Microstructure Measurements from an Underwater Glider in the Turbulent Faroe Bank Channel Overflow

Measurements of ocean microstructure are made in the turbulent Faroe Bank Channel overflow using a turbulence instrument attached to an underwater glider. Dissipation rate of turbulent kinetic energy epsilon is measured using airfoil shear probes. A comparison is made between 152 profiles from dive and climb cycles of the glider during a 1-week mission in June 2012 and 90 profiles collected from the ship using a vertical microstructure profiler (VMP). Approximately one-half of the profiles are collocated. For 96% of the dataset, measurements are of high quality with no systematic differences between dives and climbs. The noise level is less than 5 x 10(-11) W kg(-1), comparable to the best microstructure profilers. The shear probe data are contaminated and unreliable at the turning depth of the glider and for U/u(t) < 20, where U is the flow past the sensor, u(t) = (epsilon/N)(1/2) is an estimate of the turbulent velocity scale, and N is the buoyancy frequency. Averaged profiles of epsilon from the VMP and the glider agree to better than a factor of 2 in the turbulent bottom layer of the overflow plume, and beneath the stratified and sheared plume-ambient interface. The glider average values are approximately a factor of 3 and 9 times larger than the VMP values in the layers defined by the isotherms 3 degrees-6 degrees and 6 degrees-9 degrees C, respectively, corresponding to the upper part of the interface and above. The discrepancy is attributed to a different sampling scheme and the intermittency of turbulence. The glider offers a noise-free platform suitable for ocean microstructure measurements.