Statistical Characteristics Of Winter Extreme Cold Events And Diagnostic Analysis Of Circulation Anomalies Over China In Recent 60 Years

Based on 1629 winter daily and monthly mean temperature data and reanalysis data from1960/1961 to 2017/2018 in China,this paper conducted a statistical study on the changes of winter temperature and Regional Extreme Cold Events(RECEs)in recent 60 years in China.In fact,the observations further reveal the evolution characteristics of winter temperature and RECEs in China;and deeply analyze the influence of decadal variation of atmospheric circulation on RECEs;At the same time,the differences of the atmospheric circulation in the upper and lower layers of the three periods were compared and analyzed,and the differences of the tropospheric and stratospheric coupling circulation system during the winter RECEs outbreak in different periods were summarized.The results showed that:In the past 60 years,under the general trend of global warming,the winter temperature in China on the whole showed an upward trend,but it did not continue to rise,which can be divided into cold period(1960-1985),warm period(1986-2005)and hiatus period(since2006).Since the hiatus period,the winter temperature in China has leveled off or even decreased slightly,while the RECEs have increased and strengthened.Regionally,there is a significant north-south difference in winter temperature and RECEs in China.Since the hiatus period,the temperature in most parts of the north has decreased,and the RECEs have increased significantly,while the temperature in the south has not decreased significantly.At the same time,each index of RECEs in winter in China has the strongest performance in cold period and the weakest performance in warm period.RECEs in hiatus period occur more frequently,have larger influence range,longer duration and stronger intensity than those in warm period.At the same time,the warm period of national type RECEs and middle and eastern type RECEs increased during hiatus period,which influenced the cold air in northwest of RECEs increased,while the cold air in east of RECEs decreased.In addition to the global warming trend,the winter mean temperature and RECEs frequency in China also have obvious interdecadal variations,which indicates that the natural climate forcing factor is also one of the main factors affecting the winter temperature in China,and it plays an important modulating role in the winter temperature changes in China.In recent 60 years,the variation characteristics of winter RECEs in China were influenced by AO and East Asian winter monsoon,and the interdecadal variation of AO and East Asian Winter monsoon during the hiatus period enhanced the RECEs.During cold period,warm period and hiatus period,the changes of atmospheric circulation in stratosphere and troposphere and the changes of sea level pressure are significantly different,and the influences on various aspects of winter RECEs in China are also different.Compared with the warm period,the East Asian trough is strengthened during hiatus period,the Ural Mountains blocking high and the Okhotsk blocking high are strengthened,the Arctic vortex is weakened,and the Siberian high is strengthened.The intensities of the East Asian winter monsoon and the Aleutian low lead to more frequent,stronger,wider and longer duration of winter RECEs in China during the hiatus period than the warm period.The variation of winter RECEs in China is influenced by the northward eastward propagation of the Rossby wave source excited over the North Atlantic Ocean.Especially in the hiatus period,the abnormal wave flux first propagates the energy of Rossby wave northward to the North Pole,resulting in the weakening of the North Pole vortex.Then,the energy of Rossby wave continues to propagate eastward and southward from the polar regions to the Ural Plateau region.The Ural blocking was enhanced,which further affected the East Asian circulation and resulted in the increase of winter RECEs in China.Although there are more RECEs in both cold and stagnant periods in China,the two periods indicate that the atmospheric circulation characteristics of RECEs eruptions are not completely the same.The changes of Rossby waves over the North Atlantic are the reason for the adjustment of the East Asian circulation with the increase of regional extreme cold events during stagnant periods.At the same time,the variation of winter RECEs in China is affected by the interdecadal oscillation of SST in the three oceans.When IPO/PDO or AMO is negative,the increase and enhancement of winter RECEs in China are more favorable.At the same time,when the Indian Ocean SST is abnormally cold,the RECEs are enhanced.The variation of atmospheric circulation before the RECEs eruption was significantly different in three periods.In the stratosphere,in the week before the eruption of RECEs,the polar vorticity is the most active in the hiatus period,and the polar vorticity tends to split towards dipole type.In the troposphere,before the RECEs eruption,the circulation adjustment was the most obvious in the hiatus period,and the development of the Okhotsk obstructions was the strongest,and a typical East Asian inverted Ω flow pattern was established.The Ural obstructions and Echai obstructions continued to strengthen and develop towards the polar region,and the polar vortex split southward.The tropospheric atmospheric circulation changed from 3-wave pattern to 2-wave pattern,while the 3-wave pattern before the RECEs eruption in cold and warm periods.In the sea level pressure layer,the Siberian high is stronger in the cold period and hiatus period,and the position of the Siberian high is more east-south in the hiatus period than in the cold period.The Siberian high began to strengthen 2 weeks before the RECEs outbreak and reached its strongest about 3 days before the RECEs outbreak,while the cold and warm periods began to strengthen within 1 week before the cold event outbreak.