Study On The Change Characteristics And Mechanism Of Ural Blocking Highs Under The Global Warming

The abnormal activities of the Urals blocking highs(UB)often cause the cold wave weather in East Asia during winter.Under the background of global warming,the cold events occur frequently in East Asia,which should to be related to the change of characteristics and mechanism of the UB.The change characteristics of UB in winter under the background of global warming are analyzed,and the evolution mechanism is deeply understood from perspective of large-scale atmospheric waves,which is of great significance to improve the prediction of extreme cold weather in East Asia.Based on the reanalysis data provided by National Centre from Environmental Prediction-Department of Energy(NCEP-DOE)from 1979 to 2021,five blocking high indexes are defined,including the cumulative frequency index,the central latitude index,the blocking intensity index,the duration index and the north rim latitude index.On this basis,a new comprehensive blocking index(CBI)is established,which can comprehensively summarize the characteristics of UB.Then the change characteristics of these sixes indexes of UB and their relationship with the atmospheric circulation are analyzed.After that,the influence of large-scale waves,including the planetary waves and Rossby waves on the evolution mechanism of UB from establishment to maintenance to decline and then to collapse in winter is investigated by using dynamic diagnosis method,such as quasi-geostrophic potential equation and energy propagation theory.Finally,the CMIP6 model outputs are used to test the simulation ability of the UB characteristics in winter,then the reasons for the simulation bias are explored from the perspective of planetary waves and Rossby waves,and the possible change trends of UB and their relationship with weather and climate in East Asia is projected under different scenarios in the 21st century.The main conclusions are as follows:1.A new CBI is established to describe the characteristics of UB totally,and relationship between the UB and atmospheric circulation anomalies in NH during winter is revealed.The maintenance of this relationship is adjusted by the planetary waves.By studying the distribution characteristics of each blocking high index,it is found that the center of UB is mainly concentrated in 60°N?67.5°N,and about 1/3 of the total UB with the north rim latitude reach 72.5°N.The intensity is about 0.04,and the duration is generally 5?9 days.The UB are most likely to occur in January in winter,with longer duration and larger variation of intensity.Only the duration of UB shows an obvious upward trend in winter from 1979 to 2018.The six blocking indexes change interdecadally in 2002.Specifically,the UB occur more frequently with the norther location,stronger intensity and longer duration and larger CBI after 2002.When the UB occur more frequently or their locations more northward or their intensities strong or their duration longer in winter,the geopotential heights(GPH)of 100 and 500 hPa over the Urals are higher,and the polar vortex deforms and even splits.The GPH of upstream in Europe and downstream in East Asia are lower by exciting the Eurasian teleconnection,so the European trough and East Asian trough are deepened.The zonal winds weaken on 200 hPa over the Eurasia,which are beneficial for the strengthening of mid-latitude atmospheric meridional circulation.Meanwhile the subtropical jets over the Mediterranean and China are stronger and move northward.The CBI can well reflect relationship between the UB and atmospheric circulation anomalies in the NH during winter,and highlights that the effect of large-scale planetary waves on providing a good atmospheric background for development of the UB.2.There is a good indication from the establishment to decline of the UB by thermal and dynamic processes of the planetary waves,and the stationary part plays a key role on southward movement of the UB entering the decline stage.The GPH in the north of Urals around 70°N is increased by the influence of the negative vorticity advection and the positive vertical gradient of temperature advection caused by planetary-scale wave from-4day to Oday,which is a signal of establishment of the UB.The prophase maintenance of UB(Oday to+4day)depends more on the dynamic process of planetary wave.After+4day,the GPH in the south of Urals around 60°N is increased under the combined thermal and dynamics actions of planetary wave,while in the north of Urals around 60°N,the GPH is decreased,and the UB moves southward and begins to decline.By regarding the planetary-scale waves as large-scale stationary vortices and only considering local effects,it is found that the{u*v*} and-?/?p?