Seasonal forecasts and climate drivers
Learn about the climate drivers that influence seasonal forecasts and the impacts they can have on UK and global weather.
Seasonal forecasts are shaped by aspects of the global weather and climate system many of which are predictable. These are known as climate drivers, examples include tropical sea-surface temperatures and pressure patterns over the North Atlantic. While these drivers help us understand what may occur the atmosphere remains a major influence and this is inherently unpredictable, meaning there is always a wide range of possible outcomes. As such, seasonal forecasts give an indication of which types of weather are more likely to occur whilst acknowledging many different scenarios are possible. Different drivers can exert competing influences adding complexity to the interpretation of seasonal forecasts. At the Met Office, we use advanced climate models that represent the effects of all these drivers to provide the best possible overall picture.
Climate drivers
There are several climate drivers you might see mentioned in the UK 3-month outlook and seasonal forecasts. These include the following. Click on each one to jump to the relevant part of the page below:
Each driver has different influences on the UK and global weather with impacts often varying depending on the time of year. These influences are known as ‘teleconnections’ and are an intense area of ongoing scientific research including here at the Met Office.
El Niño Southern Oscillation – ENSO - and La Niña
The El Niño Southern Oscillation (ENSO) is a variation in climate in the tropical Pacific Ocean with three phases, the neutral phase, El Niño and La Niña. These phases alternate in an irregular multi-year cycle called the ENSO cycle. Peak El Niño and La Niña phases tend to occur in Northern Hemisphere winter. Each phase describes sea surface temperature and accompanying atmospheric circulation patterns over the tropical Pacific Ocean.
UK weather impacts
ENSO can also impact the UK weather. The impacts of El Niño and La Niña on the UK are moderated by competing global climate drivers, however in general:
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La Niña increases the risk of a cold start and mild end to the winter. During the latter part of winter La Niña increases the chances of westerly winds.
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Whereas El Niño increases the risk of a windy and mild start and cold end to winter.
Global weather impacts
ENSO can affect weather around the world, changing the chances of floods, drought, heatwaves and cold seasons for different regions. La Niña tends to have opposite effects to El Niño. The main impacts can be found around the tropics and vary with season. In general:
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El Niño effects include increased risks of floods in Peru and droughts in Indonesia, India and parts of Brazil.
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El Niño can also push up global temperatures, which is why El Niño years often feature amongst the hottest years on record.
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During La Niña, parts of Australia, Indonesia and equatorial South America are typically wetter than normal. Whereas the southern USA often experiences drought.
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El Niño is also thought to limit development of tropical storms in the North Atlantic, whereas La Niña can enhance development.
Video explainer - El Niño - What is is?
Video explainer - El Niño and La Niña
North Atlantic Oscillation – NAO
The North Atlantic Oscillation (NAO) is a large-scale atmospheric pressure see-saw in the North Atlantic region. The NAO describes the difference in pressure between Iceland and the Azores islands to the west of Portugal. It has two phases, the positive and negative phase. Sometimes the pressure difference is larger than it is on average (positive NAO) and at other times the difference isn’t so strong (negative NAO).
Research by the Met Office Hadley Centre shows that the winter NAO and related climate patterns in Europe and North America are now predictable months in advance, thanks to a new global climate model. This breakthrough enables earlier preparation for weather events like winter storms, high winds, and extreme temperatures.
UK weather impacts
The NAO is an important tool for forecasting long term weather patterns across the UK in autumn, winter and spring. It acts as a broad indicator of the dominant pressure regime over weeks or months giving strong clues of the type of winter we can expect in the UK, it is one of the key features in the Met Office’s winter forecast. Weather systems operate on much shorter timescales, driven by transient features like fronts, low-pressure systems, and local dynamics.
In general:
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During a positive NAO phase winds from the west are stronger. They bring mild air from the Atlantic in the winter months this leads to mild, wet and potentially stormy conditions.
