Consequences of the Aral Sea restoration for its present physical state: temperature, mixing, and oxygen regime

Abstract. 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 (2006) to 3400 km² (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 a series of expeditions to the North Aral Sea in 2016–2019 and year-long continuous monitoring of the annual thermal and oxygen regime by an autonomous mooring station, we present the first comprehensive analysis of the North Aral system behavior on seasonal to interannual scales after its "cold restart". We demonstrate that the new seasonal mixing regime 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 but a stable salinity stratification can develop during ice melt in late winter. 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.