Impact of climatic factors on permafrost of the Qinghai–Xizang Plateau in the time-frequency domain

A combined time-frequency analysis provides a more profound understanding of the impacts of various climatic factors on permafrost. Morlet wavelet transformation was used to detect multiple periodicities in monthly soil temperatures, as well as data for five climatic factors over the interval from 1996 to 2010 on the Qinghai–Xizang Plateau. These periodic relationships were further analyzed in the time-frequency domain. Our results indicated that four of the five climatic factors have multiple periods: air temperature has an 18 months period, downward longwave radiation (LWD) had periods of 9, 18,142 and 176 months, downward shortwave radiation (SWD) had periods of 9, 18, 122 and 174 months, precipitation had periods of 9, 18, 68 and 138 months, while wind speed had periods of 9, 18, 28, 58, 122 and 178 months. Soil temperatures had quasi-periodic variations of 9, 18, 41, 110 and 176 months at 0.5 m depth; and had periods of 9, 18, 68–71, 120 and 169–173 months at 6 m depth. A relationship analysis of these periodicities indicated that the multiple periodicities in soil temperature were mainly controlled by periodicities in climate change. In particular, the periodicity in air temperature had the greatest effect on soil temperatures at 0.5 m depth, while LWD had the greatest effect on permafrost temperatures at 6 m depth based on a grey incidence analysis.