Modeling deep-water renewal in Lake Baikal

Temperature, dissolved oxygen, nutrients, and chlorofluorocarbon-12 data obtained from Lake Baikal are used to describe deep-water renewal in a deep, temperate-latitude lake. Observations are used to propose the physical mechanism governing convection and to formulate a model of deep ventilation. The key physical mechanism governing deep-water renewal is the so-called thermobaric instability. Because the temperature of maximum density decreases with depth, a lake can become conditionally unstable if the base of the cold mixed layer is displaced to a depth at which its temperature matches the local temperature of maximum density, thereby resulting in sinking plumes. An important consequence of this phenomenon is that deep temperate lakes such as Baikal do not completely mix twice yearly; instead, deep ventilation is episodic. A two-dimensional model of a wind- and buoyancy-driven lake shows many strong mixing events and a fairly realistic seasonal cycle, indicating that the hypothesis is physically realizable. A filling-box model is used to deduce the annually averaged fluxes necessary to produce a steady vertical distribution of tracers as observed. Good fits are obtained to oxygen and chlorofluorocarbon distributions by this model.