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Dust!

May 22, 2016 — 3 Comments

Saharan dust reaching the UK gets in the news quite regularly, usually unfavourably in connection with pollution events. Desert dust is one of several types of minute particles, called aerosols, that are emitted into the atmosphere including salt, carbon and volcanic ash. Human made aerosols, such as CFCs and sulfate aerosols from the burning of fossil fuels, are infamous for destroying the ozone layer and causing climate change but 90% of atmospheric aerosols have natural origins and they all contribute in major ways to the global weather machine. The aerosols constantly floating around in the air that we breathe are made up of a complex mix of particles clumping together in an invisible soup that we are unaware of most of the time.  On occasions dust concentration becomes “thick” enough to become visible and reveals itself as haze. Here is some information about desert dust to help get to grips with this impressive weather phenomenon and hopefully clear the air of those mysterious arid particles!  This post concentrates mostly on Saharan desert dust but volcanic dust is mentioned as a comparison and is worthy of a separate post at a later date.

Where does it come from?

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Saharan dust blown across the Atlantic reaches South America

Dust is naturally lifted into the atmosphere from deserts and is an important component of global weather and climate processes and nutrient transport to ecosystems.  Global mineral / desert dust emissions into the atmosphere are estimated to be up to 1500-1800 Tg/year (teragrams) per year and emerge from numerous arid and semi-arid source regions.  For comparison, the average global annual volcanic output of ash from average scale small eruptions has been estimated as an average of only 20 Tg/yr (20 million tonnes per year).  Less frequent, larger eruptions, inject much more ash into the atmosphere. The Icelandic volcano, Eyjafjallajökull, erupted 250 million tons of volcanic ash during the eruption in 2011. This was still small compared with the largest mega eruptions which blast huge volumes of volcanic dust higher into the atmosphere. Desert dust is usually swept up by winds only as high as the planetary boundary layer 2-4km (PBL), whereas volcanic ash can be injected into the high troposphere or even the stratosphere where it encircles the Earth quickly. Nevertheless, in average conditions desert dust usually dwarfs volcanic ash in the atmosphere, unless there is a colossal eruption. (1 Tg = 1 million tonnes ) and albeit desert dust resides at lower altitudes.

global dust source regions

global dust source regions

The biggest global source of atmospheric dust is the Sahara Desert, a huge area of sand dunes, stone and gravel plateaus, dry valleys and salt plains creating nearly 5 million sq km of potential dust producing terrain.  Within the Sahara Desert the Bodele Depression in Chad is thought to contribute half of all Saharan dust.

How does dust get into the air?

Dust is lifted by strong surface winds produced at different scales, from small local convective processes such as dust devils to meso-scale convective systems such as large thunderstorms through to regional scale frontal depressions. Importantly, rainfall in arid areas contributes to available dust by causing flash floods that wash fine debris into river and lake beds. These rivers and lakes then dry out and provide an important source of desert dust when the wind blows. A good example is the Bodélé Depression in Chad, which is part of the dried out Lake Chad.  This area has dust storms on average of 100 days per year and can loft 700,000 tonnes of dust into the atmosphere every day.

http://en.wikipedia.org/wiki/Bod%C3%A9l%C3%A9_Depression

Meso-scale convective weather systems in deserts can cause strong cold downdrafts of out-flowing evaporatively cooled air descending from cumulonimbus storm clouds that can entrain particles and lift them vertically into powerful upward thermals. Sandstorms known locally as haboobs are created in this way and appear as frightening “Hollywood”style dust fronts in Africa, Australia, China, the USA and recently in the film Interstellar.

haboob dust storm formation

haboob dust storm formation

Dust can also be lifted from the surface by powerful winds covering a large area associated with troughs and fronts sweeping across, or near to, desert regions.  One such wind is called the Sirocco which occurs in eastward tracking Mediterranean lows where the warm sector produces strong southerly winds which can bring dusty conditions into Europe especially in Spring and Autumn.

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A significant sirocco event occurred 23 March 2016.  The event shows up well on a synoptic chart and satellite photo.

Various other synoptic scale meteorological scenarios bringing European / UK dust events are discussed below.  Once elevated, coarse dust (sand) falls out nearest to the origin but fine dust (clay), less than 0.002mm in diameter, can be lifted high into the troposphere, up to 10km, where it can remain aloft for weeks and be driven thousands of miles across oceans by jetstreams.  Saharan dust routinely travels to the Caribbean in the summer on an easterly jetstream.  Dust is eventually deposited in light winds, usually in anticyclonic high pressure systems, or is washed out in rainfall.  In this way some 40 million tons of dust is transported from the Sahara and deposited in the Amazon rainforest every year.

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desert dust entrainment and transport

There are broadly two types of dust storm.

  1. Dust plumes have a streaky linear point pattern of dust emerging from a point source and spreading into a cone.
  2. Dust fronts are walls of dust rising on an extensive, frequently curved path.

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Desertification of environments in China and Africa seem likely to be increasing the area of global dust producing regions and potentially making the planet more dusty. However, it is not certain whether global atmospheric dustiness will increase or decrease due to expected climate change in source regions like North Africa.  The world has certainly been more dusty in the past. It is understood that during past glacial periods (last glacial maximum 18,000 years before present) water was locked up in glaciers creating drier conditions particularly in periglacial mid-latitudes.  In Europe, China (Yellow River) and the US (Idaho, Washington, Iowa and Mississippi), huge areas of wind-born dust deposited thick aeolian sediments, one of which extends across the North European plain which now forms very fertile soil called loess.  Loess has become some of the most productive agricultural terrain in the world.  “Dust to dust” seems more apposite than ever considering our reliance on natural dust transport for our food.

