Microstructure observations during the spring 2011 STRATIPHYT-II cruise in the northeast Atlantic

Small-scale temperature and conductivity variations have been measured in the upper 100 m of the northeast Atlantic during the STRATIPHYT-II cruise (Las Palmas-Reykjavik, 6 April-3 May 2011). The measurements were done at midday and comprised 2 to 15 vertical profiles at each station. The derived turbulent quantities show a transition between weakly-stratified (mixed layer depth, MLD, <100) and well-mixed waters (MLD > 100), which was centered at about 48°N. The temperature eddy diffusivities, K_T, range from 10 ^-5 to 100 m² s^-1 in the weakly-stratified stations, and range from 3 × 10^-4 to 2 × 100 m ² s^-1 in the well-mixed stations. The turbulent kinetic energy dissipation rates, μ, range from 3 × 10^-8 to 2 × 10^-6 m² s^-3 south of the transition zone, and from 10^-7 to 10^-5 m² s^-3 north of the transition zone. The station-averaged K_T values throughout the mixed layer increase exponentially with the wind speed. The station-averaged μ values throughout the mixed layer scale with the wind stress similarity variable with a scaling factor of about 1.8 in the wind-dominated stations (epsilon approx; 1.8 ≈ ³(-κz)). The values of K_T and μ are on average 10 times higher compared to the values measured at the same stations in July 2009. The results presented here constitute a unique data set giving large spatial coverage of upper ocean spring turbulence quantities.