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ABSTRACT
Vaned, internally recording instruments that measure temperature fluctuations using FP07 thermistors, including fluctuations in the turbulence wavenumber band, have been built, tested, and deployed on a Tropical Atmosphere Ocean (TAO) mooring at 0°, 140°W. These were supplemented with motion packages that measure linear accelerations, from which an assessment of cable displacement and speed was made. Motions due to vortex-induced vibrations caused by interaction of the mean flow with the cable are small (rms < 0.15 cable diameters) and unlikely to affect estimates of the temperature variance dissipation rate χ^sub T^. Surface wave-induced cable motions are significant, commonly resulting in vertical displacements of ±1 m and vertical speeds of ±0.5 m s^sup -1^ on 2-10-s periods. These motions produce an enhancement to the measurement of temperature gradient in the surface wave band herein that is equal to the product of the vertical cable speed and the vertical temperature gradient (i.e., dT/dt ~ w^sub c^dT/dz). However, the temperature gradient spectrum is largely unaltered at higher and lower frequencies; in particular, there exists a clear scale separation between frequencies contaminated by surface waves and the turbulence subrange. The effect of cable motions on spectral estimates of χ^sub T^ is evaluated and determined to result in acceptably small uncertainties (< a factor of two 95% of the time, based on 60-s averages). Time series of χ^sub T^ and the inferred turbulent kinetic energy dissipation rate ε are consistent with historical data from the same equatorial location.
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1. Motivation
The need for extended observations of mixing is highlighted by the potentially prominent role played by mixing in changing equatorial SST on interannual (El Niño) time scales. Coincident with the passage of a downwelling Kelvin wave observed prior to the 1991-93 El Niño, reduced mixing observed by Lien et al. (1995) may have provided positive feedback toward increasing SST in the central Pacific. At the other extreme, enhanced subsurface mixing is a prime (but unproven) candidate for the 8°C surface cooling (in 1 month) at 0 125°W to abruptly conclude the strong 1997-98 El Niño (McPhaden 1999; Wang and McPhaden 2001). This enhanced mixing, inferred from bulk estimates, must also be highly intermittent (characteristic of naturally occurring turbulence) and not amenable to observation from...