Main Flow Patterns Of The Winter Stratospheric Polar Vortex And Their Relationship With Eurasian Climate Anomalie

The possible influence of changes in the Arctic stratospheric polar vortex on the extratropical troposphere especially in the mid-to-high latitudes of the Northern Hemisphere is still not well understood.Using the ERA5 reanalysis and different scenario experiments by the CESM2-WACCM model,the northern winter stratospheric polar vortex is categorized into several regimes,which mainly reflect the role of the intensity and position changes of the polar vortex in the northern hemisphere on the tropospheric circulation and near-surface climate.Considering that the weak polar vortex has a better correspondence with the continental cold anomaly,this paper focuses on the simulation and scenario project of the weak polar vortex event represented by the sudden stratospheric warming（SSW）event,and examines the downward coupling characteristics of SSW under different future emission scenarios and the future possible changes.The main conclusions are as follows:（1）As a consequence,the stratospheric polar vortex can be clustered into six groups based on the k-mean clustering algorithm,including the homogeneously-intensified（HI）,homogeneously-weakened（HW）,North America-intensified（NAI）,North America-weakened（NAW）,Eurasia-intensified（EUI）,and Eurasia-weakened（EUW）shapes.Statistics of each polar vortex clustering confirms that the yearly frequency of the HI state shows a decreasing trend in past decades,while the HW increases as inferred from the long-term trend.（2）The close relationship between the state of stratospheric polar vortex and tropospheric teleconnection is explored.The stratosphere-troposphere coupling in most stratospheric polar vortex circulation patterns is realized by the downward propagation of the North Atlantic Oscillation（NAO）or Arctic Oscillation（AO）signals.The Pacific North American pattern（PNA）is closely related to the movement of the stratospheric polar vortex towards North America.Eurasian pattern:EU is sensitive to HI and NAW modes.The positive（negative）abnormal mode of HI corresponds to the negative（positive）phase of EU pattern.The relationship between Western Pacific teleconnection（WP）and most stratospheric polar vortex patterns is very weak（except HW）.（3）The patterns of precipitation and temperature anomalies under the six polar vortex regimes are also different:the HI（HW）lower tropospheric temperature anomaly is stronger and more persistent than the NAI（NAW）regime,and the mid-latitude anomaly is larger than other regimes;HI（HW）has a strong impact on precipitation in Europe and the North Atlantic,and the dry（wet）anomaly is only formed in the middle of the North Atlantic for NAI（NAW）regime,and less in other places.It shows that the intensity and location of the stratospheric polar vortex have a significant regulatory effect on the tropospheric circulation and the near-surface climate.The results also show that the continental cold anomaly is stronger and more significant under the three types of weak polar vortex.（4）Based on the reanalysis results,the stratospheric polar vortex circulation pattern,evolution characteristics,low-level temperature and precipitation response simulated by CESM2-WACCM model,but the simulation and reanalysis of HI and EUW circulation patterns are different（EUW polar vortex is more east）.The negative and positive anomalies in the EUW tropospheric circulation mode are distributed in northwestern Russia and Alaska respectively.For the simulation,HI,NAI and EUW regimes are affected by weakened wave activity,while HW,NAW and EUI are affected by upward planetary wave.There are persistent warm（cold）temperature anomalies in Eurasia and the mid-latitude region of North America.The precipitation anomaly pattern also changes with the change of the stratospheric polar vortex morphology.（5）The typical representative of the weak polar vortex regimes is the SSW event.Using the simulation and projection of the SSW and its impact on the near surface are explored as compared with the ERA5 and NCEP/NCAR reanalyses.The SSW frequency ranges from 4 to7 per decade in CESM2-WACCM experiments,comparable to ERA5（6 per decade）.Projected relative change in the displacement and split SSWs is much more uncertain due to the underestimation of the SSW frequency in the model and uncertainty in the greenhouse gas emission pathways.Both the model and reanalysis show that after the displacement SSW,there is a symmetric banded negative height anomaly in the middle latitude and a positive potential height anomaly over Greenland and the Arctic in the short lag period of the troposphere.During the long lag period,the NAM response of the downward propagation changes slightly,and the mid-latitude negative potential height anomaly is only over the North Atlantic and Northeast Asia.After the occurrence of the split SSW,the negative NAM signal continued to propagate downward.The tropospheric response at short lag is in annular mode.During the long lag period,NAO-like negative response will appear over the North Atlantic,and no significant change is expected in the CO₂ increase experiments.（6）The prediction results of SSW show that the model underestimates the frequency of SSW and the impact of greenhouse gas emission path on it has great uncertainty,so there is great uncertainty in the projection of SSW.In all CO₂ increase experiments,the downward propagation of annular stratospheric signals at short lags associated with displacement SSWs likely reinforces,whereas the downward coupling is projected to change little at long lags for displacements and at all lags for splits.CESM2-WACCM also projects a weakening of the wavenumber-2 forcing for split SSW in the future.Enhanced tropospheric negative annular mode response is projected at short lags for displacements in all future scenarios.In contrast,the projected change in the tropospheric response to splits is zonally heterogeneous at short lags,resembling a wave train pattern with the East Asian trough deepening,but the NAO-like response to splits at long lags is not projected to change significantly.The cold pattern over North Eurasia following displacement SSWs might expand further equatorward in the future projections,whereas cold anomalies over North America following splits might enhance in the future.