Modeling extreme Black Sea and Caspian Sea levels of the past 21,000 years with general circulation models

This paper describes the relationship between sea levels and climate based on the links between sea-level variations and river runoff. During the final late Pleistocene and postglacial periods, the Caspian Sea fluctuated between regression and transgression stages. The Black Sea experienced fluctuations as well, but these were mainly controlled by the world ocean due to water exchange through the Bosporus Strait. Sometimes, the Caspian Sea overflowed into the Black Sea through the Manych Strait, and they periodically coalesced. Change in the level of both seas could be interpreted as responses to the regional-scale water budget (the balance between inflow and outflow components). These components can be calculated from atmospheric general circulation models. This approach uses climate modeling data to reproduce river runoff changes, and, consequently, variations in seawater and sea level under contrasting climate conditions. In response to glacial conditions of the last cold Pleistocene event, the lowering levels of the Black Sea (post-Karangatian regression stage) and the Caspian Sea (Atelian regression stage) are simulated simultaneously. This lends credence to the idea of the connection between deep regression states of the Caspian and Black Seas and mature stages of the late Quaternary glacial/cooling/drying planetary events. Analysis of observed information allows us to conclude--taking into account the uncertainties of reconstructed data--that at least two regression stages occurred simultaneously with late Quaternary glacial planetary events. The simulation of transgression stages (their onset and duration) remains a very difficult problem. Results of modeling have shown that during the warm periods (taking as examples the mid-Holocene and Allerod events), simulated river runoff did not increase to the extent needed for a strong transgression and overflow of the Caspian Sea into the Black Sea through the Manych Strait. Thus, there is no clear understanding about the source of "additional" water volume necessary to elevate the level of the Caspian Sea to a point that would permit overflow into the Black Sea.