Will spring 2013 be the coldest in 30 years?
Early figures released by the Met Office suggest that spring (March, April and May) 2013 is on course to be the coldest spring in the UK since 1979.
Observed data from 1 March until 15 May has been extrapolated with an assumption of average temperatures for the rest of May, which looks likely given the latest forecast.
Whilst the figures may change slightly at the end of the month once all of the information has been processed, they still illustrate well the distinct chill we’ve experienced in the past few months.
The estimate suggests that the average temperature for this spring will be around 6.1C, which would place it as the 6th coldest spring in records that go back to 1910, and well below the normal spring average of 7.7C.
A huge influence on this year’s cold spring was an exceptionally cold March, when average temperatures were 3.3C below normal, making it the coldest in 50 years.
What was also remarkable about March this year, is that it was colder than each of the winter months – December, January and February. The last time that this happened was back in 1975.
What caused the start of spring to be so cold?
For much of March, the jet stream was stuck unusually far south of the UK, across Iberia, northern Africa and the Balkans.
This meant that our usual spring weather – warmth, rain and occasional sunshine, was steered to these parts of Europe instead, leaving us under the influence of a large blocking area of high pressure, bringing persistent spells of cold easterly winds and snow.
Although April and May haven’t been as anomalously cold, they have still been a bit colder than average, with a distinct lack of any notable warmth.
Why does the jet stream keep getting stuck?
The jet stream is driven by the temperature contrast between the cold poles and warm equator, normally rapidly moving from west to east at mid-latitudes.
However, recent years have seen a significant warming of the Arctic, leading to reduced amounts of Arctic sea ice. As a result, more heat is absorbed by the exposed oceans, rather than being reflected back to space by ice.
Subsequently, this warming of the Arctic lessens the temperature contrast between the poles and the equator, with scientists suggesting that this causes a weakening of the jet stream.
When the jet stream is weaker, it tends to meander around more and becomes prone to getting stuck in the same place for longer periods of time – bringing extended spells of the same weather and increasing the risk of extremes.
It is worth pointing that this is one of a few possible reasons for the jet stream movement, with sea surface temperatures and weather patterns in the tropics other possible influencers.
If you want regular updates about the position of the jet stream and what it means for our weather, follow me on Twitter – @liamdutton
Map graphic courtesy of the Met Office.