Changes of climate and seasonally frozen ground over the past 30 years in Qinghai–Xizang (Tibetan) Plateau, China

Air temperature, ground surface temperature (GST; 0 cm at depth), precipitation and freezing depth data at 50 meteorological stations in the Qinghai–Tibet Plateau (QTP) were analyzed to examine changes of climate and seasonally frozen ground (SFG) in the past 30 years. The latitude, longitude, elevation, mean annual air temperature (MAAT), annual precipitation (AP) and maximum freezing depth at each station were used as the criterions to group the stations by the Hierarchical Cluster Analysis method. Fifty stations were grouped into four clusters, which are distributed in different regions of QTP. The most significant climate warming occurred in northeastern QTP, and the warming trend was greater in the cold season than in the warm season. Annual precipitation (AP) increased in the northwestern, inland and southeastern regions of QTP, but decreased in the northeastern QTP. The most significant changes of seasonally frozen ground (SFG) occurred in regions with thickest SFG, i.e., inland QTP, then northeastern and northwestern QTP. The duration of SFG shortened differently in different regions. Significant changes also occurred in the inland and northeastern regions of QTP. The cold season air temperature is the main factor controlling SFG change. The warming trends of ground surface temperatures are more significant than air temperature, and the warm season warming is greater than cold season warming. Changes of SFG depth, duration and surface temperature are likely to enhance heat exchanges between ground and atmosphere, in favor of stronger plateau monsoons.