What is El Niño? What is La Niña?

El Niño is officially here now - What this means for you (as of Tuesday 19 September 2023)

Credit: NIWA & WeatherWatch

El Niño Southern Oscillation: what is it?

El Niño and La Niña are opposite phases of a naturally occurring global climate cycle known as the El Niño Southern Oscillation, or ENSO for short. ENSO influences rainfall, temperature, and wind patterns around the world, including New Zealand. El Niño and La Niña episodes occur on average every few years and last up to around a year or two.

Although ENSO has an important influence on New Zealand’s climate, it accounts for less than 25 percent of the year-to-year variance in seasonal rainfall and temperatures at most locations. Nevertheless, its effects can be significant. Watach video What is El Niño? What is La Niña?

El Niño

During an El Niño event, ocean water from off the coast of South America (near Ecuador and Peru) to the central tropical Pacific warm above average. The warming takes place as trade winds (the permanent east-to-west prevailing winds that flow around the equator) weaken or even reverse, blowing warm water from the western Pacific toward the east. As a result, sea temperatures in the far western Pacific can cool below average. The unusually warm water in the eastern Pacific then influences the Walker Circulation, acting as a focal point for cloud, rainfall, and thunderstorms. It is this change in the Walker Circulation that impacts weather patterns around the world.

El Niño’s average influence on New Zealand

It’s important to bear in mind that while we know the average outcome of El Niño because of historical data, no El Niño is average—each comes with a unique set of climate characteristics and therefore can be expected to influence the weather differently.

During El Niño, New Zealand tends to experience stronger or more frequent winds from the west in summer, which can encourage dryness in eastern areas and more rain in the west. In winter, the winds tend to blow more from the south, causing colder temperatures across the country. In spring and autumn, southwesterly winds are more common.

La Niña

During a La Niña event, ocean water from off the coast of South America to the central tropical Pacific cools to below average temperatures. This cooling occurs because of stronger than normal easterly trade winds, which churns cooler, deeper sea water up to the ocean’s surface. Sea temperatures can warm above average in the far western Pacific when this happens. The unusually cool water in the eastern Pacific influences the Walker Circulation and suppresses cloud, rain, and thunderstorms. This change impacts weather patterns around the world, but in a different way than El Niño does.

Watch video What does La Niña mean for New Zealand?

El Niño is officially here now - What this means for you.

It’s official, an El Niño weather pattern is now in place over New Zealand and Australia as conditions in the tropical Pacific are finally confirmed by the Bureau of Meteorology.

In today’s video we break down what this means for where you live over the coming weeks and months. This isn’t our usual ClimateWatch update - it’s like ClimateWatch LITE - and briefly covers expected rainfall and temperature patterns across both New Zealand and Australia in the coming months.

NZ’s location on earth means some wet weather will still come in from the Southern Ocean - like we see this weekend - but the El Niño announcement does mean eastern and northern parts of both main islands are at the highest risk for drier and hotter weather, much like how it’s likely to be hotter and drier in Australia’s east and inland.

We also have a 15 day rainfall outlook for both NZ and Australia - which we’ll break down in more detail in next week's October ClimateWatch update. Our next ClimateWatch update will be out by the end of September, drilling into more detail for the rest of spring and how summer is shaping up.

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