A Preliminary Study On The Variation Characteristics And Mechanisms Of Spring Extreme Low Temperature Events In Northeast China-Eastern North China From 1961 To 2016

Extreme low-temperature events have serious a influence on socio-economic development.Spring is the season that cold and warm air-parcles frequently interact over China,thus extreme low-temperature events often happen.Less attention has been paid to spring extreme low-temperature(SELT)events over China.Based on the daily minimum temperature data from 829 stations in China,the spatio-temporal features of SELT events over China are investigated in 1961–2016.Further,the variability in the SELT events over Northeast-eastern North China and the possible mechanisms are explored.The main results are concluded as follows:(1)The rotated empirical orthogonal function(REOF)analysis indicates that,according to the variability of the SELT frequency,China can be divided into objectively five separate regions,which are Northeast China-eastern North China,the Jiangnan region,eastern Northwest China-western North China,Southwest China,and northern Xinjiang.The SELT frequencies over the five regions have significant interannual and interdecadal variability.Commonly,the SELT frequencies over the five regions exhibit a 2-4-year quasi-periodicity according to the Morlet wavelet analysis results.Among the five region,the Jiangnan region,eastern Northwest China-western North China and northern Xinjiang exhibit a quasi-periodicity with 2-4-year over the whole period,Northeast China-eastern North China exhibits such a quasi-periodicity before the early 1980 s,and Southwest China shows such a quasi-periodicity over the period between early 1980 s and mid–1990s.For the long-term variation,all of the five regions have decreasing trends of the SELT frequency.However,their abrupt change years are significantly different: The abrupt change year of the SELT frequency is around 1987/1988 over Northeast China-eastern North China and Southwest China,1995/1996 over the Jiangnan region,1990/1991 over eastern Northwest China-western North China,and 1997/1998 over northern Xinjiang.At the same time,the SELT intensity over the five regions weakened in 1961–2016.Although during the past half-century,the frequency and intensity of SELT are generally decreased over China,they are increased in recent 10 years over eastern China(east of 115°E).(2)The first REOF mode of SELT frequency over China mainly reflects the variability over Northeast-eastern North China.Then,the possible mechanisms that cause the change of this mode are analyzed.The composite differences of atmospheric circulations between periods before and after the interdecadal change suggest that the anomalous high pressure over the Lake Baikal is a substantial circulation factor that influences the interdecadal change of SELT frequency over Northeast-eastern North China.Such anomalous high pressure is significantly enhanced around 1987/1988,which coincides with that of SELT frequency over Northeast-eastern North China.Further analysis indicates that,the North Atlantic mid-latitude SST warming is the key factor for the abrupt change of SELT frequency over Northeast-eastern North China.The North Atlantic mid-latitude SST is significantly warmed around the 1980 s,which could excite Eurasian wave teleconnection and further induce anomalous high pressure over the Lake Baikal,further decreasing the SELT frequency Northeast-eastern North China.(3)The relationship between the interannual variabilities of Northeast-eastern North China SELT frequency and atmospheric circulation indicates that there is an enhanced relationship of spring North Atlantic Oscillation(NAO)on the Northeast-eastern North China SELT frequency after the late 1970 s.The relationship between the two is weak before the late 1970 s but strong after the late 1970 s.The atmospheric circulation analysis indicates that,during the period of 1961–1976,the influence region of the spring NAO is mainly confined to the North Atlantic region and the spring NAO has a weak impact on the East Asian circulations.In contrast,during 1977–2016,the influence region of the spring NAO is extended and the spring NAO could excite Eurasian mid-latitude wave train pattern,inducing anomalous high pressure over the Lake Baikal.Therefore,the spring NAO exerts a significant influence on the variability of the Northeast China-eastern North China is controlled by such anomalous high pressure,which is unfavorable for Northeast-eastern North China SELT frequency.