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Changes in ocean circulation, especially perturbations and regular large scale changes like El Nino and La Nina, are significant in controlling climate and weather systems around the planet.  How El Nino Southern Oscillation (ENSO) events influence European weather is still not entirely clear.  Nevertheless it is known that Europe is not nearly as impacted by ENSO events as other large land masses.  In any case, the entire mid-latitude belt around the northern hemisphere has suffered a cold 2013 Spring so it seems unlikely that ENSO or other circulation perturbations in different oceans could alone be to blame for the cooling of the whole Northern Hemisphere.  In addition, the current ENSO status is in “neutral” which means we are between La Nina and El Nino events … blaming either would be churlish, so ocean circulation does not seem to be a prime suspect for killing Spring 2013… so we should set El Nino and La Nina free!
Nevertheless, it is interesting to note that, whilst Europe and North America have frozen, the North Atlantic has experienced higher than “normal” temperatures. So, whereas it could be partly to blame for the European Little Ice Age, this time the North Atlantic Drift warm ocean current also seems to be entirely innocent of killing off Spring 2013.

point to note: additional considerations regarding longer term thermal changes in oceans such as the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) are currently controversial, though widely used by global warming sceptics to indicate the relative importance of cycles in ocean circulation over CO2 contributions to climate change on a human time scale.  Sceptics consider Arctic ice loss and CO2 to be insignificant in comparison to orbital wobbles and ocean circulation cycles.  THIS is an excellent website which debunks many of the climate skeptic arguments.

Climate Cluedo!

Volcanoes erupt ash and gas high into the atmosphere. A volcanic dust veil injected into the atmosphere can reduce solar radiation receipt at the surface for a few years and lower global average temperatures sometimes by a few degrees.  A cooler atmosphere also tends to sink and create high pressure at the surface, a prolonged feature of our 2013 winter. Finally, sulphuric acid from volcanic eruptions can get into the stratosphere where it is particularly effective at reflecting solar radiation which would induce cooling.  So, volcanic eruptions near Polar regions could produce cooler winters for a few years and several high latitude volcanoes have erupted fairly recently: Kasatochi in Alaska in 2008, Mt. Redoubt in Alaska and Sarychev in Russia in 2009 and Eyjafjallajokull Iceland in 2010.

volcanic eruptions and global temperatureHowever, none of these have been terribly big eruptions, Eyjafjallajokull was only VEI4 (volcanic explosivity index).  In fact, this eruption was insignificant compared to super-eruptions of the past which did have a climatic cooling effect: Tambora was 10,000 times bigger!  Hekla in Iceland is rumbling right now but that, along with these other examples, seems too little and too late to be a true suspect in the crime of Spring 2013!

Climate Cluedo!

Whilst the output of the sun varies very little (it is called “solar constant” for good reason), various other measurable factors such as solar magnetic activity, sunspot cycles and irradiance have all been raised as possible suspects for the crime of killing Spring 2013 across the northern hemisphere. Low solar energy receipt has been correlated with a colder northern hemisphere by influencing northern blocking.  A low point in sunspot activity called the “Maunder Minimum” has been closely associated with the onset of the Little Ice Age when the Thames regularly froze over every winter. Solar flares have been associated with warming of the upper atmosphere which has, in turn, been linked to higher pressure in the polar regions.
Links (correlations) have also been found between sunspot cycles and the extent of sea ice in the North Atlantic. Nevertheless, correlating solar activity and the climate on Earth has been surprisingly difficult to pin down.  Importantly, correlating sunspot activity with any climatic variable does not indicate a causal link. Wiggly lines on graphs may coincide but does one cause the other?  It is all still full of controversy. Even the IPCC recognise that, whilst there is some evidence to suggest a link between warming and increases in solar activity between 1750 and 1950, the measurable increase in global warming far exceeds any change in solar output. Since 1979, when measurements of sun output started in space, there has been no long-term significant increase in solar energy.  Sunspots are frequently hauled in to account for weather and climate changes but the evidence is hard to pin on them and almost entirely circumstantial!  Sunspots are intertwined with the greatest driver of our weather, the Sun, however, with such flimsy evidence the jury is still out on this suspect. With other major suspects under questioning it seems reasonable to let solar energy go free.

Climate Cluedo!

