Surface mixed and mixing layer depths

In order to understand the daily cycle of heat storage within the surface mixed layer it is necessary to distinguish between the mixed layer, the zone of relatively homogeneous water formed by the history of mixing, and the mixing layer, the zone in which mixing is currently active. We compare surface layer definitions based on density (or temperature) with turbulence measurements to evaluate their skill in finding mixed and mixing layer depths, using definitions based on density increase from the surface, and on density gradients. Both types of definition are capable of finding the mixed layer depth, with some tuning for local conditions. Neither definition, however, gives mixing layer depths consistently matching the turbulence measurements, although density differences give more stable results. Measurements of turbulent dissipation rates or overturning length scales often yield consistent estimates of mixing layer depths, but there are cases where overturning lengths give distinctly better results. We conclude that overturning length scales give the most reliable measure of mixing layer depth, although conventional shipborne CTDs are seldom capable of sufficiently resolving the overturns.