Abstract
Mongolia is one of the most sensitive regions to climate change, located in the transition of several natural and permafrost zones. Long-term trends in air freezing and thawing indices can therefore enhance our understanding of climate change. This study focuses on changes of the spatiotemporal patterns in air freezing and thawing indices over Mongolia from 1960 to 2020, using observations at 30 meteorological stations. Our results shows that the freezing index ranges from −945.5 to −4,793.6°C day, while the thawing index ranges from 1,164.4 to 4,021.3°C day over Mongolia, and their spatial patterns clearly link to the latitude and altitude. During the study period, the trend in the thawing index (14.4°C-day per year) was larger than the trend in the freezing index (up to −10.1°C-day per year), which results in the net increase of air temperature by 2.4°C across Mongolia. Overall, the increase in the thawing index was larger in the low latitudes and altitudes (e.g., the Gobi-desert, steppes, the Great lake depression and major river valleys) than in high latitudes and altitudes (mountain regions), while it was the opposite for the freezing index. The highest values for both thawing index and freezing index (i.e. the least negative values) have occurred during the last 2 decades. As the trends in the freezing and thawing indices and mean annual air temperature confirm intensive climate warming, increased permafrost degradation and shallower seasonally frozen ground are expected throughout Mongolia.
Highlights
Permafrost is an important component in mid and high-latitude regions, as well as high altitude environments
In agreement with previous studies (Wu et al, 2011; Shi et al, 2019), our results suggest that the decrease of the freezing index and increase of the thawing index in Mongolia are noticeable since the beginning of the 1980s and 1990s
The results from this study indicate that the decrease in the freezing index is usually higher in mountain regions and the north of Mongolia featuring permafrost, compared to steppes and the Gobidesert dominated by seasonally frozen ground
Summary
Permafrost is an important component in mid and high-latitude regions, as well as high altitude environments. The mean annual air temperature (MAAT) ranges from −1.0°C to −6.5°C in the high altitudes of the Altai, Khangai, Khuvsgul, and Khentii mountains (generally featuring permafrost) and from −1.0 to 8.0°C in the Great Lake depression, the steppe region and the Gobi-desert with seasonal frozen ground regions (MARCC, 2014; Ishikawa et al, 2018). We selected 30 meteorological stations with long-term climate data sets, which are distributed throughout all typical eco- and permafrost zones in Mongolia, with elevations ranging from 688 m a.s.l. at the KH station to 2,222 m a.s.l. at the TO station (Figure 1) These two stations are located in the warmest and coldest regions of Mongolia, respectively. There are no missing values in the current version of the defined domain in CRU TS 4.03 (Harris et al, 2020)
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