Turbulence in the Summertime Pycnocline
Iossif Lozovatsky, University of Notre Dame, Civil & Environmental Engineering & Earth Sciences, Notre Dame, IN, United States, Jae-Hak Lee, KORDI Korea Ocean Research and Development Institute, Ansan, Korea, Republic of (South) and Harindra Joseph Fernando, University of Notre Dame, Department of Aerospace and Mechanical Engineering, Notre Dame, IN, United States
Contact First Author: Iossif Lozovatsky; i.lozovatsky@nd.edu
English Abstract:
Microstructure measurements in the East China Sea allowed examination of turbulence influenced by summertime stratification in shallow (52 to 62 m depth) tidal basin, using 134 CTD and the dissipation rate profiles obtained during a 25-hour drift over a slightly slopping bottom. Turbulence in the pycnocline was highly intermittent, with spatially alternating layers of low (~ 10
-9) and enhanced (~ 10
-7) W/kg dissipation rate. An analysis of temporal variability of ε, eddy diffusivity K
N and squared buoyancy frequency N
2. Near the upper boundary of the pycnocline, majority of the dissipation and diffusivity extrema coincide with the extrema of buoyancy frequency, showing positive and negative (min-max) correlations. A negative correlation was mainly found in weakly stratified layers of the pycnocline and in a stratified upper portion of the BBL. The dependence of the dissipation on the normalized squared buoyancy frequency was very strong (the “-3” power) in a relatively narrow range of weak stratification when turbulence not only dissipated but also produced mixing that reduces the local buoyancy (density) gradient.
Analysis of the cumulative distribution functions (CDF) of the logarithm of the dissipation rate in the test area showed that the empirical CDFs can be successfully approximated by the generalized extreme value distribution for the areas of sharp pycnocline as well as less stratified transitional layers between the pycnocline and BBL and between the pycnocline and surface layer. This suggests that the generation and dissipation of turbulence in the summer pycnocline of ECS and adjacent startified layers can be considered as a result of extreme events such as random internal-wave breaking and sporadic instabilities. This turbulence occurred under light winds, being influenced by tidal currents in the BBL and internal waves in the pycnocline.
