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

The main characteristics of spatial and temporal variability of the precipitation regime in Sweden were studied by using the long-term monthly precipitation amount (1800-1990) at 33 stations. The data were filtered by using Empirical Orthogonal Function (TOT) analysis, which provides principal modes of both spatial variability and time coefficient series describing the dominant temporal variability. Canonical correlation analysis (CCA) was used to reveal association between the atmospheric circulation and the characteristics of the climate variability. Statistically significant upward shifts in the mean precipitation have been found during cold months (March, September, November and December) and only a downward shift (less significant) for August. Simultaneous changes in the time series associated to the optimally correlated circulation patterns were found, indicating an important role of the circulation. The circulation patterns are given by the North Atlantic Oscillation (NAO) in March and December and a cyclonic structure centred over southern Scandinavia in September and November. These changes may have induced changes in the mean precipitation seasonality reflected by a shift of the maximum precipitation from August to July (after 1931 for western p;irt and alter 1961 for the 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 variability were objectively identified. For these regions the standardised monthly precipitation anomalies were computed. The frequency of the extreme events (very dry;wet and dry/wet months) over 5-year consecutive intervals was analysed. It has been concluded that extreme wet months were more frequent than extreme dry months over the entire country, especially in the northern and southeastern part.