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大気ブロッキングの持続メカニズムについて

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概要 A new eddy-feedback mechanism, the Selective Absorption Mechanism (SAM), to explain the block maintenance is proposed. In this mechanism which is based on the vortex-vortex interaction, that is, the i...nteraction mechanism of a blocking anticyclone and synoptic eddies with the same polarity, a blocking anticyclone actively and selectively absorbs synoptic anticyclones. The blocking anticyclone thus supplied with low potential vorticity can last for a long time against dissipation. The SAM is verified by two main analyses. The first is to examine ten real cases of blocking using trajectory analysis. Trajectories are calculated by tracing parcels originating from synoptic anticyclones and cyclones upstream of blocking. Parcels starting from anticyclones are attracted and absorbed by the blocking anticyclone, whereas parcels from cyclones are repelled by it and attracted by the blocking cyclone if it exists. The second analysis is to conduct numerical experiments using the nonlinear equivalentbarotropic potential vorticity equation with some different conditions, which are in shape and amplitude of blocking and in variabilities (displacement and strength) of stormtracks. These experiments indicate that the SAM effectively works to maintain the blocking without depending on these conditions. These results show that the SAM is the general maintenance mechanism of blocking. Next, some possible applications of the SAM are mentioned. First, the SAM is applied to a quantitative estimation of the interaction between a blocking anticyclone and synoptic anticyclones in real situations. The effect of the forcing by the interaction between the blocking anticyclone and synoptic ones is estimated. The response to this forcing in a barotropic model well explains the pattern and amplitude of real blocking. Thus, the interaction with synoptic anticyclones plays an important role in the block maintenance. Second, the SAM is adapted to a block in summer when the activity of synoptic eddies is thought to be the minimum during a year. However, it is found that the variability of potential vorticity is climatologically the maximum in summer, which implies that the SAM can also be adopted to summer blocking. Then, a case study for the 2010 summertime blocking is performed for investigating whether the SAM actually contributes to the maintenance of this blocking. Through the trajectory analysis, an analysis of ensemble forecast data, and a barotropic model experiment, the maintenance of this summertime blocking is found to be also largely contributed by the selective absorption of synoptic anticyclones. Thus, even in summer, the SAM can be essential for the maintenance of blocking.続きを見る
目次 1 Introduction
2 The Selective Absorption Mechanism
3 Eddy Straining Mechanism versus Selective Absorption Mechanism
4 Case study: Trajectory analysis
5 Numerical experiments
6 Discussion
7 Toward quantification of the SAM
8 Application of the SAM to summertime blocking
9 Conclusions and remarks
Acknowledgments
Appendix A : Derivations of modon, rider, and spherical modon solutions
Appendix B : Linear stability analysis on a β-plane channel model
Appendix C : Method of rotation along the great circle
Appendix D : Note on the PV-inversion technique for isolating disturbances
References
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登録日 2013.07.12
更新日 2020.10.09

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