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Whereas, during a negative NAO phase winds from the west are weaker so there is a greater chance for winds from the east or north-east. In winter, winds from the east bring cold air so there is a greater chance of a snowier winter in the UK.
It is important to note while the NAO provide an indication of the most likely pressure patterns over the winter period, the actual pressure chart for any given day could differ significantly. Even within a positive or negative NAO phase, individual storms and pressure systems can vary greatly from day to day.
Global weather impacts
During a positive NAO phase, winds from the west dominate, bringing with them warm air, while the position of the jet stream enables stronger and more frequent storms to travel across the Atlantic. These support mild, stormy and wet winter conditions in northern Europe and eastern USA. Meanwhile, northern Canada, Greenland and southern Europe are prone to cold and dry winter conditions.
Whereas, during a negative NAO phase, winds from the east and north-east are more frequent, bringing with them cold air, while the adjusted position of the jet stream leads to weaker and less frequent storms. As a result, Europe and eastern USA are more likely to experience cold, calm and dry winters. Whilst northern Canada and Greenland will tend to be mild and wet.
Video explainer - What is the North Atlantic Oscillation (NAO)?
Indian Ocean Dipole - IOD
The Indian Ocean Dipole (IOD) is a change in sea surface temperatures and accompanying surface pressure patterns across the Indian Ocean. The IOD changes year to year between three phases: positive, negative, neutral. It most often reaches is peak in the northern hemisphere autumn, but its effects can be felt into the winter season. The IOD is like the El Niño Southern Oscillation (ENSO) cycle in the Pacific Ocean. The ENSO cycle and IOD are often linked although IOD events can occur without ENSO being active.
UK weather impacts
A positive IOD can increase the likelihood of westerly winds over the UK in winter. This often works in tandem with ENSO activity, which typically drives the IOD. However, the IOD can exert a similar influence even when acting independently of ENSO.
One such independent event occurred during 2019, when the IOD was strongly positive and significantly affected weather patterns across many countries. In the UK, winter 2019–2020 was notably wetter and windier, with February bringing a series of named storms that made it the wettest February on record, although other factors likely contibuted.
Global weather impacts
Each phase of the IOD has different effects on weather patterns. The closer a country is to the Indian Ocean the stronger the link between the weather patterns and the IOD.
Positive phase:
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Easterly winds across the Indian Ocean develop.
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Over the western side of the Indian Ocean low pressure develops and there is increased rainfall and flooding in eastern Africa.
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Meanwhile over the eastern side of the Indian Ocean, surface pressure is higher. Reduced rainfall and droughts can occur in Indonesia and Australia.
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Increased rainfall during the southwest Monsoon season across the Indian subcontinent can also occur.
Negative phase – the impacts are reversed:
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Westerly winds across the Indian Ocean.
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More rainfall and increased risk of flooding over Indonesia and Australia.
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Reduced rainfall and droughts in eastern Africa.
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Reduced rainfall during the southwest Monsoon season across the Indian subcontinent can also occur.
During the neutral phase the IOD has no impact on weather patterns.
Video explainer - What is the Indian Ocean Dipole?
Madden-Julian Oscillation - MJO
The Madden–Julian Oscillation (MJO) is characterised by an eastward-moving region of enhanced tropical rainfall, mainly observed over the Indian and Pacific Ocean. Preceding and following this area of tropical thunderstorms are regions where rainfall is suppressed. A cycle of the MJO takes 30–60 days, but it is not continuously active – there are periods when no MJO activity is apparent.
An area of enhanced tropical rainfall is first apparent over the western Indian Oceans, which spreads eastwards into the warm waters of the tropical Pacific. This pattern of tropical rainfall tends to lose its identity as it moves over the cooler waters of the eastern Pacific, before reappearing at some point over the Indian Ocean again.
A wet phase of enhanced rainfall is followed by a dry phase, where thunderstorm activity is suppressed (less rainfall). Each cycle lasts approximately 30–60 days and MJO activity is often described as being in one of 8 phases.
