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!
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.
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”.
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.
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.
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.
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.
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.
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.
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:
…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.
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:
Warmer ENSO years exceeding +0.50 Nov AO on left (10 events). 5 out of 10 had lots of Eurasian Snow (right). pic.twitter.com/TWw3MfGAZV
— Anthony Masiello (@antmasiello) November 3, 2015
//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).
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.
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.
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.
winter 2006 – 7 http://news.bbc.co.uk/1/hi/sci/tech/6401063.stm
winter 2013 http://www.metoffice.gov.uk/about-us/how/case-studies/january-2013-snow