What is El Niño?

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.