Influence Of Arctic Sea-ice Variation On Northern Hemispheric Weather And Climate And The Role Of Stratosphere-troposphere Coupling

Influenced by global warming,Arctic sea-ice has been continuously declining.As one of the most important components of cryosphere,Arctic sea-ice loss could not only greatly influence the polar biosphere,but could also have some impacts on global weather and climate.Using reanalysis datasets and climate model,this thesis analyses the influence of Arctic sea-ice loss on Northern Hemispheric winter weather and climate,and reveals the potential role of stratosphere-troposphere dynamical coupling in this Arctic-mid-latitude connection.Moreover,this thesis also predicts the possible influence of future 2-degree-global-warming regional sea-ice loss on the Arctic stratospheric polar vortex(SPV)using the model data provided by Coupled Model Intercomparison Project Phase 6(CMIP6)-Polar Amplification Model Intercomparision Project(PAMIP).The major conclusions of the thesis could be summarized as follows:Firstly,the possible influence of Arctic sea-ice loss on the vertical structure of Arctic warming and the potential role of stratosphere-troposphere coupling are analysed.Results show that there exists a deep warming in winter over Arctic throughout the whole troposphere associated with the sea-ice loss over Barents-Kara-Seas(BKS).The undelying mechanism is further investigated and the results show that BKS sea-ice loss could enhance the upward propagating waves,leading to the wave convergence anomalies in the sub-polar upper troposphere and lower stratosphere.The consequent eddy feedback leads to the clockwise residual overturning anomalies in the sub-polar mid-to-upper troposphere,inducing the anomalous descent and consequent adiabatic warming in the polar mid-to-upper troposphere.Using climate model and “nudge” technique to suppress the stratosphere-troposphere coupling processes caused by sea-ice loss,only the Arctic lower troposphere is warmed in response to sea-ice loss.This indicates that stratosphere-troposphere coupling plays an important role in the Arctic deep warming induced by sea-ice loss.This thesis further quantifies the relative contribution of tropospheric and stratospheric pathways to the winter Eurasian cooling induced by Arctic sea-ice loss and analyses the underlying mechanisms,respectively.Results show that the winter Eurasian cooling induced by the reduced sea-ice centers over eastern Asia and northern Europe,and tropospheric and stratospheric pathways contribute roughly equally to the Eurasian cooling.In the tropospheric pathway,the weakened meridional temperature gradient over the northern Eurasia due to the reduced sea-ice increases the Ural blocking frequency and enhances the Siberian High.The enhanced Siberian High favors two streams of cold air-mass,reaching northern Europe and eastern Asia,respectively.In the stratospheric pathway,the enhanced upward-propagating planetary wave 1 causes the shift of the SPV towards Eurasia and the extension of vortex edge towards northern Europe and eastern Asia.The positive potential vorticity anomalies over the two regions induce the tropospheric cyclonic anomalies below through potential vorticity conservation,further enhancing the surface cold anomalies over the two regions.This thesis further analyses the impacts of Arctic sea-ice reduction on the East Asian Trough(EAT)and the underlying mechanisms.Results show that the BKS sea-ice reduction leads to a deepened EAT in late winter(especially in February),while the EAT axis tilt is not sensitive to the BKS sea-ice reduction.Results from a linearized barotropic model and Rossby wave energy ray tracing model reveal that long Rossby wave trains stimulated by the BKS sea-ice loss propagate downstream to the North Pacific,strengthening the EAT.In addition,the BKS sea ice reduction enhances the planetary waves entering the stratosphere and further weakens the late-winter SPV.The circumpolar westerly in the upper troposphere is decelerated and shifted equatorward.According to the theory of wave-mean flow interaction,the poleward transport of eddy momentum flux is also shifted southward.Hence,tropospheric westerly maintained by eddy momentum is also decelerated and shifted southward,inducing the cyclonic anomalies over the North Pacific and the deepening of the EAT in late winter.Finally,this thesis predicts the possible influence of future2-degree-global-warming regional sea-ice loss on the Arctic SPV,using the multi-model large ensemble simulations provided by PAMIP project.Results show that BKS sea-ice loss could weaken the SPV during winter(November-February)through enhancing the upward propagating planetary wave-1,while sea-ice loss over Okhotsk Sea could strengthen the SPV in late winter and early spring(January-April)through weakening the upward propagating planetary wave-2.Further analysis shows that the divergent responses of SPV across different models can be well explained by the divergent planetary-scale wave responses to BKS sea-ice loss,but not so for Okhotsk sea-ice loss.Moreover,this thesis also found that the frequency of Arctic stratospheric sudden warming(SSW)increases(decreases)associated with the future sea-ice loss over BKS(Okhotsk).The life cycle of SSW event is not sensitive to the future sea-ice loss over the two regions,but Okhotsk sea-ice loss could enhance the downward migration of SSW signal.