Seasonal variability of circulation and air-sea interaction in the Caspian Sea based on a high resolution circulation model

Abstract

The Caspian Sea is the largest closed basin on Earth, with unique physical characteristics and complex bathymetry. Sparse measurements and coarse resolution models have not yet provided a full picture of its dynamics. In this study, a high resolution (∼3.3 km) Regional Ocean Modeling System (ROMS) was used to study its annual and seasonal circulation patterns, and distributions of salinity and temperature. Temperature and salinity nudging were implemented which forced the model to follow in situ observations. This approach effectively limited the model error in predicting the temperature and salinity. The model successfully reproduced sea surface temperature and salinity over the entire basin. The simulated sea surface salinity showed three salinity fronts over the Caspian Sea. The salinity profile was vertically uniform in deep waters of the Caspian Sea. In contrast, the vertical thermal structure formed the seasonal thermocline at a depth of 15–40 m which was intensified in summer. In general, the seasonal variability of water temperature or salinity was confined to the upper 100 m of the water column. Simulations revealed an impermanent weak northward current along the eastern coasts of the Middle Caspian Sea, a persistent strong southward current along its western coasts and a permanent deep anticyclonic gyre over the Southern Caspian Sea. Using a high resolution model, this study also reports several mesoscale and small scale structures in the Caspian Sea which have not been reported before.