Turbulent mixing induced by nonlinear internal waves in Mono Lake, California

Describing the spatial and temporal occurrence of turbulent mixing and the mechanisms of turbulence production are fundamental problems in physical limnology and oceanography. Here we present results from Mono Lake, California, that illustrate pronounced steepening of the pycnocline upon cessation of strong wind forcing and the enhanced turbulence resulting from this nonlinearity. Three events occurred with wind speed of >10 m s(-1). Upwind, the upper pycnocline upwelled, but on relaxation of the wind, it downwelled 6-10 m and compressed. The similarity of wave amplitude during compression to mixed layer depth and Lake numbers <4 were indicative of significant nonlinearity. Energy in the internal wave field increased by two orders of magnitude, but similar to 50% of the energy was lost within 24 h. The largest energy losses occurred at low frequencies rather than via an energy flux from low to moderate and high frequencies. Eddy diffusivities, computed over 6-d intervals using the heat budget method, averaged similar to 10(-6) m(2) s(-1) in the energized pycnocline but were five to 10 times higher during the initial upwelling and downwelling periods. Microstructure casts, taken 1-2 d after the first event, revealed that the percentage of the pycnocline that was turbulent varied with bottom slope, being 80% turbulent where bottom slopes were up to 10%, 60% where bottom slopes were 3%, and less than 7% where bottom slopes were similar to 0.1%. The threshold for turbulent transport was exceeded only 10% of the time and only where bottom slopes were highest.