Attribution Of Winter Temperature And Precipitation Change Over Mid-to-high Latitude In Northern Hemisphere

Surface air temperature and precipitation are the two fundamental variables in climate,but play the great roles in climate change.Changes in temperature and precipitation not only affect the climate and ecosystem,but are closely related to human activities.On the century scale,surface air temperature is majorly characterized by warming trend,but the warming trend differs significantly by season and region,the most significant warming is observed in cold season over the arid and semi-arid regions in mid-to-high latitude of Northern Hemisphere.On the short time period,however,it shows obvious decadal(multi-decadal)variability.The representative example is the global warming hiatus in the 21~stt century.In the warming climate,the precipitation has changed with the significant difference by regions.Climate change is not only influenced by anthropogenic forcing,the internally generated atmospheric circulation variability also makes contribution,particularly at local and regional scales.However,due to the internally generated variability,estimation of anthropogenic influence on climate change in observation is a challenge.In this study,we applied dynamical adjustment to the temperature and precipitation datasets,in which the sea level pressure was employed as the atmospheric circulation indicator.Dynamical adjustment could separate the circulation induced component of temperature(precipitation).Subtracting the circulation induced dynamical component,the residual part is attributable to the thermodynamic influence,which is dominated by human activities.Firstly,we explored the roles of anthropogenic forcing and atmospheric circulation change in the enhanced warming over the arid and semi-arid regions in mid-to-high latitude of Northern Hemisphere by dynamical adjusting observed temperature datasets.Then,by combining observation and model simulations,we examined the contribution of internally generated atmospheric circulation,the sea surface temperature(SST)anomaly in the tropical Pacific ocean and the external radiative forcing to the recent global warming hiatus,especially revealed the causes of the regional cooling of winter temperature during hiatus.Finally,the impact of atmospheric circulation on the winter precipitation changes over the mid-to-high latitude continents in Northern Hemisphere in the past~100years was examined,and thus the anthropogenic influence on the precipitation change was estimated.The main conclusion are summarized as following:(1)In the past century scale,the atmospheric circulation induced dynamical temperature shows obvious interannual and decadal variability,while the thermodynamic temperature dominate the warming trend of surface air temperature in the past~100 yeas.From the view of spatial distribution,dynamical temperature trend shows regional warming and cooling with relatively uniform amplitude.However,the thermodynamic temperature shows warming over the study area,but the warming trend is largely varied by regions.The thermodynamic temperature warming trend is significantly higher in the arid and semi-arid regions than humid regions,but the dynamical temperature exhibits uniform trend over dry and humid regions with small amplitude.The contribution of thermodynamic warming in arid and semi-arid regions to the whole warming of the study area is up to 65%,which is much higher than the contribution of dynamical temperature.Therefore,the enhanced winter warming in arid and semi-arid regions over the mid-to-high latitude of Northern Hemisphere is dominated by the thermodynamic temperature,implying the important role of anthropogenic forcing in the enhanced drylands warming.(2)The recent warming hiatus is characterized by regional cooling trend in winter temperature.There are three cooling centers:the strongest cooling is observed over Eurasian continent,with the cooling trend up to-6?/12 years,and another two cooling centers are over North American continent and tropical Pacific ocean.We examined the warming hiatus from the regional perspective using 10-member initial-condition ensembles with global coupled climate model CESM1 in which observed tropical Pacific sea surface temperature anomalies(TPAC SSTAs)and radiative forcings are specified.We found that the cooling anomalies in tropical Pacific cause part of the cooling over Eurasia and North America,but most of the cooling over North America and Eurasia is induced by intrinsic atmospheric circulation anomalies.There is an anomalous positive atmospheric circulation centered over western Eurasia and the North Pacific,which is indicative of cold air advection and causes the cooling trends over Eurasia and North America.In addition,the radiative forcing have a weak cooling trend over Eurasia,which augments the cooling over there.However,averaged over the global,external radiative forcing induced a warming trend,the intrinsic atmospheric dynamics and teleconnections forced by TPAC SSTAs caused the global mean warming hiatus,and they contribute almost equally to the slowdown trend of global mean winter temperature.(3)In the past century scale,winter precipitation changes exhibit noticeable regional discrepancy.In the northern Europe,the significant increase trend in precipitation is observed.By applying dynamical adjustment method,we found that the atmospheric circulation changes account for approximately 40%of the wetting over northern Eurasia,driven by the cyclonic SLP trends over the Northern Eurasia.The residual thermodynamic trend,which is an estimate of anthropogenic forcing,contributes about 60%to this wetting.Land around the Mediterranean shows drying trend,which is dominated by the anticyclonic SLP trends over the Mediterranean and central of the Northern Atlantic ocean.Precipitation trends over eastern North America are generally positive but statistically insignificant,because the significant drying trend induced by anticyclonic circulation trends and the significant wetting trend caused by the anthropogenic forcing cancelled out.Dynamical adjustment largely reconciles precipitation trend in observations and climate model simulations.The geographical pattern and amplitude of observed thermodynamic precipitation trend are in good agreement with anthropogenically-forced trends obtained from ensembles of historical simulation in CMIP5 and CESM1,suggesting the effectiveness of dynamical adjustment method in estimating the human influence on winter precipitation changes.