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loess hills, fertile farmland courtesy of dust!

What are the impacts of dust on climate and environment?

Over long time scales dustiness increases during cold climatic periods (glacials or ice ages). Evidence for this comes from ice cores in Antarctica and Greenland shown below.

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Reasons for increased dustiness during cold glacial periods includes:

  • Increase desertification (less rainfall generally in cold periods as more water locked up as ice)
  • Increased land area (due to falling sea levels, so more dust sources available)
  • Increased winds

Over shorter time scales, dust plays various complex and sometimes contradictory roles in atmospheric processes, including modification of solar energy receipt, temperature, cloud formation and influencing rainfall.  Dust also has impacts on ecosystems and human activities which can be beneficial or detrimental and even hazardous. So, what can atmospheric dust do exactly?

DUST…

  • absorbs and scatters incoming sunshine causing surface cooling
  • increases cloud condensation nuclei enhancing rainfall or…
  • increases cloud condensation nuclei enhancing condensation of small droplets which stay aloft so reducing rainfall
  • causes “blood / red / mud rain” events creating dirty cars and windows
  • neutralizes acid rain: dominant minerals in dust are usually >pH7 and include acid neutralizing carbonates
  • imports important beneficial minerals and nutrients such as nitrates, phosphates, iron, calcium, silicates etc to ecosystems like the Amazon rainforest. 200 million tons of fertilizing dust is transported from Africa to the Amazon each year of which about 40 million tons is deposited directly into the forest ecosystem: this is possibly the main nutrient source for the forest. Marine ecosystems also benefit from dust inputs e.g. stimulating growth of phytoplankton and subsequent food chain.
  • imports pernicious alien spores and soil fungus to coral reefs potentially causing coral death events
  • reduces Atlantic hurricane formation: enhanced dust from the Saharan Air Layer (SAL) over the Atlantic during the hurricane season has been correlated with reduced numbers of hurricanes, possibly due to the dust reducing sunshine which suppresses Atlantic sea surface temperatures in the hurricane development zone.  The EUMETSAT satellite image below shows a dust veil (pink) killing off convection cells (brown and green) as it moves across the Atlantic towards the Caribbean.  The Saharan Air Layer is a hot, dry and dusty stream of upper air emanating from West Africa, especially during summer.  The SAL could also inhibit convection, and hurricane formation, by creating an inversion preventing updrafts necessary to kick-start tropical storms.

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  • Dust also impacts human activities and health.  Severe dust storms impact activities requiring good visibility such as air travel and some sports. It can also carry organisms such as spores, fungus, bacteria and viruses which could introduce disease far away from the origin of the dust.  Serious cardiovascular and respiratory problems might also be aggravated by fine airborne dust. http://rt.com/news/smog-britain-sahara-pollution-981/

So, dust is clearly a critical part of the weather machine and can bring both benefits and problems.  The next section attempts to explain how desert dust can get all the way to Britain from the Sahara.

What weather patterns typically bring dust to the UK and Europe?

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In Winter, the subtropical Saharan HIGH pressure is strong with winds wanting to spill away in all directions, potentially carrying dust.  However, with a more southerly jetstream and visits by low pressure systems, the Mediterranean is often unsettled and wet during winter. Despite the Sahara being dusty in winter, dust events extending to Europe in winter tend to be restricted because particles are washed-out by winter rainfall before it gets very far north.

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The transitional seasons of Spring and Autumn can produce the most significant dust episodes in Europe. The desert heats up and dries out creating ideal conditions for dust to be elevated by strong winds.  Low pressure can still dip south on meridional jetstreams and create Genoa low pressure which typically increases wind speeds across North Africa.  A cooler Mediterranean Sea surface temperature means that less convection occurs and creates less wash out opportunities as any dust travels north. Therefore, springtime is potentially more dusty for Europe given the right conditions.

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In Summer the Mediterranean  HIGH pressure develops as a semi-permanent feature.  This inhibits transport of dust from the Sahara.  Nevertheless, occasional heat lows over Iberia or cut-off lows can create the right southerly wind on a Spanish Plume to bring dust as far as the UK.

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Saharan dust moves north associated with Spanish Plume

Dust events in Europe vary in scale and can occur at any time of year but it seems usually and most effectively in transition seasons, especially Spring. In 1901 an historic dust event created the first recorded “blood rain” across Europe.  In this single dust event, well documented, some 50,000 tonnes of dust was deposited across Europe (this would have required a 250km long convoy of 2500 20-tonnes lorries to transport). It has been estimated that dust build up across Europe is 4-5mm per century.

Some case studies of European dust events

Here are some examples of past European dust events showing the synoptic evolution of how dust gets to Europe. Note that meso-scale convective systems (MCS) typically producing dust storms in the Sahara are sub-synoptic and sometimes the dust lofting event barely shows up on these charts.  Nevertheless, the synoptic patterns transporting the dust into populated parts of Europe are well illustrated in these examples.

