The article discusses the entire causal chain in the North Atlantic – atmosphere –Volga basin – Caspian Sea level system. Its component part is the “Caspian Sea” hydrological system, interpreted as the sea itself, its drainage basin, and the atmosphere above the sea and the basin. Its analysis has demonstrated that changes in the sea water volume are modeled by a set of hydrological and meteorological factors of different nature, with different spatio-temporal variability, and determined at significantly different accuracies. The main factor for the interannual variation of the sea water volume, and hence the increment of the Caspian level, is the annual runoff via the Volga. The Volga streamflow variation is fully controlled by precipitation in the drainage zone of the catchment. In turn, interannual precipitation fluctuations are largely defined by the inflow (outflow) of a vertically integrated horizontal flow of water vapor across the lateral boundaries of the runoff-producing zone of the Volga basin. The zonal transfer of water vapor from the North Atlantic is shown to be the determinant in the among-year variation of precipitation in the Volga basin both in winter and in summer. It has been established that evaporation is growing and the zonal transfer of water vapor to Europe and then to the Volga basin is being intensified as a result of a higher cyclonic activity in the North Atlantic, the Norwegian Sea in particular, due to large-scale ocean-atmosphere interactions. Consequently, more rainfall occurs in the runoff-producing zone of the basin, annual streamflow of the Volga increases, and the Caspian level rises. The reverse is observed when cyclonic activity in the North Atlantic weakens. Therefore, the Caspian Sea level is an integral indicator of large-scale moisture exchange in the ocean-atmosphere-land system.