High-resolution open-ocean temperature spectra

Abstract

Accurate (<1 mK) temperature sensors have been stiffly moored at ∼1450 m in the open Canary Basin for 1.5 years while sampling at 1 Hz. The sensors were in an area where regular density steps occur. In this article, we investigate the variability of internal waves in such "steppy" environment. The waves vertically move layers of persistent temperature gradients for particular temperatures. The frequency (σ) spectra of temperature, and more clearly those of inferred vertical currents w, show an internal wave band IWB that extends from 0.97f < σ < Nt, where f denotes the inertial frequency or vertical Coriolis parameter. Transition frequency Nt is higher than buoyancy frequency N, computed over large vertical scales Δz = O(100) m. The extension of IWB beyond the traditional bounds [f,N] is probably due to small-vertical-scale layering (Δz ≈ 1 m). The associated weak stratification between such thin layers provides small-scale Ns ≈ 4fh, where fh, denotes the horizontal Coriolis parameter. This minimum stable stratification Ns is associated with tilting of vorticity away from gravity, which causes the subinertial spectral extent to 0.97f. Isothermal smoothing reveals details of coherent vertical internal wave and incoherent motions. The present w spectrum continuum of coherent IWB motions is not flat, as in previous near-surface observations, but linearly increases from σ = f to a peak at σ ∼ 0.8N. The coherence spectrum shows a weak, significant peak at approximately twice the local buoyancy frequency for 2.5 < Δz < 100 m. Instead of dominant mode 1, zero phase difference, observed in IWB across the 130 m of observations, these super-buoyancy motions show mode 2 dominance, π phase difference. Copyright 2009 by the American Geophysical Union.