European Dust March 2014

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European Dust April 2011

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So what is the future for dust?

There is no certainty on the impact of climate change on the future of dustiness in the atmosphere.  There have been press articles suggest there is increasing Saharan dust emission due to population increase, intensive farming and land degradation in North Africa.

“There has been a dramatic increase in some aspects of dust flux [emissions], which have doubled over the last 50 years. Population pressure alone is likely to exacerbate the problem and if current trends continue the amount could double again over the next 50 years,” said Dr Bryant, a Reader in Dryland Processes at the University of Sheffield.

Nevertheless, the impact of these activities is not certain and others suggest dust emissions are not increasing. For example, despite human desertification and degradation of semi-arid environments causing increased potential source areas of dust, it appears that the most significant dust source globally, the Sahara  desert, has not in fact been perturbed by human activities since the major dust sources are mostly in uninhabited areas and in true-deserts.

The IPCC predict that North Africa will get drier and therefore presumably more dusty.  However, models suggest that specific dust source regions could become wetter.  There are significant uncertainties over African dust and climate change and there seems to be no clear correlation over recent decades between measurable climate change and dust load in the atmosphere. Models cannot agree on rainfall changes in North Africa.

Here are some links for further information on dust…

aerosol http://www.nasa.gov/centers/langley/news/factsheets/Aerosols.html

excellent detail:  http://www.goes-r.gov/users/comet/EUMETSAT/at_dust/print.htm#page_1.3.0

Greek forecasting dust: http://forecast.uoa.gr/dustindx.php?domain=med

Barcelona dust forecast centre http://dust.aemet.es/forecast

satellite dust over Western Europe: http://oiswww.eumetsat.org/IPPS/html/MSG/RGB/DUST/WESTERNEUROPE/index.htm

cross-sections of dust across Europe: http://charadmexp.gr/instruments/16/

ecosystem impacts http://gallery.usgs.gov/videos/223#.VS1CZfnF_To

http://earthobservatory.nasa.gov/Features/Aerosols/

http://www.independent.co.uk/environment/saharan-dust-six-things-you-didnt-know-about-red-dust-storms-9232879.html

more info: https://books.google.co.uk/books?id=RyYyWQrn-D8C&pg=PA288&lpg=PA288&dq=dust+events+europe&source=bl&ots=EAi_LZA045&sig=7lvbivQoXeOXESgsqRaI8ERn27w&hl=en&sa=X&ei=h78rVf-2AeWQ7Abqx4GgDg&ved=0CDgQ6AEwAw#v=onepage&q=dust%20events%20europe&f=false

http://www.hindawi.com/journals/isrn/2013/245076/

http://www.climatenewsnetwork.net/desert-dust-feeds-deep-ocean-life/

https://news.cnrs.fr/articles/dust-blowing-in-the-winds-of-climate-change?utm_content=buffercc37b&utm_medium=social&utm_source=twitter.com&utm_campaign=buffer

notes

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El Nino: massive Pacific heat engine

El Niño has no obvious or strong effect on UK winter weather.  Historically, El Niño years have coincided with both mild/wet and cold/dry winters in the UK.  By itself, El Niño does not directly drive our winters in any single, simple direction. For example, El Niño winter 2009/10 was the coldest winter for 30 years with a notable “Big Freeze”, while the El Niño winter 2006/2007 was the second warmest winter on record.  The strongest recent “Mega” El Niño in 1997-98 turned out to be a stormy and mild December in the UK, with just two minor snow events and then a notably mild January and February, with Tmax even reaching 17C on occasions.  However, despite weak and ambiguous El Niño signals for UK winters, when other weather drivers and teleconnections are combined with El Niño there is some research to suggest that stronger impacts such as stormy early winters and cold dry late winters are possible. The current El Niño could turn out to be one of the most powerful in 50 years (though recent measurements still show it 3rd in the league table of Mega-El Niños).  Of course, wintry weather can also occur completely independently of any El Niño event, such as the snow of January 2013.

“In Britain, the impact of El Niño is nowhere near as marked as in other parts of the world. But it does tip the balance a little bit more in favour of wet and windy weather. It makes it more probable,”

Jeff Knight, a climate modeller at the Met Office’s Hadley Centre, Exeter.

Read on for more details on how this fascinating and topical weather story might or might not impact our winter weather!

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Some drivers, teleconnections and indicators of UK winter weather

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Winter 2015-16 most powerful El Nino in 50 years?

El Niño: What is it?

El Niño is a natural change in the atmospheric pressure and wind patterns and flow of ocean currents in the Equatorial Pacific.  In normal “non-El Niño” conditions, a 200 metre deep pool of the world’s warmest sea surface water builds up in the West Pacific.

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Average Pacific Ocean sea surface temperatures non-El Nino conditions

This is known as the West Pacific Warm Pool and it is formed by intense insolation, a piling up of warm water driven by the easterly trade winds and low evaporation in light winds found round Indonesia.  Moist warm air over the West Pacific Warm Pool creates an area of instability and convergence in low pressure systems where air rises forming deep tropical clouds, heavy rain and thunderstorms and sometimes typhoons.  A contrasting cold pool exists in the East Pacific where upwelling of deep ocean water reaches the surface off the coast of Peru courtesy of the cold Humboldt current bringing Antarctic water up from the ocean depths . Cool dry air subsiding over the cold pool in the East Pacific condenses moist air and forms low cloud and fog that acts as a feedback loop by reducing insolation and creating cooler conditions. Normally, brisk Easterly Trade Winds drive this cool tongue of ocean water west and, on it’s journey along the Equator, the sea surface warms up.  This normal Pacific pattern is known as the “Walker Circulation”.

