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Accueil du site → Doctorat → États-Unis → 1996 → Stratification and mixing in hypersaline Mono Lake, California

University of California at Santa Barbara (1996)

Stratification and mixing in hypersaline Mono Lake, California

Romero, Jose R

Titre : Stratification and mixing in hypersaline Mono Lake, California

Auteur : Romero, Jose R

Université de soutenance : University of California at Santa Barbara.

Grade : Doctor of Philosophy (PhD) 1996

Application of a one-dimensional (1D) vertical mixing model to hypersaline (about 94 grams per liter) Mono Lake during 1 year reproduced mixed-layer dynamics well, but hypolimnetic heating was underestimated. One possible source of hypolimnetic heating is vertical mixing by methane bubble plumes rising from the sediments. Simulations with the inclusion of a bubble plume algorithm required an ebullition rate 300 times greater than the maximum estimate to simulate observed hypolimnetic heating.
The influence of lake level and salinity changes on seasonal mixing was modeled with the 1D model where the diffusivity was based on the Lake Number (LN). The simulation reproduced salinity dynamics well for 2 years of monomixis and 6 years of meromixis. Assuming climate change causes less precipitation, the frequency and duration of meromixis for 100, 87.5, and 75% of the freshwater inputs over a 50-year period (1940-1990) was simulatedwith the assumption of no stream flow diversion. Simulations indicate the lake is susceptible to meromixis over a large lake level range for all scenarios during large runoff years.
The effect of freshwater inputs on stratification, vertical mixing, and upward ammonia flux was evaluated during a 6-year (1989-1994) monomictic period. Five years had falling lake level and periods of inverse salinity stratification with double diffusive salt fingering conditions during the last several months of thermal stratification. Bi-weekly to monthly summer (June-September) vertical diffusivity estimates in the thermocline from the heat-flux gradient method ranged from 9.5x10(-7) square meters per second during wet 1993 to 4.2x10(-6) meters per second during a drought in 1989. Estimated seasonal and interannual differences in the upward ammonia flux can be partly explained by variations in freshwater inputs and wind forcing.
The 1D model simulated mixed-layer dynamics adequately for 5 years from 1989-1994. The destabilizing influence of inverse salinity stratification resulted in the inapplicability of the 1D assumption during 1989. During the other 5-year modeled diffusivities within the pycnocline were underestimated by 10-20 times. Three approaches were tested to increase mixing : (1) sub-daily wind speed input, (2) benthic boundary layer turbulence, and (3) the LN as an index of mixing. The LN parameterization yielded the best results and suggests boundary mixing along the margins and perhaps shear mixing in the interior during high wind forcing predominate as the major vertical transport mechanisms.

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