Seasonality of multidecadal and centennial variability in European temperatures: The wavelet approach

Temperature variability on multidecadal and longer timescales is studied by application of the wavelet transform (WT) to seven seasonal early instrumental temperature series at locations in Europe. The WT spectra are computed for all four seasons of each record. A few spatiallly coherent components of temperature variability are detected on timescales longer than 60 years. All components appear seasonally dependent; the major differences in time-frequency patterns occur between late winter and early autumn. The WT-estimated monotonic trends indicate warming in winter at all locations, with the mean rate 0.4 degreesC per century, and little change or cooling in summer. Superimposed on this monotonic trend, a quarter-millennial oscillation tends to show the opposite phases in winter and summer, enhancing both the winter warming and the summer cooling trends. A regular oscillatory signal with a timescale of 60-80 years detected earlier in annual mean European temperatures impacts central and eastern Europe during summer and autumn; over northwestern Europe and in Scandinavia the warm-season multidecadal signal is either weak or irregular. The cold-season temperature variations are dominated by the centennial (similar to 120 years) rather than multidecadal timescales. The present WT analysis extends the earlier results based on multichannel singular spectrum analysis of the same data set and confirms the presence of long timescales of variability in European temperatures; it further indicates the crucial role of seasonality in the spatiotemporal structure of low-frequency temperature variability.