Normal Pacific Walker Circulation

Normal Pacific Walker Circulation (RGSweather diagram)

Strong El Niño episodes result in a reversal of the normal pattern of Pacific Ocean wind and ocean currents and dramatically changes the sea surface temperatures across the Pacific.

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Pacific Ocean sea surface temperatures during warm El Nino phase

The reversal of winds and currents causes the West Pacific Warm Pool to move to the Central and East Pacific where there is normally cold ocean water, hence El Niño are known as “warm phases”.  Strong El Niño phases produce a tongue of above average sea surface temperatures extending 13,000 km long and 1000 km wide across the Equatorial Pacific and this has a major impact on weather patterns across parts of the world, especially during the Northern Hemisphere winter when El Niño usually reaches a peak of intensity.

El Nino phase Pacific Ocean

El Nino phase Pacific Ocean (RGSweather diagram)

During El Niño episodes the Pacific trade winds weaken, the subtropical jetstream can reverse and strengthen and wind driven upwelling slackens.  As a result the Equatorial ocean current reverses as warm water starts moving to the east. Whilst it is not known what causes an El Niño, a key change is in the pressure pattern across the Pacific basin.

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Tahiti sea level pressure correlates negatively with Darwin i.e. when one is high, the other is low. ENSO warm phase yields HIGH pressure over Darwin.

ENSO: El Niño Southern Oscillation or Pacific Pressure See-Saw

In El Niño phases the normally LOW pressure measured over Darwin, Australia changes to higher pressure and the reverse goes for pressure over the east Pacific, measured in Tahiti, where pressure falls.  The changing fortunes of these pressure cells is known as the Southern Oscillation.  The reversal of pressure gradient weakens or reverses the trade winds and allows the West Pacific warm pool to “slosh” east across the Pacific towards South America.  The resulting thermal expansion and the reversal of ocean currents, actually raises the sea level in the East Pacific.  This all takes months and the coupling of the ocean currents and atmospheric winds is critical in creating a complete El Niño Southern Oscillation (ENSO). This video explains the phenomenon well:

So during an El Niño event, the easterly trade winds converging across the equatorial Pacific weaken.  This in turn slows the ocean current that draws surface water away from the western coast of South America and reduces the upwelling of cold, nutrient–rich water from the deeper ocean, flattening out the thermocline (boundary between deep cold water and surface warm water) and allowing warm surface water to build in the eastern part of the Pacific.  Once the ocean currents and atmospheric winds “couple-up” then a positive feedback loop is established which causes further sea surface warming in the East Pacific.  Here the air is warmed above, becomes more buoyant and rises, lowering pressure so further drawing in more westerly winds.  These changes transport enormous amounts of heat and energy to the East Pacific which alters the subtropical jetstream which transfers changes in the atmosphere further “downstream” to other parts of the world.

El Nino

El Nino sea surface temperature anomalies October 2015

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El Nino sea surface temperature anomalies November 5th 2015 (courtesy xmetman wordpress)

Significant global Impacts

Due to the release of immense amounts of heat from the Pacific Ocean, El Niño years often become record-breakers for global average temperature.  The energy and moisture released flows “downstream” into the global circulation and has significant impacts on weather elsewhere.  El Niño reaches a peak around Christmas, hence the name “Christ Child” bestowed on the phenomenon by Peruvian fishermen who suffer from the collapse of their fisheries during warm episodes as the upwelling of nutrient rich bottom waters are capped by the invasion of the nutrient poor warm pool.  This causes a temporary collapse in sea life in the East Pacific.  El Niño occur periodically but irregularly over a cycle of 3 to 7 years, they differ in strength and are sometimes followed by a corresponding reversal to a strengthened “normal” flow called La Nina. The last mega-El Niño was 1997-1998 and our 2015-2016 El Niño looks like matching that strength or possibly exceeding it (update November: not so likely now* see Xmetman blog post in refs at foot of page)

The effects of El Niño around the Pacific and neighbouring continents are the most obvious and well correlated with the event, for example wetter and stormier conditions in South America, drier drought conditions with more wildfires in Indonesia and Australia and NE Brazil and a weaker SE Asian monsoon and wet winters in SE USA.  El Niño years also correlate with 44% fewer Atlantic hurricanes due to the enhanced subtropical jetstream shearing the heads off developing thunderstorms and enhanced Pacific hurricanes due to warmer SSTs e.g. Patricia October 2015.  Some of these effects have already occurred in the 2015 El Niño with hurricane activity correlating well with expected changes and an Indian heatwave with reduced monsoon.

The chart below shows a composite of analogue surface temperature anomalies for October in El Niño years (source JMA) compared to the actual conditions measured for mid-October 2015.  The patterns match surprisingly well, especially for the more significant and more strongly correlated locations.  This hints at how patterns for this El Niño might expect to map out as expected. Note the lack of any significant impact in NW Europe.

