Space-time variations of climatic seasons and their correlation with the phenological development of nature in Estonia

Abstract

In this paper the spatial and temporal variability of climatic seasons and phenological phases in Estonia are estimated, and the correlation between them is analysed. The start dates and durations of 8 climatic seasons at 23 stations and of 16 spring and summer phenological phases in Estonia between 1946 and 1998 were used for data analysis. The homogeneity of long-term series of climatic seasons was tested using an SNHT (standard normal homogeneity test). The time series can be considered nearly homogeneous. The phenological calendar method was applied. Significant spa- tial and temporal variability of climatic seasons was detected. The thermal influence of the Baltic Sea is the main factor in the formation of spatial differences. Due to the comparatively cold sea surface, the spring season lasts longer in coastal regions. On average, beginning of summer is observed first in southeast Estonia, at the end of May. In North Estonia summer starts at the beginning of June, and in coastal areas in the middle of June. The onset of the springtime phenological phases has a pattern similar to that of the start dates of spring and summer. The onset migrates over Estonia from south- east to northwest. In coastal areas, climatic seasons during the autumn period start 1 to 5 wk later than in continental Estonia. This spatial difference increases from the beginning of autumn to the beginning of winter. Climatic seasons in the spring period have tended to start earlier over the period between 1891 and 1998. At the same time, seasons in the autumn period tended to start later. In con- clusion, the summer season has lengthened significantly (by 11 d), while winter has contracted by 30 d. Correlation between start dates of climatic seasons and phenological phases is moderate in Estonia. In most cases, correlation coefficients are statistically significant at the 0.05 confidence level. The highest values are typical for seasons and phases that occur nearly at the same time. In general, the space-time variability of phenological phases is much lower than that of climatic seasons in the same period.