What are El Niño and La Niña?

THE UK Met Office explains that El Niño and La Niña are terms which describe the biggest fluctuation in the Earth’s climate system and can have consequences across the globe.

The name ‘El Niño’ describes the warming of sea surface temperature, typically concentrated in the central-east equatorial Pacific, which occurs every two to seven years.

Watch the Met Office explainer video

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 droughts, floods, heat waves, and cold seasons for different regions, and even raising global temperatures.

The Met Office video says, that to explain El Nino we first need to know what’s happening in the tropical Pacific. This vast stretch of ocean sees consistent winds, called trade winds, which blow from east to West. These winds push warm water near the surface towards the west, 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, cooler water replaces the warmer water that’s been pushed away. The cooler water is pulled up from deeper down in the ocean.

This creates a temperature difference across the tropical Pacific, with warmer water piled up in the West and cooler water in the east.

Warming water adds extra heat in the air which causes temperatures to rise with more vigour, and it’s this rising air that creates an area of more unsettled weather with more cloud and 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, drier 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 – 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, cancelling 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 large-scale wind patterns. It’s this change in winds which has a knock-on effect, changing temperatures and rainfall in locations around the world.

An El Niño is declared when sea temperatures in the tropical eastern Pacific rise 0.5 °C above the long-term average for three consecutive months. El Niño is felt strongly in the tropical eastern Pacific with warmer than average weather, but effects are transmitted around the world.

The effects of El Niño often peak during December; it’s name “the boy” is thought to have originated as “El Niño de Navidad” centuries ago when Peruvian fishermen named the weather phenomenon after the new born Christ.

La Nina is effectively a reversal of El Nino conditions.

‘La Niña’ or “the girl” is the term adopted for the opposite side of the fluctuation, which sees episodes of cooler than average sea surface temperature in the equatorial Pacific. The conditions for declaring ‘La Niña’ differ between different agencies, but during an event sea temperatures can often fall 3-5 °C below average. Cooler, drier than average weather is experienced in the tropical eastern Pacific.

There are also neutral phases of the cycle when conditions are closer to the long-term average (within +/- 0.5 °C). These may be within a period of warming or cooling in the cycle. Approximately half of all years are described as neutral.

These episodes alternate in an irregular inter-annual cycle called the ENSO cycle. ‘ENSO’ stands for ‘El Niño Southern Oscillation’, where ‘Southern Oscillation’ is the term for atmospheric pressure changes between the east and west tropical Pacific that accompany both El Niño and La Niña episodes in the ocean.

The name ‘ENSO’ is a reminder that close interaction between the atmosphere and ocean is an essential part of the process. While the global climate system contains many processes, ENSO is by far the dominant feature of climate variability on inter-annual timescales.

El Niño years are one factor that can increase the risk of colder winters and hotter summers in the UK.

Watch the Met Office explainer video

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