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Phase 1 – Enhanced rainfall develops over the western Indian Ocean.
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Phase 2 and 3 – Enhanced convection (rainfall) moves slowly eastwards over the Indian Ocean.
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Phase 4 and 5 – Enhanced rainfall has reached Indonesia and the west Pacific.
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Phase 6, 7 and 8 – Enhanced rainfall moves further eastward over the western Pacific, eventually dying out in the central Pacific.
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The next MJO cycle begins.
UK weather impacts
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When the MJO is in its active phase over Indonesia and the West Pacific, it tends to drive a negative North Atlantic Oscillation (NAO) 2 to 3 weeks later.
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A positive NAO index tends to be preceded by phase 3 and 4 of the MJO, which brings milder and wetter weather across the UK.
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A negative NAO index tends to be preceded by phase 6 and 7, which influences a ‘blocked’ weather pattern and is often associated with colder and drier weather across the UK.
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The timescale of the MJO having an influence on North Atlantic weather regimes is usually 10 to 12 days.
Global weather impacts
- The MJO creates favourable conditions for tropical cyclone activity, including Atlantic hurricanes.
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The enhanced rainfall phase of the MJO can also bring the onset of the Monsoon seasons around the globe. Conversely, the suppressed convection phase can delay the onset of the Monsoon season.
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There is evidence that the MJO influences the El Niño Southern Oscillation (ENSO) cycle. It does not cause El Niño or La Niña, but it can contribute to the speed of development and intensity of El Niño and La Niña episodes. The MJO appears to be more active during neutral and weak ENSO years.
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There is also evidence to suggest that the MJO can influence the onset of a Sudden Stratospheric Warming (SSW) event.
Video explainer - What is the Madden-Julian Oscillation?
Quasi-Biennial Oscillation - QBO
The Quasi–Biennial Oscillation (QBO) is a regular shift in wind ditrection in the stratosphere high above the equator. It alternates between two phases, an easterly phase, where winds blow from east to west and a westerly phase, where qwinds blow from west to east. The winds change direction roughly every 14 so a full cycle takes around 28 months. Because of this consistency, the QBO is one of the most predictable atmospheric oscillations.
UK weather impacts
The QBO influences the winter stratosphere at high latitudes, including the stratospheric polar vortex (SPV). This affects the strength of the jet stream which in turn influences the weather in the UK. However, its impact is not straightforward because it interacts with other climate drivers.
In general:
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Westerly QBO sees an increased chance of a strong jet stream and mild, wet and stormy conditions in winter.
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Easterly QBO sees an increased chance of a weaker jet stream, reducing the flow of mild Atlantic air and increasing the likelihood of very cold northerly or easterly winds in winter from the Arctic and continental Europe.
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When the QBO is easterly there is also an increased chance of a sudden stratospheric warming event.
Video explainer - What is the Quasi-Biennial Oscillation?
Stratospheric polar vortex (SPV) and sudden stratospheric warming (SSW)
The stratospheric polar vortex (SPV) is a circulation of strong winds high in the stratosphere, up to 50km above the surface of the Earth, circulatng around a mass of cold air above the North Pole during the Northern Hempishere winter. Its strength varies during the winter and these changes can influence the atmosphere below and ultimately UK weather.
A stronger stratospheric polar vortex favours a strong jet stream, often bringing mild and wet conditions to the UK. Conversely, when the stratospheric polar vortex weakens, the jet stream tends to weaken and meander, increasing the chance of colder outbreaks.
Sometimes the stratospheric polar vortex can break down entirely in an event called a sudden stratospheric warming (SSW). Major sudden stratospheric warming events happen when the SPV is displaced completely off the north pole or is split into two parts, both of which can severely weaken and even temporarily destroy the SPV. It can take up to a month for the SPV to recover following such an event.
SSW can sometimes cause the jet stream to become more wavy allowing blocking high pressure systems to form at the surface.
