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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 ϵχ are estimated.\ud\udThe narrowband oscillations are characterized by the following:\ud\ud    *\ud\udgroupiness—packets consist of O(10) oscillations;\ud    *\ud\udspectral peaks of up to two orders of magnitude above background;\ud    *\ud\uda clear day–night cycle with more intensive activity at night;\ud    *\ud\udenhanced mixing rates;\ud    *\ud\udfrequencies of 1–2 × 10⁻³ Hz, close to both the local buoyancy and shear frequencies, N/2π and S/2π, which vary slowly in time;\ud    *\ud\udhigh vertical coherence over at least 30-m scales; and\ud    *\ud\udabrupt vertical phase change (π/2 over <20 m).\ud\udThe 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. This correspondence suggests 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

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