Spatiotemporal variations in freezing and thawing indices on the Tibetan Plateau during 1978−2017

The Tibetan Plateau is the largest expanse of alpine frozen soil in China. The study of freezing and thawing indices can provide some implications for ecological environmental evaluation. Based on the daily air temperature data obtained from 34 meteorological stations, freezing and thawing indices and their spatiotemporal variation trends from 1978 to 2017 on the Tibetan Plateau were calculated and analyzed, and some key factors that may affect these indices are discussed. The results showed that during 1978–2017, the freezing index of the Tibetan Plateau ranged from 386.8 to 886.1 ℃·d, and the thawing index ranged from 1 960.3 to 2 521.5 ℃·d; the temporal variation trends of the two indices were nearly negatively correlated, and significant downward (−9.2 ℃·d·a⁻¹) and upward trends (11.1 ℃·d·a⁻¹) were observed for the time series of the freezing index and thawing index, respectively. The spatial distribution of the mean value of the freezing index showed an increase from southeast to northwest, whereas the thawing index was high in the south-central and eastern areas. The rate of decline of the freezing index in the northwest of the Tibetan Plateau was faster than in the southeast, whereas the thawing index showed a rapid upward trend in most areas on the Tibetan Plateau. Freezing and thawing indices were affected by the latitude, longitude, altitude, and some socio-economic factors; however, altitude was the dominant factor.