Laboratory investigation of seasonal circulation in a brackish lake subject to ice cover

Laboratory experiments were conducted in a walk-in freezer containing an open tank filled with brackish water (S=0.4 g/L). We describe the circulation starting with convection and cooling in the ‘fall’ when the air temperature was below freezing (-20 °C). As the water cooled toward the temperature of maximum density, we observed the decay of the convective currents.  Then we observed the formation of reverse stratification and the onset of ice cover. The salt excluded at the base of the growing ice generated salt-fingers that transported salt downward (Olsthoorn et al., 2022).

Five hours after ice-on, the freezer was set to 10 °C, and air temperature began to rise.  As the air temperature approached 0 °C, the salt-fingering decayed.  After the air temperature rose above freezing, the circulation was dominated by warming through the side-walls of the tank; this warming generated relatively fresh water flowing downward along the side walls to the bottom of the tank. This freshened the bottom of the water column and initiated inverse salt-fingers or ‘fresh-fingers’. After the ice melted, warming convection was observed.  As the water warmed toward the temperature of maximum density, the convective currents decayed. These experiments simulated the expected circulation in a brackish lake subject to ice cover.