Sea ice cover in the Caspian and Aral seas from historical and satellite data
Résumé
Time and space variations of ice cover in the Caspian and Aral Seas from historical and satellite data are discussed. Existing published continuous time series of ice cover parameters for these seas stop in 1984-1985; the results of observations for later periods are heterogeneous and mostly unpublished. The current lack of time series for ice cover parameters since the mid-1980s may be filled successfully by using microwave satellite observations that provide reliable, regular and weather-independent data on ice cover. In our study, we use the synergy of two types of satellite observations: (i) passive microwave data from SSMR and SSM/I sensors (since 1978) as well as (ii) a combination of simultaneous data from active (radar altimeter) and passive (radiometer) microwave instruments onboard the TOPEX/Poseidon satellite (since 1992). An assessment of ice conditions from historical observations is given to provide a background to compare with satellite-derived data. The ice algorithms were applied for satellite data; series of dates of first and last observation of ice cover, duration of ice season as well as ice extent area have been computed for various regions of the Caspian and Aral Seas. These time series show pronounced regional, seasonal and interannual variability. There is a marked decrease of both duration of ice season and ice extent since the winter 1993/1994. Several factors that may explain this warming signal are discussed, as well as existing and potential implications of changes in ice conditions for industrial activity and the sea ecosystems.