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Atmospheric Blocking and Extreme Weather Events

Reference
Kreienkamp, F., Spekat, A. and Enke, W. 2010. Stationarity of Atmospheric Waves and Blocking over Europe - Based on a Reanalysis Dataset and Two Climate Scenarios. Theory of Applied Climatology 102: 205-212.
One phenomenon not often discussed in weather and climate studies is atmospheric blocking. Blocking is a stationary ridge of high pressure in the mid-latitude jet stream. These can be associated with unusually warm and dry weather in areas where they form, and cooler or wetter conditions upstream and downstream of where they occur. Some recent examples of blocking and its impact on regional weather are; 1) the Western European heat wave of 2003, or 2) the extreme heat in Russia in 2010 and the downstream flooding in Pakistan, or 3) the recent cold temperatures over most of North America and Europe during December 2010.

In the present study, Kreiencamp et al. (2010) used the National Centers for Atmospheric Research (NCEP) re-analyses to examine the occurrence of blocking events over Europe since the 1950s using a well-known blocking index (Tibaldi and Molteni, 1990). They then employed the atmospheric general circulation model (ECHAM) used by the Intergovernmental Panel on Climate Change (IPCC) in an effort to determine how well the models were able to simulate such blocking. Lastly, they examined two climate warming scenarios (A1B and B1) for the 21st century in order to infer whether blocking will become more or less common in the 21st century based on model projections. So what did they find?

With respect to the re-analysis data, Kreiencamp et al. found little evidence of a statistically significant trend over the period 1951 - 2007 apart from a weak decrease in the European region, which decrease suggests that extreme weather events caused by blocking events have probably also declined. With respect to model simulations, they found the models showed little change in the frequency, seasonality, or interannual variability of blocking for the Atlantic/ European region as a whole, but a significant decrease in the Central European region frequency. Although we are cautious about placing too much emphasis on model projections, this finding is also good news, for it suggests the number of heat waves and/or cold waves which can be attributed to atmospheric blocking will not increase in number for the Atlantic/European region during the 21st century. In fact, the model output suggests fewer of these occurrences and/or a shorter duration of such events.

Additional Reference
Tibaldi, S. and Molteni, F. 1990. On the operational predictability of blocking. Tellus 42A: 343-365.

Archived 5 January 2011