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El Nino - What is it?
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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.