An investigation of thermal structure in two fresh-water lakes

The temperature fine structure of the thermocline in two stratified fresh water lakes is investigated using data from repeated vertical temperature profiles, and temperature time series from moored thermistor chains. The data show that much of the structure in the thermocline migrates through the water column with time. Analysis suggests that the migrating aspect of the structure is associated with the natural oscillations of the lake.A theory for the wind forced internal oscillations of a viscous, non-rotating, long-narrow lake of constant stratification is developed. The introduction of viscosity gives a smooth phase change with depth near the nodes of the vertical displacement profile, and enables the theory to model the observed vertical migration of features in the lake.The theory is compared to several aspects of the data from the two lakes. Phase profiles calculated from thermistor chain data are compared to the predicted phase profiles, and in the. diurnal frequency band where the wind forcing is strongest the agreement is good for values of the viscosity of about 0.1 cm2 sec 1. The amplitude of the vertical displacement, in the diurnal frequency band, agree best with values predicted by the theory for a viscosity of 0.2 cm2 sec 1, which, considering the errors involved, is not significantly different from the value obtained from the phase profiles. Comparisons are also made between the theory and observations of the energy as a function of the' forcing in various frequency bands, of the phase profiles as a function of frequency, and of the phase of the wind relative to the internal oscillations. The agreement in all cases is sufficient to warrant the description of the migrating structures in terms of damped internal oscillations of the lake.</p