The 1997 El Nino seen by TOPEX/Poseidon. Source NASA

What are El Niño and La Niña?

What is El Niño?

The name 'El Niño' is widely used to describe the warming of sea surface temperature that occurs every few years, typically concentrated in the central-east equatorial Pacific.

An El Niño is declared when sea temperatures in the tropical eastern Pacific rise 0.5 °C above the long-term average. El Niño is felt strongly in the tropical eastern Pacific with warmer than average weather.

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 newborn Christ.

What is La Niña?

'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.

Video: What is El Niño?

ENSO - an interaction with the atmosphere and the ocean 

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.

Our research helped show that the El Niño and La Niña cycle has impacts all over the world. For example, El Niño years are one factor that can increase the risk of colder winters in the UK.

El Niño is also thought to limit development of tropical storms in the North Atlantic, likewise La Niña can enhance development. We now better understand these impacts and reproduce many of them in our climate models.

These events are associated with widespread changes in the climate system that last several months, and can lead to significant human impacts affecting things such as infrastructure, agriculture, health and energy sectors.