| The deviations from the long-term averages of the atmospheric circulation are referred to as atmospheric circulation anomalies.In East Asia,for the anomalous weather and climate events,such as extreme low temperature,extensive warm winters,continuous rainstorms,the key precursor and one of the immediate causes are the recurrent,persistent positive and negative circulation anomalies over the Ural Mountains.To improve the prediction level of large-scale anomalous weather and climate events over East Asia,investigating the mechanism of recurrent,persistent positive and negative circulation anomalies over the Ural Mountains will be significant.So far,the persistent positive and negative circulation anomalies over the Ural Mountains,are used to be regarded as different atmospheric responses to different forcing factors.However,this viewpoint cannot give good explanations for the mechanism of positive and negative circulation anomalies.In this thesis,the author tries to explain the mechanism of positive and negative circulation anomalies over the Ural Mountains by using multiple flow equilibria theory,which is a nonlinear dynamics perspective.The conventional multiple flow equilibria theory is associated with the high-index flow and low-index flow equilibria,the blocking high is considered a near-resonant low-index equilibrium.The persistent positive anomaly and negative anomaly of the Ural atmospheric circulation are usually corresponding to the blocking high and cut-off low over the Ural Mountains,respectively.The formation of the positive and negative Ural circulation anomalies is evidently local,the frequencies of occurrence for the positive and negative anomaly events are roughly equal.For these characteristics of the positive and negative Ural circulation anomalies,the conventional multiple flow equilibria theory cannot give reasonable explanations.Thus,it is necessary to develop a new multiple flow equilibria theory to explain the mechanism of the positive and negative Ural circulation anomalies.The simplified models that people used to study the multiple flow equilibria theory generally are uncoupled,that is the feedback from the atmosphere to the“thermal forcing”is ignored.To overcome this shortcoming,a simplified coupled land-atmosphere model,which consists of a two-layer quasi-geostrophic channel model and a land-atmosphere energy balance model,is used in this thesis to revisit the multiple flow equilibria in the atmosphere.Combining the theoretical study,numerical experiments and diagnosis of observations,a new theory on local multiple flow equilibria is proposed in this thesis,the theory generally explains the mechanism of positive and negative Ural circulation anomalies.The major results of this thesis are as following:?1?In the 9-variables lower-order truncated system of the coupled land-atmosphere model,the same solar irradiance and orography forcing may produce two stable equilibria with opposite wave phase.In the lower-layer flow fields of the model,one?the other?equilibrium is characterized by ridges?troughs?over the mountains and large-scale easterlies?westerlies?,hence,called ridge-type?trough-type?equilibrium.The ridge-type and trough-type equilibria originate from the supercritical pitchfork bifurcation of the large-scale zonal flow interacting with the orography.Therefore,these two equilibria are dual equilibrium states which are equally likely to occur.The land-atmospheric coupling greatly decreases the atmospheric baroclinicity and increases the stability of ridge-type equilibria,so that the stable ridge-type and trough-type equilibria can coexist over the realistic range of model parameters.?2?In the 630-variables higher-order truncated system of the coupled land-atmosphere model,under the same solar irradiance and orographic forcing,there may coexist the ridge-type and trough-type circulation regimes.The key features of the ridge-type and trough-type circulation regimes are generally like those of the ridge-type and trough-type equilibria,respectively.The coexistence of the ridge-type and trough-type circulation regimes are fundamentally due to the presence of the orography.Thus,the existence of the ridge-type and trough-type equilibria are verified by the results from numerical experiments of the higher-order truncated model.?3?The atmospheric circulation patterns over the Ural Mountains can be classified into three categories,namely,neutral type,positive anomaly and negative anomaly.Based on the statistical analysis on the observations of 180 winter months from 1958to 2017,the results show:i)On the daily and monthly time scales,all three Ural circulation patterns are persistent and have high frequencies of occurrence.ii)1360.9W m-2 is a critical value of the total solar irradiance?TSI?,when TSI is above the critical value,the neutral type pattern is the single circulation regime;when TSI is below the critical value,positive and negative anomaly patterns are dual circulation regimes,in addition,positive and negative anomaly events always have nearly the same frequencies of occurrence.These results suggest that there exist three equilibria of the atmospheric circulation over Ural Mountains,which follows the supercritical pitchfork bifurcation theory,the neutral type pattern represents an intermediate equilibrium,the positive and negative anomaly patterns represent two symmetric equilibria.?4?The physical mechanism study shows that the positive and negative anomaly patterns,as two circulation regimes,result from the supercritical pitchfork bifurcation of the large-scale zonal flow interacting with the Ural Mountains.Solar activity can affect the large-scale zonal flow in the lower-middle troposphere over the northern region of Ural Mountains,during the high?low?TSI months,the zonal flow tends to be weakened?strengthened?.Thus,solar activity can regulate the occurrence of supercritical pitchfork bifurcation,hence,regulating the transitions between neutral type pattern and anomaly type patterns of the Ural atmospheric circulation.Therefore,the positive anomaly and negative anomaly,as two types of recurrent and persistent circulation anomaly patterns over the Ural Mountains,can be regarded in dynamics as dual equilibrium states which are equally likely to occur.In this thesis,the classic study on multiple flow equilibria by using quasi-geostrophic channel atmospheric model is extended to the coupled land-atmosphere model,two flow equilibria theory associated with the supercritical pitchfork bifurcation is proposed,the theory generally explains the mechanism of positive and negative Ural circulation anomalies.Moreover,the important influence of solar activity on the formation of Ural circulation anomalies is also a new finding. |
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