Shear and Temperature Microstructure Measurements from APEX Floats

Abstract Temperature and shear microstructure data were collected by the epsilometer, a custom turbulence package, integrated on to a Teledyne Webb APEX float. Profiles were conducted in both a controlled-ascent mode similar to the Argo mission (∼0.08 m s −1 rise rate) and at the float’s maximum rise rate of ∼0.3 m s −1 , which might be used for shorter missions supporting process studies. In both modes, profiles of turbulent dissipation rate of kinetic energy ε and thermal dissipation rate χ were computed. Valid ε profiles were obtained in both profiling modes, though platform vibration was significant at the faster ascent rate, and probe sensitivity was reduced at the slower rise rate. Shear spectra resolve the viscous rolloff well, giving a noise floor for ε of about 3 × 10 −10 W kg −1 in both modes. Individual temperature spectra also resolve the diffusive rolloff for χ about 2 × 10 −11 K 2 s −1 despite the presence of significant fine-scale structure in temperature, which impacted the quality of the temperature gradient spectra at low wavenumber. Following published best practices, each ε and χ estimate was kept or rejected based on a figure of merit measuring the goodness of fit of the measured spectrum to the theoretical form. The ε cannot be computed when the buoyancy pump is active (3% of the time) due to excessive vibrations. At other times, about >65% and 75% of raw estimates of the turbulent dissipation rates ε and χ met our quality control criteria, respectively. Significance Statement Widespread turbulence measurements will improve our understanding of the ocean but are only possible with large arrays of autonomous profiling instruments. Here, the feasibility of making direct shear and temperature turbulence measurements from a profiling float is demonstrated.