Abstract. Internal standing waves (seiches) in the South Aral Sea are studied for the first time. The study, based on numerical simulations and field data, focuses on two different campaigns: the first in autumn 2006, when the stratification was weak and there was a mild prevailing northeasterly wind, and the second in autumn 2013, when the stratification was strong and there was a mild easterly wind. Between these two campaigns, the sea surface level decreased by 3.2 m. The periods of the fundamental modes were identified as 36 and 14 h, respectively. In both years, either second or third vertical modes were found. In general, the vertical modes in 2013 were higher because of the broad and strong pycnocline. For both years, it was found that the deep quasi-homogeneous mixed upper layer could sustain internal waves under mild wind conditions. The observed first and second vertical modes in 2006 are the first and second horizontal modes and the second and third vertical modes in 2013 are the second and third horizontal modes. The results suggest that, due to sea level variations, the neck connecting the Chernyshev Bay to the main body of the lake can become a critical location for the development of a nodal line for all principal oscillation modes. Rotation effects on waves were not analyzed in this study.
The theoretical models of Batchelor and Kraichnan, which account for the smallest scales of a scalar field passively advected by a turbulent fluid (Prandtl . 1), have been validated using shear and temperature profiles measured with a microstructure profiler in a lake. The value of the rate of dissipation of turbulent kinetic energy « has been computed by fitting the shear spectra to the Panchev and Kesich theoretical model and the one-dimensional spectra of the temperature gradient, once « is known, to the Batchelor and Kraichnan models and from it determining the value of the turbulent parameter q. The goodness of the fit between the spectra corresponding to these models and the measured data shows a very clear dependence on the degree of isotropy, which is estimated by the Cox number. The Kraichnan model adjusts better to the measured data than the Batchelor model, and the values of the turbulent parameter that better fit the experimental data are q B 5 4.4 6 0.8 and q K 5 7.9 6 2.5 for Batchelor and Kraichnan, respectively, when Cox $ 50. Once the turbulent parameter is fixed, a comparison of the value of « determined from fitting the thermal gradient spectra to the value obtained after fitting the shear spectra shows that the Kraichnan model gives a very good estimate of the dissipation, which the Batchelor model underestimates.
Abstract. Internal standing waves (seiches) in the South Aral Sea are studied for the first time. The study, based on numerical simulations and field data and, focuses on two different campaigns: the first in autumn 2006, when the stratification was weak, and the second in autumn 2013, when it was strong. During this time, the sea surface level decreased 3.2 m but differences in stratification depend only on the time when the campaigns were carried out because the maximum density gradient decreases even though the sea continues to become saltier. In 2006 there was a mild prevailing northeasterly wind and in 2013 a mild easterly wind. The fundamental modes were identified as 36 h and 14 h, respectively. For both years we focused on the sub-inertial modes which were found to be either second or third vertical modes. In general, the vertical modes in 2013 are higher because of the strong stratification. For both years, it was found that the quasi homogeneous mixed deep upper layer could sustain internal waves under mild wind conditions. Regarding the horizontal structure, in 2006 they are first and second modes and in 2013 second and third horizontal modes. The results suggest that, due to sea level variations, the neck connecting the Chernyshev Bay to the main body of the lake can become critical for the development a nodal line in that neck.
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