Response And Mechanism Of Summer Droughts In Northern China To Barents Sea Ice Melt In Spring

Since the mid-1990 s,the trend of drought in North China and Northeast China has been severe and extreme drought events have increased.This paper focuses on the external forcing factor(Arctic sea ice)and atmospheric circulation factor(summer North Atlantic Oscillation)to explore the effects of spring Barents Sea ice and summer NAO on extreme drought in northern China.At the same time,it also analyzes the influence of spring Barents Sea ice melting on the pattern of summer NAO.To summarize the paper,the main research conclusions are as follows:(1)After the mid-1990 s in northern China,the drought in the eastern northwest,north China,and northeastern regions intensified,while the drought in the northwest did not change significantly,even with the characteristics of humidification.Compared with July,there are more areas affected by the drought and more serious drought events in August.From the spatial distribution of drought and extreme drought in northern China,the areas with more frequent droughts(sc-PDSI<=-2)in summer are mainly in the eastern part of Northwest China,northern China and southern part of Northeast China.The frequency of occurrence in 40 years approximately 15-25 times,and extreme drought(sc-PDSI<=-4)occur more frequently in North China and Northeast China,the extreme drought frequency of occurrences is 10-15 times in 40 years.Compared with July,the frequency of drought and extreme drought occur more frequently in August.(2)From the perspective of atmospheric circulation,there exists a significant teleconnection between the eastward shift in the negative-phase NAO and drought over northern China.Negative NAO events(NAO index<-1.0)tend to give rise to extreme drought over northern China.When the NAO is in the negative phase,the cyclonic eddy-driven jet enhances the eastward extension of the North Atlantic Jet Stream(NAJS)in the jet exit at 200 h Pa,which leads to the connection between the NAJS and Asian-African jet and strengthens the waveguide;the occurrence of barotropic instability results in the transformation of barotropic kinetic energy to seasonal-scale kinetic energy,which provides adequate kinetic energy for cyclonic eddy growth and maintenance of the downstream wave train;the eastward extension of the NAJS strengthens the zonal wind shear in the jet exit at 200 h Pa.In general,upward motion caused by cyclonic circulation at 500 h Pa and upper-level wind shear are beneficial to precipitation in the jet exit;The Rossby wave excited by NAO in summer travels eastward along a wave train similar to SRP,passing to the Caspian Sea,Central Asia,and converging over northern China,further strengthening the high-level anticyclonic circulation anomaly,and the low-level 850 h Pa circulation field.There are abnormal northerly winds in the water vapor flux field(1000 h Pa-300 h Pa),which is not conducive to the northward transportation of water vapor in the southwest of the summer background field.And this can further lead to the reduction of precipitation and the occurrence of extreme drought.(3)The decrease of sea ice in the Barents Sea during spring mainly leads to the extreme drought in Northeast China in summer by two ways.First of all,affected by the melting of sea ice,there is a wave train that starts from the position of the southern activity center of NAO in the negative phase in summer,and passes through Central Europe and Central Asia.The northern part of China exhibits a wavelet pattern of "negative-positive-negative-positive".In addition,a similar Polar/Eurasian(POL)teleconnection appears over the Pakistan-Lents Sea-Kara Sea points to the northeastern part of China in the middle and high latitudes.These two wave trains converge over the northeastern China,which can further enhance the anticyclone anomaly.In the upper downstream at 200 h Pa,the northern China is controlled by a strong anticyclonic circulation.The easterly wind on the south side of the anticyclonic circulation weakens the high-level westerly jet.The mid-level 500 h Pa over the northeastern China shows positive geopotential height anomalies,which are conducive to the anomalous sinking movement in the northeast China.The combined influence of the low-level atmospheric circulation led to the occurrence of drought and extreme drought in the Northeast.(4)The numerical simulation experiment of sea ice melting is designed using the atmosphere module CAM5 in CESM1.0.4.The results show that pronounced sea ice decrease in the Barents Sea-Kara Sea leads to the northern and southern active centers of the North Atlantic Oscillation(NAO)gradually shift.On the other hand,the positive geopotential height anomaly gradually forms and strengthen along with season,and then activity center over the south of NNAO shift eastward remarkably.Concurrently,the positive geopotential height anomaly is strengthening,and the negative geopotential height of the southern active center and positive geopotential height over European region locate on the climatological trough ridge,which could amplify the wave amplitude.Focus on the downstream,the sea ice melting experiment induced the upper,middle and lower atmospheric circulation anomalies are roughly consistent with our reanalysis results over Northern China,which is further verified our studies.