significant echo amplitude variations of week‐to‐month‐long duration were observed, which were only partially related to horizontal flow variations associated with the monsoons. Projection of the strong horizontal currents (exceeding 150 cm/s at times) into the vertical component was not observed, attesting to fairly exact orientation of the four beams and tilt meters. This indicates that the vertical current measurement from ADCPs can be potentially useful for phenomena with vertical velocities exceeding a few millimeters per second. However, an analysis of echo amplitude and vertical current variations at the diurnal period suggests that the measured vertical velocity is, at least at that period, probably dominated by active vertical migration of biological scatterers through the water column.

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