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.
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 sub-surface 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. 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 moisture 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.
We consider the numerical scheme of the three-dimensional model of enclosed and semienclosed sea hydrodynamics with variable sea surface topography described by a linear free-surface equation. We derive equations integrated over the upper sea layer and consider the formulation of the finite difference scheme of the model in the spherical coordinate system and the derivation of the sea free-surface equation which is approximated by an implicit time scheme. A peculiarity of the model is that it directly takes into account real water fluxes at the upper boundary in a mass balance equation, in this case we need not specify sources or artificial salt fluxes in a salt balance equation. We give an example of the model efficiency in the problem of modelling the seasonal variability of the Caspian Sea circulation and level.
Abstract. Decadal variability in Caspian Sea thermohaline properties is
investigated using a high-resolution ocean general circulation model
including sea ice thermodynamics and air–sea interaction forced by prescribed
realistic atmospheric conditions and riverine runoff. The model describes
synoptic, seasonal and climatic variations of sea thermohaline structure,
water balance, and sea level. A reconstruction experiment was conducted for
the period of 1961–2001, covering a major regime shift in the global climate
during 1976–1978, which allowed for an investigation of the Caspian Sea response to
such significant episodes of climate variability. The model reproduced sea
level evolution reasonably well despite the fact that many factors (such as possible
seabed changes and insufficiently explored underground water
infiltration) were not taken into account in the numerical reconstruction.
This supports the hypothesis relating rapid Caspian Sea level rise in
1978–1995 with global climate change, which caused variation in local
atmospheric conditions and riverine discharge reflected in the external
forcing data used, as is shown in the paper. Other effects of the climatic shift
are investigated, including a decrease in salinity in the active layer,
strengthening of its stratification and corresponding diminishing of
convection. It is also demonstrated that water exchange between the three
Caspian basins (northern, middle and southern) plays a crucial role in the
formation of their thermohaline regime. The reconstructed long-term trends in
seawater salinity (general downtrend after 1978), temperature (overall
increase) and density (general downtrend) are studied, including an
assessment of the influence of main surface circulation patterns and model
error accumulation.
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