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

Temperature variability on multidecadal and longer timescales is studied by application of 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. The major seasonal differences in time-frequency patterns on timescales longer than 60 years refer to winter/spring versus summer/autumn. The WT analysis indicates that multidecadal oscillatory signal with a timescale 60-80 years, detected earlier in European temperatures, originates mainly from summer and autumn temperature series from central and eastern Europe. Over the northwestern and northern Europe the multidecadal oscillation is hardly detectable, showing strong phase modulation and time dependence. In winter and spring, the low-frequency temperature variability is dominated by centennial timescales. The WT-estimated monotonic trends indicate warming in winter at all locations, with mean rate of 0.4 oC/century, and little changes or cooling in summer. A quarter-millennial oscillation, superimposed on these monotonic trends, tends to show opposite phases in summer and winter, enhancing winter warming in the 20th century. The present WT analysis extends earlier results based on multichannel singular spectrum analysis of the same dataset and confirms major role of seasonality in spatio-temporal structure of temperature variability on low-frequency range; it further indicates the presence of multidecadal oscillation in summer and autumn temperatures from sites in central and eastern Europe.