Seasonal stratification and basin-scale internal waves in the North Aral Sea

The restoration of the North Aral was an unprecedented effort to save a large water basin by construction of a dam that separates it from the rest of the desiccating Aral Sea area. As a result, the lake volume has stabilized at 27.5 km³, the area has increased from 2800 km² in 2006 to 3400 km² in 2020, and the salinity has dropped from 18 to 10 g kg^-1. The consequences of this unique experiment include highly dynamic changes of the thermal conditions, seasonal stratification, ice regime, and dissolved oxygen content and remain not fully quantified to date. We analyze the current state of the North Aral Sea with regard to stabilization of its long term dynamics, as well as consider the possible future projections in view of the global change effects on the regional hydrological regime and potential water management measures. Using data from two year-long observations, we analyze the current seasonal mixing regime and sub-seasonal oscillations due to lake-scale internal waves in the North Aral. We found that the seasonal stratification pattern is intermediate between dimictic and polymictic, with relatively weak summer thermal stratification occupying only a small deep part of the lake. Salinity does not contribute to the summer density stratification.   On  the background of weak thermal stratification,  highly energetic internal waves with periods of 4.5 days  dominate the near-bottom dynamics and facilitate mixing at the lake bottom. As a result, the bulk of the water column remains well saturated with oxygen throughout the year. However, low-oxygen conditions may develop in the deepest part of the lake in mid-summer. In summary, the mixing regime of the restarted lake favors vertical transport of dissolved matter and  water-sediment mass exchange ensuring oxygenation of deep waters and supply of nutrients to the upper water column. While the North Aral Sea is restored to the well-mixed state similar to that before its desiccation started, its seasonal mixing regime is currently in unstable equilibrium, wobbling between polymictic and dimictic conditions. The fragility of this seasonal pattern is demonstrated by modeling results: slight changes of the water level or transparency may turn the Aral Sea to steadily dimictic or polymictic state.