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Weather and Climate Dynamics An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/wcd-2019-19
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/wcd-2019-19
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 08 Jan 2020

Submitted as: research article | 08 Jan 2020

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This discussion paper is a preprint. It is a manuscript under review for the journal Weather and Climate Dynamics (WCD).

The representation of Northern Hemisphere blocking in current global climate models

Reinhard Schiemann1, Panos Athanasiadis2, David Barriopedro3, Francisco Doblas-Reyes4, Katja Lohmann5, Malcolm J. Roberts6, Dmitry Sein7, Christopher D. Roberts8, Laurent Terray9, and Pier Luigi Vidale1 Reinhard Schiemann et al.
  • 1National Centre for Atmospheric Science, Department of Meteorology, University of Reading, Reading, United Kingdom
  • 2Euro-Mediterranean Centre for Climate Change (CMCC), Bologna, Italy
  • 3Department of Physics of the Earth, Astronomy and Astrophysics, Complutense University of Madrid, Madrid, Spain
  • 4Earth Sciences Departement, Barcelona Supercomputing Center (BSC), Barcelona, Spain
  • 5Max Planck Institute for Meteorology, Hamburg, Germany
  • 6Met Office Hadley Centre, Exeter, United Kingdom
  • 7Alfred Wegener Institute, Bremerhaven, Germany
  • 8European Centre for Medium Range Weather Forecasting (ECMWF), Reading, United Kingdom
  • 9Climat, Environnement, Couplages, Incertitudes, CECI, Université de Toulouse, CNRS, Cerfacs, Toulouse, France

Abstract. Global Climate Models (GCMs) are known to suffer from biases in the simulation of atmospheric blocking, and this study provides an assessment of how blocking is represented by the latest generation of GCMs. It is evaluated (i) how historical CMIP6 (Climate Model Intercomparison Project Phase 6) simulations perform compared to CMIP5 simulations, and (ii) how horizontal model resolution affects the simulation of blocking in the CMIP6-HighResMIP (PRIMAVERA) model ensemble, which is designed to address this type of question. Two blocking indices are used to evaluate the simulated mean blocking frequency and blocking persistence for the Euro-Atlantic and Pacific regions in winter and summer against the corresponding estimates from atmospheric reanalysis data. There is robust evidence that CMIP6 models simulate blocking frequency and persistence better than CMIP5 models in the Atlantic and Pacific and in winter and summer. This improvement is sizeable so that, for example, winter blocking frequency in the median CMIP5 model in a large Euro-Atlantic domain is underestimated by 32 % using the absolute geopotential height (AGP) blocking index, whereas the same number is 19 % for the median CMIP6 model. As for the sensitivity of simulated blocking to resolution, it is found that the resolution increase, from typically 100 km to 20 km grid spacing, in the PRIMAVERA models, which are not re-tuned at the higher resolutions, benefits the mean blocking frequency in the Atlantic in winter and summer, and in the Pacific in summer. Simulated blocking persistence, however, is not seen to improve with resolution. Our results are consistent with previous studies suggesting that resolution is one of a number of interacting factors necessary for an adequate simulation of blocking in GCMs. The improvements reported in this study hold promise for further reductions in blocking biases as model development continues.

Reinhard Schiemann et al.
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Short summary
In blocking situations the typical westerly atmospheric flow in the midlatitudes is blocked by near-stationary high pressure systems. These blocking events can be associated with extremes at the surface such as cold spells in winter and summer heatwaves. Climate models have long been known to underestimate blocking occurrence. In this study, we assess the latest generation of models and find sizeable improvements in how blocking is represented, in part due to increases in model resolution.
In blocking situations the typical westerly atmospheric flow in the midlatitudes is blocked by...
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