Recently Increased Cold Air Outbreaks Over Mongolia And Their Specific Synoptic Pattern

In recent years, extreme cold air outbreak events (COEs) in winter have tended to occur more frequently over the mid-latitudes of Eurasia, including Mongolia, despite marked warming across the continent. In the 2000s, an increase in COEs contributed to huge losses of livestock throughout Mongolia, causing socioeconomic stagnation. This study diagnosed the interdecadal variability of the frequency of COEs during cold season (November-March) using observations from 70 meteorological stations in Mongolia by contrasting a past period (PP:1981-1999) and a recent period (RP:2000-2016). Then, this analysis was extended to characterize the synoptic pattern of each period using ERA-Interim reanalysis data in terms of the ridge and trough system and cold advection. We defined a COE as an event having at least five continuous days with a daily average air temperature lower than minus one standard deviation from the daily climatology during cold season in 1981-2016. The results revealed that totally 112 COEs could be identified over Mongolia during 1981-2016 with a higher number of events (59) during the RP. Regionally, a slight increase in COEs was observed in the north, whereas no clear change was evident in the south. Composite analyses for each period demonstrated that a trough in the central Eurasian mid-troposphere was deepened and southwestward advection of cold air enhanced during the RP, in comparison with the PP, which changed the wind direction over northern Mongolia from westerly to northwesterly. In addition, the number of snow cover days have increased slightly in the RP in northern Mongolia, likely acting as a favourable condition for low air temperatures with intensifying surface cooling. These findings suggest that a deepened trough over central Eurasia, strengthened northerly cold advection into Mongolia, and cooling from the snow-covered surface are most likely to have resulted in the increased frequency of COEs overnorthern Mongolia in the RP.