The Impact Of Autumn Arctic Sea Ice On East Asian Wintertime Transient Eddy Activities

The Arctic is the region with the strongest warming under the background of global warming.With the unusual warming of the Arctic and the accelerated melting of Arctic sea ice in recent years,its climate variability has increased and its impact has become stronger,which induces more effects to the weather and climate in the northern hemi-sphere,especially on weather extremes.However,due to the uncertainty brought by different definitions,discriminant criteria and analytical methods of the weather ex-tremes and circulation changes,the impact of Arctic climate change on the middle and low latitudes,and its dynamic processes and mechanisms are still the focus and diffi-culty of current climatic researches.This paper investigates the effects of Arctic sea ice on East Asian winter atmosphere and the mechanisms from the perspective of tran-sient eddy energy.Based on the Arctic sea ice concentration data from HadlSST1 and NSIDC and the atmospheric reanalysis data from NCEP/NCAR and ERA-Interim,the coupled relationship between summer and autumn Arctic sea ice and East Asian winter transient eddy activities from 1979 to 2014 was first analyzed.And the possible effects and mechanisms of autumn sea ice on East Asian winter transient eddy activities were discussed in detail.Afterwards,through the analysis and comparison of the historical scenario simulations of 15 CMIP5 models,the Arctic sea ice,winter transient eddy activities and their relationship were assessed and the key process in the influence of Arctic sea ice on East Asian winter transient eddy activities was detected.The main conclusions of the paper are as follows:1.There exists a significant connection between the autumn Arctic sea ice and the East Asian winter transient eddy activities on the interannual time scaleThis paper first analyzes the relationship between East Asian winter transient ac-tivity eddy field(represented by transient eddy kinetic energy,EKE)and monthly aver-aged Arctic sea ice concentration from July to December through S VD(singular value decomposition)analysis.Meanwhile,the EKE and SIC fields are divided into three groups,including the raw fields,the linearly detrended fields and the interannual incre-mental fields.It is found that the detrended fields and interannual incremental fields show similar coupled relationship between the summer and autumn(especially Octo-ber)Arctic sea ice and the subsequent East Asian wintertime transient eddy activities on the interannual time scale,i.e.with the reduction of October sea ice in Barents,Kara,Laptev,and East Siberia Seas,the East Asian winter 250 hPa EKE increases signifi-cantly over the vicinity of the polar region,the north of Balkhash Lake and Lake Baikal in northern Eurasia,and weakens over the Black Sea,Iranian plateau and the southeast of Tibet Plateau.The SVD analysis of the raw fields shows that a significant correlation between the Arctic sea ice and the transient activities exists only for November and De-cember sea ice,and is different from the results of detrended fields.With the reduction of November and December sea ice in Barents and Kara seas,the EKE decreases over northern Eurasia,including Northeast Asia.Further analysis suggests that the above correlation between the November sea ice and winter EKE in the raw fields represents their linear trend over the past 36 years intrinsically instead of their relationship on the interannual time scale.2.The possible mechanism of the impact of autumn Arctic sea ice on East Asian winter transient eddy activities on the interannual time scale is revealedWith the reduction of October Arctic sea ice,the winter EKE and eddy available potential energy(EAPE)over northern Eurasia increase significantly,and the region of the increased EAPE is more extensive.The increase of the low-frequency eddy en-ergy of 10 to 90 days time scale,including sub-monthly time scale of 10 to 30 days and sub-seasonal time scale of 30 to 90 days,plays a major role,while high-frequency eddy energy of 2 to 10 days decreases over northern Eurasia.Further analysis of mech-anism indicates that there exists an increase of low-frequency waves over the vicinity of Barents and Kara Seas and a southward propagation of low-frequency waves from the polar regions to the northern part of Eurasia in winter when the Arctic summer and autumn sea ice is significantly less in that year.With the more meandering background flow and stronger meridional transport,the southward low-frequency eddy energy fur-ther enhances,corresponding to the increase of low-frequency transient activities over the north of Balkhash Lake and Lake Baikal in northern Eurasia,and the more occur-rence of persistent weather extremes.At the same time,with the warming of air aloft sea and land near Arctic Circle,the low-level baroclinicity weakens in the northern part of Eurasia,which may explain the decrease of the transient eddy activities on synoptic time scale and the reduction of the fast moving weather events.3.The key role of the simulation of the low-frequency eddy in simulating the relationship between Arctic sea ice and East Asian transient activities is discoveredThe assessment of the 15 CMIP5 models indicates that most of the models can simulate the climatological distribution of sea ice area from the Barents sea to the East Siberia sea in summer and autumn well,but the simulated sea ice reduction trend is larger than the observation in general.A statistical analysis of the spatial distribution of the interannual variation of the East Asian winter 250 hPa EKE shows that the simu-lation of the low-frequency EKE is slightly better than that of the high-frequency EKE in some models,e.g.ACCESS1.3,GFDL-CM3,GFDL-ESM2M and HadGEM2-CC can simulate the regional interannual variation of the low-frequency EKE well.The as-sessment of the coupled changes between autumn Arctic sea ice and East Asian winter EKE shows that MIROC-ESM-CHEM,MPI-ESM-LR,etc.have good skills in sim-ulating the spatial distribution of sea ice in the leading SVD mode,GFDL-ESM2M and HadGEM2-CC have good skills in simulating the spatial distribution of EKE in the leading SVD mode.Combining the respective assessment of sea ice and EKE,it is found that the model that can better simulate the interannual variation mode of low-frequency EKE in East Asia can better reproduce the response of East Asian winter EKE to the previous autumn sea ice changes.Further analysis of the low-frequency eddy energy balance equation shows that the simulation of the low-frequency EKE mainly depends on that of the barotropic conversion process(referred to as BT—the conversion of kinetic energy from time mean flow to low-frequency eddy),the sim-ulation of low-frequency EAPE mainly depends on that of the baroclinic generation process(referred to as BG,the conversion of potential energy from time mean flow to low-frequency eddy).The results indicate that improving the simulation of these two processes may be an effective way to improve the simulation of low-frequency transient activities in the atmosphere and further help ameliorate the response of atmosphere to the changes of Arctic sea ice in CMIP5 models.