Cold Air Outbreak - 10 to 20 December 2009

Global Circulation

From Western Europe to the midst of Siberia the winter of 2010 was a bit colder than usual. However, Greenland and in north-eastern Canada it was much less colder than normally in winter. This pattern of climate variability was caused by the Arctic Oscillation.

Figure 2.1: Temperature anomalies relative to 1971-2000 in the winter of 2010. Source: NCEP/NCAR R1 reanalysis.

Figure 2.1 show the temperature anomalies of the winter of 2009 - 2010 (December - February) relatively to the 1971-2000 normals. The regional differences were large. The northern half of Europe was 2 to 4 °C colder than the 1971-2000 average. In northeastern Canada and on Greenland the temperatures were more than 10(!) °C above normal.

This pattern of temperature anomalies is not uncommon and is caused by a well-known pattern of circulation anomalies known as the Arctic Oscillation. The pressure anomalies in the winter of 2010 are shown in Figure 2.2. In the Arctic the air pressure was higher than normal, whereas the subtropical highs in the Atlantic and Pacific were weaker.

Figure 2.2: Sea-level pressure anomalies relative to 1971-2000 in the winter of 2010. Source: NCEP/NCAR R1 reanalysis.

The high pressure situated over Iceland and Greenland caused the jet to flow south and drag cold arctic air with it south. The high pressure system north of Siberia brought cold air to the South, which continued westwards into northern Europe for most of the winter. The flow continued as warm southerly winds going North to the west of Iceland and by this explains the temperatures we observe in Figure 2.1.

The deviations that we observe in Figure 2.1 and 2.2 during the winter of 2010 are large in the fact that the AO index was the largest anomaly since the beginning of the series in 1900.

The synoptic situation during the second half of December also resembles this script with the buildup of a high pressure belt from Greenland towards Russia (Fig. 2.3).

Figure 2.3: Fig 2.3 - ECMWF Geopotential Height 1000 hPa. from 10 to 22 December 2009, 00UTC

The above animation shows the strenghtening of a high pressure system over the Brith Isles and Skandinavia that sets course towards Greenland and Iceland. This ending position of the high pressure system explains us why the temperatures were a bit colder than normal, but do not yet explain the snowfall that was observed. An answer to this is also found in the animation.

Along the southern flank of the anti-cyclone over Russia there was a continuous flow of cold and dry air over Poland, Germany, Netherlands into England. On the 16th/17th and also on the 20th depressions develop over Skandinavia. With the high pressure system over Iceland and Greenland these depressions receive a southerly impuls and as moving over the relatively warm sea water pick up moisture. As the depressions move over the colder landsurfaces they produces severe snowfall.

During the same period Mediterranean region was undergoing strong cyclonic activity. From 10 to 13 December a deep low over the eastern Mediterranean brought intensive precipitation to the Balkan peninsula. After that the whole chain of cyclones, from 14th to 21th December, accompanied with cold air from the north or north east and moist and warmer air from the south resulted in huge amounts of precipitation over the Mediterranean area.

Most of the depressions were entering Mediterranean Sea from the Atlantic ocean and moving further to the east. Some of them were deepening over Genoa bay and, accompanied with strong cold advection from the north, brought heavy precipitation - rain and often snow. That was also the case on 19th December when cold air from the north, on the leading side of the extensive blocking high with centre over Iceland and Greenland, came into the western Mediterranean. There a Genoa cyclone was developed, picking up warm and humid Mediterranean air. These two processes combined and produced heavy snow over Croatia.

The following three chapters will go more in depth to explain the synoptic and mesoscale situations leading to the snowfall in the Netherlands, England and Croatia.