Spatial and temporal variability of Canadian seasonal precipitation (1900-2000)

Wavelet and cross-wavelet analysis are used to identify and describe spatial and temporal variability in Canadian seasonal precipitation, and to gain further insights into the dynamical relationship between the seasonal precipitation and the dominant modes of climate variability in the Northern Hemisphere. Results from applying continuous wavelet transform to seasonal precipitation series from 201 stations selected from Environment Canada Meteorological Network reveal striking climate-related features before and after the 1940s. The span of available observations, 1900-2000, allows for depicting variance and covariance for periods up to 12 years. Scale-averaged wavelet power spectra are used to simultaneously assess the temporal and spatial variability in each set of 201 seasonal precipitation time series. The most striking feature, in the 2-3-year period and in the 3-6-year period-the 6-12-year period is dominated by white noise and is not considered further-is a net distinction between the timing and intensity of the temporal variability in autumn, winter and spring-summer precipitation. It is found that the autumn season exhibits the most intense activity (or variance) in both the 2-3 year and the 3-6 year periods. The winter season corresponds to the least intense activity for the 2-3 year period, but it exhibits more activity than the spring-summer for the 3-6 year period. Cross-wavelet analysis is provided between the seasonal precipitation and four selected climatic indices: the Pacific North America (PNA), the North Atlantic Oscillation (NAO), the Northern Hemisphere Annular Mode (NAM) originally called the Arctic Oscillation, and the sea surface temperature series over the Nino-3 region (ENSO). The wavelet cross-spectra revealed coherent space-time variability of the climate-precipitation relationship throughout Canada. It is shown that strong climate/precipitation activity (or covariance) in the 2-6 year period starts after 1940 whatever the climatic index and the season. Prior to year 1940, only local and weaker 2-6 year activity is revealed in western Canada essentially in winter and autumn, but overall a non-significant precipitation/climate relationship is observed prior to 1940. Correlation analysis in the 2-6 year band between the seasonal precipitation and the selected climatic indices revealed strong positive correlations with the ENSO, the NAO, and the NAM in eastern and western Canada for the post-1940 period. For the period prior to 1940, the correlation tend be negative for all the indices whatever the region. A particular feature in the correlation analysis results is the consistently stronger and positive NAM-precipitation correlations in all the regions since 1940. The cross-wavelet spectra and the correlation analysis in the 2-6 year band suggest the presence of a change point around 1940 in Canadian seasonal precipitation-that is found to be more likely related to NAM dynamics. (c) 2006 Elsevier Ltd. All rights reserved.