Abstract: How anisotropic is ocean turbulence at dissipative scales? An observational approach.



Authors: Tim Fischer, Marcus Dengler

Estimates of diapycnal mixing by microstructure probes mainly focus on dissipation rate as central variable and usually take local isotropy of turbulence as granted. In its most general form, the dissipation rate must be calculated from 12 strain rate correlations; assumption of local isotropy reduces this requirement to only one strain rate correlation needed. In conditions of strong shear and/or strong stratification, as frequently encountered in near-shore- and/or confined waters, deviations from local isotropy are known and may thus lead to errors in the calculated dissipation rate. Here a concept of a measurement technique for dissipation rate under anisotropic conditions, as well as first results of its application are presented. A standard microstructure profiler equipped with 4 shear sensors is used to measure relations of strain rate correlations while sinking. Making use of the direction of sensitivity of the airfoil shear sensors, several sensor configurations are possible that deliver different sets of up to 5 strain rate correlations. The measured relations between them are then compared to theoretically derived relations that represent turbulence regimes of different symmetries. Knowing the symmetry to be applied forms the base for given estimates of dissipation rate.