Update 23/04/2013: UK Met Office now backtrack on loss of Arctic Sea Ice as a possible cause. Read it here. Nevertheless, Arctic Sea ice loss could be a future cause of climate change so we will leave it here as a suspect!

Scientists have been monitoring the shrinking mass of sea ice which covers the Arctic Ocean.  Arctic sea ice shrank by 2.7%per decade between 1979-2006 and continues to do so at a faster rate now.  The 2013 ice maximum was reached on March 15 and was the sixth lowest on record at 15.13 million sq km, 733,000 sq km below the 1979-2000 average.  The lowest maximum extent occurred in 2011.  Sea ice naturally expands during winter as more sea freezes over. By March it is at its maximum size, extending furthest from the North Pole. The ice naturally shrinks to a minimum area around the Pole by late summer / Autumn which is when scientists usually take another measurement of sea ice extent.  The Arctic has warmed TWICE as fast as the rest of the northern hemisphere (Arctic Amplification) and the Arctic sea ice has shrunk in area.  In summer 2007 the North West Passage north of Canada became ice free for the first time. Sea ice has a high ALBEDO which means it is bright and white and reflects solar energy (the Arctic gets 24 hours of sunlight during the summer). Most sunshine is reflected back into space off the ice and does not heat the atmosphere or the sea, the energy is more or less “lost” from the earth-atmosphere system.   Less sea ice means more dark ocean water is exposed and, with a lower albedo, this readily “absorbs” solar energy i.e. warms up.  The Polar atmosphere is warmed up by this extra heat flux from the ocean. The “warmer” atmosphere contains more energy and water vapour and this could potentially deliver more humid Polar air carrying more snow to mid-latitudes.
There is a final twist to this theory: warmer Arctic air reduces the difference between Polar temperatures and those further south. This reduced “thermal gradient” weakens the jet stream which feeds off big differences in temperature. A weaker jet stream is less able to BUST the blocking HIGH pressure built by Polar air.  All this adds up to slower moving weather systems in the mid-latitudes which prolongs any extreme weather: the cold gets further south and lasts longer.  There are many factors which control climate but loss of Arctic sea ice and the almost prophetic predictions made by scientists studying the effects it would induce, seems to keep Arctic Sea ice as a key suspect! The video below concerns all types of ice loss in the Arctic: land based and sea ice.

Climate Cluedo!


global atmospheric circulationCold air sinks and this builds HIGH pressure over the Polar regions as part of the “normal” global atmospheric circulation.  Hot air rises over the Equator creating LOW pressure.  Tropical and Polar air flows in a series of complex but fairly predictable patterns in between.  These cells power global weather.  Polar air meets Tropical air in the mid-latitudes at the Polar Front.  Fronts, low pressure systems and jet streams are all a product of this unhappy meeting of two the different air masses.  The jet stream (fast flowing ribbons of westerly winds at altitudes of 15 km) usually acts like a belt and keeps the Polar air inside the high latitudes.  The mid-latitude jet usually sits somewhere near the UK during “normal” winters and brings in relatively mild westerly winds circulating around depressions with rain and wind.  2013 has seen higher than usual pressure over the Polar regions and these have pushed the jet stream and attendant LOW pressure systems further south than usual, somewhere over the Mediterranean which has seen more rain and wind than usual.  Winds blow from HIGH to low pressure and, without the belt-like effect of jetstreams to keep them in, frigid polar air has flooded out across the mid-latitudes.

blocking highs and AO

Anywhere located north of the jetstream has been left exposed to these incursions of the Arctic air mass.  Winds from the Pole tend to blow from the NE or East rather than straight from the North to South because the spin of the Earth deflects them to the right … the coriolis force.  Hence, “the Beast from the East” in the UK.




Arctic Oscillation NAOThe index measuring the balance between HIGH pressure in the north and LOW to the south is called the Arctic Oscillation and this has been at record breaking negative figures this Spring: meaning the pressure over the Pole is unusually high compared to the low pressure over the mid-latitudes.  Blocking HIGH pressure prevented warm air getting to the UK on several occasions this Spring and, with this fresh in our minds, we must consider BLOCKING HIGHS as “caught red handed” in the act of causing our cold spring.  However, it seems that there is always a bigger fish and we must find the master criminal controlling the BLOCKS to get nearer the real killer of Spring2013!  We must find the cause behind these increased heights over the Pole? The answer might be the loss of Arctic sea ice.

