FY 2000

Correcting moored ADCP data for fish-bias errors at 0°, 110°W and 0°, 140°W from 1993 to 1995

Plimpton, P.E., H.P. Freitag, and M.J. McPhaden

NOAA Tech. Memo. OAR PMEL-117, NTIS: PB2000-106363, NOAA/Pacific Marine Environmental Laboratory, Seattle, WA, 35 pp (2000)

Moorings in the Tropical Atmosphere Ocean (TAO) array monitor upper ocean velocity, temperature, and salinity as well as atmospheric parameters. Beginning in 1990, PROTEUS (PROfile TElemetry of Upper ocean currentS) moorings were equipped with a 153.6-kHz RD Instruments acoustic Doppler current profiler (ADCP). With this instrument, ocean currents to a depth of 250 m could be measured and data transmitted to shore in real time via Service Argos. However, due to the reflection of acoustic energy from fish in the vicinity of surface moorings, large bias errors were at times found in the ADCP velocities. This report describes the correction of the ADCP velocity errors at 110°W and 140°W from 1993 to 1995 using coincident mechanical current meter (MCM) data. A method is developed that uses empirical orthogonal functions of ADCP-MCM speed differences to correct the ADCP velocities in post processing. Corrected speeds are expected to be accurate to about 5 cm s^–1. Beginning in August 1995, the ADCPs at these locations were deployed on subsurface moorings that were free of the fish-bias errors encountered on the surface mooring deployments.