Abstract
To quantify the effects of recent and potential future decreases in surface wind speeds on lake thermal stratification, we apply the one-dimensional process-based model MyLake to a large, shallow, polymictic lake, Võrtsjärv. The model is validated for a 3-year period and run separately for 28 years using long-term daily atmospheric forcing data from a nearby meteorological station. Model simulations show exceptionally good agreement with observed surface and bottom water temperatures during the 3-year period. Similarly, simulated surface water temperatures for 28 years show remarkably good agreement with long-term in situ water temperatures. Sensitivity analysis demonstrates that decreasing wind speeds has resulted in substantial changes in stratification dynamics since 1982, while increasing air temperatures during the same period had a negligible effect. Atmospheric stilling is a phenomenon observed globally, and in addition to recent increases in surface air temperature, needs to be considered when evaluating the influence of climate change on lake ecosystems.
Highlights
Thermal stratification is a natural phenomenon that occurs in lakes as a result of the thermal expansion properties of water
Over the last 28 years, wind speed has been declining in all seven of the meteorological stations in Estonia (Fig. 1). This is seen in the raw daily data from Tartu-Tõravere (Fig. 1b) and in the annually averaged wind speeds from all seven meteorological stations situated throughout Estonia (Fig. 1c)
Based on long-term meteorological data available from in situ meteorological stations in Estonia, we found a large-scale decrease in surface wind speeds during the past ∼30 years, consistent with other locations globally (Vautard et al 2010)
Summary
Thermal stratification is a natural phenomenon that occurs in lakes as a result of the thermal expansion properties of water. It is determined by the balance between turbulence, which acts to enhance mixing, and buoyancy forces, which act to suppress turbulence and result in a vertical layering (Boehrer and Schultze 2008). The vertical layering that exists during stratification has several implications for the ecosystem, as it inhibits the downward penetration of direct vertical mixing and influences temperatures at depth (Livingstone 2003) but it separates processes of production and nutrient depletion in the epilimnion from processes of decomposition and nutrient regeneration in the hypolimnion and sediment
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