On the lake effects of the Caspian Sea

The impact of the Caspian Sea on precipitation and circulation is investigated. The control regional climate model simulation accurately represents the spatial and temporal pattern of observed precipitation and the upper atmospheric jet. From a simulation without the Caspian, it can be deduced that it causes a year‐round increase in evaporation and increases in precipitation in autumn and winter with little change in spring and summer. Over half of the over‐lake precipitation is attributable to the lake effect. These findings are confirmed through coupled simulation with a three‐dimensional lake model. The Caspian Sea's effect on surface air temperatures (an increase in winter and decrease in summer) causes an increase in cyclonicity in winter and anticyclonicity in spring and summer. These surface air temperature effects also penetrate higher up in the atmosphere, causing a perturbation of up to 25 m in the geopotential height at 200 hPa, leading to an increase in the maximum speed of the subtropical jet over western Asia during summer of 6.5%. An accurate description of the Caspian Sea in general circulation models is thus important for both local and regional climates.