| As one of the most serious weather systems which extrodinarily impact on climate and weather of East Asia,the blocking highs in key Eurasian regions will result in anomalously hot weather,flood,freezing,and other severe weather climate events and pose a great threat to people's lives and property.By using reanalysis data in 1979-2016 and methods of T&M,objective analysis which are widely accepted to identify blocking highs,we study the climatic characteristics of blockings in key Eurasian areas,the relationships between blockings and internal factor(ultra long waves),external forcing(two types of El Ni?o Oscillation(ENSO)),and thermodynamic characteristics of Urals blocking highs systematically.The main conclusions are as follows:(1)The Ural and Okhotsk regions were areas of high probability of blocking highs,while there was relatively low frequency of blocking highs in the Baikal regions.The “twin blocking type” was the main situation of atmospheric circulation at mid-latitudes.The average duration of blocking highs in the Ural and Okhotsk regions was relatively long with about 6.6 days,and that was only 4.9 days in Baikal regions during winter.As a whole,the blockings in three key Eurasian regions during the winter showed a decreased trend from cumulative days,cumulative frequency,or average duration during 1979-2005,especially since the 1990 s.However,during nearly 10 years there was an upward trend of frequency of blocking highs.Significant interannual change was observed in the cumulative days of blockings in Ural and Okhotsk regions,yet there was no obvious change in the cycle blockings of the Baikal areas.There was a clear 4~6 year cycle of changes of Ural blocking highs in the early 1980 s to the end of the 1990 s,while 5~7 year cycle of changes of blocking highs in Okhotsk regions was more active from the late 1980 s to the early 21 st century.(2)The Eastern Pacific-La Ni?a(EP-LN)event would lead to more transient blockings in Eurasian regions,while East Pacific-El Ni?o(EP-EN)would cause transient blocking highs fewer.However,the instantaneous blocking highs in west of Ural Mountains(60°E)and the eastern part of Sea of Okhotsk(140°E)were greatly affected by the Central Pacific-El Ni?o(CP-EN),wherethere were more and stronger blockings.The EP-LN incident and the Central Pacific-La Ni?a(CP-LN)event led to the average duration of blockings in Ural regions increase by 28% and 25%,respectively,however caused blocking highs in Okhotsk areas to decrease.The EP-EN and CP-EN incidents made the average duration of the blocking highs in Okhotsk areas increase by more than 14% and 18%.The blockings of Ural regions would reduce in EP-EN and CP-EN incidents by some degree.In winter,when the EP-EN event occurred,the geopotential height(GPH)in the Ural and Okhotsk regions was negative anomaly,indicating the blocking highs were weak.The positive anomaly of 500 hPa GPH in the Ural and north of Baikal areas occurred which made the blockings higher under EP-LN event.When the CP-EN events occured,the 500 hPa GPH in the three key regions of Eurasian was stronger in different degrees,indicating that the CP-EN event were conducive to the generation of blocking highs.Under the CP-LN incident,the little higher GPH of the Ural and Baikal areas provided good conditions for the occurrence and development of the blocking highs.(3)The proportion of the 1-8 wave spectrum in the mid-latitudes of the Northern Hemisphere varied from 0.93 to 0.96,and the proportion of 1-3 wave spectrum exceeded 0.5,which demonstrate that the ultra-long wave is the most important component of atmospheric motions.There was a quasi-two-month periodic variation of proportions of the 1-8 wave and 1-3 wave spectra at 30°N or 60°N,which oscillation in winter was more significant than it in summer.When the proportion of 1-3 wave spectrum was strong,the GPH in Ural areas was low,and the GPH in the Baikal areas was high,but the changes of GPH in the Okhotsk areas were not significant.Conversely,as the proportion of 1-3 wave spectrum was weak,the GPH in the Okhotsk areas was high,yet the changes of GPH in other two regions were not obvious.In addition,when the 1-3 wave spectrum was abnormal at 60°N,the anomalies of GPH were more significant.The amplitude of 1 wave was positively related to the blocking indexes of the Ural regions,and negatively related to blocking indexes in Okhotsk areas.But the correlation between amplitude of 3 waves and the blocking highs in Ural Mountains regions was negative,yet amplitude of the 3 waves was positively related to the blockings in the Okhotsk areas.When the duration of blockings in Ural areas increased from 4 days to 7 days,the amplitude of 1 wave increased,but the amplitude of 2 waves decreased at 30°N,meanwhile the amplitude of 1 wave and 2 waves at 60°N increased by 43% and 1.86 times.The amplitude of 1 wave was reduced by 25% and the amplitude of 2 waves was increased from 112 gpm to 130 gpm as the duration of blockings in Ural regions increased from 4 days to 7 days in winter.The sharp increase of the amplitude of 2 waves might lead to a longer duration of blocking highs.(4)When we studied the thermodynamic characteristics of blockings in Ural regions during the whole life process in winter,it was found that before the “Day-3”,the stationary temperature T* at middle latitudes decreased with height.The high-level T* was transported downward along the southern edge of the blockings.Before the "Day-3" the stationary heat flux in overall troposphere at the mid-latitudes was distributed as "dumb-bell" shape,i.e.At the same time,the areas of 20°-70°N were controlled by positive(northward)stationary momentum flux {u*v*},and the two maximum centers were located at 30°N,250 hPa and 55°N,300 hPa,respectively.From “Day-3” to “Day 0” the temperature gradient(-?T/?y)below 400 hPa decreases,which led to thermal winds weak.The westerly winds at mid-latitudes would also be reduced.Meanwhile,the negative(southward)stationary heat flux {v*T*} occurred at mid-latitudes in the middle of troposphere,which showed that heat flux was converged at the mid-latitudes.That provided sufficient heat for blocking establishment.At the same time,the positive stationary momentum flux at mid-latitude became stronger with maximum value center located near 55°N.The stationary momentum flux {u*v*} was transported southward in north of 70°N.That showed that there was a convergence of momentum flux at 70°N,which contributed to the strengthening of the polar front jet.The transportation of heat flux led to a decrease in the temperature gradient at mid latitudes,which made a decrease of westerly.The stationary momentum flux at mid-latitudes was transported northward that caused the polar front jet to be stronger.Until “Day +3”,the cold air from upper troposphere was transported down the ground to strengthen the Siberian High.The negative stationary heat flux in the middle atmosphere disappeared at the same time.The stationary momentum flux rapidly decreased to return to the normal.At the time of establishment of the blockings at 500 hPa,the transportation direction of anomalous heat and momentum flux was northward,while direction in collapse periods was southward.In addition,the non-linear effects played the most important role in the anomalous heat flux {v*T*}' transportation.However,linear and non-linear both were siginificant in the transportation of anomalous momentum flux {u*v*}'. |
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