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Around the Pacific, very roughly, places that are normally wet and stormy become drier and more settled but can also suffer drought and fires e.g. Indonesia and Australia, while those places which are normally dry become stormy and wet and suffer from flash floods and landslides e.g. Peru and California. The most extreme weather impacts occur during the cold winter season of each El Niño in the Pacific but the knock-on effects can last into the following summer and link with places over great distances. The charts below show some of the recognised El Niño impacts. Note the complete absence of any reliable or linear teleconnections in Europe recognised by NOAA.

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Impacts of El Nino

Weak UK and European Impacts…

Impacts on the weather further away from the Pacific mostly consist of weaker signals that are often reversible due to other stronger weather drivers.  The impact of El Niños on European weather, especially the UK, fits into this category because there are no strong, reliable impacts based solely on El Niño episodes on UK weather.

“There is really no effect in the U.K. that we can say is definitely caused by El Niño” AccuWeather Meteorologist Tyler Ros states.

Other drivers of weather become more significant because the UK is located further downstream and along way from action in the Pacific.  Research by Judah Cohen, Atmospheric and Environmental Research (AER), suggests El Niño warm years overall bring warmer winters to the Northern Hemisphere. However, other research has picked up on some weak “teleconnections” between El Niño events and colder European winters. Some of the connections are illustrated below:

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El Nino connections to European winter weather (diagram RGSweather)

…But some possible El Niño signals for UK and Europe?

In Europe, research shows that any El Niño signals are “strongest” in middle and late winter and they approximate to a negative North Atlantic Oscillation.  A negative NAO corresponds with higher pressure over Iceland and a weaker meridional (wiggly) jetstream.  This situation can lead to cold outbreaks for the UK as a sinuous jet can provide chances for Arctic air to leak out of the Poles. In addition, El Niño is associated with low temperatures and decreased precipitation over NE Europe, connected with higher than normal pressure here. This provides the UK with the risk of cold North Easterly winds coming from Russia… so called “Beast from the East”. Some modest El Niño signals emerging from research for European winter weather are listed below but it is important to point out that these are weak signals and other research finds no reliable El Niño winter signals at all!

  • Atlantic storm tracks shifted south taking storms over Mediterranean
  • More cyclonic weather patterns over Central Europe
  • Pressure over Scandinavia HIGH; or western Russia anticyclone expanded over Europe: this would increase the chances of cold Easterly winds
  • High sea level pressure over Iceland across to Scandinavia and NE Europe
  • NE Baltic cold impact: but not in very strong El Nino events when warm impact may occur (UK MetOffice)
  • LOW sea level pressure across central Europe and Western Europe: higher precipitation
  • Some El Ninos have cold winters in NE Europe and enhanced precipitation in Central Western Europe
  • Positive NAO in Nov-Dec : this would mean a stronger jetstream with milder conditions for much of Europe, especially NW
  • Negative NAO late winter into Spring: this would mean a weaker more meridional jetstream with the possibility of blocked patterns and potential Arctic outbreaks or easterlies (other things coming into play)
  • High rainfall in the Mediterranean and decreased precipitation over NW Europe and Scandinavia
  • Frequency of upper troughs over central Europe was very high
  • Temperatures and precipitation over Turkey are high.
  • Israel high rainfall

There are mixed messages regarding the El Niño signal for European weather. Overall, the signal is most consistent in late winter and resembles the negative phase of the North Atlantic Oscillation which itself links to higher chances of cold winter episodes with northern blocking.  The prolonged 1940–1942 El Niño was accompanied in northeastern Europe by three of the coldest winters of the 20th century. In early winter the signal is almost the opposite with a positive NAO, stronger jetstream which brings milder stormier conditions to Europe.  Variability between El Niño impacts on Europe is also large and range much larger than the impacts themselves. Some research shows such variability might be due to volcanic eruptions in the tropics prior to El Niño events.  There is some evidence that pronounced El Niño impacts on European weather follows major volcanic eruptions e.g. El Chichon 1982, Pinatubo 1991.

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Calbuco Volcano erupted April 2015 but is this eruption big enough to trigger stronger El Nino teleconnections this winter?

Links between El Niño  and other atmospheric drivers

In the 20th Century all three of the strong El Niño events followed major volcanic eruptions. Even so, the signals were not consistent between these events.  Some studies also show a connection of El Niño events to stratospheric conditions. Warming of the stratosphere (sudden stratospheric warming) and subsequent weakening of the Polar Vortex have been linked to increased chances of cold winter weather in Europe (due to a weakening of upper westerly zonal winds propagating down into the Troposphere, allowing cold easterlies to break out into Europe).  Some research finds an increased frequency of such stratospheric events, especially in late winter, during El Niño years. Additionally, volcanic eruptions might also play a role in warming the lower stratosphere and encourage SSW (sudden stratospheric warming) events but further research is needed to establish any firm connection.

