Spatial variability of deep-ocean motions above an abyssal plain

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 similar to5000 m), the current differences (similar toshear) 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 (Napproximate to2f) the subinertial frequency peak was well within the combined internal inertio-gravity wave band. At 650 mab, Napproximate to4f, and smaller peaks at similar to1.00f 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: similar to30 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 similar to300 km, due to small bottom relief. Results are discussed relevant for matching records from nearby moorings, to generate extra long time series.