Spatiotemporal Features Of Extreme Cooling Events In East China And Their Relationships With Arctic Oscillation During Boreal Winter Half Year

Extreme cooling events are common extreme weather events in China during winter,which often have significant impacts on national economic activities and people's lives.It is significant for us to deepen understanding the changes of extreme weather and improve our prediction capabilities with research on the spatiotemporal characteristics and causes of extreme cooling events in China.Based on the daily mean data including air temperature of Chinese stations,NCEP / NCAR reanalysis and Arctic Oscillation Index(AOI)from NOAA,the extreme cooling events during boreal winter half year were defined,their characteristics and causes and the relationships with Arctic Oscillation(AO)were investigated.Results show that despite large spatial differences in the total frequencies and average intensities of extreme cooling events,their long term trends are weak with barely changes in the past decades.Extreme cooling events occur most frequently in the south of Northeast China,most part of North China,the north of Middle China and least in the south of Middle China.The climatic average intensities appear a strong-weak-strong pattern from north to south.Significant interannual and interdecadal variabilities in the frequency of extreme cooling events are found with highest frequencies in 1965?2009 and lowest in 1983.The characteristics of circulation anomalies before and after the extreme cooling process of different regions are obvious.Before the extreme cooling event,middle-level geopotential height anomalies and low-level temperature anomalies above the mid-high latitudes of Eurasian continent showed “+”“-” “+” wave train patterns from northwest to southeast.However,when an extreme cooling event occurs,the circulation above the extreme cooling area is equivalent barotropic in the north of 40°N but baroclinic over the south of 40°N with significant northerly anomalies over the cooling area.The propagation of wave activity flux impacts the occurrence and eastward propagation of circulation anomalies before and after the occurrence of extreme cooling events.The most critical factor for the extreme cooling process in each region is the effect of dynamic cooling from cold advection,but the contributions of dynamic cooling caused by vertical movement and non-adiabatic heating to local temperature changes have distinct regional differences.The AOI in boreal winter has remarkable interannual and interdecadal variabilities.It exhibits significantly negative correlations between AOI and the frequencies and intensities of extreme cooling events East China.The slight decrease of extreme cooling events in recent decades may be related to the increase of the AOI.The change of AO phase regulates the atmospheric circulation in the mid-high latitudes of the northern hemisphere.When the AO is positive(negative),the Ural ridge,the East Asian trough,and the Siberian high are weak(strong),it appears a less(more)meridional circulation which is unfavorable(favorable)for the southward of cold air and could results in the reduction(increase)of extreme cooling events.