Effects Of The Quasi-Biennial Oscillation And Its Zonal Asymmetry On East Asian Climate

The quasi-biennial oscillation（QBO）is the dominant mode of interannual variation in the tropical stratosphere,making the dynamic coupling between the stratosphere and troposphere via changing the atmospheric circulation and stratospheric thermal structure.It also has an essential impact on global weather and climate.Therefore,this study focuses on analyzing the spatial-temporal characteristics of the QBO and its zonal asymmetry and examines their influence on the East Asia climate,aiming to understand the physical mechanism of the climate variation and providing a reliable diagnostic for improving East Asian climate prediction and skills.Since QBO is a relatively regular downward propagation phenomenon,reasonably representing QBO characteristics,especially considering its vertical propagation structure,is the key to understanding QBO changes and effects.Thus,this thesis uses the Complex Empirical Orthogonal Function（CEOF）method to establish a new QBO index,which can identify the propagation patterns of the QBO for phase division.The new index can quantitatively feature the QBO amplitude,phase,period,and downward propagation speed.It provides a helpful supplement for relevant QBO monitoring,prediction,and studying its impacts.In addition,this thesis uses observation and reanalysis data to reveal the asymmetric zonal distribution of equatorial stratospheric zonal winds and systematically analyzes its basial characteristics,impacts on the East Asian climate,and involved physical mechanisms.This thesis also comprehensively and quantitatively simulates and evaluation of the temporal and spatial characteristics of the QBO vertical structure in present climate models and projects QBO future changes and its dynamic variation,the results of which provide a reliable reference for improving the simulation ability of climate models to the stratospheric QBO signal and effectively predicting variations of stratosphere responding to the climate change.The main conclusions are as follows:（1）ERA5 best features the QBO spatial and temporal properties among all the reanalysis data and has a high phase consistency with the observation.It also reflects the vertical difference of the QBO amplitude trend,showing an increasing trend of the upper QBO amplitude but a decreasing trend in the lower levels.The results of momentum budget analysis show that the total advection term tends to damp the QBO,thereby weakening the QBO.On the contrary,the resolvable wave forcing term mainly promotes the downward propagation of the QBO and maintains its amplitude.（2）Spatio-temporal characteristics of the QBO can be reasonably captured by the first complex empirical orthogonal function（CEOF）and its derived time series.With this method,alternate variations of the typical barotropic（easterly/westerly wind）and baroclinic（easterly/westerly wind shear）components in the QBO can be adequately reflected by decomposing the QBO downward propagation structure in terms of the CEOF-based diagnostics.The QBO properties（amplitude and downward propagation speed）have a clear phase asymmetry and are larger in the westerly downward propagating phase than the easterly one.They show the largest difference between the two shear phases and their differences are about 7.8 m s^-1 and 0.38 km month^-1.These properties also feature a clear seasonality and are larger in boreal spring-summer but smallest in winter with the difference about 3.1 m s^-1and 0.25 km month^-1,which is much truer for the baroclinic component of the QBO than the barotropic one.Compared to those existing QBO indices,the CEOF-derived index can provide the representative QBO stages,much closer to realistic values,and thus better obtain the stable typical spatial structure of the QBO,contributing to a unified phase division for various QBO stages.In terms of the CEOF-based phase division,the QBO shows a strong modulation on the Arctic Oscillation and Madden-Julian Oscillation activities.The evolution of the QBO phase reveals a connection with the El Ni?o-Southern Oscillation.（3）Both the observation and reanalysis data illustrate that the equatorial stratospheric zonal winds have zonal asymmetry（QBOa）,where the difference increases with height.The most significant variation is in summer but relatively stable in autumn.In addition,the spring QBOa index is highly correlated with the summer precipitation in China,resulting in an opposite distribution of precipitation anomalies on the north and south sides of the Yangtze River and Huaihe River Basin.The effects of QBOa phases are opposite,but the intensity difference is slight.In the positive（negative）phase,the southern China region produces a robust descending（ascending）motion and the divergence（convergence）of water vapor flux,resulting in a reduced（increased）water vapor.In contrast,the northwestern China region presents a significant ascending（descending）motion,and water vapor is wetter（dryer）.The reasons for differences between the north and south precipitation are different.The large-scale circulation indicates the largest difference,especially the influence of the vertical water vapor transport in the south China region is very significant,while the dynamic and nonlinear terms have the greatest impact on the vertical water vapor transport.（4）We evaluated CMIP6 models with QBO properties,and it reveals that current climate models have differences in simulating the vertical variation of the QBO amplitude.However,they can capture the main propagating structure and spatio-temporal characteristics of the QBO.Under climate change,most models indicate that QBO shows a significantly weakened trend in the future,and its strength is more robust with the increase in emissions.The strengthened tropical upwelling causes it in the lower stratosphere in the future,which results in a robust damping effect on the QBO,significantly weakening its lower parts.On the other hand,in the future,the upper troposphere will be more stable,which leads to less convective activity,making the reduction of vertical wave flux from the troposphere to the stratosphere.Therefore,weakened vertical wave forcing will not keep the strength of the QBO downward propagation and maintain its amplitude.