Fish effects on ocean current observations in the Cariaco Basin

Abstract

Multiple years of moored current meter observations from the Cariaco Basin show low-frequency variations along with near-inertial waves and further imply the persistent diurnal movement of fish species known to populate the basin. In agreement with short-term observations from 1979, the more recent observations with acoustic Doppler current profilers provide evidence of the multidecadal presence and behavior of these species. An unwanted corollary, however, is a bias in both the vertical and horizontal components of velocity due to the fish movements. Removal of this fish bias results in large data loss (approximately 72%); however, an interpolated, non-biased data set is developed with depth-averaged horizontal velocities comparable to the observations, demonstrating successful elimination of the bias. Further comparisons show that the interpolated data result in minimal variance density loss at low frequencies and a reduction of variance density at high frequencies such that the interpolated data in the internal wave range more closely fit the Garrett-Munk spectrum. The net result is a data set appropriate for further analysis. A mean downward velocity of 0.18 cm s-1 is a reflection of a biogenic particle flux and some residual fish contamination. The mean settling speed of particles in the Cariaco Basin is calculated, via Stokes law, to be smaller than 0.04 cm s-1. Velocity observations from acoustic current meters at depths greater than 400 m are impacted by the water clarity; therefore alternate methods should be used to make velocity measurements at depth. Copyright 2009 by the American Geophysical Union.