Interannual And Interdecadal Variations Of The Extratropical Anticyclones Over Eurasia During Boreal Winter

Anticyclones are important components of weather systems.The anticyclone's occurrence frequency is related to the regional and global weather,as well as climate change.Eurasian extratropical anticyclones may affect the winter weather and climate over China.The study on extratropical anticyclones is of great significance for safeguarding the people's daily life and production and understanding the atmospheric movement and its change rule.Based on the NCEP/NCAR reanalysis data from 1948 to 2013,this study used an improved objective computer program to identify and track the winter anticyclones in the Northern Hemisphere.A database of extratropical anticyclones was created for the investigation of the climate and long-term climate change of the anticyclones in Eurasia.The interannual and interdecadal variations of their position and intensity were analyzed by using baroclinic front theory.At the same time,the characteristics of the interdecadal variation of the anticyclones over the Mongolian Plateau and their impacts on the winter surface air temperature over East Asia were also investigated.The main results are as follows:(1)The objective method can identify and track the anticyclones and determine the occurence time,the position and central pressure of the anticyclone based on the local maximum sea level pressure and the anticyclonic circulation of the surface wind field.A database of extratropical anticyclones in the Northern Hemisphere was created for the investigation of the temporal and spatial distribution characteristics.As for the spatial variation,the high-frequency anticyclonic activity areas are in the eastern part of the North Pacific,the Northeast Atlantic,east of the Rocky Mountains and the Asian region.Over the Eurasian and North American continents,the anticyclones are more concentrated and located in the higher latitude than the North Pacific and the North Atlantic Ocean.Anticyclones are mostly formed in high latitudes,and maily moved eastward or southeastward.As for the temporal variation,the activities of the anticyclones in the Northern Hemispshere have significant interannual and interdecadal variations,and have different temporal variation characteristics in different regions.The occurence frequency of anticyclones showed a significant period of 2-3 years in most part of the Northern Hemisphere.In North Atlantic,North America and Eurasia,during the stonger years of the anticyclones,their occurrence frequency becomes lower.In North Atlantic,North America and North Pacific,the movement trajectories of stronger anticyclone are located further northward.In North America and North Atlantic,the stronger the anticyclones become,the larger latitudinal ranges their activities have.In addition,less than 10%of total anticyclones can last for more than one week.(2)The climatic characteristics of the anticyclones,especially the strong anticyclones in Eurasia were investigated.Most(strong)anticyclones were formed over the Mongolian Plateau and Iranian Plateau.The distribution of the maximum occurrence frequency is quite similar to that of the major anticyclone genesis location.Major activity areas of anticyclones correspond to the areas of large meridional gradients of temperature in the lower troposphere(850hPa)and in front of the upper-level ridges.There is a close relationship between the intensity and displacement of the anticyclones except for those originating from the Mongolian Plateau and northeastern Siberia.Stronger anticyclones are more likely to live longer.(3)Interannual and interdecadal variabilities of the occurrence frequency and intensity of the anticyclones are significant in Eurasia.The high occurrence frequency areas of the extratropical anticyclone over Eurasia during boreal winter are the areas with large climate variability of the anticyclones,especially the anticyclones over the Mongolian Plateau and the Iranian Plateau are most active.The occurrence frequency and intensity of anticyclones have significant periods of 2-4 years as well as of 16-30 years for the occurrence frequency.The occurrence frenquency distribution of the anticyclones over the Mongolian Plateau and Iranian Plateau during the period of 1948-2013 exhibits a significant dipole-like pattern,with an increasing trend in the north and a decreasing trend in the southern part.The interdecadal variations of the occurrence frequency of the anticyclones over the Monglian Plateau and the Iranian Plateau are significant,particularly in lower latitudes.The abrupt change of the anticyclones over the Mongolian Plateau occurred in the late 1970s.From 1948 to late 1970s,the main anticyclone activitites are active in further northward,whereas they are active in further southward during the 1980s to 2013.(4)The interdecadal variations of the anticyclone displacement and intensity of the anticyclones over Eurasia during boreal winter can be explained by the interdecadal variation of the baroclinic front position and intensity expressed by the meridional temperature gradients in lower troposphere.However,the impact of the baroclinic fronts on the anticyclones over Eurasia has significant locality.The dipole-like distribution of the occurence frequency variations of the anticyclones,with opposite signs of the variations in the northern and southern part of the Mongolian Plateau,is closely related to the meridional movement of the baroclinic frontal zone in the area of 80 °E-120 °E.However,the interdecadal variations of dipole-like distribution of the anticyclones over the Iranian Plateau can hardly be explained alone by the latitudinal displacement of the baroclinic front in the area of 30 °E-80 °E.Variations of intensity of anticyclones over the Mongolian Plateau and the Iranian Plateau are associated with the mean intensity of the baroclinic frontal intensity.(5)The surface air temperature over most part of East Asia,especially in middle and high latitudes,becomes higher during the years when the anticyclones over Mongolia are located further southward.The atmospheric circulation anomaly associated with the interdecadal variability of the occurrence frequency of anticyclones,a barotropic structure,showes weakening and contraction northwest of the Siberian high,the weakening of the trough over East Asia and is not beneficial for the formation and maintenance of the blockings over the Ural Mountains,which can be explained by the contribution of the synoptic-scale transient waves to the geopotential tendency.The intensification of the anticyclonic wave breaking over the regions from Mongolia to the Northeast China reflectes the high frequency poleward momentum flux is significantly enhanced.This leads to stronger westerly in middle and high latitudes over the north of the region,while the westerly becomes weaker in middle latitudes over the south of the region.With the coexisting action of these parameters,the southward moving polar cold air is weakened,and the warm air in middle and low latitudes can be transferred straight forwardly to the north,leading to warmer surface air temperature over most part of ast Asia.