Mixing on the Carolina Self and at the Gulf Stream Front near the Shelf Break during 2015 CASPER East

Results of the microstructure measurements collected from R/V Atlantic Explorer during CASPER East experiment (October-November, 2015) are discussed. The measurements were made on the Carolina shelf and at the Gulf Stream front. The turbulent kinetic energy (TKE) dissipation rate was evaluated using the multi-cast VMP data. Daily recurrent measurements were concentrated at two locations – about 2 and 30 miles from the coast. The surface layer stratification was dominated by highly variable salinity (28-31 psu near the coast) and (31-34 psu offshore), determined by water advected to the test sites by local winds. The ADCP transects of R/V Sharp revealed strong variability of shelf currents affected by tides, stormy winds and near-bottom intrusions of warmer and saltier waters of Gulf Stream origin. Turbulence in the surface layer was mostly governed by wind stress and to a lesser extent by buoyancy flux. Below the surface layer, the TKE dissipation rate varied in a relatively narrow range between 10^-9 and 3x10^-8 W/kg. Only once, after a day-long storm (hourly averaged winds up to 14 m/s), the entire 30 m water column became turbulent with a characteristic dissipation ε ~ 10^-7 W/kg at the depths > ~ 5 m. On the shelf, and in the Gulf Stream pycnocline, the empirical probability of log₁₀ε could be well approximated to the generalized extreme value distribution. This finding is in agreement with our previous studies on the dissipation rate distributions in strongly stratified waters of the East China Sea and Bay of Bengal. A more traditional log-normal probability model fits well for empirical distributions of eddy diffusivity K_N. The estimated parameters of the probability model for K_N can be used to characterize the intermittency of mixing in advanced regional numerical models. A functional dependence K_N(Ri) will also be presented for mixing in the pycnocline at the cold wall of the Gulf Stream.