UK weather impacts
Typically, when an SSW occurs a large area of blocking high pressure will form over the North Atlantic and Scandinavia. This typically brings colder, drier spells to northern Europe, including the UK, while southern Europe becomes milder and wetter. Cold easterly winds can develop on the edge of the blocking high, sometimes leading to snow.
However, major SSW events occur in roughly half of winters, and even when they happen, they don’t always result to intensely cold spells of weather in the UK.
Video explainer - What is the Polar Vortex?
Video explainer - Sudden Stratospheric Warming
A transcript for this video can be found here.
What is the Atlantic Meridional Overturning Circulation?
The Atlantic Meridional Overturning Circulation (AMOC) is a large system of ocean currents that transports warm water from the tropics northwards into the North Atlantic. This ‘conveyor belt’ is powered by differences in water temperature and salt content, which affect the water’s density. As warm water moves north, it cools and becomes saltier due to evaporation. The resulting cold, salty water is denser and sinks deep into the ocean, flowing southwards beneath the surface. Eventually, this deep water rises again through a process called upwelling, completing the circulation. This global mixing of ocean waters helps distribute heat and energy around the planet, shaping the climate we experience today.
Is the AMOC Changing?
Continuous measurements of the AMOC began in 2004. These show that the AMOC varies from year to year, likely influencing UK weather, though it’s too soon to confirm long-term trends. Before 2004, measurements were rare, so scientists rely on indirect evidence like sea floor sediments to study past changes. While details vary, it’s probable that the AMOC has experienced large, rapid changes in the distant past, such as at the end of the last ice age.
Video explainer - The ocean circulation that keeps Europe warm
What will be the effect of climate change on the AMOC?
Climate models predict that the AMOC will weaken during the 21st century as greenhouse gas levels rise. Warmer air causes the ocean surface to retain more heat, while increased rainfall and ice melt make the water fresher and lighter. These changes reduce the sinking of dense water, slowing the ‘conveyor belt’. According to the IPCC Sixth Assessment Report, there is medium confidence that the AMOC will not collapse abruptly before 2100.
A weaker AMOC means less warm water will reach northern regions, partially offsetting the warming effect of greenhouse gases over western Europe. However, even with a gradual weakening likely over the 21st Century, the overall trend for the UK and surrounding areas is still warming.
For more on the AMOC and climate tipping points, visit the Climate Dashboard and read the Met Office blog: What do we mean by a climate tipping point?.
What is the Pacific Decadal Oscillation?
The Pacific Decadal Oscillation (PDO) is a long-term climatic event affecting vast areas of the Pacific Ocean over periods of 20 to 30 years. Positive PDO phases are linked to periods of more rapid global warming whilst it’s thought that negative phases could be linked to times of slower global warming.
The PDO can amplify or offset the effects of El Niño or La Niña, depending on whether they are in the same or opposite phases. The exact causes of PDO shifts remain uncertain, with research exploring factors such as El Niño/La Niña cycles, atmospheric pressure changes, industrial pollution, and natural variability.
What Is the jet stream?
The jet stream is a band of strong winds about 5 to 7 miles above the Earth’s surface, blowing from west to east. It plays a crucial role in shaping weather patterns by influencing wind and pressure systems below.
The jet stream can flow smoothly like a straight river or meander and loop. When it is straight, weather systems move quickly; when it buckles, systems can stall or behave unpredictably. It can intensify low-pressure systems (leading to stormier weather) by drawing air out from above, lowering the pressure further. Conversely, a slower, more curved jet stream can allow high-pressure systems to dominate, resulting in calmer, drier weather.
What causes the jet stream?
The jet stream exists mainly due to temperature contrasts between the equator and the poles. In the northern hemisphere, cold air lies to the north of the jet stream, while warmer air is to the south. During winter, the temperature difference is greater, making the jet stream stronger and more likely to bring wet, stormy weather. In summer, the difference is smaller, so the jet stream shifts north, often resulting in calmer, drier conditions.