Vertical turbulent diffusion and upwelling in Lake Baikal estimated by inverse modeling of transient tracers

Abstract

Vertical turbulent diffusion coefficients, upwelling velocities, and oxygen depletion rates are estimated by inverse modeling of the concentrations of CFC-ll (CCI3F), CFC-12 (CCI2F2), 3H, 3He, and dissolved oxygen for the southern, central, and northern basin of Lake Baikal. A model is developed that considers two regions in each basin of Lake Baikal: (1) a surface mixed layer (SML) 400 m thick and (2) a dcepwater column (DWC) below 400 m. The SMLs are assumed to be well mixed. In each of the DWCs, passive tracers are transported by vertical turbulent diffusion and upwelling. Upwelling is generated by a depth-dependent source of water because of density plumes propagating from the SML downward to larger depths. This water is considered to contain the same tracer concentrations as the SML. The tracer concentrations in the SMLs of the three basins are coupled to the atmosphere by gas exchange (including water vapor transport) and precipitation to the catchment by river inflow and outflow and to the neighboring basins via diffusive exchange and advection. SMLs and DWCs of the same basin are connected by vertical turbulent diffusion, density-driven water transport, and upwelling. Beginning at the turn of this century, the tracers CFC-II, CFC-12, 3H and 3He are modeled simultaneously to predict modern concentrations. On the basis of the tracer data the vertical diffusion coefficient K2 is determined to be 4.6x 10 high minus 4 (Marion H.)m2 S-I ± 10% for the southern, 6.3x10high minus4 m2 s high minus 1, ± 10% for the central, and 1.7x 10.high minus 4 m2 s·high minus 1 ± 25% for tbe northern basin. The vertical advective flux of water at 400 m water depth is calculated as 110 km high 3 yr high minus1 in the southern, 70 km3 yr high minus1 in the central, and 290 km3 yr high minus 1 in the northern basin. Concentration of dissolved molecular oxygen is modeled by using the estimated transport parameters and by fitting for the unknown consumption rate. Inverse modeling of oxygen suggests that 02 depletion in the DWC can be described by a volume sink of44 ± 3 mg02 m high minus3 yr high minus1 combined with an areal sink at the sediment water interface of 17000 ± 3000 mg02 m-2 yr high minus1.