Articles | Volume 21, issue 2
Research article
21 Feb 2017
Research article |  | 21 Feb 2017

The internal seiche field in the changing South Aral Sea (2006–2013)

Elena Roget, Elizaveta Khimchenko, Francesc Forcat, and Peter Zavialov

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.

Short summary
This is the first work on the internal seiches of the South Aral Sea, which is also of general interest as the lake is considered a paradigm for the lakes whose surface levels are decreasing. It is based on field data and numerical simulations. For autumn 2006 and 2013, the fundamental mode is discussed together with other modes which are found to be higher vertical modes. The importance of sea level variation on the development of higher horizontal modes is also discussed.