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.