Spatial variability of deep‐ocean motions above an abyssal plain

Abstract

Near‐bottom current observations are discussed from an array of moorings separated by 8–40 km horizontally in the Madeira abyssal plain, North Atlantic Ocean. The area is topographically smooth, but observed currents varied strongly over short distances. Across horizontal scales O(10 km), “incoherent” internal wave properties were retrieved such as conservation of relative frequency bandwidth and internal wave polarization at inertial, tidal, and higher harmonics frequencies. In the vertical (between 10–110, 110–650 m above the bottom (mab) at ∼5000 m), the current differences (∼shear) were predominantly at the local inertial frequency f. Near f the largest variance was found at 110 mab, with peaks in power at 0.99f and 1.03f. As stratification was weak (N ≈ 2f) the subinertial frequency peak was well within the combined internal inertio‐gravity wave band. At 650 mab, N ≈ 4f, and smaller peaks at ∼1.00f and 1.03f were observed. Toward the bottom subinertial currents increased in magnitude. The horizontal scale of variation of subinertial and inertial motions was small: ∼30 km. The magnitude of horizontal current differences in these frequency bands was equal to that in the tidal band. Although the energetically dominant, highly deterministic tidal currents were predominantly barotropic, confirming a previous study, their horizontal scale of variation was ∼300 km, due to small bottom relief. Results are discussed relevant for matching records from nearby moorings, to generate extra long time series.