Currents in homogenous bodies and the upper mixed layers of stratified lakes are mainly the result of wind forcing applied at the free surface. The specific basin-scale patterns of motion that develop in response to wind depend on the specific spatial and temporal patterns characterizing the wind field (the frequency, intensity, duration, and location at the time of the events), interacting with the topography, internal waves, and Coriolis effects. By using a simplified and linear set of governing equations and boundary conditions, we have analyzed the mechanisms by which some specific patterns of motion, observed in the field, are generated. However, the governing equations of motion are not linear and the description presented here is, at most, approximate. In shallow lakes, stratification is weak and one cannot easily determine where the bottom surface of the upper mixed layer is. Even if one could decide where that surface is, upwelling events frequently occur in these lakes, when bottom water surfaces, and the upper mixed layer disappears from the upwind end of the lake. These are cases when the nonlinear effects become important. In those cases, an appropriate description of currents involves using the fully nonlinear governing equations, which can only be solved with numerical methods. Even if the response of lakes were linear (or one had a 'perfect' numerical model), one major difficulty that needs to be faced in interpreting and analyzing quantitatively circulation and currents in HB and SML is to characterize the wind stress field (i.e., define realistic boundary conditions). Partly, because of this difficulty, there continues to be a considerable amount of uncertainty on what is the relative importance of wind stress curl, topography, and internal waves in generating the large-scale circulation in lakes. Another difficulty that we also face in analyzing the velocity field in lakes is the fact that any description (or observation) of such fields is, at most, incomplete. Many of the techniques used are based on the study of discrete points (Eulerian current meters, Lagrangian drifters, etc.), at single locations or at a series of locations along a transect.
One of the most promising techniques in describing and analyzing the currents in lakes consists in integrating the numerical solution of nonlinear equations and field observations (data-assimilation) in creating a coherent description of circulation. These techniques are now used in the ocean but very few studies have applied this technique to lakes.
See also: The Benthic Boundary Layer (in Rivers, Lakes, and Reservoirs); Currents in Stratified Water Bodies 1: Density-Driven Flows; Currents in Stratified Water Bodies 2: Internal Waves; Currents in Stratified Water Bodies 3: Effects of Rotation; Small-Scale Turbulence and Mixing: Energy Fluxes in Stratified Lakes.
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