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.