Autonomous microstructure EM-APEX floats

Abstract

Fast responding FP-07 thermistors have been incorporated on autonomous profiling EM-APEX floats to measure microscale ocean temperature fluctuations produced by turbulence. In this implementation, the FP-07 thermistor generates an electrical signal corresponding to ocean temperature fluctuations, which is conditioned by an analog circuit board, and digitized and recorded on a custom data acquisition and storage board. The raw and processed temperature observations are stored on a microSD card. Results from eight microstructure EM-APEX floats deployed in the Sargasso Sea are presented here. The slow profiling speed of EM-APEX floats enables them to resolve the higher wavenumber regime of the microscale temperature gradient spectrum, beyond the roll-off wavenumber. The temperature variance dissipation rate $\chi$ is computed directly by integrating the observed temperature gradient spectrum over the turbulence wavenumber region without the need to fit the observed temperature gradient spectrum to the empirical spectral form. The accuracy of $\chi$ estimated from microstructure EM-APEX floats is confirmed by the agreement, within a factor of 2, between the temperature diffusivity $K_T$ computed from our estimates of $\chi$ and estimates of diapcynal diffusivity computed from simultaneous tracer measurements. The observed temperature gradient spectra averaged over many realizations resemble the Batchelor spectral form, though individual spectra often do not fit the empirical prediction. Estimates of $\chi$ from different floats have similar temporal fluctuations and vertical profiles, further supporting measurement quality. Estimates of $\chi$ exhibit a lognormal distribution, as expected for statistically homogeneous isotropic turbulence. Turbulence measurements derived from FP-07 sensors on autonomous profiling floats are of comparable quality to those on conventional free-fall microstructure profilers.