Benthic Observations on the Madeira Abyssal Plain: Currents and Dispersion

Abstract

An experiment to measure near-bottom currents on the Madeira Abyssal Plain is described. The moorings placed near 33°N, 22°W were separated by 5?40 km with instruments at 10, 100 and 600 m above the bottom (depth ?5300 m). Rotor stalling occurred ¼ to ? of the time but does not hinder the analysis which separated currents into tidal (3 cm s?1, inertial (1.3 cm s?1) and low frequency (2.5 cm s?1) components. The M2 tide is found to be principally barotropic with magnitude, ellipticity, orientation and phase adequately predicted by the tidal model of Schwiderski (1979). Oscillations of near-inertial frequency are found to be bottom intensified, have a wavelength of 100 km directed nearly due south and 3 km vertically: their phase velocity is directed downward suggesting the bottom as the source. The vertical group velocity is estimated at ?150 m day?1 upward and corresponds to the 4?6 day lag observed between 10 and 600 m for the envelope of inertial amplitude. Low-frequency statistics are presented also revealing bottom intensification. The array mean current can not be distinguished from zero; the kinetic energy, 3.6 cm2 s?2 at 10 m, decreases to 1.6 cm2 s?2 by 600 m, and rms east currents exceed rms north currents by 50%. Despite the dominance of long periods, 50?150 days, the integral time scale derived from the array autocorrelation data is 7 ± 2 days. By assuming equality of Eulerian and Lagrangian time scales we estimate the abyssal (horizontal) diffusivity as 2.5 ? 106 and 1.5 ? 106 cm2 s?1, east and north, respectively. Spatial correlations are calculated for the array, and the transverse velocity function is found to have a zero-crossing near 35 km. streamfunction maps are made at 10-day intervals by the method of optimum linear interpolation. trajectories calculated over a 6?12 day period agree well with the drift of two groups of four neutrally buoyant floats at depths 4600?4900 m despite very weak flows, circa 1.5 cm s?1. Trajectories have also been calculated of the behavior of 170 pairs of particles at initial separations 1?20 km for up to 10 days. Their statistics reveal mean dispersion rates which yield pair diffusivity estimates of 5 ? 103 cm2 s?1 at 1 km separation increasing to 2 ? 106 cm2 s?1 at 20 km separation. Over this range the average time for the squared separation of particles to double is 10?15 days. The float cluster growths are quite comparable. The merits and limitations of the methods employed are discussed.