An ensemble probability forecast of rainfall > 10 mm

Seasonal forecasts and climate drivers resources

Learn about the climate drivers that influence seasonal forecasts and the impacts they can have on UK and global weather.

Seasonal forecasts rely on aspects of the global weather and climate system that are predictable. These are known as climate drivers and include tropical sea-surface temperatures and pressure patterns over the North Atlantic. The predictable effects of climate drivers act alongside inherently unpredictable variability. As such, seasonal forecasts give an indication of which outcomes are more likely to occur whilst acknowledging a wide range of outcomes are possible. In addition, different drivers can have competing influences on the seasonal forecast, making interpretation more complex. At the Met Office, we use suites of climate models that represent the effects of drivers to help provide an integrated view.   

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 varying depending on the time of year. These influences – acting on UK climate a long distance from their source – are  known as ‘teleconnections’ and are an intense area of ongoing scientific research including here at the Met Office. 

El Niño Southern Oscillation – ENSO

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.  

Global weather impacts UK 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: 

  • El Niño effects include increased risks of floods in Peru and droughts in Indonesia, India and parts of Brazil. 

  • El Niño can also push up global temperatures, which is why El Niño years often feature amongst the hottest years on record. 

  • During La Niña, parts of Australia, Indonesia and equatorial South America are typically wetter than normal. Whereas the southern USA often experiences drought.  

  • El Niño is also thought to limit development of tropical storms in the North Atlantic, whereas La Niña can enhance development.  

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: 

 

  • 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. 

  • Whereas El Niño increases the risk of a windy and mild start and cold end to winter. 

Video explainers

El Niño – What is it?  
La Niña  – What is it? 

You can find out more about ENSO on our webpage: What are 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).  

Global weather impacts UK 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. 

 

Changing local pressure patterns strongly influence local weather patterns such as temperature, rainfall, wind strength and direction. 

The NAO is an important tool for forecasting long term weather patterns across the UK in autumn, winter and spring. As the NAO gives strong clues for the types of winter we can expect in the UK, it is one of the key features in the Met Office’s winter forecast. In general: 

  • 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. 

  • 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 whilst the NAO gives an indication of the most likely pressure set-up over the winter period, any pressure chart for a given day would be different. 

Video explainer

What is the North Atlantic Oscillation (NAO)?

Find out more about the NAO on our webpage: North Atlantic Oscillation. 

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.  

Global weather impacts UK weather impacts

Each phase of the IOD has different effects on weather patterns. The closer the country is to the Indian Ocean the stronger the link between the weather patterns and the IOD. 

Positive phase:  

  • Easterly winds across the Indian Ocean develop. 

  • Over the western side of the Indian Ocean low pressure develops and there is increased rainfall and flooding in eastern Africa. 

  • Meanwhile over the eastern side of the Indian Ocean, surface pressure is higher. Reduced rainfall and droughts can occur in Indonesia and Australia. 

  • Increased rainfall during the southwest Monsoon season across the Indian subcontinent can also occur. 

Negative phase – the impacts are reversed: 

  • Westerly winds across the Indian Ocean. 

  • More rainfall and increased risk of flooding over Indonesia and Australia. 

  • Reduced rainfall and droughts in eastern Africa. 

  • 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.

A positive IOD can increase the likelihood of westerly winds over the UK in winter. Often this simply acts jointly with ENSO activity that is driving the IOD in the first place, although IOD tends to have a similar influence when acting independently to ENSO.

An example of the latter kind of event occurred during 2019, when the IOD was extremely positive and had large impacts on weather patterns across many countries. Although other factors were likely at play, wetter and windier conditions were observed in the UK in the winter of 2019-2020 with February seeing a series of named storms travelling across the UK leading to it being the wettest February on record.

Video explainer

What is the Indian Ocean Dipole?

Madden-Julian Oscillation - MJO

The Madden–Julian Oscillation (MJO) is characterised by 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. 

  • Phase 1 – Enhanced rainfall develops over the western Indian Ocean. 

