In one of my last posts I promised more information about atmospheric blocks. I have gotten quite into the matter of these interesting obstacles during my work so far. What are they, what are they blocking, why are they interesting? What is an atmospheric block? Well, the blocking community does not really agree on one definition. So let’s start with what is nowadays considered the classical definition from Rex (1950):
Within the sector lying between 120° W and 60° E longitude and at the 500 mb level, the normal circulation pattern may be described as a relatively broad westerly current (jet) […] A particular longitudinal variation in this circulation pattern frequently appears on the synoptic chart irrespective of season. This anomaly expresses itself as follows: the flow over the North American continent and western Atlantic, which is an essentially zonal motion at a relatively high energy level, subsequently “breaks down” over the European continent or eastern Atlantic into a more cellular motion at a lower energy level. […]
A usual feature […] is the formation of a quasi-stationary warm ridge or anti-cyclone just downstream from the point of “breakdown”. […] Situations, showing the pronounced development of such a quasi-stationary anticyclone were first noted by GARRIOTT (1904), and are now commonly referred to as examples of “blocking action” […]
In other words: Here in Europe a lot of our synoptic-scale (=large-scale; ~1000 km) weather is influenced by westerly (=from the west) flow. Alongside the so called storm tracks, cyclones (=low-pressure systems) move into Europe and bring weather fronts with them. During a blocking event in the classical sense the cyclones are blocked from moving eastwards by an anti-cyclone (=high-pressure system). In Europe blocking is preferentially located west of the British Isles over the north-eastern Atlantic or over Scandinavia. Different subtypes of blocks can be distinguished depending on the relation of the high and low pressure fields. A blocking high just to the north of a low pressure system is termed Rex block (Fig. 1).
A so called Omega consists of a high pressure system in the north and two low pressures systems on the southern flanks. In the pressure surface they look similar to the Greek letter Ω. Fig. 2 shows an idealized Omega Block and Fig. 3 shows a real blocking event in summer 2010 over eastern Europe and Russia as derived from satellite data. Reddish colors indicate higher pressures, bluish colors lower pressures. (In case you are wondering why pressure-systems are indicated in meter you can click here.)
So these arrangements of highs and lows influence the weather in Europe quite significantly. So far, so unexciting. Why are they interesting? Let’s add one more aspect by looking at the word quasi-stationary: blocking anti-cyclones do not move with the normal westerly flow — they disrupt it. For up to several weeks. These stationary conditions are very favorable for the development of all kinds of extremes which can severely affect weather and thus society. Pfahl and Wernli (2012) showed that up to 80% of summer hot temperature extremes in northern Europe are associated with blocking. Buehler et al. (2011) connected winter cold spells with blocking. More specifically single extreme events like the Russian heatwave, the severe flooding in Pakistan, and the cold European winter all in 2010 have been connected to blocking.
Fig. 4 depicts a time series of temperature anomalies throughout the atmosphere at the location of the blocking from Fig. 3. While Fig. 3 shows the entire northern hemisphere Fig. 4 only shows the average of a small region. The 500 hPa level is indicated in Fig. 4 as dashed line, the times signs on the line mark days with blocking. During the Russian blocking high pressure systems were affecting the region for almost 2 months in total. The build up of heat in the atmosphere is clearly visible towards the end of the time series, were the temperature anomaly on the surface reaches 10 K (i.e., it was 10K or 10°C warmer than normal). In the troposphere the temperature is also more then 2 K warmer than normal. The black line indicates the tropopause (transition between troposphere and stratosphere at about 10 km altitude). In the stratosphere temperatures during blocking are actually between 2 K to 4 K colder than normal.
The severe impacts of these extreme events have raised the interest in blocking in the last years. Recently, the first dedicated workshop on atmospheric blocking was held in Reading, UK. It brought together most of the experts in the field and I was fortunate enough to be able to join them. This brings me back to the beginning: confronted with question of what could be a generally accepted blocking definition soon a consensus was reached: there is none nor should there be one.
And this brings us to the fundamental issue with blocking: so far nobody really knows why blocks do what they do (which is chiefly: not move). Probably connected to that also current NWP (=numerical weather prediction) models don’t do the best of jobs in getting blocking right (neither do climate models nor some re-analyses by the way). With different definitions of what the essence of a block is (classical blocking of the westerly flow? a stationary anti-cyclone? a high pressure ridge that leads to extremes?) also come different indices to detect blocking in atmospheric data. The anti-cyclonic motion of air around a block leads to east winds in the west-wind zone, so the reversal of wind direction can be used to define/detect blocking. A high pressure system is visible as a local maximum in the GPH field, so gradients of GPH are a common way to find blocks. More recently potential vorticity (you really don’t have to know what that is to continue) has become a new way of identifying blocking highs. With these different ways of defining and detecting blocking come different blocking frequencies and different temporal/spatial distributions of blocking occurrence.
