Variations Of The Winter North Atlantic Jet Stream In The Past 2500 Years And Its Impact On The Climate Of Europe

The North Atlantic jet stream（NAJ）is a relatively narrow band of strong westerly air flow that crosses the North Atlantic with a southwest-northeast tilt,which is the dominant atmospheric circulation over the North Atlantic-European sector.Variations of the NAJ strongly control the temperature,precipitation,and storm activity around the Atlantic Basin and beyond,particularly in winter when the NAJ and the associated storm tracks are strongest.Recent studies showed that anthropogenic warming has seriously affected the NAJ.However,it remains controversial whether the changes in the NAJ under anthropogenic warming have deviated from the natural variability.The lack of long-term records of the NAJ covering past warming periods prevents a deeper understanding of its natural variability.Furthermore,the state-of-the-art numerical models（Coupled Model Intercomparison Project Phase 6/CMIP6）fail to capture the magnitude of the observed trend of NAJ strength,thereby making it impossible to make a convincing attribution.This makes it difficult to predict the possible changes in the NAJ and their resulting climate impacts within the context of global warming.Stalagmiteδ¹⁸O from Europe may provide important information on changes in winter NAJ.This is because precipitation during the cold season accounts for the majority of the total annual infiltration into caves,and changes in the NAJ may be recorded in stalagmiteδ¹⁸O by affecting regional precipitation,temperature,and water vapor transport.In this study,we first investigate the relationship between the winter NAJ and European precipitation/stalagmiteδ¹⁸O by analyzing observed precipitationδ¹⁸O data from the Global Network of Isotopes in Precipitation（GNIP）stations,simulated precipitationδ¹⁸O data from the ECHAM5-wiso atmospheric circulation model,and 11high-resolution stalagmiteδ¹⁸O records spanning the observational period.Then we present a 2500-year-long record of winter NAJ strength based on well-dated,high-resolution stalagmiteδ¹⁸O records from southeastern Europe.This long record provides a rare opportunity to understand the impact of anthropogenic warming on NAJ strength,based on the comparison of modern warming with past warming periods.Finally,we collected 24 hydrological-sensitive records spanning the past 2000 years in Europe and discussed the impact of the NAJ on hydrological changes.These results provide a valuable basis for predicting future trends in European hydrology.Based on the above work,the main conclusions are as follows:（1）The NAJ plays a dominant role in influencing European precipitation and stalagmiteδ¹⁸O.On interannual to decadal time scale,the NAJ latitude has a more pronounced effect.On multi-decadal to longer time scale,the NAJ strength becomes the primary factor.（2）The stalagmiteδ¹⁸O records from southeastern Europe show a significant negative correlation with the winter NAJ strength.The winter NAJ strength controls stalagmiteδ¹⁸O in the region by regulating the relative contribution of water vapor between the ¹⁸O-depleted North Atlantic Ocean and the ¹⁸O-enriched Mediterranean.During years when the NAJ is strong,NAJ extends eastward and the relative contribution of moisture from the North Atlantic increases,resulting in depleted precipitationδ¹⁸O,and vice versa.Thisδ¹⁸O-NAJ relationship supports the use of stalagmiteδ¹⁸O as an index of winter NAJ strength,with more negativeδ¹⁸O values indicating stronger NAJ,and vice versa.（3）The winter NAJ weakened during both the Roman Warm Period（～300 BC–200AD）and the Medieval Warm Period（900–1250 AD）.However,the NAJ has strengthened under anthropogenic warming（since～1850 AD）,indicating that its current trend has already deviated from the natural variation.The best explanation for this anomalous trend of NAJ strength since～1850 AD is the increasing sea-surface-temperature gradient in the North Atlantic,potentially driven by the cooling trends in the subpolar North Atlantic（known as the North Atlantic Warming Hole）under anthropogenic forcing.This anomalous trend suggests that continued global warming may further strengthen the NAJ in the future.（4）The hydrological changes in Europe are largely controlled by the latitude and strength of the NAJ.The latitude position of the NAJ determines the trajectory of water vapor towards Europe,while the strength of the NAJ impacts the amount of water vapor transported.During the Dark Age Cold Period（300–700 AD）,Europe experienced relatively mild hydrological conditions,due to the moderate intensity and centered latitude position of the NAJ.During the Medieval Warm Period（900–1250 AD）,northern Europe become wetter than the south.Although the NAJ weakened during this interval,it shifted northward and primarily transported water vapor to the northern regions of Europe.Conversely,during the Little Ice Age（1450-1800 AD）,the southern Europe was wetter than the northern,as the NAJ significantly strengthened and shifted southward,leading to a large amount of moisture being transported to southern Europe.The north-south dry-wet difference increased compared to the Medieval Warm Period.（5）Within the context of the Current Warm Period,the NAJ has shown a tendency to shift towards higher latitudes,which aligns with the northerly position of the NAJ during the Medieval Warm Period.Our findings provide evidence supporting the prediction of models that the NAJ will shift northward as warming continues.These results suggest that with ongoing global warming,the NAJ may shift northward and strengthen,resulting in further increased precipitation in northern Europe and decreased precipitation in southern Europe.