Climate Cluedo!

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!

Weather is always interesting, of course, but this week is extra-fascinating! 3 weather things to watch this week in Reigate:
1. Watch out for… highly unstable cool air moving off the Channel onto warming surfaces during the daytime leading to convection, cumulonimbus clouds and heavy showers, maybe risk of thunder and hail over Reigate: especially Monday and Tuesday.  April showers in a cold March?
2. Keep an eye on … the big show-down between the powerful Polar North Easterlies spilling out from huge blocking HIGH pressure over the Arctic and the, so far, rather feeble South Westerly maritime air that has made little impression recently.  They are due to have a face-off on Thursday. The leading edge of fronts might be snow…but will the warm air reach Reigate or will the LOW retreat out into the Atlantic?
3. You can’t see this in the sky but… note the off-the-scale negative Arctic Oscillation graph that indicates a weak jetstream and high pressure over the Pole which allows Polar air to flood out over the UK easily. Will this mean another fake start to Spring this weekend (check the temperature graph leaping up at the weekend!) and then a return to cold next?
Reigate will mostly stay on the “warm” side of this LOW and avoid the very coldest easterlies this week which will stay up north (except mid-week which will be cooler down here) but the cold air will never be far away so frosty nights are certain, cool temps in slacker winds are definite, heavy showers hit and miss and a minor risk of snow anytime are still worth watching for!
Hope you enjoy the week weather watchers!

quick update Monday: cold weather continues to next week… polar easterly winds return Monday-Tues. after warmer spell this weekend but poss SNOW preceding warm front Saturday- update later.  Heavy rain with snowfall/sleet poss in between for SE: on weds, thurs & Fri. Very active, very odd, potentially extreme weather…watch this space for updates. So, for the moment the Easterlies win! some are calling this the MIA: Mini-Ice Age!

quick update Weds: snow threat Fri/Sat as Atlantic fronts on a vigorous depression move in from the west and move up against the cold air.  This may dump snow on Reigate Saturday.  Next week looks COLD … back in the freezer high threat: snowy Easter possible.  NOT the start of Spring by any stretch…someone tell those birds.

Here is a summary of the causes of the March 11 2013 “Channel Blizzard” which brought extra-ordinary “Spring” weather to SE England and the Channel Islands and N France. At RGS we had record low wind chill temperatures of -10°C at 9.30am, 36mph gusts and sub-zero temperatures all day.  Follow the numbers on the map to get a quick view of why it all happened!

1. Cold source region for Polar Air: the NE winds originated from the Polar regions with temperatures below -15°C and traveled across an extremely cold continent to reach the UK.

2. On their journey, the North Easterlies warmed a little over the North Sea (by now +4-5°C) which caused showers to form in unstable air (warming causes air to rise, clouds form and it snows). These showers formed lines called “snow streamers” which fed snow to the SE for most of the day and into Tuesday morning.

3. Much warmer SW winds at +10°C met the frigid cold Polar air mass somewhere over the Bay of Biscay but they didn’t mix well and they certainly didn’t get on!  In fact, the cold, dense polar air pushed the lighter, less dense tropical air right up off the ground, to over 4000m, where all the moisture condensed, formed cloud and snowed!  The high winds experienced across SE England were “squeezed” like toothpaste between the HIGH over Iceland the LOW over France (a high “pressure gradient”: look how close together the isobars are over SE England and the Channel!).

4. Next? Things will stay cold during mid-week as the UK remains firmly in Polar air and enjoys some dry weather courtesy of a HIGH over Iceland (unusual).  Clearer skies and frosts.  However, a Polar Low forming SE of Greenland at the moment is due to break through the Icelandic HIGH later in the week and bring unsettled conditions to the end of the week / weekend: as Polar air is still involved this may bring further snow and sleet.

Greenwich Lightship is a UK Met Office weather station in the middle of the English Channel. 3 metre waves, 50mph winds, sow and 10 foot waves were recorded.  Unbelievably horrendous conditions which the Channel Islands experienced as a blizzard.

Watch the satellite animation below and spot the storm winding up in the Bay of Biscay before it hits the Channel and says “Hello” to France and the UK!