In short: the stage is set for a interesting field of climate science. The combination of poor predictability and process understanding with high potential impacts explains the attention blocking has received in recent years. With increasing computational power, better (mostly meaning: higher resolution) models can be run with effects on blocking representation. Pfahl et al. (2015) (paywall) recently suggested new important mechanisms for development and maintenance of blocking. And finally I can’t help but mention that we recently introduced a new data set for blocking research, based on global satellite observations.
Meanwhile there is also still a lot of research done on the blocking temperature connection especially in the prospect of climate change (e.g., Cassou and Cattiaux (2016) (paywall)). I’m currently working on connecting atmospheric blocking to European spring temperatures — a topic that recently received special relevance due to the cold spell that hit central Europe in late April 2016 and lead to severe damages in agriculture also in Austria. So in this sense there is at least one thing that is not blocked by this atmospheric feature: research interest.
 Rex, D. F., 1950: Blocking Action in the Middle Troposphere and its Effect upon Regional Climate I: An aerological study of blocking action, Tellus, 2, 196–211, doi:10.1111/j.2153-3490.1950.tb00331.x.
 Pfahl, S., and H. Wernli, 2012: Quantifying the relevance of atmospheric blocking for co-located temperature extremes in the Northern Hemisphere on (sub-)daily time scales. Geophys. Res. Lett., 39 (12), doi:10.1029/2012GL052261.
 Buehler, T., C. C. Raible, and T. F. Stocker, 2011: The relationship of winter season North Atlantic blocking frequencies to extreme cold or dry spells in the ERA-40. Tellus A, 63 (2), 212–222, doi:10.1111/j.1600-0870.2010.00492.x.
 Matsueda, M., 2011: Predictability of Euro-Russian blocking in summer of 2010. Geophys. Res. Lett., 38 (6), L06801, doi:10.1029/2010GL046557.
 Galarneau Jr., T. J., Hamill, T. M., Dole, R. M., and Perlwitz, J., 2012: A Multiscale Analysis of the Extreme Weather Events over Western Russia and Northern Pakistan during July 2010, Mon. Wea. Rev., 140, 1639–1664, doi:10.1175/MWR-D-11-00191.1.
 Cattiaux, J., R. Vautard, C. Cassou, P. Yiou, V. Masson-Delmotte, and F. Codron, 2010: Winter 2010 in Europe: A cold extreme in a warming climate. Geophys. Res. Lett., 37 (20), L20704, doi:10.1029/2010GL044613.
 Anstey, J. A., Davini, P., Gray, L. J., Woollings, T. J., Butchart, N., Cagnazzo, C., Christiansen, B., Hardiman, S. C., Osprey, S. M., and Yang, S., 2013: Multi-model analysis of Northern Hemisphere winter blocking: Model biases and the role of resolution, J. Geophys. Res., 118, 3956–3971, doi:10.1002/jgrd.50231.
 Pfahl, S., C. Schwierz, M. Croci-Maspoli, C. M. Grams, and H. Wernli, 2015: Importance of latent heat release in ascending air streams for atmospheric blocking. Nature Geoscience, 8, 610–614, doi:10.1038/ngeo2487.
 Brunner, L., A. K. Steiner, B. Scherllin-Pirscher, and M. W. Jury, 2016: Exploring atmospheric blocking with GPS radio occultation observations. Atmos. Chem. Phys., 16 (7), 4593–4604, doi:10.5194/acp-16-4593-2016.
 Cassuo, C., and J. Cattiaux, 2016: Disruption of the European climate seasonal clock in a warming world. Nature Climate Change, doi:10.1038/nclimate2969.
 AGRI4CAST, 2016: JRC MARS Bulletin – Crop monitoring in Europe. European Comission/Joined Research Centre, URL https://ec.europa.eu/jrc/sites/default/files/jrc-mars-bulletin-vol24-no5.pdf.
Figures 1 & 2 are by the COMET® Website at http://meted.ucar.edu/ of the University Corporation for Atmospheric Research (UCAR), sponsored in part through cooperative agreement(s) with the National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce (DOC). ©1997-2016 University Corporation for Atmospheric Research. All Rights Reserved.
If not stated otherwise all pictures/figures are by Lukas Brunner and licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.