Responses of Soil Freeze-Thaw Processes to Climate on the Tibetan Plateau from 1980 to 2016

Soil freeze-thaw processes are remarkable features of the land surface across the Tibetan Plateau (TP). Soil moisture and temperature fluctuate during the freeze-thaw cycle, affecting the soil water and energy exchange between the land and atmosphere. This study investigates variations in the soil temperature, humidity, and freeze-thaw state and their responses to air temperature and precipitation on the TP from 1981 to 2016. Regional simulations of the TP using Community Land Model version 4.5 demonstrate that the climate of the TP has become warmer and wetter over the past 37 years, with increases in both regional average temperature and precipitation. Using empirical orthogonal function analysis and the Mann-Kendall trend test of air temperature, we show that 1980-1998 was relatively cold, and 1999-2016 was relatively warm. Soil temperature and moisture in most areas of the TP were affected by air temperature and precipitation, and both showed an upward trend during the past 37 years. Overall, from 1981 to 2016, the freezing date of the TP has become delayed, the thawing date has been hastened, and the duration of the freeze-thaw state has shortened. The surface soil freezes and thaws first, and these processes pervade deeper soil with the passage of time; freeze-thaw processes have an obvious hysteresis. Precipitation and air temperature had marked effects on the freeze-thaw processes. Higher air temperatures delay the freezing date, hasten the thawing date, and shorten the freeze-thaw period. Areas with the highest precipitation saw late soil freeze, early thaw, and the shortest freeze-thaw duration. Areas with less vegetation froze earlier and thawed later. The freeze-thaw duration increased in the northwest of the plateau and decreased on the rest of the plateau. This article informs research on frozen soil change in the context of global warming.