El Niño has started... what does that mean for UK weather?

The effects of El Niño on the Pacific Ocean... and the world. Credit: MET OFFICE

Forecast centres around the world have now declared that an El Niño, the most powerful fluctuation in our climate system, has begun in the tropical Pacific.

El Niño is a warming of the Pacific Ocean as part of a complex cycle linking atmosphere and ocean. It can disrupt weather patterns around the world, due to releasing a huge amount of heat from the Pacific Ocean into the atmosphere.

For more than a year, scientists have been talking of an increased risk of the start of El Niño conditions in the tropical Pacific.

This phenomenon can change the odds of floods, droughts, heat waves and cold seasons for different regions around the world and can even raise global temperatures... so you can see why Scientists have been keeping a close eye on it.

First of all, let's explore what NORMALLY happens in the tropical Pacific...

Trade winds usually blow from east to west across this vast expanse of ocean.

These winds push warm water across the surface in their direction of travel so that the warm water piles up on the western side of the ocean around Asia and Australasia.

On the other side of the ocean around South and Central America, as the warmer water gets pushed away from the coast it's replaced by cold water which is pulled up from the ocean depths - this process is called "upwelling"This creates a temperature difference across the tropical Pacific - warmer water piled up in the west and cooler water in the east.

Warmer water adds extra heat to the air which causes the air to rise with more vigor, and it's this rising air that creates an area of more unsettled weather, with more cloud and rainfall.

Temperature difference allows air to rise vigorously over the warmer expanse of water. Credit: MET OFFICE

That rising air in the west sets up atmospheric circulation across this part of the world - warm moist air rises on one side of the ocean - cooler, drier air descends on the other.

This circulation reinforces the easterly winds so this part of the world sits in a self-perpetuating state until El Niño begins...

Warm moist air rises on one side of the ocean - cooler, drier air descends on the other. Credit: MET OFFICE

Early signs of an El Niño last year failed to fully develop and atmospheric conditions remained close to neutral into the start of 2015.

Now, however, observations from the tropical Pacific show that we have moved to weak El Niño conditions for the first time in five years.

CHANGES...this is what is happening in May 2015...

If conditions are right, tropical Pacific weather systems - or slow changes in the ocean around the Equator - can set off a chain of events which weaken or sometimes reverse the usual trade winds.

With weakened trade winds, there's less push of warm surface water to the western side of the ocean - - and less upwelling of cold water on the eastern side.

This allows the usually colder parts of the ocean to warm, cancelling out the normal temperature difference.

When El Niño starts, more warm surface water spreads to the western side of the ocean - less temperature difference. Credit: MET OFFICE

Because the area of warmest water moves, so does the associated wet and unsettled weather. This changes rainfall patterns over the Equatorial Pacific - as well as the large scale wind patterns.

The area of warmest water moves, so does the associated wet and unsettled weather, changing rainfall patterns. Credit: MET OFFICE

It's this change in winds which has a knock on effect, changing temperature and rainfall in locations around the world.

The main impacts are around the Tropics - but wherever you are El Nino has the potential to affect you directly.

So, what now?

While it is still too early to determine with confidence how strong this El Niño might be, forecast models from centres around the world – including the Met Office – suggest this El Nino could strengthen from September onwards.

Predicting exactly how an El Niño might develop remains difficult, but as we move a few months ahead it’s likely forecast models will provide a higher level of certainty about what will happen.

The current outlook suggests that at least a moderate El Nino is likely and there is a risk of a substantial event.

What does this mean for the UK?

There has been some media speculation about how the El Niño conditions could impact our weather over the coming months.

However, even a strong El Niño only slightly changes the risk of extreme UK spring and summer weather, and we wouldn’t expect it to be the dominant driver of our weather over the next few months.

Looking further ahead, there are a number of factors that affect winter conditions in Britain. The increase in risk of a colder winter this year from the developing El Niño is currently considered small.

Worldwide droughts, floods...?

It can be linked with poor monsoons in Southeast Asia, droughts in southern Australia, the Philippines and Ecuador, blizzards in the United States, heatwaves in Brazil and extreme flooding in Mexico.

The consequences of El Niño are much less clear for Europe and the UK.

Each El Niño event is unique however, so it’s not possible to say exactly what the consequences will be for any given year.