Diurnal Variability and Its Quantification of Subsurface Sound Scatterers in the Western Equatorial Pacific

摘要

A downward-looking acoustic Doppler current profiler (ADCP), suspended by a series of surface and subsurface floats and connected to an anchored ship, provided a quite stable platform to measure the vertical profiles of backscatter strength (BS) and three components of the velocity from 12 to 22 November 1992 at 1°30′ S and 156°15′ E, in the Intensive Flux Array (IFA) of TOGA/COARE. While the variability of the horizontal velocity was controlled by the semi-diurnal tide, BS and vertical velocity were dominated by diurnal variability probably caused by the diel migration of zooplankton. The downward migration occurred early in the morning (0500-0700 in local time) and the upward one late in the afternoon (1700-1900). The average values of about 4 cm s~(-1) for the sinking and rising speed were estimated from Doppler shift and BS isopleth displacements. The subsurface chlorophyll maximum (SCM) coincident with the top of the thermocline at 80-100 m was also detectable in the BS data during daytime when almost no migrating zooplankton remained in the upper 300 m. Backscatter signals from the SCM and thermocline were separated by correlating the BS data with the chlorophyll a and temperature data. The maximum contribution of the migrating zooplankton, passively drifting phytoplankton and temperature gradient on BS was estimated to be 14.8, 7.0, 5.1 dB, respectively.