Revisiting issues in estimating spectra of ocean temperature microstructure

This study revisits two distinct issues causing spectral distortion in temperature microstructure to better deal with recent widespread measurements from fast-response thermistors. First, the platform dependence of the thermistor response, which has not been well recognized before, is examined. Using simultaneously measured velocity shear and temperature microstructure data from a vertical microstructure profiler (VMP) and autonomous underwater gliders, this study estimates the shape of the thermistor response by treating the Kraichnan spectrum as the true spectrum. The estimated response of thermistors mounted on gliders exhibits clear vehicle speed dependence and attenuates more severely than those mounted on a VMP, while individual thermistor irregularities have only a minor effect. It remains unclear how platform vehicles influence the water boundary layer over a moving thermistor, but users are cautioned to take vehicle type into account when correcting thermistor responses. Second, it is explored how response-corrected temperature gradient spectra deviate from the Kraichnan spectrum at low Reynolds numbers, with a special focus on poorly understood anisotropic spectra associated with salt fingers. Although signals of salt fingers are successfully isolated from those of shear-driven turbulence using criteria based on the density ratio R ρ and the buoyancy Reynolds number Re b , given by 1 < R ρ < 2 and Re b ~ o (1), any specific spectral deviations are not observed, apart from finestructure contamination characterized by a negative spectral slope, even in glider microstructure data containing a horizontal component. The evidence for the anisotropic salt-fingering +2 spectral slope, proposed exclusively in the 1980s, remains unconfirmed.