Climatic variability of water circulation in the Caspian Sea based on satellite altimetry data

Abstract

ABSTRACTOscillations of the Caspian Sea level is the result of interrelated hydrometeorological processes and climate change not only in the catchment area of the sea but also far beyond it. The change in the tendency of the mean sea level variations in the middle 1970s, when the long-term sea level fall was replaced by its rapid and significant rise, is an important indicator of the changes in the natural regime of the Caspian Sea. Now, monitoring of the Caspian Sea level is based on tide gauge and satellite altimetry data. However, climatic variability of water circulation in the Caspian Sea remains unclear. Traditionally, currents in the Caspian Sea have been investigated by numerical methods. Instrumental observations of the currents in the Caspian Sea are mostly carried out in the shelf zone. Available data cover very short periods and reflect variability only in synoptic and higher frequency of the sea dynamics. This paper presents the results of the application of satellite altimetry methods for investigation of interannual variability of sea water circulation in different parts of the Caspian Sea and in the sea as a whole using the data of TOPEX/Poseidon, Jason-1and Jason-2 missions from 1993 to 2012. Satellite altimetry data provide a unique possibility to analyze climatic variability of water circulation in the Caspian Sea. The author discusses the technique of calculating synoptic dynamic topography as a superposition of the annual average climatic dynamic topography calculated by the thermohydrodynamic model for the time period of 1948‒2004 and sea level anomalies calculated by satellite altimetry data. The analysis of vorticity of geostrophic currents confirms predominance of cyclonic circulation in the Northern and Middle Caspian Sea and anticyclonic circulation in the Southern Caspian Sea. The analysis shows that interannual variability in average current velocity and vorticity are opposite to each other. From 1993 to 2007, vorticity was decreasing at the rate of −0.17 ± 0.02 10–7 per year, and average current velocity was increasing at the rate of +0.11 ± 0.06 cm s−1 per year. After 2008, the tendency of variability has changed to the opposite. Vorticity was increasing at the rate of +0.75 ± 0.12 10–7 per year, but velocity of currents was decreasing at the rate of −0.47 ± 0.19 cm s−1 per year.