Field Investigation of the Geostrophic Motions in the Ice-covered Lake Paajarvi

Field measurements during two winter seasons were performed in the ice-covered Lake Paajarvi that (i) has no significant in/outflows and (ii) is larger than the baroclinic Rossby radius of deformation, to identify the controlling parameters on the mean currents for such physical systems. Typical Rossby numbers calculated from velocity data, combined with a scaling analysis of the governing equations of motion, reveal that these currents are geostrophic. Continuous temperature readings from high spatial and temporal resolution thermistor chain data demonstrate, in general, downward and upward isotherm tilting in the upper and lower sections respectively of the fluid column (convex) for one winter campaign and roughly the opposite to the other. The form of the hydrostatic pressure gradients, and deformation of the interior thermal field associated with these, in each case provides evidence of anti-cyclonic/cyclonic gyres propagating around the lake edge due to the fundamental control of the Coriolis acceleration. One of these modes of physical behaviour confirms the direction of circulation hypothesised by previous numerical modelling studies but comparisons between two winters suggest that a strong effect on the direction of the gyres is due to the initial thermal stratification during the frozen period.