The World Meteorological Organisation reported that the past decade has been the warmest observed, with rising greenhouse gas emissions driving record ocean heat, melting ice and more extreme weather.

On top of those findings comes the growing prospect of El Niño, which could emerge by New Zealand’s spring and amplify the effects of global warming in the months beyond.

At this stage, the event is expected to be moderate, resulting in global mean sea surface temperatures of around 1C warmer than average. However, there is still considerable uncertainty in how it will develop.

For the planet, El Niño can bring a wide range of damaging impacts, from wildfires and drought in some regions to heavy downpours and flooding in others.

It can also have significant impacts for New Zealand and its primary sector. Some stand-out events – such as in 1982–83, 1997–98 and 2015–16 – caused severe droughts that cost the economy hundreds of millions of dollars.

Notably, it follows a rain-soaked start to the year across much of New Zealand, partly owing to the lingering, warm-and-wet influence of El Niño’s climate driver counterpart: La Niña.

As that La Niña fades out over coming weeks, as it is forecast to, scientists will be monitoring the potential seesaw-like shift to El Niño.

But, in New Zealand’s part of the world particularly, such swings do not play out in isolation: other large-scale patterns are likely to influence how El Niño’s effects are felt.

El Niño explained

The name El Niño – meaning “Christ Child” – was given by Peruvian fishers who noticed warm waters appearing off their coast around Christmas. Its global significance was recognised over time.

It occurs when the usual trade winds across the tropical Pacific weaken, sometimes reversing direction, allowing warmer water to spread eastward and raise sea surface temperatures. In contrast, La Niña features stronger trade winds and cooler waters.

Together, these phases form the El Niño/Southern Oscillation (ENSO), which typically cycles every two to seven years and is measured by sea surface temperatures in the central equatorial Pacific.

At a global scale, El Niño events make for warmer years by raising air temperatures and reducing the rate at which the ocean absorbs heat.

They tend to bring warmer conditions to parts of Asia and North America, wetter weather to parts of the United States and South America, and drier conditions in Australia and parts of Africa.

In New Zealand, El Niño typically shifts atmospheric circulation patterns. Winters tend to feature more southerly winds, spring more south-westerlies and summer more westerlies.

This generally leads to cooler conditions overall, although eastern areas such as Wairarapa, Hawke’s Bay and Gisborne can still experience warmer than average summers.

Rainfall patterns also change. El Niño is often associated with drier conditions in the north and east of both islands, and wetter conditions in Southland and along the West Coast of the South Island.

By contrast, La Niña tends to bring warmer temperatures and different regional rainfall patterns, often with wetter conditions in the north and east. Because El Niño produces a clearer signal for New Zealand, it is usually more predictable.

The wider picture

Alongside ENSO, longer-term climate variability also plays a role.

Another cycle called the Interdecadal Pacific Oscillation (IPO) resembles ENSO and similarly influences patterns across the Pacific Basin, but operates on timescales of 20 to 30 years.

In its positive phase, warmer-than-average sea surface temperatures occur in the central equatorial Pacific, while cooler conditions prevail elsewhere in the basin. The opposite occurs in the negative phase.

The IPO can either amplify or dampen the effects of global warming. In New Zealand, positive phases have tended to slow regional warming, while negative phases have accelerated it.

Since 1998, the IPO has been in a negative phase. During this period, New Zealand’s warming rate has increased markedly, from about 0.14°C per decade in earlier decades to around 0.27°C per decade more recently.

Another important influence is the Southern Annular Mode (SAM), which affects the strength and position of westerly winds around New Zealand.

In its positive phase, high-pressure systems tend to dominate, pushing the westerly winds further south and bringing calmer, warmer conditions. This can lead to drier weather in the west and south, but wetter conditions in the north.

In its negative phase, lower pressure dominates, bringing stronger westerlies, cooler temperatures and wetter conditions, particularly in western regions.

SAM operates year-round but has its strongest effects in summer, interacting with both ENSO and the IPO.

How these three drivers play off one another will ultimately influence what unfolds in the coming seasons.

With El Niño expected to develop over winter, warmer sea surface temperatures will spread across the Pacific. The IPO, meanwhile, remains firmly in its negative phase, meaning warmer conditions are likely to persist around New Zealand.

At the same time, SAM is expected to fluctuate between positive and negative phases, although it has shown a tendency toward more positive conditions as the climate warms.

Together, these influences point toward the possibility of a significant El Niño event unfolding against the backdrop of a steadily warming planet.

Whether this evolves into another severe event for New Zealand remains to be seen. The climate of each season, after all, carries its own distinct personality.

But, coming on the back of the world’s warmest period on record, it warrants our close attention.

Jim Salinger, Adjunct Research Fellow, Te Herenga Waka — Victoria University of Wellington

This article is republished from The Conversation under a Creative Commons license. Read the original article.