Research On The Characteristics And Mechanism Of Interannual And Interdecadal Anomalies Of Strong Winter Cooling Events In North China

Based on the minimum temperature data,the NCEP / NCAR reanalysis data,the NOAA rebuilds monthly sea surface temperature data and the sea ice density data of Hadley Center in UK from 1980 to 2016,the frequency distribution of winter extreme cold events in North China is selected according to the definition.The prediction signals are found by analyzing the interannual and interdecadal components of its frequency,its correlation with the atmospheric circulation,the external forcing factors of sea surface temperature and sea ice,so as to provide reference for short-term climate prediction in North China.The main conclusions are as follows:(1)The frequency of single-station winter extreme cold events in North China increases gradually from southeast to northwest in winter,with a decrease before 2008 and then a significant increase.It is caused by the frequency of winter extreme cold events in later winter(January and February).The frequency's spatial distribution of regional winter extreme cold events is similar to that of single station.(2)The mid-latitude sea temperature in the northern hemisphere has a significant impact on the interannual anomalies of the frequency of single station winter extreme cold events in North China.The main affected areas are eastern Shanxi,southern Hebei and Shandong.When the SST in the North Pacific shows an “El-Ni(?)o-like” anomaly in winter,it triggers(-AO),(+PNA)and(+EUP)atmospheric teleconnection patterns in the mid-lower troposphere.As a result,the ridge to south of Lake Baikal becomes stronger.Meanwhile,SST in the North Atlantic shows a "reversed C" negative anomaly with(+NAO),(+PNA)-like and(+EUP)-like patterns,and the ridge to southwest of Lake Baikal becomes stronger.Furthermore,both cause the Siberian high become weaker in the north and stronger in the south.With the weaker East Asia subtropical jet and stronger East Asia winter monsoon,these factors lead to a significant increase of the frequency of winter extreme cold events in North China.When the SSTA shows an “El Ni(?)o-like” developing pattern from summer to autumn in the North Pacific,the frequency will be higher.The purported mechanism between the mid-latitude SSTA and the frequency in North China is the following: the SSTA in the North Pacific in summer and autumn excites atmospheric teleconnection wave trains,and the Atlantic stores these anomaly signals.In winter,the interaction between the SSTAs in the North Pacific and the North Atlantic enhances the Eurasian teleconnection wave train.As the upstream wave energy spreads downstream,the center of the wave train moves eastward with the seasons,leading to an increase of the frequency of single-station winter extreme cold events in North China.The Arctic sea ice in early autumn also has a certain impact on the interannual anomalies of the frequency of single-station winter extreme cold events in North China.The influence of sea ice on the frequency is mainly concentrated in the sea ice near Hudson Bay in early autumn.When there are more sea ice anomalies in the key area,the frequency is more.Meanwhile,AO is in a negative phase,leading a large range of cold air to control Eurasia.And wind field is weak,leading cold air trapped in Eurasia,which makes the single station winter extreme cold events occur frequently in North China.(3)The mid-latitude sea temperature anomaly in the northern hemisphere also has a significant impact on the interdecadal anomaly of the frequency of the single station winter extreme cold events in North China.Through the SVD decomposition of sea temperature and frequency,it is found there was an interdecadal transition in 1995.Before 1995,the frequency was less,and then more.The area with significant influence of sea temperature was in the central part of North China,namely Shanxi and Hebei.Before 1995,when the North Pacific in winter was "like El Ni(?)o" and the North Atlantic SST anomalies were negative anomaly,it triggered(+ EU),(+PNA-like)and(+NAO)atmospheric teleconnection patterns in the sea level pressure field.The 500 h Pa geopotential height field was similar.The trough-and-ridge system was deep.The cold air from the west of Novaya Zemlya in high latitudes-near the Barents Sea was guided southward by the northwest airflow in front of the trough ridge and transmitted to Central Siberia,and then along the east of Asia.The coast invades North China,which has led to an increase of the frequency of single station winter extreme cold events in North China.The opposite was true after 1995.The impact of sea ice in early autumn on the interdecadal anomaly of the frequency of single station winter extreme cold events in North China is mainly located in Xindi Island and the East Siberian Sea-Beaufort Sea.Decomposing the interdecadal components of sea ice and frequency by SVD,it can be found that there is an interdecadal transition around 1997.The more sea ice in early autumn corresponds to frequent single station winter extreme cold events in North China.Tthe opposite is the case in the later period.Before 1997,when sea ice in autumn was more,the Siberian High was weak in the north and strong in the south and the ridge between Lake Bay and Bahu strengthened.The upper level corresponds to the weaker subtropical westerly jet,which is conducive to the cold air going south,which makes single station winter extreme cold events in North China occur frequently.After 1997,the situation was the opposite.