Asymmetric vertical internal wave propagation

Abstract

Yearlong data from a single ADCP above the northern extension of the Mid-Atlantic Ridge are used to investigate vertical internal wave propagation in the open ocean (395-1025 m). Whilst 80% of incoherent semidiurnal tidal (D2) phase propagates downward, 90% of near-inertial (f) phase propagates upward. Most upward propagating f-phase is found at sub-f frequencies σmin(N = f) ≈ 0.74f < σ ≤ 0.99f, N the buoyancy frequency and σ min the lower bound of the inertio-gravity (IG) wave band following the 'non-traditional approach', which includes the horizontal component of the Earth's rotation in momentum equations. The upper bound of mostly downward propagating D2-phase coincides with the upper IG-bound σmax(N = f) ≈ 1.35f. The limited band [σmin, σmax] demonstrates that f and D2 contain special internal waves concerning vertical propagation. It is suggested that this is partially attributable to small-scale step layering in N, with N = f separating 'trapping' from 'classic propagating' wave regimes, rendering the mean large-scale N ≈ 18f less important. Copyright 2006 by the American Geophysical Union.