The present paper concentrates on the movement of the spring thermal bar with time, and a simple theoretical model is derived for the case of an axisymmetric circular lake. The lake is considered to consist of three different zones: 1) a nearshore stably stratified warm water zone, 2) the thermal bar zone of descending water with temperature approximately equal to that of maximum density, and 3) an offshore cold unstably stratified zone. The basis for the model is the heat budget equation, where horizontal dynamical heat exchange in the vicinity of the bar zone is taken into account. In comparison with a previous model (Zilitinkevich et al. 1991), describing an infinite wedge of water, the present model is more general and takes into account the effect of shore-line curvature. This effect is characterized by a non-dimensional number and causes a marked increase in the displacement rate of the bar at the final stage of its existence. Model predictions are also compared with field data taken from the literature.

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