Research On The Impact Of Atmospheric Low-frequency Oscillations On Strong Winter Cooling Events In North China

Based on the daily minimum temperature data of 419 stations in North China from 1980 to 2016,this paper gives the definition of regional extremely cold event(Regional Extremely Cold Event,RECE),counts the frequency of the RECE in the past 37 years and analyzes the low-frequency characteristics of the RECE.Multiple RECEs are selected to systematically study the impact of atmospheric low-lrequency oscillation on them,and to find the extended period forecast signal.Then the differences of the effects of the low-frequency oscillation on RECEs under different SST backgrounds are discussed.Finally,the two RECEs in winter of2015 and their possible causes are compared and analyzed.The main conclusions are as follows:(1)During the winter of 1980-2016,29 RECEs occurred in North China,with the longest(shortest)duration of 10 days(2 days).The winter minimum temperature in North China mainly oscillates in 10-20 d and 20-30 d,and the most significant oscillation is in 10-20 d.And the RECEs show a strong quasibiweekly oscillation in 10-20 d.The 10-20 d component of winter minimum temperature in North China roughly corresponds to the original sequence,which can reflect the actual cooling process.(2)Twenty-four RECEs with the low-frequency period of 10-20 d are selected to study.The result shows that in the middle troposphere,the low-frequency high in the north of Lake Baikal and East Asian Trough are stronger.The strong low-frequency Siberian high and its east to south is also one of the important reasons for the extreme cooling in North China.In the low-level wind field,the low-frequency anticyclone in Nova Zembla and the low-frequency cyclone near Lake Baikal are the main low-frequency influence systems.In the upper troposphere,the weak East Asian temperate jet and the strong East Asian subtropical jet cooperate with each other.The north of Lake Baikal is the direct source of low-frequency cold air,and the high vortex pool near Lake Baikal is the indirect source in thestratosphere.The positive(negative)correlation in the-14d(-10d)located in the east of Nova Zembla(the north of the East European Plain)corresponds to the low-frequency negative(positive)anomaly of500 h Pa height field,which has an important reference role in predicting the evolution of the low-frequency East Asian Trough(Ba-Bei Lake high)and further predicting the extreme cooling in North China.(3)According to the different SST backgrounds,the 24 RECEs are divided into three types: normal(5),El Ni(?)o(9)and La Ni(?)a(10),to further compare the differences of low-frequency circulation characteristics of different types of events.Compared with the normal years,the range of low-frequency high-pressure in the 500 h Pa height field is wider and the north boundary is more northward in El Ni(?)o and La Ni(?)a years,and the low-frequency high-pressure system in La Ni(?)a years is the most easterly,showing northeastsouthwest distribution.In normal years,the range of low-frequency Siberian high is the smallest;the range of El Ni(?)o is the second largest and the eastern part of the southern boundary of the high is to the south;the range of La Ni(?)a is the largest and obviously to the east and south.In normal years,the direct source of cold air is located in Lake Balkhashcentral Siberia,and the source of high-level cold air is located near the Lake Baikal-Kamchatk;in El Ni(?)o years,the direct source of cold air is to the north of 60°N in the north of Lake Baikal,and the high-level vortex pool in the stratosphere is located near Lake Baikal.In the years of La Ni(?)a,the low level was mainly affected by the cold air from Lake Baikal,and the West Siberian Plain was the main source of the cold air in the high level.In normal years,the negative correlation in the-6d in the Aral Sea is mainly concerned;in El Ni(?)o years,the positive(negative)correlation signal is located in the Kara Sea(west of Eastern European Plain)in the-14d(-10d);in La Ni(?)a years,the negative correlation in the-12 d in the Norwegian Sea first moves eastward and then southward to Ba-Bei Lake;in the-10 d,the positive correlation in the Severnaya Zemlya-East Siberian sea first spreads southwest,and then southeast to East Asia.(4)The two RECEs in winter of 2015 are corresponding to the changes of the 10?20d low-frequency component of the minimum temperature from peak to valley.In the first(second)process,the Lake Balkhash-Lake Baikal(referred to as Ba-Bei Lake)high pressure is located to the west(northeast),the Siberian High pressure is to the south(north),the East Asian trough is northeast to southwest(south to north),the Aleutian Low is deeper(westward and deeper),and there is no(a)negative Arctic Oscillation anomaly.The source of cold air in the upper and lower layers is located near 60°N in the northern part of Lake Baikal(above the north of Lake Balkhash and North China,respectively)and cooling is slower(faster)for the first(second)process.The continuous transmission of low-frequency disturbance energy in the Mediterranean Sea is one of the reasons for the difference in the duration of the two cooling processes.The low-frequency negative(positive)anomaly in Western Europe at 500 h Pa height field in the-18d(-12d)is a valuable reference for predicting RECEs in North China.