Mixed layer dynamics in a lake exposed to a spatially variable wind field1

Field measurements are described detailing the formation of frontal structures in the surface mixed layer near the entrance of an embayment in a medium‐sized lake. The observed evolution of the fronts is interpreted in terms of three well defined dynamical regimes. The first, differential heating, is defined as the variable heat capture due to decreased surface heat losses and warmer surface water in the sheltered embayments. The second, variable deepening of the mixed layer, is induced by a sudden increase in wind speed in the open central areas of the lake, with only minimal mixing in the sheltered parts of the lake. The third is the horizontal gravitational motion driven by the horizontal density gradients set up by the active deepening of the mixed layer in the exposed parts of the lake. Although detailed wind data were not available, observations of the temperature and salinity fields show the existence of fronts in lakes and strongly suggest the importance of the above mechanisms in determining the horizontal variability of these fields in the surface mixed layer of medium‐sized lakes.