Comparison of Ellison and Thorpe scales from Eulerian ocean temperature observations

TL;DR

This study introduces a new method to estimate ocean turbulence scales using temperature variance from moored instruments, offering an alternative to traditional density-based techniques with promising accuracy in bottom-intensified turbulence regions.

Contribution

The paper presents a novel approach to estimate the Ellison length scale from temperature data, providing an alternative to Thorpe scales for turbulence analysis in the ocean.

Findings

Temperature variance correlates well with Thorpe scale.

Ellison scale can estimate turbulence dissipation from temperature data.

Method effective with high time resolution measurements.

Abstract

Ocean turbulence dissipation rate is estimated either by means of microstructure shear measurements, or by adiabatically reordering vertical profiles of density. The latter technique leads to the estimate of the Thorpe scale, which in turn can be used to obtain average turbulence dissipation rate by comparing the Thorpe scale to the Ozmidov scale. In both cases, the turbulence dissipation rate can be estimated using single vertical profiles from shipborne instrumentation. We present here an alternative method to estimate the length scale of overturns by using the Ellison length scale. The Ellison scale is estimated from temperature variance just beyond the internal wave band, measured by moored instruments. We apply the method to high resolution temperature data from two moorings deployed at different locations around the Josephine seamount (North Eastern Atlantic Ocean), in a region of bottom-intensified turbulence. The variance of the temperature time series just above the internal wave frequency band is well correlated with the Thorpe scale. The method is based on the time--frequency decomposition of variance called "maximum overlap discrete wavelet transform". The results show that the Ellison length scale can be a viable alternative to the Thorpe scale for indirectly estimating turbulence dissipation rate from moored instruments in the ocean if time resolution is sufficiently high. We suggest that fine structure contaminated temperature measurements can provide reliable information on turbulence intensity.