Publication: Blocking and its Response to Climate Change
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2018-09
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Springer Heidelberg
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Abstract
Purpose of Review Atmospheric blocking events represent some of the most high-impact weather patterns in the mid-latitudes, yet they have often been a cause for concern in future climate projections. There has been low confidence in predicted future changes in blocking, despite relatively good agreement between climate models on a decline in blocking. This is due to the lack of a comprehensive theory of blocking and a pervasive underestimation of blocking occurrence by models. This paper reviews the state of knowledge regarding blocking under climate change, with the aim of providing an overview for those working in related fields. Recent Findings Several avenues have been identified by which blocking can be improved in numerical models, though a fully reliable simulation remains elusive (at least, beyond a few days lead time). Models are therefore starting to provide some useful information on how blocking and its impacts may change in the future, although deeper understanding of the processes at play will be needed to increase confidence in model projections. There are still major uncertainties regarding the processes most important to the onset, maintenance and decay of blocking and advances in our understanding of atmospheric dynamics, for example in the role of diabatic processes, continue to inform the modelling and prediction efforts. Summary The term ‘blocking’ covers a diverse array of synoptic patterns, and hence a bewildering range of indices has been developed to identify events. Results are hence not considered fully trustworthy until they have been found using several different methods. Examples of such robust results are the underestimation of blocking by models, and an overall decline in future occurrence, albeit with a complex regional and seasonal variation. In contrast, hemispheric trends in blocking over the recent historical period are not supported by different methods, and natural variability will likely dominate regional variations over the next few decades.
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© The Author(s) 2018. This paper was stimulated by an international workshop on atmospheric blocking in April 2016 with over 100 participants. Giacomo Masato instigated the workshop and it was taken forward by members of the WMO World Weather Research Programme’s WG on Predictability, Dynamics and Ensemble Forecasting (PDEF) with sponsorship from WCRP/SPARC, IAMAS/ICDM, NCAS-Climate, ERC, and the Universities of Bern and Reading. We thank Ben Harvey as the local organiser for the workshop, hosted by the University of Reading, and Ted Shepherd for his ERC support. We would like to acknowledge David Barriopedro for the analysis presented in Figs. 2, 3, and 4, and to thank two anonymous reviewers for their helpful comments. Reanalysis data was kindly provided by NCEP/NCAR, EMCWF and JRA. We acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modelling groups for producing and making available their model output. For CMIP the U.S. Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. TW was supported by UK NERC grant NE/ N01815X. JS is supported by the Norwegian Research Council funded project ClimateXL (grant no.243953). OM received funding from the Swiss National Science Foundation Grant Nr. 200021_156059. S.-W. Son is supported by the Basic Science Research Program through the National Research Foundation (NRF) funded by the Ministry of Science and ICT (2017R1E1A1A01074889).