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The Connection Between The Arctic Stratospheric Polar Vortex Variability And The Anomalous Tropospheric Weather During Northern Winter

Posted on:2020-09-09Degree:DoctorType:Dissertation
Country:ChinaCandidate:J L HuangFull Text:PDF
GTID:1360330596486656Subject:Atmospheric Science
Abstract/Summary:PDF Full Text Request
The Arctic stratospheric polar vortex plays an important role in the dynamical coupling between the stratosphere and troposphere.Any changes in the strength,location and shape of the polar vortex can extend deep into the polar lower stratosphere and thence to the surface,affecting not only the tropospheric circulation patterns but also the longevity of weather patterns associated with it.Using various reanalysis data and climate models participating in the Coupled Model Intercomparison Project Phase 5(CMIP5),this thesis analyzes the connection between the anomalous Arctic stratospheric polar vortex and the tropospheric weather anomalies.The main conclusions of this thesis can be summarized as follows.1.Firstly,the variations of tropospheric blockings and their connections with the stratospheric polar vortex during different stages of the life cycle of extreme stratospheric polar vortex events(i.e.,strong vortex(SV)events and weak vortex(WV)events)are investigated.The blocking frequency decreases over the Euro-Atlantic sector and increases over the western North Pacific during the onset and maturation stages of SV events.There are more blocking days over the western North Pacific and weakened upward planetary wave fluxes in the stratosphere during the maturation stage of shorter timescale SV events than during longer timescale SV events and the weakening of the upward planetary wave fluxes is mainly resulted from the linear wave interference effect.The blocking frequency is increased over the Euro-Atlantic sector during the decline stage of SV events.This increase is found to be related to the descending stratospheric zonal wind anomalies and the poleward displacement of eddy-driven jet stream.The increased blocking frequency leads to an enhancement of the planetary wave fluxes in the stratosphere via both the linear wave interference(for shorter timescale SV events)and nonlinear wave interference(for longer timescale SV events)effect.The changes of the blocking frequency during the growth stage of WV events are almost opposite to that observed during the onset and maturation stages of SV events.The blocking frequency decreases over the Euro-Atlantic after the central date of WV events,which are related to the descending stratospheric zonal wind anomalies and the equatorward displacement of eddy-driven jet stream.The nonlinear wave interference begins to make a considerable contribution to the weakening of the upward wave fluxes in the stratosphere after the central date of WV events.2.This study analyzes the differences and similarities of Eurasian cold air outbreaks(CAOs)under the weak(CAOW),strong(CAOS),and neutral(CAON)stratospheric polar vortex states and examines the potential links between the polar vortex and Eurasian CAOs.The results indicate that the colder surface air temperature(SAT)over Europe in the earlier stages of CAOW events is likely due to that the amplitude of pre-existing negative North Atlantic Oscillation pattern is larger in CAOW events than those in CAON and CAOS events.Marked by the considerably negative stratospheric Arctic Oscillation signals entering the troposphere,the SAT at middle latitudes over eastern Eurasia in CAOW events is colder than that in CAON events.Larger diabatic heating rate related to positive sensible heat flux anomaly in CAOW events likely offsets,to some degree,the cooling effect caused by the stronger cold advection and makes the differences in area-averaged SAT anomalies over northern Eurasia between the CAOW and CAON events look insignificant in most stages.Massive anomalous waves from the low-latitude western Pacific merge over northeastern Eurasia,and then weaken the westerly wind over this region and create favorable conditions for southward advection of cold air masses in the earlier stages of all three types of CAOs.This study further analyzes the interannual relationship between the stratospheric polar vortex strength and the intensity of Eurasian CAOs and finds that CMIP5 climate models relative to the reanalysis dataset tend to underestimate the correlation between them.The relationship between them is strengthened under RCP4.5 and RCP8.5 scenarios over the period 2006-2060.In addition,the intensity of Eurasian CAOs exhibits a decreasing trend in the past and in the future.3.This study further investigates the influence of ENSO and QBO on the interannual relationship between the Arctic stratospheric polar vortex and Eurasian CAOs.The correlation analysis indicates that the correlation between the polar vortex strength and Eurasian CAOs strength is stronger under El Ni?o winters than under La Ni?a winters.In addition,the correlation between them is stronger under the easterly phase of QBO(QBO-E)than that under the westerly phase of QBO(QBO-W).When removing the winters in strong QBO-W and QBO-E,the correlation between them remains significant under El Ni?o winters.Similarly,with El Ni?o and La Ni?a winters removed,the correlation between them is still stronger under QBO-E than under QBO-W.The results of CMIP5 simulations are consistent with those of reanalysis dataset.There exist decadal changes in the annual correlation between the polar vortex and Eurasian CAOs.The influence of ENSO on the polar vortex strengthens the annual correlation between them for the period 1958-1976.The influences of both 50-hPa and 30-hPa QBO signals on the polar vortex weaken the annual correlation between them for the period 1958-1976.For the period 1998-2016,the 50-hPa QBO signal weakens the annual correlation,whereas the 30-hPa QBO signal strengthens the annual correlation.4.Using composite analysis,this study examines the preconditioning of events in which the Arctic stratospheric polar vortex shifts towards Eurasia(EUR events),North America(NA events)and the Atlantic(ATL events).An increase in blocking days over northern Europe and a decrease in blocking days over the Bering Strait favor the movement of the vortex towards Eurasia,while the opposite changes in blocking days over those regions favor the movement of the vortex towards North America.An increase in blocking days over the eastern North Atlantic and a decrease in blocking days over the Bering Strait are conducive to movement of the stratospheric polar vortex towards the Atlantic.The polar vortex shift events are further classified into events with small and large polar vortex deformation,since the two types of events are likely to have a different impact at the surface.A significant difference in the zonal wave-2 heat flux into the lower stratosphere exists prior to the two types of events and this is linked to anomalous blocking patterns.This study further defines three types of tropospheric blocking events in which the spatial patterns of blocking frequency anomalies are similar to the blocking patterns prior to EUR,NA and ATL events,respectively,and our reanalysis reveals that the polar vortex is indeed more likely to shift towards Eurasia(a 20.8% increase),North America(a 9.4% increase)and the Atlantic(a 4.3% increase)in the presence of the above three defined tropospheric blocking events.These shifts of the polar vortex towards Eurasia,North America and the Atlantic lead to statistically significant negative height anomalies near the tropopause and corresponding surface cooling anomalies over these three regions.
Keywords/Search Tags:The stratospheric polar vortex, stratosphere-troposphere coupling, stratospheric planetary wave, tropospheric blocking, tropospheric jet stream, cold air outbreaks
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