Observations of the Diurnal Cycle of Near Surface Dissipation and Shear

The STRASSE field experiment was conducted on the R/V Thalassa in the Sub-Tropical North Atlantic in August/September 2012 as part of the Salinity Processes in the Upper Ocean Regional Study (SPURS) campaign. During this cruise we deployed the Air-Sea Interaction Profiler (ASIP), an autonomous upwardly-rising microstructure instrument capable of resolving small-scale processes close to the air-sea interface. ASIP provides direct estimates of the dissipation rate of turbulent kinetic energy, temperature, salinity, and PAR at timescales suitable for the study of diurnal processes. In combination with the ASIP data, we had shipboard meteorological data for calculation of air-sea fluxes, and a surface mounted Lagrangian ADCP for determination of the near-surface velocity and shear. The average conditions during STRASSE were low wind and high insolation, which are typical for the generation of near-surface diurnal warming.

The temperature and dissipation data from ASIP, as well as the shear data from the ADCP exhibit a strong diurnal signal, with the mixing layer shoaling to <10 m during the daytime restratification, and breaking down at nighttime due to convection. The magnitude of the dissipation during restratification is enhanced near the surface but suppressed below the mixed layer. The ADCP data indicate a descending shear layer which leads to the destruction of the stratified surface layer.

Here observed near-surface shear and stratification will be compared with predictions from LES boundary layer models that account for Langmuir turbulence.