North Atlantic Oscillation - Winter of 2009-10 in Europe

Winter of 2009-10 in Europe

The winter of 2009-10 in Europe was unusually cold. It is theorized that this may be due to solar activity. The Met Office reported that the UK, for example, had experienced its coldest winter for 30 years. This coincided with an exceptionally negative phase of the NAO. Analysis published in mid-2010 suggests it was caused by a freak combination of an 'El Niño' event and the rare occurrence of an extremely negative NAO.

However, during the winter of 2010-11 in Northern and Western Europe, the Icelandic low, typically positioned west of Iceland and east of Greenland, appeared regularly to the east of Iceland and so allowed exceptionally cold air into Europe from the Arctic. A strong area of high pressure was initially situated over Greenland, reversing the normal wind pattern in the northwestern Atlantic, creating a blocking pattern driving warm air into northeastern Canada and cold air into Western Europe, as was the case during the previous winter. This occurred during a La Niña season, and is connected to the rare Arctic dipole anomaly.

On the other side of the Atlantic both of these winters were mild, especially 2009-2010, which was the record warmest in Canada. The winter of 2010-2011 was particularly above normal in the northern Arctic regions of the country.

The probability of cold winters with much snow in Central Europe rises when the Arctic is covered by less sea ice in summer. Scientists of the Research Unit Potsdam of the Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association have decrypted a mechanism in which a shrinking summertime sea ice cover changes the air pressure zones in the Arctic atmosphere and impacts on European winter weather.

If there is a particularly large-scale melt of Arctic sea ice in summer, as observed in recent years, two important effects are intensified. Firstly, the retreat of the light ice surface reveals the darker ocean, causing it to warm up more in summer from the solar radiation (ice-albedo feedback mechanism). Secondly, the diminished ice cover can no longer prevent the heat stored in the ocean being released into the atmosphere (lid effect). As a result of the decreased sea ice cover the air is warmed more greatly than it used to be particularly in autumn and winter because during this period the ocean is warmer than the atmosphere.

The warming of the air near to the ground leads to rising movements and the atmosphere becomes less stable. One of these patterns is the air pressure difference between the Arctic and mid-latitudes: the so-called Arctic oscillation with the Azores highs and Iceland lows known from the weather reports. If this difference is high, a strong westerly wind will result which in winter carries warm and humid Atlantic air masses right down to Europe. In the negative phase when pressure differences are low, cold Arctic air can then easily penetrate southward through Europe without being interrupted by the usual westerlies, as has been the case frequently over the last three winters. Model calculations show that the air pressure difference with decreased sea ice cover in the Arctic summer is weakened in the following winter, enabling Arctic cold to push down to mid-latitudes.