/?y {v*T*} are less than 0 in the north of 70°N from-6day to+6day and positive in the vicinity of 50°N?60°N from-2day,with the positive area gradually moving southward,especially after+2day,making the increasing area of GPH gradually move southward and the UB declines.It is revealed that the stationary part of thermal and dynamic processes by planetary-scale waves plays an important role in the southward movement of the UB into the decline stage.But the thermal and dynamic processes of planetary waves can not well explain the collapse of the UB.3.The adjustment of barotropic and baroclinic energy caused by thermal and dynamic processes also plays a key role in establishment,prophase maintenance(0 to+4day)and decline of the UB.The analysis shows that barotropic energy is the core energy of UB maintenance,which source is from external transportation,barotropic process generation and atmospheric baroclinic instability generating baroclinic energy then transformation.When analyzing the barotropic and baroclinic energy distribution in the Eurasian region during-6day to+6day,it is found that on-4day,due to the propagation of barotropic energy over the Atlantic to the downstream,the barotropic energy in the Urals and its upstream European region is relatively strong,and the barotropic process promotes and increases the barotropic energy.At the same time,the enhancement of atmospheric baroclinic instability caused by temperature advection promotes the generation of barotropic energy,and the transformation of barotropic energy to barotropic energy further increases barotropic energy.In the stage of UB prophase maintenance(0day to+4day),there is a large amount of barotropic energy over the Urals,that is the energy source of blocking high maintaining.However,in this stage,the generation of baroclinic energy is relatively small,so the increase of baroclinic energy is from the outside,and the barotropic structure of UB can be maintained by the conversion of baroclinic energy to barotropic energy.After+4day,the barotropic anticyclone winds over the Urals decrease,especially at high latitudes(north of 70°N),and the decrease of barotropic energy indicates the decline of the blocking highs.The barotropic and baroclinic energy adjustment can not explain how to collapse for the UB.4.The upstream energy of Rossby wave packet(RWP)to blocking area can provide energy for maintenance of UB,especially play a role of energy relay during later maintenance(+4day to collapse).The sharply reduction of the RWP energy in blocking area is the key factor to promote the collapse of blocking high.The energy transmitted from the upstream to the blocking area is less and the energy is dissipated from the blocking area outward so that the energy in the blocking area is less.Or the RWP in the blocking region suddenly expand outward to release energy,and the energy transport direction is not unified,which reduces the energy in the blocking region.They can cause the blocking high to collapse.For the mid-latitude UB case,the RWP is mainly concentrated in the key blocking region(around 60°E,60°N)and its upstream.And the energy is propagated toward the northwest,and the phase moves toward the west,resulting in the morphology of UB from northeast-southwest" to"northwest-southeast" on Oday.As the UB grow to+6day,the upstream energy is propagated to the blocking area,which gradually increases the wave packet value and accumulates energy,which is conducive to the maintenance of UB.Until+8day,the energy in the Urals area is small and the blocking collapses.The energy is transported into upstream of Urals deepening the European trough.For the high-latitude blocking high,the RWP is mainly concentrated in the key area of blocking(near 60°E and 70°N)and southeast of the UB(near 60°E?90°E and 45°N),and it is quasi-stationary with the phase velocity only of 200?300 km·day-1.To+4day,the upstream RWP energy is transported to the blocking accumulating energy which provides energy support for the maintenance of blocking high.Until+6day,the high-latitude RWP releases energy southward,resulting in strong energy in the whole Eurasian region.The RWP energy propagates to the southeast and northeast,resulting in energy dispersion and collapse of blocking high.5.CMIP6 models verify that the planetary waves and Rossby waves play an important influence on the UB.It is found that 11 climate models can basically simulate the distribution characteristics of UB indexes in winter,and there are the best simulation effects on the north rim latitude and CBI especially.However,the model generally underestimates all blocking indexes,especially the blocking intensity index,which is only 1/2 of that by the NCEP-DOE.There are interdecadal differences in simulation ability of the occurrence frequency of the UB in winter:the occurrence frequency of UB is overestimated and underestimated before and after 2002,respectively.The accuracy and empty prediction rate of the simulation of the occurrence frequency of UB before 2002 are better than those after 2002.Through a variety of quantitative comparison,it is determined that there is a better simulation effect on UB by the CanESM5,EC-Earth3,FGOALS-g3 and IPSL-CM6A-LR in winter.The above four models also reproduce activity of planetary and Rossby waves well.The prediction of possible changes of the UB and their impact on weather and climate in East in winter during the 21st century by using the above four model data is as follows:the UB decrease in frequency,move southward,weaken in intensity,shortens in duration,and decrease in the CBI in winter of the 21st century.However,the relationship between the UB and East Asian winter monsoon will be closer under SSP5 8 5 in the 21st century.