Impact Of Declining Arctic Sea Ice On The Summer Air Temperature In Northern Hemisphere Midlatitudes

Rapid Arctic warming,sea ice decline,and weakening summer circulation have coincided with an increasing number of extreme heat waves and other destructive weather events in the Northern Hemisphere midlatitudes in summer.A topic of increasing interest is whether Arctic warming and sea ice loss induces midlatitude cold spells and heat waves via changes in the large-scale circulation.In particular,the hypothesis that Arctic sea ice loss remotely causes more frequent extreme hot midlatitude summer weather is not confirmed in both observation and modeling studies and the possible mechanism has instead often been criticized.Based on the data of observed sea ice,the NOAA/ERA-Interim reanalysis data of,and two models of CAM4 and ECHAM5 in 1979-2012,we study on the possible effect of declining Arctic sea ice on summer temperature and circulation in northern hemisphere through observed data.Furthermore,the effects of are verified by the model design of two groups of experiments,one including sea ice forcing and the other excluding sea ice forcing.Regressing the Northern Hemisphere sea ice index of the early autumn,winter and spring on the summer surface air temperature,the result shows that the cooling signal in Eurasia is becoming more and more significant with the reduction of sea ice.Especially in the spring,the whole Eurasian continent presents a cooling signal,with the most significant regions in the Mediterranean and the northern Eurasia,and the emergence of cooling signal in southern North America.It is suggested that the spring sea ice signal can be a leading indicator to predict summer temperatures in the northern hemisphere to some extent.The relationship between sea ice and circulation field and temperature field are barotropic.Secondly,with the reduction of sea ice in spring,the summer Z500 in Eurasia is a low,with lows located in the Mediterranean region and the southwest North America.Finally,it is inferred that the possible effect of spring sea ice on summer Z500 is to weaken the highs of Eurasia and the North American continent,which provides some basis for the hypothesis that declining sea ice can cause cooling trends in the Northern Hemisphere midlatitudes in summer.The model results shows that the reduction of sea ice has led to a wide range of cooling trends in midlatitude,especially in three regions of northern Russia,Europe and North America,and the cooling trend caused by sea ice forcing is about-0.07?/10yr to-0.11?/10yr,offsetting the warming trend caused by the warming sea surface temperature by20?30%.The cooling trend of tropospheric temperature(925hPa and 500hPa)is more significant than that of the surface.And the cooling trend may be dominated by spring sea ice variability.In concern of the circulation field,summer Z500 showed a negative phase of AO due to sea ice forcing,which means that the Arctic region significantly increased,while the a wide range of mid-latitudes declined.This suggests that sea ice loss has reduced thickness gradients between the midlatitudes and Arctic and thus weakened the summer polar vortex.The 300 hPa wind response exhibits a strengthened midlatitude jet stream from East Asia to the middle Atlantic,but weakened upper level zonal winds in the north side of the climatological position of the midlatitude jet stream in the North America and Atlantic sector.Anomalous easterly flows along much of the coastal Arctic and enhanced meridional flows at high-middle latitudes allow Arctic cold air to more frequently reach well into the midlatitudes of northwestern North America,Europe,and northern Russia.