Lake Vostok, isolated from direct exchange with the atmosphere by about 4 km of ice for millions of years, provides a unique environment. This inaccessibility raises the importance of numerical models to investigate the physical conditions within the lake. A topographic ridge splits the lake into a northern and southern part. Basic considerations reveal that the high pressure leads to convective flow in the lake. Using a three dimensional numerical model and the best available geometry, we analyse the baroclinic flow and the tracer dispersion within the lake. From our model experiments we find a different representation of the flow regime in the northern and southern basins. In the north and the northern part of the southern basin, where melting at the ice base dominates,convection provides a vertically well-mixed water column. In the south, where Vostok Station is located, basal freezing across about 3500 km^2 provides a vertically stable stratification of the water column's upper half. The different vertical stratifications lead to tracer concentration gradients in the water column which will influence the information retrieved from the Vostok ice core. The time needed for tracers to dissipate across the whole lake is strongly dependent on the location where they are released and amounts from years to decades.