| On PATCHEX, off the coast of California, we observed daytime stratification and turbulent decay within the surface mixed layer during a period of generally light winds. Absorption of solar radiation strongly stratified water near the surface, but also weakly stratified all depths in the remnant mixed layer below the diurnal thermocline. In addition, lateral advection contributed significantly to restratification. Within the remnant layer, dissipation remained nearly constant for about one convective period after the end of convective forcing and then decayed by a factor of 40 in 4 hours, during which, turbulence in the remnant layer could be modelled in accordance with a balance between dissipation and the storage term for turbulent kinetic energy.; On the TOGA-COARE microstructure pilot cruise, in the western Pacific warm pool, we profiled for seventeen days at 0{dollar}spcirc{dollar}N, and for five days at 2{dollar}spcirc{dollar}N. Winds were mostly light, but rainfall was often quite intense. Strong daytime restratification often prevented nightly convective deepening down to the seasonal thermocline. Following rainfall, we observed pools of fresh water, that typically disappeared within a few hours, leaving the mixed layer nearly well-mixed in salinity; thus we did not observe a permanent barrier layer. Modelling such an event using the Price-Weller-Pinkel model suggests a fresh pool will be mixed away on time scales of a few days.; Using data from the above two cruises, we 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 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 type, however, gives mixing layer depths consistently matching the turbulence measurements. Measurements of turbulent dissipation rates or overturning length scales often yield consistent estimates of mixing layer depths, but at times overturning lengths give distinctly better results. |