In addition to volcanic activity and stratospheric behaviour, other drivers and atmospheric behaviours can have significant influences on UK winters and these might enhance or reduce any El Niño signal or overwhelm it completely. Examples of some drivers / indicators and teleconnections that seasonal forecasters use include:

  • Solar activity: low sunspot numbers connect to northern blocking.  Currently low.
  • Atlantic hurricane activity: more hurricane activity injects heat to the Poles that increases the chance of northern blocking and cold winters. 2015 season very low hurricane activity.
  • October Siberian snow cover: high and rapid expansion of Eurasian snow cover in October links to increased chance of sudden stratospheric warming later in the winter which can cause cold late winters. Current Siberian snow cover is more than more recent recorded years.
  • October weather patterns: recent research shows that an anticyclonic October in the UK (dry) can link to cold winters with LOW pressure in Europe and northern blocking at high latitudes. This enhances a negative NAO.
  • Quasi-Biennial-Oscillation: westerly upper tropical wind pattern surpresses chances of cold outbreaks in mid latitudes. Currently westerly QBO.
  • Atlantic sea surface temperatures: tripole of warm/cold/warm pattern hints at potential for -ve north atlantic oscillation in winter.  Currently no tripole but cold pool anomaly in central North Atlantic could cool NW flow a little more than usual.

How the different teleconnections work together is complicated and many are at the cutting edge of climate long range forecasting and research.

The search for ENSO / winter correlation

Reanalysis of groups of strong El Niño years can be correlated with years exhibiting current atmospheric patterns from the list above and these show interesting results for UK winters illustrated below.  The following recent twitter chat is an example of such reanalysis widely undertaken by weather experts and enthusiasts:

//platform.twitter.com/widgets.jsThese can then be rolled forward to see how things pan out through the winter. Here is an interesting example from consultant meteorologist Anthony Masiello.  He has reanalysed El Niño winters with years with November positive Arctic Oscillations (as now).

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December enso: westerly influence, positive NAO

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January enso: with Siberian snow cover: Atlantic blocking with -Ve AO and potential for Arctic incursions

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February enso: Siberian snow cover years yield a beast from the east

The results above seem to match the idea of warmer unsettled Atlantic driven conditions before Christmas and colder blocked patterns after Christmas i.e. January and February have a decidedly blocked patterns to the north with low pressure to the south… as predicted in El Niño years as a possibility. HOWEVER.. if you check the number of years represented there are only 10 for El Niño years with +NAO and only 5 with widespread October Siberian snow cover. This is therefore not a significant finding, as Anthony himself points out on twitter.

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Too much inter-event variability for headline news

Globally, there is great inter-event variability between different El Niño years. The charts above, from JMA, show composite impacts of El Niño events and their significance over several decades. These appear to show some warmer and wetter than average winter conditions in Europe which agrees with Cohen et al but is in contrast to other findings.  To complicate matters further there are also different types of El Niño such as Central Pacific (Modoki) events that are correlated with different impacts on global weather e.g. colder winters in USA.  The latest 2015-16 El Niño appears to be turning out as a “standard” East Pacific mega-El Niño event with a long continuous tongue of warm SST anomalies stretching across the east Pacific.  Even so, no two El Niño events are the same.

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annual nonsense headlines from Daily Express about winter weather

One thing is for sure, recent newspaper reports touting confident headlines suggesting certainty over severe winter weather impacts in the UK and Europe “caused” by El Niño are not based on the findings of climate research or historic precedent which show only tentative and conflicting connections with our winter weather. It might be more accurate to suggest that no one really knows how El Niño  mixing with all the other connections will play out this winter!  Nevertheless, this should not stop the efforts of scientists trying to find clues for long range forecasts.

The last word should go to the UK MetOffice who state the following for the UK winter outlook with regard to El Niño 2015:

What does El Niño imply for the UK this winter?

Unlike some parts of the world, the effect of El Niño on Europe is relatively subtle. In El Niño years there is a tendency for early winter to be warmer and wetter than usual and late winter to be colder and drier. Despite this, it is just one of the factors that influence our winters, so other influences can overwhelm this signal – it is relatively straightforward, for example, to find years where these general trends were not followed.

El Niño moderately increases the probability of the positive phase of the North Atlantic Oscillation (NAO) in late autumn and early winter and the negative phase of the NAO in late winter. (In winter) the positive phase of the NAO is associated with milder- and wetter-than-average conditions, whilst the negative phase is associated with colder- and drier-than-average conditions.

useful references:

https://bobtisdale.wordpress.com/2014/02/22/el-nino-and-la-nina-basics-introduction-to-the-pacific-trade-winds/

http://www.vox.com/cards/el-nino-2014/why-is-everyone-talking-about-el-nino

http://www.metoffice.gov.uk/research/climate/seasonal-to-decadal/gpc-outlooks/el-nino-la-nina/ENSO-impacts

http://onlinelibrary.wiley.com/doi/10.1029/2006RG000199/full

http://www.weatherwest.com/archives/3566

http://blog.metoffice.gov.uk/tag/el-nino/

http://www.metoffice.gov.uk/media/pdf/n/b/A3_plots-temp-NDJ_v2.pdf

http://news.sky.com/story/1552290/what-is-an-el-nino-and-how-will-it-affect-us

http://www.accuweather.com/en/weather-news/godzilla-el-nino-not-to-blame-for-uk-cold/53117537

https://xmetman.wordpress.com/2015/11/07/latest-el-nino-still-only-3rd-strongest/

winter 2006 – 7 http://news.bbc.co.uk/1/hi/sci/tech/6401063.stm

winter 2009-10 http://www.metoffice.gov.uk/about-us/who/how/case-studies/winter09-10

winter 2013 http://www.metoffice.gov.uk/about-us/how/case-studies/january-2013-snow

http://www.independent.co.uk/news/science/el-nino-how-the-phenomenon-triggers-major-changes-in-weather-patterns-across-the-world-a6725806.html?utm_source=&utm_medium=&utm_campaign=

http://www.telegraph.co.uk/news/weather/11999259/British-winter-could-feel-longer-and-colder-because-of-El-Nino.html

Soil moisture: possibly the most under-rated meteorological measurement!  Rarely do weathermen get animated about the extent of wet sod across the country.  Nevertheless, soil moisture, usually measured in centimetres of water in the top two metres of soil or as % saturation (see maps below), has been found to control continental scale weather patterns, summer maximum temperatures and even heat waves and the extent of droughts.  So we ignore soil moisture at our peril, especially as soil moisture also controls vegetation growth and death and the ability of farmers to grow food.

