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

Abstract. 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 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 K_T values throughout the mixed layer increase exponentially with the wind speed. The station-averaged 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 ≈ 1.8 u_☆³/(−κz)). The values of K_T 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.