Where's winter?

  • Illustration of the North Atlantic jet stream valid for Sunday 23rd February. Image shows the enhanced upper levels winds.

    Illustration of the temperature anomaly. This is how far above or below the climatic average the current temperature is.

    A property in Laleham Reach, Surrey continues to be surrounded by water. Steve Parsons/PA Wire/Press Association Images

  • Where's winter?
    20.02.2014 16:23

    Over this winter season the UK has been continually bombarded by Atlantic storms. These have bought extreme conditions to the UK with flooding in many parts. Along with this, temperatures have been relatively high compared to recent winters. On the night of the 16th/17th of February the lowest temperature of the season so far was recorded at -7.7C in Altnaharra, Sutherland, Scotland; which is substantially above the lowest temperature last year which was -13.6C in Buntingsford, Hertforshire, England. Although the winter season is not yet over, trying to understand why we have experienced these conditions seems to be a hot topic at the moment.

    The position and strength of the North Atlantic jet stream is the reason for recent stormy conditions over the UK. This season the jet stream has been particularly strong and positioned such that the UK is directly in the path of the North Atlantic storms. The jet stream is located near the tropopause (the transition between the troposphere and the stratosphere) and is a fast, narrow air current. The position and strength of the jet stream is a dominant factor in weather conditions around the globe.

    Over much of this season the jet stream has been over or to the north-west of the UK. With the Atlantic Ocean being unusually warm, this has helped to allow deep Atlantic depressions (low pressure systems) to form, which have then followed the path of the jet stream north-eastwards making landfall on the south-western edge of the UK. In this position not only has the UK been directly in the path of this stormy weather but it has been under the influence of the warmer tropical air. In contrast, last winter (2012-2013) the UK spent much of the season to the north of the jet stream where cold air from the Arctic flowed southwards, resulting in much colder conditions.

    So is this winter season a taste of things to come in the future? The answer is complex and unclear. Although we would expect similar conditions to this in a warmer atmosphere, it is very difficult to say that this is exactly what will happen. A slight shift in the position of the North Atlantic jet stream and the UK would be to the north of it again and would be experiencing much colder conditions. Although the strengthening of the North Atlantic jet stream this year has been loosely linked to an unusually prolonged wet period over Indonesia the mechanism behind the event is also unclear.

    In conclusion, while the general cause of the UK's mild winter is clear, the overall mechanism behind the strengthening of the North Atlantic jet stream, remains unclear. As to whether it is an indication of future weather conditions, only time will tell. Climatic variations on this scale are not evident from year to year, but decade to decade.

    By: Lara Gunn