Relationship between Extreme Low Temperature Events and Atmospheric Circulation in Yunnan during Winter Half Year

Abstract

The regional extreme low temperature events（RELTE） in winter half year of Yunnan from 1961 to 2019 are examined by using a simplified objective identification method.The results are shown as following： the frequency of Yunnan RELTE、 the cumulative impact intensity and the number of stations affected by the Yunnan RELTE showed a trend of decline in the past 59 years， but the frequency of extreme events and the number of sites affected increased in recent 10 years.The RELTE duration mainly ranged from 1 to 4 days， the occurrence probability was higher in November and March of the winter half year.The possibility of RELTE was higher in the eastern part of Yunnan， but in the narrow area west of Nujiang River the possibility was lower.Since the mid-1980s， there was no year in which all three seasons of winter half year RELTE are occurred in Yunnan.The frequency of late autumn RELTE decreased rapidly and there aren’t occur in recent 10 years.From the analysis of circulation characteristics， the atmospheric circulations affecting the Yunnan RELTE are different in different seasons： In late autumn， the Rossby wave over North America are spreaded eastward on the upper 300 hPa， the 500 hPa atmospheric pressure field over East Asia is "high in the west and low in the east" and the 850 hPa northeast wind leads the cold advection southward， which easily causes the Yunnan RELTE to be active.The winter RELTE are caused by the eastward Rossby wave over the Azores Islands in the North Atlantic Ocean at 300 hPa， the "high in the north and low in the south" air pressure field over East Asia at 500 hPa， the cold advection guided by the mid-latitude east wind at 850 hPa.In early spring， the Rossby wave of 300 hPa on the northeast Pacific Ocean travels eastward from the Scandinavian Peninsula to East Asia and the pressure field of 500 hPa on East Asia is "high in the north and low in the south"， east wind and north wind of 850 hPa lead the cold advection southward together and meets southern branch trough in Yunnan， which is prone to RELTE.

Keywords

• yunnan
• winter half year
• extreme low temperature events
• cold advection
• rossby wave