Spatial and temporal variability of climatic seasons on the East European Plain in relation to large-scale atmospheric circulation

Abstract

Start date and duration of climatic seasons on the East European Plain (EEP) are analysed. Climatic seasons are determined using daily mean temperatures. Data on climatic seasons are created for 73 stations in the period 1946-1995 and for 12 stations in the period 1881-1995. Cli- matic seasons of the first half of the year begin first on the northern coast of the Black Sea and last at the northeastern stations. The start of early spring is observed in March and April, the start of spring in April and May, and the start of summer in May and June. Climatic seasons in the second half of the year begin first in the northeast and last in the south. The influence of the Black and Baltic Seas on the retardation of the onset of climatic seasons is remarkable. Autumn usually starts in August and September, late autumn in September and October, and winter in October-December. Duration of summer extends from 5 mo in the south to less than 1 mo in the north, and the duration of winter from 7 mo in the northeast to less than 3 mo in the south. The duration of the intermediate seasons is longer in the western EEP and shorter in the eastern EEP. Some time series of start date and duration of cli- matic seasons on the EEP have a significant trend. In the western part, the beginning of early spring has shifted up to 1 mo earlier. Correspondingly, the duration of winter has decreased, and the dura- tions of early spring and spring have increased. Using principal component and cluster analyses, regionalisation schemes of the EEP, based on start dates of climatic seasons, are worked out for every season separately and for all the seasons together. The summary regionalisation distinguished Tem- perate and Sub-Arctic climatic zones, boreal and sub-boreal sub-zones within the Temperate Zone and regions in every zone and sub-zone. Start dates of climatic seasons are correlated with a great number of parameters of atmospheric circulation. Dependence of climatic seasons on atmospheric circulation is analysed and discussed.