Turbulence in the Sicily Channel from microstructure measurements

Abstract

Small scale turbulence in the two main deep passages of the Sicily Channel was characterised for the first time with microstructure measurements collected during four cruises spanning a two year period. Large turbulent kinetic energy dissipation rates (ε) were observed, with averaged values below the mixed layer reaching 10−7 W kg−1, confirming that the Sicily Channel is a hotspot for turbulence. Contrasted depth-averaged ε were observed between the two passages below the mixed layer: enhanced ε in the northeastern passage ranging from 1.3 × 10−8 - 2.7 × 10−7 W kg−1 over the different cruises, and much weaker ε in the southwestern passage ranging from 3.5 × 10−9 to 7.7 × 10−9 W kg−1. This contrast in ε occurs due to a stronger deep flow at the northeastern passage, resulting in larger shear and stronger turbulence. Internal tides act as another important source of turbulence in both passages, modulating the subinertial flow and inducing shear instabilities. Enhanced turbulence was also revealed by additional measurements made downstream (with respect to the deep flow) in the northeastern passage towards the deeper Tyrrhenian Sea, as dense waters overflow above steep topography.A wave-wave parameterisation was tested for ε, which showed a reasonable consistency for the less turbulent southwestern passage, but not for the more turbulent northeastern passage, suggesting a difference in the mechanism of turbulence.