Drivers of Caspian Sea Level Decline: Volga Runoff, Wind Regime, and Climate Variability

The Caspian Sea is the largest enclosed water body on Earth, and it is undergoing rapid fluctuations in water level that pose significant risks to ecosystems, infrastructure, and socio‑economic activities across its littoral states. This study integrates hydrological and atmospheric perspectives to identify the primary drivers of recent and projected sea level changes.Long‑term hydrological records (1938–2020) from the Volga River, the dominant freshwater contributor, reveal a strong historical correlation between high runoff and increased atmospheric precipitation in its basin. However, since 2005, a marked decline in the runoff coefficient at the Verkhneye Lebyazhie hydrological station has been observed, attributable to regional warming that exceeds global temperature anomalies. This decline contributed to a 133 cm reduction in Caspian Sea level between 1977 and 2020. Importantly, while sea level changes historically mirrored Volga runoff fluctuations, since 2006 the relationship has decoupled, suggesting additional climatic drivers beyond river inflow.To investigate these drivers, wind regime variability was analysed using Modern‑Era Retrospective analysis for Research and Applications, Version 2 (MERRA‑2) reanalysis data spanning 1980–2023. Statistical tests revealed no significant differences in average wind speed between the phase of sea level rise (1984–2004) and decline (2005–2022). However, the resultant wind speed increased by 10.3%, accompanied by a 9.5° shift in the predominant direction, indicating a reorganisation of atmospheric circulation over the basin. Comparison with the Southern Oscillation Index (SOI) and North Atlantic Oscillation (NAO) demonstrated moderate correlations, underscoring the role of global teleconnections in shaping evaporation and water balance dynamics.As a broader context, near‑surface air temperature across the Caspian region increased by ~1.3 ± 0.5 °C since the mid‑20th century, with ERA5 showing stronger warming in the north (~1.6 °C) and peaks up to 3.0 °C in Iran’s mountainous areas. This regional warming amplifies evaporation and interacts with wind regime changes, further accelerating sea level decline.These findings highlight that Caspian Sea level dynamics cannot be explained by river runoff alone, but are mediated through atmospheric circulation shifts and regional climate variability. By integrating hydrological records, wind regime analysis, and climate context, this study advances the scientific basis for forecasting and adaptation strategies. The results emphasise the need for coordinated climate‑hydrology assessments to anticipate future risks in the Caspian basin, where natural variability interacts with anthropogenic pressures.