Use of numerical models and satellite data to study physical processes in Lake Baikal

摘要

The survival of Lake Baikal's unique floral and fauna, of over 1500 endemic species, depends on water circulation carrying oxygen to all depths. However, the mechanisms by which this takes place are not well understood. A three-dimensional model of the lake has been developed, which is the first to be discussed in the open literature. The model, in conjunction with satellite data, is demonstrated to be particularly useful for investigating mixing processes occurring in the upper layer when the lake surface is frozen. Model generated temperature-depth profiles show that mixing extends to a depth of 40 m under regions of snow-free ice, compared to 15 m under snow-covered ice (after a period of 8 days). The model can, therefore, provide an indication of diatom populations (a vital part of the food chain), as these are larger in regions where convective mixing is most active.;Satellite data are analysed to assess the spatial and temporal distribution of snow and ice cover, in order to study density driven currents caused by non-uniform solar heating through the ice and snow. Incorporating these into the model generates current speeds of about 5x10-3 m s-1, an order of magnitude small than those observed in the lake. This suggests that current flow in the real lake may not be purely density driven, as was previously thought.;Other features observed in satellite images include thermal bars and gyres, both possible mechanisms for deep circulation of water. Thermal bars occur primarily along the south cost of the Central Basin, during June. Numerical modelling experiments suggest that gyre formation is due to the combination of a north-west wind, the Coriolis effect, and the presence of the steep Academician Ridge separating the Central and North Basins.;Finally, as regards the ice-free lake, it has been suggested that deep water ventilation might occur if internal waves are able to displace the thermocline downwards to a depth sufficient to cause 'thermobaric instability'. Model results show that wind speeds greater than 40 m s-1 must be uniformly applied to the surface of the lake for 1 to 2 days for the depth criterion to be met. It therefore seems unlikely that deep water ventilation is due to wind-induced internal waves.