Influence of North Atlantic SST Variability and Changes in Atmospheric Circulation on the Frequency Of Summer Droughts in the East European Plain

Based on the Standardized Precipitation Index (SPI) data for June-August in 1950-2016, the frequency of severe atmospheric droughts in the south of the East European Plain is analyzed. It is found that droughts were observed more often in the periods of warmer SST in the North Atlantic as compared to its colder period. It was revealed that the frequency of droughts in the Volga region and in the north-west of Kazakhstan was the greatest (to 19 droughts/100 years) in the years when the SST of the North Atlantic was extremely warm. The least frequency of severe droughts (4 droughts/100 years) in the considered regions was observed in the years of the extremely cold North Atlantic. Favorable conditions for the development of severe summer droughts in the south of the East European Plain in the period of the warm North Atlantic were registered when regional zonal circulation in the Atlantic-European sector weakened and the frequency of days with atmospheric blocking in the east of the East European Plain increased. Such conditions were related mainly to the increasing frequency of negative phases of the North-Atlantic Oscillation and the East Atlantic/Western Russia (EA/WR) pattern. The greatest (three-fold) increase in the blocking frequency was registered during the negative phase of the EA/WR pattern as compared to its positive phase.