Observations of turbulent dynamics in the ocean surface boundary layer
Sutherland, Graigory John
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Turbulence and mixing processes are investigated in the ocean surface boundary layer (OSBL) using microstructure measurements from the Air-Sea Interaction Profiler (ASIP) a vertically rising, autonomous instrument equipped with a suite of sensors designed to study the dynamics and energetics of the OSBL. These are presented in conjunction with observations of the atmospheric forcing and surface gravity wave fields to test various hypotheses for the vertical profile of the dissipation rate of turbulent kinetic energy. The influence of surface gravity waves were not restricted to the upper few metres, but in fact extend throughout the OSBL. This large depth of influence is consistent with the presence of Langmuir circulations, which are expected to fill the OSBL. Although we didn't have the instrument to detect whether Langmuir cells formed, observations of turbulent dissipation were found to be consistent with results from Large Eddy Simulations (LES) which included effects of Langmuir circulations. These are some of the first observations to confirm LES studies of Langmuir turbulence in the open ocean. The accuracy of scaling depends on the definition for the OSBL. Improved agreement with LES results were found when the depth of the active mixing layer (XLD), determined by locating the depth at which turbulent dissipation fell to a background dissipation rate, were used for the boundary layer depth rather than the depth of the mixed layer (MLD), which is the depth at which the density exceeds a threshold value relative to the near surface density. These results show the importance of defining the OSBL as the region of active mixing and not just the region with a quasi- homogeneous density structure. The variations of the MLD and XLD are investigated in in the subtropical Atlantic for a buoyancy-driven regime with both the MLD and XLD responding with a predictable diurnal structure. Observations show the density threshold used to estimate the MLD can be adapted to obtain the XLD, but this has a clear variation as a function of the local time of day and hence surface buoyancy flux. Mean values for the density threshold of the XLD are consistent with experiments, which suggest larger density thresholds near the equator.