What are El Niño and La Niña?

The El Niño - Southern Oscillation (ENSO) is a natural feature of the global climate system that influences air pressure, rainfall, temperature, and wind patterns around the world. 

It’s a coupled ocean-atmosphere phenomenon, meaning that the ocean and atmosphere work in tandem to cause fluctuations in the global climate.

ENSO has three phases: El Niño, La Niña, and neutral. El Niño and La Niña events occur, on average, every 3-5 years. El Niño events tend to last for up to a year, while La Niña can linger on for 1-3 years.

NIWA issues ENSO predictions every month in its Seasonal Climate Outlook.

El Niño – warm seas in the equatorial Pacific

During El Niño events, easterly winds along the equator, called trade winds, weaken. This leads to a rise in sea surface temperatures in the central and eastern equatorial Pacific near South America as warmer water moves in from the west.

This unusually warm ocean water puts more energy into the atmosphere to form clouds, rainfall, and thunderstorms – ultimately influencing weather patterns around the world. 

During El Niño, New Zealand tends to experience stronger, more frequent winds from the west in summer, which can encourage dryness in eastern areas like Hawke's Bay, Gisborne, Canterbury, and Otago, and more rain in the west, like along the West Coast. In winter, the wind tends to blow more from the south, causing colder temperatures across the country. 

Elsewhere, the odds for heavy rainfall and flooding increase over Peru, as does the risk for drought over Indonesia and Australia. In the tropical South Pacific the pattern of occurrence of tropical cyclones shifts eastward, so there are more cyclones than normal in areas such as the Cook Islands and French Polynesia.

La Niña – cool seas in the equatorial Pacific

During a La Niña event, the trade winds strengthen, allowing ocean water off the coast of South America to the central equatorial Pacific to cool. This happens because stronger winds churn cooler, deeper sea water up to the ocean’s surface – a process known as upwelling.

During La Niña, New Zealand tends to experience more north-easterly winds, bringing moist, rainy conditions to north-eastern areas of the North Island and less rainfall to the lower and western South Island. La Niña summers also tend to be warmer and can come with a risk for marine heatwave conditions in New Zealand’s coastal seas.

For New Zealand, north-easterly winds become more common during La Niña events, bringing moist, rainy conditions to north-eastern areas of the North Island and less rainfall to the lower and western South Island.

How is ENSO measured?

Because ENSO is a coupled ocean-atmosphere phenomenon, it is monitored by both oceanic and atmospheric indicators.

In the atmosphere – the Southern Oscillation Index

One key atmospheric indicator is called the Southern Oscillation Index or SOI.

Sir Gilbert Walker documented and named the Southern Oscillation in the 1930s. The clearest sign of the Southern Oscillation is the inverse relationship between surface air pressure at two sites: Darwin, Australia, and the South Pacific island of Tahiti.

Over periods of a month or longer, higher pressure than normal at one site is almost always concurrent with lower pressure at the other, and vice versa. The pattern reverses every few years. It represents a "seesaw", a mass of air oscillating back and forth across the International Date Line in the tropics and subtropics.

When the Southern Oscillation Index is positive, air pressure at Tahiti tends to be higher and lower at Darwin. The reverse is true when the index is negative.

Over a period of three months or more, SOI values below -1 correspond to El Niño conditions while values above 1 correspond to La Niña conditions. Values between -0.5 and -1.0 lean toward El Niño, while values between 0.5 and 1.0 lean toward La Niña. Values between -0.5 and 0.5 are considered neutral.

In the ocean – the NINO34 Index

An important oceanic indicator is called the NINO34 Index. The NINO34 Index represents the sea surface temperature in a region in the central equatorial Pacific whose conditions are strongly influenced by ENSO.

When the NINO34 Index has been 0.7˚C warmer than average for several months, it can be said that oceanic El Niño conditions are occurring.

When the NINO34 Index has been 0.7˚C cooler than average for several months, oceanic La Niña conditions are occurring.

NIWA synthesises both SOI data and NINO34 Index in its Seasonal Climate Outlook, with updates issued once a month.