Observations of the thermal structure of Langmuir circulation

A near-vertical array of thermistors has been towed through the water beneath several hundred well-defined wind rows formed on the surface of a fresh-water loch in winds of 7–9 m s−1. The loch was stably stratified in the mean, temperatures decreasing with depth, and the air warmer than the water. The temperatures at ten levels in the water and the positions of the wind rows were recorded. These measurements were supplemented by photographs of the wind rows and observations of bubbles, using sonar. The data has been analysed to produce average temperature sections across the wind rows. Near the surface a core of warm water can be distinguished beneath the wind row in the average section (although not in general in the unaveraged records) and colder water between rows, consistent with there being a Langmuir circulation pattern. At depths greater than about 0·4 times the average distances between rows, it is, however, apparent that the temperature anomalies slope to the left of the wind direction as depth increases. The anomalies (although generally significantly different from the mean) are much less than the standard deviation of the local fluctuations in loch temperature and also much smaller than the temperature changes at the ‘ramps’ described earlier (Thorpe & Hall 1980), here observed to extend to within 0·4 m of the surface. There are thus two co-existing, but possibly independent, thermal structures which can be recognized in the turbulent mixing layer.