Spring Circulation Associated with the Thermal Bar in Large Temperate Lakes

The overall circulation pattern in spring is rather specific as density-induced currents may be of significance. The density-driven circulation perpendicular to the shore can be described as consisting of two circulation cells, with a zone of convergence, referred to as thermal bar, in between. The thermal bar, which coincides with the 4°C isotherm (the temperature of maximum density), inhibits horizontal water exchange, implying its practical importance. In this paper, a hydrodynamic numerical model is used to study the relative influence of wind- and density-driven currents in a large temperate lake during spring. The study shows that the general density-driven circulation is strongly dependent on the bottom topography, with a more pronounced circulation and considerable descending motions in the thermal bar zone in lakes with steep sloping bottoms. In shallow lakes, the wind-driven circulation dominates, and the effect of density-induced currents is marginal, except at locations with a drastic change in bottom depth.