Seasonal variability of the Caspian Sea three-dimensional circulation, sea level and air-sea interaction

Abstract

Abstract.A three-dimensional primitive equation model including sea ice thermodynamics and air-sea interaction is used to study seasonal circulation and water mass variability in the Caspian Sea under the influence of realistic mass, momentum and heat fluxes. River discharges, precipitation, radiation and wind stress are seasonally specified in the model, based on available data sets. The evaporation rate, sensible and latent heat fluxes at the sea surface are computed interactively through an atmospheric boundary layer sub-model, using the ECMWF-ERA15 re-analysis atmospheric data and model generated sea surface temperature. The model successfully simulates sea-level changes and baroclinic circulation/mixing features with forcing specified for a selected year. The results suggest that the seasonal cycle of wind stress is crucial in producing basin circulation. Seasonal cycle of sea surface currents presents three types: cyclonic gyres in December-January; Eckman south-, south-westward drift in February-July embedded by western and eastern southward coastal currents and transition type in August-November. Western and eastern northward subsurface coastal currents being a result of coastal local dynamics at the same time play an important role in meridional redistribution of water masses. An important part of the work is the simulation of sea surface topography, yielding verifiable results in terms of sea level. The model successfully reproduces sea level variability for four coastal points, where the observed data are available. Analyses of heat and water budgets confirm climatologic estimates of heat and moisCorrespondence to: R. A. Ibrayev (ibrayev@inm.ras.ru) ture fluxes at the sea surface. Experiments performed with variations in external forcing suggest a sensitive response of the circulation and the water budget to atmospheric and river forcing.

Caspian Sea surface circulation variability inferred from satellite altimeter and sea surface temperature

“…In this study, the annual mean dynamic topography was obtained from the climatologically forced Caspian Sea model of Gunduz [2008], based on the HYCOM. The nominal resolution of the model is 3.2 km in the horizontal, with 25 sigma-z levels in the vertical.…”

“…Figure 8 shows SST overlaid with geostrophic currents for the month of August in the years 1993, 1997, 1998, 1999, 2000, and 2004. It is well known from earlier studies [Sur et al, 2000;Ibrayev et al, 2010;Gunduz, 2008] that upwelling occurs in August along the eastern coast of the MCS, forced by the northerly winds in summer resulting in the particular circulation. It is observed in Figure 8 that the intensity and location of the upwelling are subject to interannual variations.…”

“…During the summer upwelling period of Figure 4e, Ekman transport along eastern coast of the MCS causes the surface water to move toward the west. The mechanism of a complete reversal of the currents from a cyclonic to an anticyclonic circulation in July is not clear, although it appears possible that the fresh and cold water flowing north along the western coast of the basin may generate this anticyclone [Gunduz, 2008]. It is also possible that change in wind direction may be responsible for the reversal.…”

“…The available hydrographic data are mostly localized on the continental shelf regions and only of sufficient detail allowing to characterize the synoptic variability of the currents [Tsytsarev, 1967;Klevtsova, 1967;Bondarenko, 1993;Ghaffari and Chegini, 2010;Jamshidi and Abu Bakar, 2012]. On the other hand, modeling studies made until now only provide information on the climatological surface circulation, without revealing details of its interannual variability [Knysh et al, 2008;Ibrayev et al, 2010;Tuzhilkin and Kosarev, 2005].…”

“…They found similar dipole pattern in the SCS with a rather wide cyclonic gyre located at the south eastern part of the Sea. A schematic description of the seasonal circulation of the Caspian Sea has been articulated by Gunduz [2008].…”