After a weekend of heavy snow across much of the UK, everyone is asking, ‘is there going to be much let up from the cold?’ Well, at the moment it’s unlikely. The UK has been experiencing it’s coldest March since 1969 and is close to becoming the coldest month of the year so far (Philip Eden, 2013), a big comparison to last year when March was the 3rd warmest on record! Across much of Europe temperatures are at least 5 degrees below the seasonal average and across parts of Eastern Europe and western Russia, temperatures are up to 14 degrees below the norm. So what is going on?
There are a couple of reasons for this unseasonably cold March we have been experiencing and what is likely to be an unseasonably cold start to April too, but they are all interlinked. The jet stream is in the wrong position. Normally during spring, the jet stream begins to push northwards across the UK so that we are positioned south of it. However, at the moment it has remained positioned across southern Europe, just north of Spain and Italy (as illustrated by the bottom image). Due to the abnormal positioning of the jet stream the Greenland high pressure system has dominated Europe’s weather and climate.
The Greenland high pressure is always positioned close to Greenland and often just north of Iceland. It balances out with a second high pressure system across the Archipelago of the Azores, known as the Azores high. The balance between the two is known as the North Atlantic Oscillation Index (NAOI). If the Greenland high is dominating, then it is described as a negative NAOI and if the Azores high is it is a positive NAOI.
During the last few weeks the NAOI has been more negative as the high pressure across Greenland has become blocking. This has caused easterly winds to influence the UK. The winds come from the polar continental air mass from northern Europe and Russia, which is associated with cold temperatures and snow during the winter months. Since 2010 the UK has been dominated by a negative NAOI with only two short periods of a positive NAOI. This unusual behaviour has lead to the cooler and more disappointing summers over the last few years. So why is this?
There could be a number of reasons for the unusual movements of the jet stream and the strong negative NAOI. The most obvious, being climate change and the increase of greenhouse gases in the atmosphere. However, it could also be due to normal variation in the climate, there have been other occurrences similar to this in the past. At the moment it is too early to put a definite reason behind this unseasonably cold start to spring, but there will be lots of research into it as time progresses.
As for it getting any warmer in the new future, the models are still suggesting it will remain cold as the Greenland high continues to build. However, as of today there are some glimmers of hope of westerly winds beginning to push back into the UK in mid-April bringing some milder and more normal spring conditions to the UK.