  • Phase 2 and 3 – Enhanced convection (rainfall) moves slowly eastwards over the Indian Ocean. 

  • Phase 4 and 5 – Enhanced rainfall has reached Indonesia and the west Pacific. 

  • Phase 6, 7 and 8 – Enhanced rainfall moves further eastward over the western Pacific, eventually dying out in the central Pacific. 

  • The next MJO cycle begins. 

Global weather impacts UK weather impacts
  • The MJO creates favourable conditions for tropical cyclone activity, including Atlantic hurricanes. 

  • 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. 

  • 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. 

  • There is also evidence to suggest that the MJO can influence the onset of a Sudden Stratospheric Warming (SSW) event.

  • 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. 

  • 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. 

  • 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. 

  • The timescale of the MJO having an influence on North Atlantic weather regimes is usually 10 to 12 days. 

 

Find out more about the MJO on our webpage: What is the Madden–Julian Oscillation? 

Quasi-Biennial Oscillation - QBO

The Quasi–Biennial Oscillation (QBO) is a regular variation of the winds that blow in the stratosphere high above the equator. The QBO has two phases, an easterly phase and a westerly phase. The winds change direction every 14 months or so, this means a full cycle takes around 28 months. 

The QBO is one of the most regular atmospheric oscillations, making it highly predictable. It has wide-reaching impacts and can influence UK weather. 

UK weather impacts 

The QBO can influence the winter stratosphere at high latitudes, including the stratospheric polar vortex (SPV). This in turn affects the strength of the jet stream which influences the weather in the UK. However, the relationship between the QBO and weather in the UK isn’t simple as the QBO happens in conjunction with other climate drivers. In general: 

  • When the QBO is in a westerly phase there is an increased chance of a strong jet stream and mild, wet and stormy conditions in winter. 

  • When the QBO is easterly there is an increased chance of the jet stream becoming weaker. A weaker jet stream means less flow of mild air off the Atlantic. So, there is an increased likelihood of very cold northerly or easterly winds in winter from the Arctic and continental Europe. 

  • When the QBO is easterly there is also an increased chance of a sudden stratospheric warming event. 

Find out more about the QBO on our webpage: Quasi-Biennial Oscillation (QBO). 

Stratospheric polar vortex (SPV) and sudden stratospheric warming (SSW) 

The stratospheric polar vortex (SPV) is present in Northern Hemisphere winter. It is a circulation of winds high in the stratosphere up to 50km above the surface of the Earth. The stratospheric polar vortex circulates around a mass of cold air above the North Pole. During the winter the polar vortex can strengthen or weaken. These changes influence the atmosphere below and ultimately UK weather. 

A stronger stratospheric polar vortex favours a strong jet stream. Conversely, when the stratospheric polar vortex weakens, the jet stream also tends to weaken and become distorted. 

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. 

Sudden stratospheric warming can sometimes cause the jet stream to ‘snake’ more and this can create a large area of blocking high pressure at the surface.  

UK weather impacts 

Typically, when an SSW occurs a large area of blocking hight pressure will form over the North Atlantic and Scandinavia. This means northern Europe, including the UK is more likely to get a long spell of drier and colder-than-average weather, whereas southern Europe will tend to be milder and wetter than average. On the boundary of the blocking high, cold easterly winds can develop and in some cases the drop in temperatures leads to snow. 

However, a major sudden stratospheric warming only happens in half of winters on average, and even when an event does occur, it doesn’t always lead to intensely cold spells of weather in the UK. 

Video explainers 

What is the Polar Vortex? 
Sudden Stratospheric Warming

Find out more about the polar vortex and sudden stratospheric warming on these webpages: Polar Vortex and Sudden Stratospheric Warming (a transcript for this video can be found here). 

Further resources on seasonal forecasting 

This video series explains how seasonal forecasts are produced. Part 3 (Interpreting seasonal forecast output) explains what terciles are and why they are used in seasonal forecasts such as the UK 3-month Outlook. 

View the whole video series and a transcript of these videos by following the links here.