Narrowband Oscillations in the Upper Equatorial Ocean. Part I: Interpretation as Shear Instabilities

Extended measurements of temperature fluctuations that include the turbulence wavenumber band have now been made using rapidly sampled fast thermistors at multiple depths above the core of the Equatorial Undercurrent on the Tropical Atmosphere-Ocean (TAO) mooring at 0◦, 140◦W. These measurements include the signature of narrowband oscillations as well as turbulence, from which the temperature variance dissipation rate (χT ) and the turbulence energy dissipation rate ( χ) are estimated. The narrowband oscillations are characterized by: • groupiness packets consist of O(10) oscillations; • spectral peaks up to two orders of magnitude above background; • a clear day/night cycle with more intensive activity at night; • enhanced mixing rates; • frequencies of 1 − 2 × 10−3Hz, close to both the local buoyancy and shear frequencies, N/2π and S/2π, and remarkably constant in time; • high vertical coherence over at least 30 m scales; and • abrupt vertical phase change (π/2 over < 20 m). The abrupt vertical phase change is consistent with instabilities formed in stratified shear flows. Linear stability analysis applied to measured velocity and density profiles leads to predicted frequencies that match those of the observed oscillations. We suggest that the observed oscillation frequencies are set by the phase speed and wavelength of instabilities as opposed to the Doppler-shifting of internal gravity waves with intrinsic frequency set by the local stratification N .