Horizontal Variability of temperature microstructure at the base of a mixed layer during MILE

Abstract

Horizontal observations of temperature microstructure made during the Mixed Layer Experiment (MILE) show that the distribution of mixing activity in the seasonal thermocline at the base of the surface mixed layer was highly intermittent in horizontal planes. The horizontal extent of the observed microstructure patches was usually less than 10 m. The patches tended to occur in groups extending over distances of a few hundred meters, although some were isolated. Many patches of temperature microstructure activity occurred at density fronts separating regions of strong but opposite horizontal temperature‐salinity (T‐S) gradients. The sequence of horizontally averaged T‐S values form a looping pattern on a T‐S diagram which suggests that horizontal stirring processes are a source of microstructure generation. Spatial averages of the Cox number 〈(∇T)2〉/〈∇T〉2 and χ, the temperature variance dissipation rate, are determined by very small ractions of the record lengths, usually less than 5%. The distribution of χ was found to be approximately lognormal with variance of log10 χ of about 1.0, mean of 1.1×10−6 °C2/s, and mode 3.9×10−10 °C2/s. Good agreement was found with X values measured by dropsondes during MILE after accounting for the strong intermittency.