El Niño and La Niña
El Niño and La Niña are opposite phases of a naturally occurring global climate cycle known as the ‘El Niño-Southern Oscillation' (ENSO). They disrupt normal patterns of wind and rainfall, in different ways, in many parts of the world — including New Zealand.
Understanding El Niño
El Niño is one phase of a naturally occurring global climate cycle known as the El Niño-Southern Oscillation (ENSO).
El Niño disrupts normal weather patterns across much of the globe and can lead to intense storms in some places and droughts in others, affecting many lives and livelihoods.
Other phases of ENSO — La Niña, and neutral conditions
ENSO’s other phases are La Niña – the extreme opposite of El Niño – and neutral (neither El Niño nor La Niña).
El Niño and La Niña occur irregularly, typically every two to seven years. Their strength can vary significantly from phase to phase.
The video below from the United Kingdom's MetOffice provides an excellent explanation of El Niño.
Global effects of El Niño
Warm sea-surface temperatures are a key factor in the development of thunderstorms and cyclones, which deliver heavy rain to tropical areas.
As a result, El Niño typically leads to much wetter and stormier conditions than usual in the eastern Pacific, particularly in coastal Peru and Ecuador, and much drier than normal conditions in the western Pacific.
Generally, more — and more intense — cyclones affect the southwest Pacific from Vanuatu east to French Polynesia, while fewer than normal affect Indonesia, Papua New Guinea and Australia.
This is reflected in the elevated likelihood of tropical storms in the southwest Pacific, accompanying this year's El Niño.
The changes in sea-surface temperature also influence the circulation of air and moisture at high levels in the atmosphere, which can significantly affect the weather experienced on the ground in many other parts of the world, including New Zealand.
While every event is different and no single impact is certain, typical far-reaching effects of a strong El Niño include above average rainfall in southeastern South America, eastern equatorial Africa and the southern US, and below average rainfall in parts of Australia and India.
El Niño and climate change
Some climate researchers have suggested the frequency and intensity of El Niño and La Niña phases will be affected by climate change. However, there is no scientific consensus on this issue, or on whether the relatively high frequency of El Niños observed over the last two decades is related to the rise in global temperatures this century.
Climate change is likely to increase the frequency and intensity of extreme weather events, such as floods and droughts, regardless of whether they are associated with El Niño.
Further information
Read this year's tropical cyclone outlook produced by NIWA
Check out the USA National Oceanic and Atmospheric Administration's El Niño portal