NUMERICAL MODELING OF LARGE-SCALE CIRCULATION IN LAKE-ONEGA AND LAKE-LAGODA

The results of numerical modelling of large-scale circulation in Lakes Onega and Ladoga are presented, with primary emphasis on the temporal variability of currents with time scales of days. Some typical circul ation patterns have been inferred from model calculations. They reflec t the existence of different dynamic regimes in the lakes, namely, for ced and free circulation regimes. The forced circulation regime is the well-known wind-induced double-gyre circulation accompanied by coasta l upwelling and downwelling. A case of double-gyre circulation in Lake Onega was investigated in particular detail. The second dynamic regim e is a free response (or a relaxation) of the stratified lake to wind cessation, and is connected closely with the evolution of wind-induced upwelling and thermal front propagation. Diagnostic calculations demo nstrate that the regime of relaxation supports the restoration of cycl onic circulation in Lake Onega. Barotropic circulation patterns in Lak e Ladoga were calculated with the emphasis on prevailing winds from we st to south-east. Our calculations show that the bottom relief of Lake Ladoga causes asymmetry in the double-gyre circulation patterns. In p articular, approximately equal cyclonic and anticyclonic circulation c ells appearing in the case of southerly wind transform to a single dom inant cyclonic cell and several small anticyclonic cells in the case o f westerly wind. We also found especially strong sensitivity of the se nse of rotation of the largest gyre to the east-west components of the wind vector.