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Atmospheric Blocking in the Northern Hemisphere
Masato, G., Hoskins, B.J. and Woollings, T. 2013. Winter and summer Northern Hemisphere blocking in CMIP5 models. Journal of Climate 26: 7044-7059.

Atmospheric blocking is a crucial dynamical phenomenon of the extratropics, where large-scale patterns develop in the atmospheric pressure field and remain nearly stationary, effectively "blocking" or redirecting migratory cyclones. They often remain in place for several days, or sometimes even weeks, causing the areas affected by them to have the same kind of weather for an extended period of time. In Europe, for example, the authors note that blocking has a huge impact on regional climate, citing Trigo et al. (2004) and Masato et al. (2012), which generally leads to "extreme cold conditions in winter and warmth during summer," citing Buehler et al. (2011).

What was done
Masato et al. analyzed characteristics of atmospheric blocking in both winter and summer throughout the latter part of the 20th century in the Northern Hemisphere, as represented by 12 models that participated in phase 5 of the Coupled Model Intercomparison Project or CMIP5, where the real-world data involved were obtained from the 40-year ERA-40 reanalysis work of Uppala et al. (2005) for the period 1958-2001.

What was learned
The three UK researchers report that in the case of winter blocking, (1) "the greatest model bias is found over Europe, where the multimodel blocking frequency is less than half of that in ERA-40," and that (2) "Atlantic blocking is overall underestimated on its southern edge," which thus suggests that (3) "most models are deficient in their representation of blocking over Europe."

In the case of summer blocking, they report that (4) "Eastern European and Russian blocking is too weak and oceanic blocking is too strong," that (5) "in the Pacific the error is mainly over the extreme subpolar region," and that (6) "there is an overall tendency for models to overestimate the Atlantic signal (80°W-0°), with an opposite behavior for the European and Asian region (0°-100°E).

What it means
In the concluding and final words of Masato et al., "all these findings suggest that there is no unique way to improve the representation of blocking," and they thus add that "further analyses will be necessary to learn more about this issue."

Buehler, T., Raible, C.C. and Stocker, T.F. 2011. The relationship of winter season North Atlantic blocking frequencies to extreme cold or dry spells in the ERA-40. Tellus 63A: 212-222.

Masato, G., Hoskins, B.J. and Woollings, T.J. 2012. Wave-breaking characteristics of mid-latitude blocking. Quarterly Journal of the Royal Meteorological Society 138: 1285-1296.

Trigo, R.M., Trigo, I.F. and DaCamara, C.C. 2004. Climate impact of the European winter blocking episodes from the NCEP/NCAR Reanalysis. Climate Dynamics 23: 17-28.

Uppala, S.M., Kallberg, P.W., Simmons, A.J., Andrae, U., Da Costa Bechtold, V., Fiorino, M., Gibson, J.K, Haseler, J., Hernandez, A., Kelly, G.A., Li, X., Onogi, K., Saarinen, S., Sokka, N., Allan, R.P., Andersson, E., Arpe, K., Balmaseda, M.A., Beljaars, A.C.M., Van De Berg, L., Bidlot, J., Bormann, N., Caires, S., Chevallier, F., Dethof, A., Dragosavac, M., Fisher, M., Fuentes, M., Hagemann, S., Holm, E., Hoskins, B.J., Isaksen, L., Janssen, P.A.E.M., Jenne, R., McNally, A.P., Mahfouf, J.-F., Morcrette, J.-J., Rayner, N.A., Saunders, R.W., Simon, P., Sterl, A., Trenberth, K.E., Untch, A., Vasiljevic, D., Viterbo, P. and Woollen, J. 2005. The ERA-40 Re-Analysis. Quarterly Journal of the Royal Meteorological Society 131: 2961-3012.

Reviewed 11 December 2013