Archives For arctic oscillation

2016-06-12_06-35-12

Reigate May 2016 summary statistics

  • Tmax 27.4C
  • Tmin 0.3C
  • Tav 13.8C (UK 11.3C)
  • total rainfall 42mm (town) 45mm (Hartswood)
  • max gust 36mph
  • average wind direction NNE
  • sunshine 181.7 hours (May 2015 161 hours)

Whilst there were fortunately no severe weather events in Reigate and few across the UK in May, the weather we experienced more widely could be linked tenuously to climate change.  Of course, caution is required with such speculative statements but attribution studies on the May floods in Paris, not so far away, have concluded that they were made 90% more likely due to climate change. The same stalled low pressure system delivered our easterly winds so we were influenced, albeit on the edges, by the same blocked weather pattern.

People attempting to climb Snowdon in North Wales in May were lucky to experience sunnier-than-usual conditions for much of the month (south wales had more thunderstorms which reduced the sunshine totals there).  Meanwhile, in Surrey, we experienced occasionally warm conditions with an unusual mean monthly wind direction from the NNE.

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Reigate pressure rose hesitantly mid-month

Pressure fell across the UK to start May but then rose mid-month, especially to the North, bringing a relatively unusual easterly flow into Reigate and the south.  Whilst there were few severe weather events during the month, this post briefly explores some of the wider factors that may have contributed to this Easterly flow and the possibility of it being linked to climate change.

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Whilst mostly dry for the UK as a whole, occasional showers, some thundery, brought Reigate rainfall totals to just above average at around 42-45mm as recorded from our two weather stations respectively in and out of town.  SE England as a whole recorded rainfall at 111% of normal rainfall, mostly falling in thundery showers, more common in SE wind regimes.

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With relatively dry Easterly winds, sunshine totals for the UK were accordingly above average given the relatively high pressure overall.  Reigate experienced 182 hours of sunshine in total for the month.

Unusually, the sunniest places in the UK were in the North and West given the easterly winds bringing occasionally cloudier conditions off the North Sea to the south and east.  May 24 shows a typical scenario with the higher pressure to the North dragging in E/NE winds across the southern part of the UK with cloud across eastern areas and clearer conditions to the west.

Some great sunny days were recorded in the mountains of Wales, Cumbria and Scotland!

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Sunny Snowdonia with kelvin-helmholtz type wave clouds trying to form over Ogwen Valley

The higher than average rainfall patches shown below in the south were associated with showers on occasionally unstable warm and humid SE winds.  Reigate reached a Tmax of over 27C in this warm flow.

The wider pressure pattern across the northern hemisphere was characterized by anomalously high heights over the Arctic and LOWER than normal pressure in mid-latitudes including Europe. This situation is called “northern blocking” and in winter could cause cold conditions in mid-latitudes.  In Spring, as the continent rapidly warms up in stronger sunshine, easterly winds can be warm or even hot for the UK.

northern blocking over Pole

northern blocking over Pole

Northern Hemisphere pressure patterns are measured by the Arctic Oscillation which, as can be seen below, remained unusually negative through much of April and May showing high pressure persisting over the Arctic relative to low pressure in the mid-latitudes.  This pressure pattern turned winds from the usual westerlies into easterlies in the UK and Europe.

negative arctic oscillation Spring 2016

negative arctic oscillation Spring 2016

The causes of this reversal of the usual mid-latitude zonal westerly wind set-up have been linked to low sea ice extent in the Arctic, especially the Kara and Arctic Gateway seas. Warmer influxes of air into the Arctic builds air pressure which then links to higher chances of Easterly winds in mid-latitudes.

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low Arctic sea ice cover March 2016

The very low sea ice extent this year was brought about by much warmer-than-usual conditions during the Polar winter, where monthly average temperatures in the Arctic (>60N) were at times 3.5C or more above average during the cold season of 2015-16. This Arctic amplification is widely accepted as being caused by human induced climate change.

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It turns out that Spring Arctic sea ice extent is some of the lowest recorded in the 38 year satellite series.

 

So, unusual sunshine in North Wales, a warm NNE mean wind direction in Reigate and cloudy conditions on the east coast can be linked to the above tele-connecting weather patterns which, in turn, can be linked to climate change in the far flung Arctic.

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Meanwhile, the strong 2015-16 El Niño declined rapidly through May and ENSO conditions were neutral by early June. Models suggest the chance of La Niña (cool Pacific) conditions by Autumn 2016 are as high as 60%.  Some forecasters bring La Nina through the summer.  La Nina, and the warmer SSTs of the tropical Atlantic, are associated with more frequent hurricanes in the Atlantic basin.  In turn, high hurricane accumulated energy transfered to the North Pole during such seasons can build Polar heights in Northern Hemisphere winters, warming the Arctic and further melting sea ice.  Whilst this is just outrageous long term amateur speculation, it is nevertheless interesting to ponder the potential for feedbacks to accelerate further climate change in the near future.

