A local wave perspective on the dynamics of atmospheric blocking over the north atlantic and europe
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Abstract
Atmospheric blocking is a large-scale phenomenon in the mid- and high latitudes and linked to the occurrence of extreme weather. This works investigates dynamical drivers of atmospheric blocking in the North Atlantic–European region. A process-based examination of the blocking life cycle is carried out in a Rossby wave-centered framework and with a particular focus on Nakamura and Huang's “traffic jam” theory of blocking onset. Rossby wave activity and waveguides are assessed based on diagnostics local in time and space and applicable during the non-linear stages of the blocking life cycle. Ensemble sensitivity analysis, compositing and a weather regime classification support the analysis of blocked regime life cycles. A novel localized atmospheric background state is constructed and its value for diagnosing waveguides illustrated.
A 2016 winter block and multi-event composites of blocked weather regimes feature important characteristics expected from the traffic jam theory such as enhanced upstream zonal wave activity flux and nonlinear suppression of zonal wave propagation at the location of the block. The traffic jam description of block onset applies in the case study and regime analysis at least regionally. An attribution of the 2016 block onset to the traffic jam mechanism is considered possible based on the presented evidence and a comparison to the theoretical model. At the same time, the suppression of zonal wave propagation is not the dominating process for all blocked weather regime composites. While Greenland Blocking is most traffic jam-like on average, zonal local wave activity flux suppression appears to be of minor relevance for the
evolution of the European Blocking composite.
A “rolling zonalization” procedure is introduced for computing an atmospheric background state of potential vorticity that is longitudinally varying and slowly evolving without a need for temporal aggregation of information. The state's waveguide occurrence statistics reproduce known climatological storm track patterns and wave propagation properties identified from the weather regime composites in the local wave activity framework well. Meaningful waveguide properties are captured even in the presence of finite-amplitude waves and closely resemble the observed stationary Rossby wave patterns in a study of the exceptional Northern Hemisphere summer of 2010, improving on traditional diagnostics. A split regional waveguide pattern diagnosed in summer 2010 is reminiscent of fundamental background state patterns frequently assumed in blocking theories.
The separation of waves from a background state is central to wave-based frameworks for understanding the synoptic- and large-scale atmosphere. The results from this work emphasize that diagnostics for both components should be local in time and space when applied to blocking. While this requirement increases the complexity of data-driven studies, it opens new possibilities for evaluating atmospheric data in terms of wave-based conceptual models for the blocking life cycle.