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UK Storminess Explained By Met Office

January 4, 2014

By Paul Homewood

 

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http://metofficenews.wordpress.com/2014/01/03/uk-weather-how-stormy-has-it-been-and-why/

 

There’s a very interesting piece from the UK Met Office, about the recent run of stormy weather in the UK.

 

 

Since the start of December the UK has seen a prolonged period of particularly unsettled weather, with a series of storms tracking in off the Atlantic bringing strong winds and heavy rain.

The windiest month since 1993

In order to compare the recent spell with the numerous stormy periods of weather in the past the Met Office’s National Climate Information Centre has done an analysis of the number of weather stations in the UK which have registered winds over certain thresholds since the start of December.

This measure suggests that December 2013 is the stormiest December in records dating back to 1969 and is one of the windiest calendar months for the UK since January 1993.

December was also a very wet month across the UK, particularly in Scotland where it was the wettest December and wettest month overall in the records dating back to 1910.

But why has this been the case?

Storms are expected in winter

First of all, we do generally expect to see stormy conditions in winter months. This is because we see a particularly big difference in temperature between the cold air in the Arctic and the warm air in the tropics at this time of year.

This contrast in temperatures means we see a strong jet stream, which is a narrow band of fast moving winds high up in the atmosphere.

The jet stream can guide storms as they come across the Atlantic, and it has been sitting in the right place to bring those storms to the UK over the past few weeks.

There’s also a close link between the jet stream and storms. The jet stream can add to the strength of storms, but then storms can also increase the strength of the jet stream. This positive feedback means storms can often cluster together over a period of time.

But why has it been particularly stormy?

Even accounting for the fact that it’s winter, the jet stream has been particularly strong over the past few weeks – but why is this the case?

It’s partly due to particularly warm and cold air being squeezed together in the mid-latitudes, where the UK sits. This could be due to nothing more than the natural variability which governs Atlantic weather.

However, looking at the broader picture, there is one factor which could increase the risk of a stormy start to winter and this is called the quasi-biennial oscillation (QBO for short).

This is a cycle, discovered by the Met Office in 1959, which involves a narrow band of fast moving winds (much like our jet stream) which sits about 15 miles up over the equator. The cycle sees these winds flip from easterly to westerly roughly every 14 months.

In 1975 Met Office researchers discovered that when the QBO is in its westerly phase, it tends to increase the westerlies in our own jet stream – meaning there’s a higher risk of a stronger, more persistent jet stream with more vigorous Atlantic storms. It has been in its westerly phase since early 2013 and we expect it to decline over the next few months.

This is just one factor among many, however, which needs to be considered – so it doesn’t mean that the westerly phase of the QBO will always bring us stormy winters.

What about climate change?

Climate models provide a broad range of projections about changes in storm track and frequency of storms. While there’s currently no evidence to suggest that the UK is increasing in storminess, this is an active area of research under the national climate capability.

 

 

So, to sum up.

  • It was the stormiest month since 1993.
  • Not only has the jet stream been in the wrong place, (or as the Met Office put it – the right place!), but it has been particularly strong in recent weeks. ( It was also noted in October that the St Jude Storm tracked very quickly across the Atlantic on a fast jet stream).
  • It was only last year that Dr Jennifer Francis, and others, were warning that a warm Arctic would lead to a weak, meridional jet stream, with consequent blocking events and harsh winters. In other words, the very opposite of what we are now seeing.
  • A strong jet stream can be due to a bigger than normal difference in temperature between the cold Arctic and warm Tropics.
  • Storms can act as a positive feedback by increasing the strength of the jet stream.
  • The pattern of the QBO is likely to be an aggravating factor.
  • There is no evidence that storminess is on the increase. Nor is there any agreement among climate models on the issue.

 

In other words, it’s not unprecedented, it’s just weather.

6 Comments
  1. John McEntee permalink
    January 4, 2014 3:32 pm

    And we must take heed of Geoffrey Lean-Britain’s Al Gore-writing in The Telegraph (4/1/14 P.24)-who states “rainfall has risen by about 5% over the past 20 years and more significantly, heavy bursts are increasingly replacing our archetypal drizzle, something expected to accelerate with climate change”
    We have been warned.

  2. January 4, 2014 4:03 pm

    Actually the jets are more meridional globally, hence the southward surges of cold air over USA and parts of Asia.

    It is just that whilst the southward surges are over USA the jets running across the Atlantic are more energised.

    In the late 20th century warming spell the greater zonality gave less intense cyclones passing well to the north of the UK.

    We currently have much bigger and more intense cyclones passing closer to us than in those days.

    It is more reminiscent of the pattern of the cooler 50’s and 60’s.

    In due course the location of the southward surges of cold air will change and western Europe could then be more affected by colder periods as over recent winters.

  3. catweazle666 permalink
    January 4, 2014 4:07 pm

    “In other words, it’s not unprecedented, it’s just weather.”

    Yes, funny that, we get a lot of weather in the British Isles. In fact, we’re known for it.

    Surprising the Met Office haven’t worked that out yet.

    Mind you, if they’re claiming their records started in1969, nothing about them would surprise me.

  4. January 4, 2014 5:13 pm

    Reblogged this on CraigM350.

  5. January 4, 2014 6:18 pm

    Prof Sir John Beddington this morning on the Today prog issued the familiar chant “It’s impossible to attribute any single event to [man-made] climate change, but [I’m going to do it anyway] events like this are expected to increase in frequency”.
    We then got Beddington’s theory that the greenhouse gases that were in the air 20 years ago are determining today’s weather. James Naughtie thinks there’s the same level of CO2 now as in the 1990s, apparently.
    Obviously it’s much better to have James Naughtie, or any of the others, challenging him than to allow a well-informed sceptic on the air.
    Beddington finished by saying that if the frequency of extreme weather events didn’t increase over the next 20 years, he’d donate his entire pension fund to charity (I made that one up – he doesn’t have that much integrity).

  6. Brian H permalink
    January 5, 2014 3:44 am

    Monte Carlo of explanations: eventually, one will get it right.

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