The turning down of the vast heat engine of the El Nino might be linked to the slightly lower May global average temperature, though confirmation from expert sources has not verified this as yet.

 

Local data for May and all months stretching back to 2012 can be found on our data page here

http://www.metoffice.gov.uk/climate/uk/summaries/2016/may

 

The ensemble charts above show that we can expect a week of change ahead.  At the start of the week fronts will bring modest episodes of rain and swings in temperature to the SE followed by a notable change mid-week as both temperature and pressure fall (see charts above) with the possibility of snow for the SE.   By next weekend there is a risk of some proper cold into the start of February.  As usual Reigate and the SE will be sheltered from most of the action but the weather will take interesting and notable swings in a predominantly downward direction nonetheless.  Models agree on how this transition will happen and it is largely based on the evolution of a LOW starting life off Canada near Newfoundland …. here are charts from the ECM that shows the story of our Canadian LOW and how it is likely to bring another taste of winter to the UK.  (Update: worth explaining that the Canada LOW mentioned here is not the same as the Nor’Easter LOW that caused blizzards in NE USA Monday-Tuesday this week: it is a LP preceding it.  If you look below at Chart 2 for 28 Jan you can spot the infamous Nor’Easter bashing the NE coast well modelled on this ECM chart from back at the weekend. The US Nor-Easter Blizzard2015 storm is responsible for building the Atlantic BLOCK helping to push Arctic winds our way but it is not travelling to the UK. The NYC storm looks to travel North up the Canadian coast, filling near S Greenland, unable to break through the Atlantic high pressure ridge extending north that it, in part, helped to build. Hope that helps!)  In any case, the weather set-up gives Reigate another flaky chance of some snow.

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1. The story starts now with the Azores HIGH pressure giving a dry and pleasant weekend for the SE, Saturday has seen brilliant blue skies as a result.  The Azores HIGH has been dominating the Atlantic recently and our Canada LOW, deepening rapidly off Newfoundland on Monday, will be forced round the HIGH to the north west to Greenland. This LOW will deepen rapidly because of the great temperature difference between frigid air pouring off Canada and humid sub-tropical air fed up from the south courtesy of the Azores high.

For us in Reigate this period sees a ridge of the Azores high pressure ebb away slightly during Sunday and this will nudge a mild SW flow to raise temperatures temporarily high into the UK and SE overnight into Monday morning.  This minor weather episode (marked by the ups and downs in temperature on the 850hPa ensemble chart top of page) will be heralded by increasing cloud on Sunday as fronts bring some patchy rain in on a warm front overnight into Monday.  Polar air will follow a cold front later on Monday and a ridge of high pressure will build quickly overnight turning the winds into the NW with a dry chilly night in store for us in Reigate and SE into Tuesday.

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2. Later on Tuesday the Canada LOW is forecast to move NW to near southern Greenland where it will feed on a brisk northerly wind of freezing cold air from the Greenland ice cap.  This freezing cold air will create a vigorous cold front that will reach the UK early on Wednesday.  This active cold front will usher in an unusually cold Polar Maritime NW’ly wind across the country reaching the SE late Wednesday with the possibility of heavier rain or even snow. One to watch carefully.

A point to note from the charts above is that the central Atlantic is anomalously COLD at the moment and so the brisk NW’ly wind will not warm up as much as usual on its journey over the Atlantic to the UK, increasing the likelihood that it will bring wintry stormy gales to the NW, with some models even showing wintry precipitation for us in the SE too, perhaps reaching us by Thursday, though these are likely to be sparse unless the cold front stalls in which case more significant falls are possible. (Update: GFS 18Z suggests this cold front could be active and bring snow across entire UK in its wake…)

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NW airflow 2m temperature anomaly

Check the temperature anomaly chart above which shows how unusually cold this NW’ly wind is going to be.  Usually NW winds do not impact SE England with snow unless fronts stall or there is an especially active undercut of unstable cold polar air.

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3. Our Canada LOW that started life just off shore from Newfoundland is forecast to cross Iceland mid-week and then slide SE into the North Sea, around the blocking high extending north through the Atlantic to meet rising pressure in Greenland, a good scenario for a COLD Europe!  As the LOW transits SE into the North Sea it will bring down Polar and then Arctic northerly winds on its back and snow for the North, NW and NE coasts.  Arctic air rarely brings snow to Surrey or Reigate … it usually runs out of moisture and lift on its transit across the cool land and instead the south and SE usually gets azure blue skies with frosty nights.  NE and E coasts can get snow showers.  Daytime temperatures everywhere may well stay near freezing if this comes off.

