A late-winter stratospheric warming can bring a European spring cooling

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  • A northern hemisphere view of the analysed temperature and wind flows in the polar stratosphere in mid-December 2015 and mid-March 2016. The green and white shading denotes very low temperatures in the polar stratosphere, while red shading is warmer. You can see how the well organised smooth vortex in December has now broken in 2 small and weak low pressure circulations, with easterly winds around the pole now. (Credit: MeteoGroup)

    Surface temperature anomalies over Europe (compared to 1981-2010 climate) between 1st and 15th March 2016, showing that SW Europe has been coldest, compared to normal. (Credit: NCEP/NCAR)

    Surface temperature anomalies over Europe (compared to 1981-2010 climate) between 1st and 31st March 2013, showing the much more extensive cold weather than 2016, following a major SSW event in January 2013. (Credit: NCEP/NCAR)

    Mid-March 2013 saw impressive snow drifts over the North Downs in Kent and Surrey. A MeteoGroup forecaster tests the quality of the snow here for making snowballs! (Credit: Paul Mott)

  • A late-winter stratospheric warming can bring a European spring cooling
    17.03.2016 15:57

    After many parts of the UK experienced their warmest December on record in 2015, this past winter will be remembered for being milder, windier and wetter than usual in most locations. The cause of the very mild December can be partly attributed to the strong El Nino event (well above normal sea surface temperatures in the central and eastern tropical Pacific) which most likely forced a weather pattern over Europe that produced an almost continuous flow of mild and moisture laden south-westerly winds throughout the month.

    Another forcing factor on the UK and European weather patterns in winter and early spring can be found in the stratosphere, the layer of the atmosphere above where most clouds develop and where medium to long range aircraft fly. Each winter, as polar latitudes receive little or no sunlight, the air in the stratosphere above the Pole cools rapidly and a strong westerly wind flow develops around this ‘cold pool’. This is called the polar vortex. During some winters, such as the most recent one, this westerly vortex can become very strong and this tends to increase the chances that the UK and Europe will have mild weather patterns, with above normal wind and rainfall.

    However, the natural variability and fluctuations in surface weather across the globe can sometimes generate large vertically moving waves in the atmosphere. As these waves travel up towards the stratosphere they tend to increase in strength and can sometimes act to slow down the polar vortex, rather like a brake being applied to a fast moving wheel. If the waves are strong enough, then the westerly vortex can distort and even break down completely, with the wind direction reversing and becoming easterly. This change in wind direction is also accompanied by a rapid rise in temperature in the stratosphere, called a sudden stratospheric warming (SSW). SSWs tend to occur about once every 2 or 3 winters, normally between mid-January and mid-March. After a strong SSW, the weather patterns over Europe and the UK can become more conducive to allowing colder wind flows to develop from the north and east, with high pressure over the North Atlantic, Scandinavia and Iceland blocking the mild westerly winds.

    At the end of this winter and into early March, there was a dramatic change in the state of the stratospheric vortex, flipping from the strongest westerly flow on record during early winter to one of the weakest on record by early March. This recent SSW event now appears to be having some influence on our weather patterns, with colder air moving in from the north and east over Central Europe, France, Spain and the UK during the last couple of weeks. Spain in particular has had an unusually cold start to spring. The UK has seen rather chilly weather recently, especially in the south, although northern Scotland has had a few warm sunny days. It looks like the UK will continue to have a few more spells of cool weather through the rest of the month. However, had this big SSW event occurred in January or February, rather than early March, then this would have increased our chances for late winter and spring to have been colder.

    In January 2013, there was a similar dramatic SSW event, but the fact that this occurred earlier in the winter allowed the weather patterns over Europe to become primed for cold weather for the rest of the winter, with extensive snow cover building up across Scandinavia and eastern Europe. So when the UK experienced an almost continuous easterly wind flow during March 2013, the air being transported in from the east was much colder than it is this year. In March 2013, the Channel Islands, Kent and Sussex, and also large parts of Wales, northern England and Scotland experienced some heavy snow and drifting, even as late as March 25th!

    By: Matt Dobson