Direct Heat Flux Estimates Using a Towed Vehicle

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Abstract
Direct estimates of vertical heat flux were computed using data collected with a towed vehicle that carried collocated velocity and temperature sensors. Horizontal wavenumbers from about 1 to 40 cpm were resolved, which excludes some potentially significant contribution from lower wavenumbers. The correlation coefficient of w and θ is 0.1 to 0.2 and statistically significant. The authors compared the cospectra obtained from three regions: (i) a well-defined turbulent interface at 400 m depth being overturned, (ii) sporadic layers of turbulence in the thermocline near a salinity maxima, and (iii) a thermal inversion at the base of a surface mixing layer. For all three regions the direct flux estimates are consistent with the notion of a flux Richardson number smaller than ∼0.2 and Osborn's method. The smallest wavenumber resolved was 0.5ko, where ko is the Ozmidov wavenumber, the largest wavenumber contributing to the flux was 6ko, and there was no significant counter-gradient flux in this range. The direct estimates and those of the method of Osborn and Cox agree within a factor of 2 when the smallest wavenumber resolved approaches ko, while for poorer spectral resolution the former is smaller than the latter. For the thermal inversion the directly estimated flux was upward.
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