FEBRUARY 2012 COLD SPELL

The cold spell in the beginning of February 2012 had a strong impact on society throughout Europe. It confronted many countries with bitterly cold temperatures, and was accompanied with strong snow fall over Balkan areas. The cold spell came as a surprise as the Netherlands had experienced record high temperatures during December and January. The onset of the cold spell and its relation to atmospheric blocking is illustrated by means of the two animations.

Circulation and Temperature anomalies in January 2012. A good illustration of the time-scale and temperature impact of atmospheric blocking can be given by showing maps of pressure and temperature anomalies. In the figure, the contours display the sea-level pressure, while the shading indicates temperature deviations from 1989-2011 DJF mean. By clicking on the image, an animated gif should start in a new window. As one can see, the first two weeks of January, there were no indications in the European area that a cold spell was getting closer. However, from 20 January onwards a large pool of very cold air moved southward, and was waiting for the blocking high to move Westward, to hit Europe. By the beginning of February, a very extensive area within Europe was more than 9.5 degrees below DJF average. This marked the onset of the cold spell that lasted for nearly two weeks, and came in the news frequently. Data source: ECMWF

Generalized Tibaldi-Molteni blocking index in January 2012. Diagnosing atmospheric blocking objectively from weather maps is not unambiguous. There are a number of metrics used to identify atmospheric blocking. In the above figure we use a two-dimensional generalization of one of the most commonly used blocking metrics, devised by Tibaldi and Molteni (1990). This index identifies large-scale reversals of the geopotential height field at 500hPa (basically requiring upperlevel Easterlies). A five-day running average value is taken (0 means not blocked, 1 means blocked). By clicking on the image, an animated gif should start in a new window. Relevant to the cold spell in Europe appears to be an atmospheric block that travelled from East Siberia towards Europe. It gradually expanded in size. Data source: ECMWF

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