Wind forcing increased in the afternoon at Toolik Lake, with maximal values early in the summer season between 5 and 6 ms^-1.  As winds increased, the thermocline downwelled and was compressed, particularly in its upper extent.  As wind speeds relaxed after midnight, the thermocline upwelled and expanded.  These motions are due to internal waves in resonance with the wind. High frequency waves were generated during upwelling and downwelling that may have contributed to turbulent
mixing in the thermocline.

Spectral analysis of isotherms in the thermocline indicate that they have the period of a second vertical, first horizontal mode wave.  When even order vertical modes are present, compression of the thermocline occurs at both its top and base, as shown here.  Similarly, expansion occurs at both locations.

While the second vertical mode waves have a period of a day; other modes are present with much shorter periods. Steepening of these internal waves, as illustrated by the 6 ºC contour on Julian Day 176.75, will lead to formation of higher frequency waves  which may break locally or at the margins of lakes.  The complex bathymetry of Toolik Lake may contribute to steepening of internal waves and increased turbulence in the metalimnion over what is expected for such a small
lake.