Patterns and frequencies of the East Asian winter monsoon variations during the past million years revealed by wavelet and spectral analyses

The wavelet and spectral analyses are used to investigate the variations of patterns and frequencies of the East Asian winter monsoon system during the past million years. Grain size of the loess–paleosol deposit at the central Chinese Loess Plateau is employed as a proxy indicator of the winter monsoon strength. On the bases of palaeomagnetic dates and a sedimentation rate model, an absolute, not orbitally tuned time scale for the loess deposit is developed. Wavelet analyses on this time series show that the patterns and frequencies of the East Asian winter monsoon climate vary with time during the past million years. The wavelet analyses also revealed a shift of climatic pattern from predominantly quasi-200- to quasi-100-ka cycle at around 500 ka BP which is also supported by previous studies using the sedimentation rate analysis as well as paleosol observations. Traditional spectral analysis on the same time series shows that the East Asian winter monsoon changes at Milankovich cycles, but there are also cycles which may not be related to orbital forcing. The predominant 41-ka cycle identified from previous studies is not evident in this time series from both wavelet and spectral analyses. Our results present new evidence that not only solar radiation, but also other boundary conditions, may have forced the monsoon system changes at the 104–105 years time scale.