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, KT , range from 10−5 to 100 m2 s−1 in the weakly-stratified stations, and range from 3×10−4 to 2×100 m2 s−1 in the well-mixed stations. The turbulent kinetic energy dissipation rates, ε, 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 KT values throughout the mixed layer increase exponentially with the wind speed. The stationaveraged ε 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 (ε ≈ 1.8u∗/(−κz)). The values of KT and ε 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.