How wet the soil obviously relates to how much it has rained recently.  During winter, in mid-latitudes, soils usually become increasingly saturated with a surplus of water building up as inputs of precipitation exceed evaporation which is reduced in the cooler months and shorter days.  During the summer, soils tend to become increasingly depleted of their moisture content as evaporation (output) exceeds precipitation (input).  This input and output of moisture forms an annual balance known as a soil moisture budget and is shown in the graph below.

2014-03-28_21-36-57

High rainfall during the winter builds up a lot of water in the soil.  In the spring time a high soil water content “uses up” more energy from the sun in the process of evaporation.  The more energy “used up” in evaporation, the more energy is lost from the system to produce sensible warming at the surface. Hidden energy, or latent heat, is required to change liquid water into water vapour.  So “latent cooling” reduces the amount of energy available to warm the atmosphere as long wave radiation.  So local temperatures can be depressed over areas of wet soil especially during a spring when wetter-than-usual soils might take a long time to dry out.  It might also be expected that, after a wet winter, there could be a cooler period until such time that the soil dries out locally and more energy becomes available to produce a sensible heat flux at the surface.

In fact, the effects of soil moisture go far beyond these micro-climatic changes and can have impacts that are continental in scale.  In a 2007 study (see below) it was found that 25% extra soil moisture could reduce continental Europe-wide temperatures by up to 2c from average summer maximums. Likewise, a 25% reduction in soil moisture could raise temperatures across continental Europe by 2c in the study period.  It was also discovered that higher winter and spring soil moisture could raise summer precipitation levels and change continent-wide pressure patterns.

The study from 2007 used reanalysis of computer weather models to investigate the impact of soil moisture on the European 2003 heat wave, the warmest for 500 years.  This heat wave killed over 20,000 people and caused crop damage.  The study found that by re-running computer models just with different soil moisture values, the maximum temperatures and heat wave intensity varied greatly.  Dry soils during the spring increased summer heat wave intensities while wet soils reduced the maximum temperatures.  The difference was significant, and in some localized regions the intensity of heat anomalies varied by 40% simply due to different soil moisture content at the outset of the model runs.  The largest differences were mainly located over central Europe.  It seems that differences in soil moisture have most impact across central continental Europe and progressively less impact on summer temps with increased distance further north.

Not only did soil moisture control temperatures, it also had a control over continental pressure patterns.  Dry soils built pressure through the middle troposphere, while wet soils could lower pressure.  This has numerous positive feedbacks: wetter soils reduce pressure which increases cloud formation and summer rainfall that enhance the wet soils.  Dry soils build pressure, reducing cloud formation, reducing summer rainfall, further drying out soils.

2014-03-28_22-22-50

So, in summary:

  • If the soil-moisture deficit is high, the dry soils raise the sensible heat flux, producing a deeper, warmer, drier low-level atmosphere: raising temperatures and enhancing surface heating and drying.  Increased drought risk.
  • If soil moisture is high, the latent heat flux by evaporation and transpiration dominates, enhancing cloud formation and a tendency for cooling.  lower temperatures and enhancing rainfall and further wetting of the soil.  Increased wet summer.

2014-03-28_21-46-22

 

The full article is here: Fischer_heat_waves_2007 (1)

Here are some charts and figures from our Vantage Pro2 weather station summarising 2013 weather in Reigate.  All data is posted on our data page for you to download freely and use, but please credit RGSweather for any subsequently produced materials.  Excel pivot tables have been used to generate some of the more fun charts and statistics below.

Our overall conclusion from 2013 is that Reigate has some of the best weather in the UK, probably!  Read on to find out why…

Reigate Temperature

Reigate topped the charts for recording the highest temperature in the UK at times during 2013 summer.

Average temperature 9.9C

Highest Tmax 33.5C on 1 August at 4:22pm. (Heathrow recorded 34.1C)

Lowest Tmin -5.9C on 22 January at 3:37am.

Lowest wind chill -11.9C on 12 March at 5:03am.

The chart below shows daily average temperatures in Reigate throughout 2013 as a full circle.  Note the annual average temperature for Reigate (10c) is shown as a red circle.

Reigate Rainfall

Despite recent experiences, it doesn’t usually rain that much in Reigate compared to elsewhere in the UK.  The extraordinary rainfall of 23-24 Dec recorded a rare 70mm for that period on our manual rain gauge. The graph shows that this rainfall total is unsual for Reigate.

Total 654mm (VP2 rain collector; uncalibrated on site; intended calibration v soon; we suspect some under-reading of totals at this stage)

Average daily rainfall 1.8mm per day

The wettest day of the week, overall, in 2013 was… Friday (but not significantly) with Thursday being a significant driest day.

