Why was December so mild?

December was very mild for many parts of North America and Europe and the warmth has continued into 2012 [EPA]

For the last month, many parts of North America and Europe have been seeing some very mild weather. This has been great news for some, but not for the skiers. Certainly it’s been a stark contrast to last year’s heavy snow.

For the month of December, many parts have now reported that their average temperatures were well above average.

For the UK, the average was only 0.5C above the usual December temperature, but when you compare this to the year before, which was 5C below the average, you realise why it’s such a hot topic of conversation.

In Sweden, the mild weather has been more pronounced. December was classed as the one of the ten warmest in the last 250 years, and much of the country is still devoid of snow. In the capital, Stockholm, the temperatures were 3C above the norm, but in northern Sweden, the contrast was far greater; Norbotten was over 7C above the average.

Across the Alps the weather has also be very mild, with snow absent across most mountainsides until the beginning of December. For Switzerland, 2011 brought the highest average temperature since 1864.

The snow has now arrived in the Alps, but it arrived in a hurry. The various layers of fresh snow haven’t yet bonded together properly, leading to a high risk of avalanches.

Across the other side of the Atlantic, it has been very mild for North America too. Many parts of Canada had to wait very late for their first snowfall of the year and the first decent lake-effect snowstorm is only now heading across the northeastern states of the USA.

But before this, it stayed mild across many parts of the Plains for weeks. In Kansas, Topeka’s December temperature was nearly 1.5C warmer than usual, and the sun continued to shine.

The question many people are asking is why?

For a start, it’s a bit of a vicious cycle. Because it has been mild and dry, there is far less snow than usual. Some of the sun’s energy would normally be used to melt the snow lying on the ground, but because there is so little around, this energy can all go directly into warming the ground and the air.

The other reason, however, is slightly more technical: it’s thanks to the North Atlantic Oscillation.

The North Atlantic Oscillation (NAO) is a natural phenomenon, much like La Nina or El Nino. Whereas La Nina sets up in the waters of the Pacific, the NAO is centred in the atmosphere above the Atlantic.

The definition of the NAO is the difference in pressure between Greenland and the Azores. When the pressure is low over Greenland and high over the Azores, the NAO is said to be in a positive phase. This set up encourages the air to flow northeast across the Atlantic into Europe. The weather for both North America and Europe is mild.

In a negative phase, the pressure over Greenland and Azores are similar. This sets up a blocking pattern across the Atlantic. The mild weather from the southwest is prevented from getting to Europe and it also makes things cooler for North America too. The weather is prevented from flowing out of eastern parts of the States, and the cold weather lingers.

As you may have guessed, in December the NAO was in a positive phase, bring mild weather to many parts. In the winter of in 2009-10 and 2010-11 the NAO remained strongly negative, bringing cold weather with plenty of snow.

Predicting the Future

Forecasting the NAO is still the cause of headaches for meteorologists.

The La Nina / El Nino cycle is slow-moving and tends to stay in one phase for several months, but the NAO can bounce between positive and negative phases at will. Sometimes, however, if the NAO is strongly positive or strongly negative, it can get stuck, and linger in the same phase for weeks at a time.

Despite having such an acute effect on our weather, the NAO can only be forecast for two weeks in advance and even then the accuracy is not perfect. The current prediction is that NAO will become negative in the next two weeks. This could lead to a burst of rather chilly weather for both sides of the Atlantic.

If the cold weather does materialise by the middle of January, it be a shock for many after the mild December weather, but at least the skiers will be happy.