Lake mixing and the turning earth

Posted on October 1, 2014

In lakes, internal waves (fluctuations in the depth of the thermocline) play a key role in mechanical transport processes. The internal waves of large lakes, i.e., those more than about 10 km across, are known to be significantly influenced by the rotation of the earth, but it’s been assumed that this effect is negligible for small water bodies. To test this assumption, researchers studied Lake Arendsee, an elliptical (1.8 x 3.2 km) German lake with a flat bottom. They collected continuous data on water density, isotherm depths and wind speed during a period extending from spring, when the lake first stratified into three layers, to autumn, when vertical mixing resumed. The average thermocline thickness increased from one metre in April to six metres in July, and then decreased again to two metres in October. Contrary to expectations, throughout the whole period of stratification, and especially in spring and autumn, wavelike fluctuations in thermocline depth were strongly influenced by rotatory effects. Internal waves were also driven by diurnal changes in wind speeds, which were lower by night than by day. At times, oscillations in the thickness of the thermocline were strongly amplified as a result of resonance with wind-driven changes. These resonant events had a diurnal frequency in spring and autumn, and a latitude-dependent rotational frequency in summer. Latitude-related rotational resonance hasn’t been recorded in previous lake studies. Resonance events of this kind may have important ecological implications, by enhancing vertical mixing, increasing the lateral transport of nutrients within the thermocline, affecting the vertical distribution of photosynthetic plankton, and increasing turbulence at the lake bed.

Reference: Bernhardt, J. & Kirillin, G. 2013. Seasonal pattern of rotation-affected internal seiches in a small temperate lake.
Limnology & Oceanography 58(4), 1344–1360.