Sunday final analysis update: crossing the Channel Monday?…be warned of v cold blizzard conditions!  LOW centre set to track thru N France most likely bringing only light snow to Reigate Monday morning but increasing risk of significant accumulation later in the day and overnight into Tuesday.  Any shift further north in the fronts could bring heavy snow to S counties pm.  Latest hi-resolution NMM model (below) shows light snow showers all day but risk increasing to heavy snow pm extending into southern counties of England, reaching Reigate late pm or evening….; UKMet Office predicts the least snow of all the models.V V tricky forecast.

So: Reigate forecast for Monday-Tuesday: emphasis on bitterly COLD, snowy day with extreme wind chill for (any) time of year with a low risk of heavier snow later and overnight to Tuesday, especially for places further South. so…accumulations 0 – 10cm by Tuesday am.

sunday 06Z NMM model

Cold weather alert! Reigate should prepare for an impressive weather change this weekend into next week.  The temperature will drop like a stone over Sunday night: shown by these Reigate maximum temperatures at midday on Saturday max +11ºC, Sunday max +5ºC, Monday max -1ºC with wind chill even as low as -18ºC (yes) possible in forecast 40 mph gusts in exposed places.  Winds will average 20 mph but even this will cause wind chill of -8ºC.  It will feel cold on Monday!  Light snowfall could start Sunday pm and continue overnight through to Monday. It may seem odd, but this snow will be caused by warm air from the South over-riding the frigid COLD Polar air blasting in from the NE: meeting at a front.  Check the temperature height diagrams to see how this works: warm, moist air circulating around the LOW in the Channel will be undercut by the dense freezing cold polar air from NE. The warm air forced to rise causes cooling, condensation, cloud formation and, as upper air temperatures are so cold… SNOW is likely to fall. Possible accumulations Sunday – Monday around Reigate could be 0-3cm (higher figures on hills) and more possible to the south of the region where precipitation could be more intense nearer the fronts.  So…positioning of the LOW is critical.  The snow maps give an idea of potential snow areas in the south on Monday.  There could be light snow showers on and off in Reigate and another moderate fall is possible Monday pm as the LOW tracks nearer to the south east of our region.  It will feel cold in strong gusts from the East: wind chills down to an outrageous -18ºC are theoretically possible if you are caught in strong easterly gusts with air temps below 0ºC all day. Temperatures will actually fall through the day on Monday as the Polar air takes a grip.  Lowest temperatures all winter are possible overnight Monday-Tuesday. Hopefully, this weather change will not take anyone by surprise: it has been on the cards for over a week and flagged up here on 2 March. The rest of the week will remain cold, but feel less cold in light winds, and drier with pressure building from the north, but more snow might precede rain later in the week as things warm a little next weekend.  More updates later as details emerge for Monday which is still v changeable!

HIGH pressure means air sinks, warms up as it descends and dries out: this should mean less cloud… right? Unfortunately not always. Reigate has seen very little sunshine in the last few weeks despite a persistent HIGH pressure. Stratocumulus cloud and stratus cloud has hung low over the North Downs. This low cloud has been trapped by a temperature inversion at 1000m, where temperatures rise with height putting a “lid” on the weather. Any rising air will hit this inversion and convection will stop. Clouds forming beneath the inversion will simply spread out, join up and cover the sky: creating overcast dull gloomy conditions. Once the cloud forms it is difficult to shift because, whilst the sunshines above the cloud warming the air, the cold air persists below exacerbating the inversion. A stronger breeze or drier air or a shift in the position of the HIGH will release us from this weather underworld. The good news is the weekend looks brighter with winds less likely to be blowing from a cool, damp North Sea. Sunday looks the brightest with temperatures reaching 9 or 10C by the afternoon: almost spring like. The rest of the week from Wednesday will become more unsettled, feeling warmer but wetter as the HIGH at last moves away to the south east allowing Atlantic westerlies back for a breath of wet air!

To Spring… early forecast models suggest wetter conditions with slightly below average temps for the south of England with possible frequent easterly winds as LOWS track to the south; and drier, warmer and slightly above average conditions in the north of the country.  Essentially HIGH to the north and LOW / jet stream to the south of the country which will keep the south cooler and unsettled with the north enjoying better weather.  In short, more of what we had in late winter, just a warmer version?! Updates later.