A phenomenon that is not often discussed in climate change studies is atmospheric blocking, a situation that develops when there is a stationary ridge of high pressure in the mid-latitude jet stream. This phenomenon is typically associated with unusually warm and dry weather in areas where these high-pressure ridges 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, (2) the extreme heat in Russia in 2010 and the downstream flooding in Pakistan, and (3) the cold temperatures over most of North America and Europe during December 2010. In investigating this phenomenon, Kreienkamp et al. (2010) used National Centers for Atmospheric Research 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 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 twenty-first century in order to infer whether blocking will become more or less common in the twenty-first century based on model projections. With respect to the re-analysis data, Kreienkamp 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 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 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 that can be attributed to atmospheric blocking will not increase for the Atlantic/European region during the twenty-first century. In fact, the model output suggests fewer of these occurrences and/or a shorter duration of such events.
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.
Tibaldi, S. and Molteni, F. 1990. On the operational predictability of blocking. Tellus 42A: 343–365.