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It’s too far ahead for any detail but some model runs show Polar LOWS sliding round the edge of the trough on the left exit side of the jetstream (where lows develop rapidly) as the trough moves east .  These daughter lows can bring snow to the SE but this is too far off to be certain.

So how long might this cold snap last?

There are indications both for and against a more prolonged cold snap but, on balance, the peak of any Arctic cold looks likely to be shortlived as the high topples east, ebbs south and allows gradually more westerly winds back across the UK.  For enduring cold we really need the LOW to move south into Europe and pressure to build to the north bringing in a blocked situation allowing cold easterlies into the UK (beast from the east).   However, this scenario looks unlikely because the Azores HIGH is likely to remain relatively dominant.  This is shown by the generally positive North Atlantic Oscillation chart above.

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North Atlantic Oscillation

The NAO is a forecast measure of the sea level pressure difference between Iceland and the Azores.  When the NAO is positive it usually indicates a strong pressure difference with a big HIGH pressure over the Azores and a LOW over Iceland.  A positive NAO correlates with a fast zonal westerly jetstream and mild wet winters for the UK.  A negative NAO indicates colder winters in which pressure rises to the north (Iceland) and allows easterly or NE winds to bring cold airmasses from the Arctic or more commonly Siberia into the UK.

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Arctic Oscillation

The Arctic Oscillation (AO) is a related cousin of the NAO.   The Arctic Oscillation is a bigger scale comparison of pressure at the Pole compared with that in the mid-latitudes.  A positive AO usually correlates with lower than normal pressure over the Poles compared to the higher pressure further south in the Atlantic.   This brings mild wet conditions to Europe (zonal westerly winds).  A negative AO sees pressure rise higher over the North Pole and this acts like a balloon to push polar air out into mid-latitudes… a cold winter scenario.  The AO forecast shows it going negative by early February and this correlates with the cold snap.  Unfortunately it looks like the AO will go positive again thereafter but this is too far off to be certain.  On the other hand…. !!

The charts above show a Polar view of the Northern Hemisphere.  They show that pressure is forecast to rise over the Pole disturbing the zonal westerly flow of the polar vortex by a displacement of the polar vortex away from the Pole (see below).  This might increase the likelihood of a prolonged cold plunge of Arctic air reaching the mid latitudes including the UK  This is good for cold weather enthusiasts in Europe!

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The other longer-term chart shown above in favour of a cold late winter is a sudden stratospheric warming forecast in early February.  A SSW can lead to pressure rising over the Polar regions a few weeks later, disrupting the upper westerly winds, potentially upsetting the jetstream and bringing cold to mid-latitudes.  This is well correlated and was significant in bringing a late winter in 2013.  So… much of interest at the moment as we enter the final third of winter 2014-2015.

note: for all decision making purposes and forecasts please consult professional agencies: e.g. MetOffice at http://www.metoffice.gov.uk/public/weather/forecast/gcpg7rs0t#?tab=fiveDay&fcTime=1422057600

http://www.nc-climate.ncsu.edu/climate/patterns/NAO.html

Next week is a real chiller for the whole of Europe. Average temperatures are 6°C below normal for this time of year in the UK.  One reason for the continued cold spring weather is exceptionally high pressure over the North Pole compared to that in the mid-latitudes which is measured by an index called the Arctic Oscillation.  The HIGH pressure BLOCKS out milder air and causes Polar air to flood further south than usual, reaching Reigate on several occasions this winter (albeit modified and warmed up along its journey).

Polar air escapes in negative AO

Polar air escapes in negative AO

The Arctic Oscillation index is currently “strongly negative”.  This indicates much higher than normal pressure over the Arctic and a weak jetstream in low pressure further south than usual.  Air flows from HIGH to LOW pressure.  This means that cold Polar air can easily push out from the north unimpeded by weak SW winds: hence the cold weather brought by Polar easterlies reaching the UK.  The jetstream also “holds in” Polar air like a belt, but the belt has slipped down well to the south of the UK causing “pants” weather for the UK.  Usually the jet migrates north of the UK at this time of year bringing in milder SW air at the surface.
The question, of course, is not “if” but “when” exactly Spring weather will arrive. The maps and ensemble graph seem to suggest that next weekend could see initial improvements with milder SW air reaching the UK and a break down of the Polar block… let’s hope this proves to be accurate!