Highest rainfall intensity 182mm/hr (briefly!) on 22 November at 8.38am.

Reigate Wind

The strongest gust recorded in 2013 was 48mph, more exposed hills and places locally recorded 60-70mph (e.g. Redhill aerodrome).  Reigate is sheltered from Northerly winds by Reigate Hill and from more common gales from the South or SW by high ground at Priory Park and woodlands.

Average highest daily wind speed  8.7mph

Max gust 48mph from WSW on 28 October at 6:15am

Dominant wind direction WSW.

The windiest months were October and December in respect of highest maximum wind gusts measured.

Wind run is a measurement of how much wind has passed a given point over a period of time. A wind blowing at three miles per hour for an entire hour would give a wind run of three miles.  Wind run is a measure of the persistence and duration of a particular wind direction.  The longest wind runs for Reigate in 2013 were associated with NE winds in March, which were persistent.  The fateful Channel Blizzard wind run that met warm SW winds and caused such unusual snowfall in the Channel Islands recorded the longest wind run of 267 miles. It’s interesting to note that our most common westerly and SW winds achieve relatively short wind runs, this is probably because our prevailing winds are associated with LOW pressure systems that cause rapid changes in wind direction as they pass through.

Rain and Wind

Most of the heaviest rainfall totals for Reigate in 2013 arrived on South (S) and West-South-West (WSW) winds.  The wind direction bringing highest average rainfall intensity (rainfall rate mm/hr) in 2013 was SSW.  There looks to be a strong correlation between rainfall rate and wind speed.  The chart shows that the heaviest rain tends to occur at lower wind speeds.  The strongest gales it seems often just precede, or arrive soon after, warm or cold fronts, prior to arrival of or just after the heaviest rain has passed.  This was certainly the case in the October StJude storm when the, now infamous, stingjet winds followed after the heaviest rain on the cold front had well and truly passed through Reigate.

Wind and Temperature

The wind direction bringing coldest temperatures to Reigate in 2013 were those bitter Easterlies in our cold 2013 Spring.  An interesting element of SSW winds also brought in cold minimum temperatures to Reigate, possibly ahead of fronts from the Atlantic when chilly continental air can be dragged in as “polar returning air” from a frigid winter continent.  Interestingly, the dominant wind bearing bringing the lowest wind CHILL temperatures was NNE, different from the winds bringing lowest average air temperature.  More on wind chill here 

Reigate Sunshine (*sunshine recorder added April 27)

Sunshine total hours *1058 hours (from April 27)

Longest daily sunshine hours 11.4 hours June 6

Shortest daily sunshine within recording period 0 hours on 21 December (shortest day)

In conclusion: Reigate, Surrey has the best weather in the UK, probably!  It is one of the driest, least windy, least stormy (we get few thunderstorms compared with many places) and sunniest and warmest places in the country.  Reigate was one of the warmest or nearly-warmest places in the UK a number of times in 2013.

Some factors making Reigate weather some of the best in the country are to do with the uniquely well-placed location of the town.  So, thanks to the Normans we are:

  • south facing: nestled at the foot of the North Downs: sheltered from cold Northerly and NE wind and rain
  • sheltered from S and SW gales by Priory hill and woods.
  • low altitude: 100m above sea level: not exposed.
  • inland from exposed coasts which experience higher winds and, sometimes, more rain
  • on the lowland East of the UK: enjoying a regionally sheltered location from cold easterly winds (Kent gets these) and westerly gales and heavy rainfall.
  • in a location that usually avoids extreme convective thundery activity (2013): Reigate missed several thunderstorm events that passed to the north of London from a SW direction; this may or may not be usual.
thunder monday

Reigate missed the big 2013 thunderstorms which often drifted from SW to NE in lines of convective activity, missing the SE

We plan to post full meta-data describing the location of our Vantage Pro and the data required to get the most our of our records in 2014. Other plans for 2014 include:

  • Calibrate the tipping bucket VP2 rain gauge before Easter.
  • Further outreach to other schools and interested parties regarding all-things weather, via this blog and @RGSweather.
  • Expand the school club and further engage students in the wonders of weather.
  • Expand the use of data in school to include more departments.
  • Provide a local service to help forecast and understand extreme weather, when we can, via Twitter.
  • Engage with the Press and media where we can provide useful information
  • Encourage readers to post comments and together contribute to the wider understanding of weather locally.

A sincere thank you to all readers for making this such a success in 2013 and please come back for more in 2014!

Here’s a copy (below) of the IPCC AR5 report that has the juicy bits highlighted for ease of reading!

In essence the IPCC have toned down some of their projections because they recognise that there has been no global warming since 1997, something their models did not expect.  The IPCC believe the 1997 – present day heat is “hidden” in the oceans and will emerge at some point.  They also point out that warming will not be regular and that some of their previous models had “forcing inadequacies” (i.e. modelled temperature rise incorrectly?) and that in some models there was an “overestimate of the response to increasing greenhouse gases and other anthropogenic forcing”.

Read Simon Keeling’s @weatherschool musing on this. here http://www.weatherweb.net/wxwebtvsimonnew.php?ID=864

Read the summary report here…

WGIAR5-SPM_Approved27Sep2013