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  • Every few years the El Niño phenomenon kicks into life in the Pacific Ocean

  • around the equator. It can affect weather around the world

  • changing the odds of floods, drought, heatwaves

  • and cold seasons for different regions even raising global temperatures.

  • But what is El Niño and how does it happen?

  • Firstly we need to know what's normally happening in the tropical Pacific.

  • This vast stretch of ocean sees consistent winds

  • called 'trade winds' that blow from east to west.

  • These winds push warm water near the surface

  • in their direction of travel, so 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 deeper down in the ocean

  • a process called upwelling. This creates a temperature difference

  • across the tropical Pacific with 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 its this rising air that creates an area of more

  • unsettled weather with more cloud in rainfall

  • That rising air in the West

  • sets up atmospheric circulation across this part of the world

  • with warm moist air rising on one side of the Ocean

  • and cooler dryer air descending 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. 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 even 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

  • canceling out the normal temperature difference.

  • 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. 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 where you see an increase in the risk of floods

  • in Peru and droughts in Indonesia, India

  • and parts of Brazil. But virtually wherever you are in the world

  • El Niño has the potential to affect you directly

  • via the weather or indirectly via socio-economic impacts

  • There's another impact from El Niño which happens because of

  • all the extra heat at the surface of the tropical Pacific.

  • This releases vast amounts of energy

  • into the atmosphere which can temporarily push up global temperatures.

  • This is why El Niño years often feature

  • among the warmest on record. Each El Niño event

  • is different so the global impacts can change.

  • You can find out more about the different impacts of El Niño

  • on our website. El Niño peaks around Christmas-time

  • and last for several months. It can dive back to neutral conditions

  • but sometimes reverses into La Niña.

  • This is the flip side of the oscillation which sees

  • a strengthening of the normal trade winds.

  • This pushes the warmest water to the far western part of the tropical Pacific

  • and increases the upwelling of cold water

  • in the east. This cooler water

  • extends out from the coast of the Americas towards the central part of the ocean

  • La Niña also impacts global weather

  • and tends to have opposite effects to El Niño.

  • You can also see more about La Niña and its impacts

  • on our website.

Every few years the El Niño phenomenon kicks into life in the Pacific Ocean

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