COMPARATIVE ANALYSIS OF MULTIDECADAL VARIABILITY OF HYDROMETEOROLOGICAL PARAMETERS IN THE PONTO-CASPIAN SEAS

Abstract

NCEP/NCAR reanalysis data for the period of 1948–2020 were used to detect, describe (in geographical sense) and compare the patterns of multidecadal variability of the hydrometeorological parameters (thermal, dynamic and humidity) in the Ponto-Caspian region (Black Sea, Sea of Azov and Caspian Sea). Polynomial approximations of the time series of the annual mean values indicated the non-linear nature and periodicity of the long-term variability of considered parameters. Being geographically closely located, these seas reveal significantly different patterns of variability. In the Black Sea two temporal intervals of an average duration of 20 and 40 years with multidirectional trends of hydrometeorological parameters changes have been detected, while in the Caspian Sea three such intervals lasting 10–25 years are identified. The North-Eastern wind regime prevails over the both basins. However, if in the Black Sea wind components act unidirectionally (air temperature rise/cooling coincides with the weakening/ strengthening of both components), then in Caspian they act the opposite way, with the major portion of heat advection provided by zonal (Eastern) transport. Phases of air warming/cooling in both basins coincide with a weakening/strengthening of this transport. The time lag between the shift of dynamic and thermal (as well as humidity) regimes is about 6–8 years, suggesting a leading role of the large-scale atmospheric forcing variations in the regional variability. Specific humidity is positively correlated with air temperature and decreases/increases during periods of cooling/warming. In contrast, relative humidity and precipitation rate variations are negatively correlated with air temperature trends. Over the Caspian Sea intensification of Eastern transport and associated cooling coincide with a situation when the North Atlantic Oscillation (NAO) index decreases to negative values, and the East Atlantic-West Russian pattern (EAWR) index is significantly positive. On the contrary, weakening of the Eastern transport and warming occur during a period of sharp strengthening of the NAO, coinciding with strongly negative values of the EAWR index. As for the Black Sea, the situation is controversial: the general pattern of variability in 1948–1992 was similar to that in the Caspian Sea and consistent with described combinations of indexes. However, after 1992 no pronounced trend in variability of NorthEastern transport have been observed.