Temporal and spatial variability of precipitation in Sweden and its link with the large-scale atmospheric circulation

Abstract

The main characteristics of spatial and temporal variability of the precipitation regime inSweden were studied by using the long-term monthly precipitation amount (1890–1990) at 33stations. The data were filtered by using Empirical Orthogonal Function (EOF) analysis, whichprovides principal modes of both spatial variability and time coefficient series describing thedominant temporal variability. Canonical correlation analysis (CCA) was used to reveal associationbetween the atmospheric circulation and the characteristics of the climate variability.Statistically significant upward shifts in the mean precipitation have been found during coldmonths (March, September, November and December) and only a downward shift (less significant)for August. Simultaneous changes in the time series associated to the optimally correlatedcirculation patterns were found, indicating an important role of the circulation. The circulationpatterns are given by the North Atlantic Oscillation (NAO) in March and December and acyclonic structure centred over southern Scandinavia in September and November. Thesechanges may have induced changes in the mean precipitation seasonality reflected by a shift ofthe maximum precipitation from August to July (after 1931 for western part and after 1961 forthe southeastern coast) and after 1961 to September, October or November for other regions.Combining rotated EOF analysis with cluster analysis, 4 regions with similar climate variabilitywere objectively identified. For these regions the standardised monthly precipitation anomalieswere computed. The frequency of the extreme events (very dry/wet and dry/wet months) over5-year consecutive intervals was analysed. It has been concluded that extreme wet months weremore frequent than extreme dry months over the entire country, especially in the northern andsoutheastern part.