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 3.2m. The periods of the fundamental modes were identified as 36 and 14h, 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 present work was carried out within the framework of the CLIMSEAS exchange project funded by the European Commission (ref. FP7-IRSES-2009-247512). Field work was supported by Russian Science Foundation grant 14-50-00095