The Inter-Decadal Variation And Atmospheric Circulation Causes Of Extensive And Persistent Extreme Cold Events In China

Under background of the global warming,the number of extreme cold events in China has increased in recent years with inter-decadal variability,indicating characteristics and causes of which need further study.In this paper,using the daily temperature observations and NCEP/NCAR reanalysis data from 1961 to 2018,we firstly investigate the frequency,cumulative days of occurrence and range of extensive and persistent extreme cold events(EPECE)in China in winter and spring.The climatic characteristics of EPECE and their inter-decadal variability are obtained by studying the frequency,cumulative number of days and extent of occurrence.Then we select two types of EPECE,the northwestern/Jiangnan type and the mid-eastern type,and compare the characteristics of the atmospheric circulation before and after the occurrence of the two types of EPECE at different inter-decade.Based on the important influence of blocking high on extreme low temperature events,the capability of the CMIP6(Coupled Model Intercomparison Project)models to simulate blocking high in Eurasia is evaluated,and the possible trends of blocking high frequency in Eurasia under two future high and low emission scenarios are predicted.The main results are as follows.(1)A total of 41 winter-spring EPECE occur in China during 1961-2018.Interdecadal variations in frequency and cumulative days of occurrence show a decreasing trend and an inter-decadal turnaround,with the cumulative days of occurrence showing a clear fluctuating downward trend before 1995,a decreasing trend slowing down after1995,and a slight increase after 2008.There are also interdecadal differences in the type and extent of EPECE occurrence.The countrywide types dominate before 1995 have a wider reach,and only two countrywide EPECE occur after 1995,with the Northwestern/Jiangnan type and the mid-eastern type being the main types.The main differences between two types of EPECE between the different years occur in eastern and southern China and in northwestern Xinjiang.The cooling range shifts south by 1.8°after 1995 compared to the previous time period,and the number of days last more than2 days increased.(2)The weather conditions of the two types of EPECE are significantly different at different times before and after 1995.We find that the intensity of the Eurasian polar vortex is strong in the early part of the two types of EPECE before 1995,reaching a maximum value of 1.27,but it is maintained for a relatively short period of time and fluctuated in intensity.The Ural blocking is established earlier before 1995,with a frequency of 21.8% and an intensity of 4.1;after 1995 the Ural blocking is established later but further east,with a frequency of 14.6% and an intensity of 4.7.The pre-1995 Siberian high is below the climatic mean in the early part of the events,with a maximum intensity level of 8.9h Pa.After 1995,Siberian high is stronger with 1039.8h Pa and decreased slowly until the 15 th day when the positive pitch disappeared.The analysis of the 315 K isentropic surface shows that anticyclonic Rossby wave fragmentation exists over Eurasia before and after the events for both types of EPECE at different times.Both the northwestern/Jiangnan type and the mid-eastern type cold air sources are located around Europe,but the northwestern/Jiangnan type is stronger,resulting in a broader area of influence.The long-lasting polar vortex and the southerly blocking and Siberian high maintain the strength of the mid-eastern type during this period.The northwestern/Jiangnan type is 4.5 days longer than before 1995 and the impact is more southerly.(3)The simulation results of 17 CMIP6 models are used to analyze the model's ability to simulate the frequency of blocking high in Eurasia.Most of the model simulations show more towards February-May and less towards June/September compared to the observed data.The temporal distribution is significantly lower than the historical observed data in June/September.The simulation results also show a spatial distribution characterized by low frequency at high latitudes.The simulation results of most of the models show that the frequency of high-blocking events decreases with longer lifespan.Most of the models show a lower frequency of height-inhibiting events in the 5-8 day lifetime range than in the 9+ day lifetime range,especially in the Okhotsk Sea which is better than the other two key areas.On average,the model ensemble is better at simulating short-lived high-inhibiting events than medium-lived ones,and most of the trends are consistent with the observed data.(4)A prediction of possible changes in the frequency of blocking events in the Eurasia under the SSP1-2.6 and SSP5-8.5 scenarios shows that the frequency of blocking events in the Eurasian critical zone is slightly higher under the SSP1-2.6 scenario than that under the SSP5-8.5 scenario.The frequency of blocking events near the midlatitudes of the Lake Baikal critical zone increases significantly in the winter months under SSP1-2.6;the frequency of blocking events in the high latitudes of the Ural Mountains critical zone increases significantly in the summer months.The frequency of altitude blocking events in the Ural Mountains during the short lifespan in winter tends to increase significantly in the SSP5-8.5 scenario,while there is no significant change in the middle and long lifespan;while the frequency of altitude blocking events in Lake Baikal and the Sea of Okhotsk during different lifespans in winter decreases to different degrees,especially in the SSP5-8.5 scenario.The frequency of height-inhibiting events in the Ural Mountains and Lake Baikal in the summer half of the year tends to increase to different degrees under different emission scenarios,especially in the Ural Mountains in the middle and long lifespan,where the increase is particularly significant under the SSP5-8.5 scenario.The change in height-inhibiting events in the Okhotsk sea is less significant,with a significant decrease in height-inhibiting events in the short lifespan only under the SSP1-2.6 scenario.The uncertainty of the change in block high under background of future warming increases,and the impact on extreme cold weather events requires continued attention.