“Record-breaking” sea conditions are going to mean more turbulence, more wind shear, and increased thunderstorm and fire risk.

By Alwyn Bakker, MetService

In New Zealand, El Niño¹ tends to generate stronger and more frequent westerly winds during summer, which brings more rainfall to western parts of the country, and dries out eastern parts.

Sea surface temperatures (SST) also tend to rise in the Tasman Sea. This increases the likelihood of low-pressure systems forming and moving towards New Zealand, as well as generally increasing convection. So the risk of thunderstorms is elevated for western parts of the country. And warmer temperatures allow air to carry more water vapour before it becomes saturated, so rainfall events can have greater impacts.

How this will affect pilots

More turbulence! And there’s a few different factors feeding into that.

Stronger winds generate stronger low-level turbulence (LLT). As a general rule, wind speed of 30kt or more at terrain height generates moderate LLT, while wind speed of 50kt or more generates severe LLT. Since we’re expecting winds to be stronger in general, instances of both moderate and severe LLT will be more common.

More and stronger westerly winds generate more mountain wave turbulence (MTW). An important component of the setup for trapped MTW is wind that’s roughly perpendicular to the range in question. Since the major mountain ranges in New Zealand run from southwest to northeast, winds from a west to northwest direction fit that roughly perpendicular requirement. It’s a less straightforward correlation than for LLT, as MTW generation also depends on atmospheric stability, the height of any inversions present, and wind shear. But provided the other criteria are met, more westerly winds lead to more MTW turbulence.

And since those stronger winds won’t be confined to a single level, wind shear is also likely to intensify. This will result in increases in frequency and severity of clear air turbulence (CAT). Part of the increase will be connected to changes in the subtropical and polar jet streams. And the (slight) increase in global temperatures can lead to lower atmospheric stability, which is also a risk factor for CAT.

Closer to home, increased warming over Australia and in the Tasman Sea will also contribute to lower atmospheric stability over New Zealand, as that air moves westwards.

Changes to average jet stream position and behaviour

The typical setup for jet streams over New Zealand differs between summer and winter.

In winter, the upper-level westerly wind usually splits into two distinct branches– one stronger subtropical, at around 25-30°S, and one weaker polar, at around 40-50°S.

In summer, the subtropical branch weakens, and can disappear entirely, while the polar branch strengthens and shifts further south, lying around 45-50°S. Note that this does not rule out the split jet setup in summer and the single jet in winter!

In El Niño conditions, the subtropical branch strengthens and moves towards the equator. During winter, this reinforces the already existing tendency towards the split jet setup, while in summer much of the weakening of the subtropical jet can be cancelled out.

This is most likely to affect international flights, especially those from New Zealand’s northern airports that travel west and northwest; more turbulence from wind shear associated with jet streams, and stronger westerly winds aloft. Since the exact position of a jet stream fluctuates on a day-to-day basis, impacts will also be variable.

Changes to rainfall distribution/intensity, and increased thunderstorm risk for some regions.

An increase in instances of poor visibility in rain, and lower cloud bases, for airports in the west and south of the South Island, like NZHK, NZWS, NZNV.

An increase in thunderstorm risks, especially in the west. It’s likely that more thunderstorm SIGMETs will need to be issued in both FIRs, and that airports will be affected by thunderstorms more often.

Rainfall reductions, especially in the upper North Island, will likely increase fire risks. As such, short-term visibility reductions due to smoke may become more common.

How long the conditions might last

The 2026/2027 ‘El Niño-Southern Oscillation’ is set to be record-breaking.

As at July 2026, anomalies of the SST in the Pacific were already large enough for this event to be classed as ‘Strong’. In September 2026, the World Meteorological Organization indicated that El Niño conditions have an “exceptionally high likelihood of nearly 100 percent” to continue through to February 2027. Further intensification is occurring as we move though spring, and is likely to peak in December 2026 – but climate effects will persist well into 2027.

For the weather nerds among you

What is El Niño?

To answer this, we’ve got to go to sea. Specifically, up around the equator, where strong easterly winds push surface water from the eastern Pacific towards the western Pacific. As the water travels west, it gets warmed up by the sun, and by the time it reaches Indonesia it tends to be around 28-30°C. Back on the other side of the Pacific, cooler water from lower in the ocean rises to fill the ‘gap’ created by those easterly winds. Here, the SST is usually around 20°C.

If those easterly winds are weaker than normal, less water is transported west, so the SSTs in the western Pacific are cooler than average. Less cold water needs to rise in the eastern Pacific to fill the gap, so the SSTs there are warmer than average. This is the phenomenon we call ‘El Niño’. If the winds are stronger than normal, the opposite happens, and we call that phenomenon ‘La Niña’. If the winds are about average, we have neither El Niño nor La Niña conditions, and we refer to the situation as ‘Neutral’.

Why we care about that

When cooler air lies over warmer water, the air warms up. That warming air expands and rises, creating an area of lower pressure. If the water is cooler than the air, the air cools and becomes denser, creating an area of higher pressure.

In El Niño conditions, the cooler-than-usual SSTs in the western Pacific mean the atmospheric pressure there is higher than usual. And the warmer-than-usual SSTs in the east mean the pressure is lower. This changes wind flows and weather patterns through the region; the western Pacific sees less rainfall, while the eastern Pacific sees more.

Part of this occurs because the South Pacific Convergence Zone (SPCZ) moves. This is a branch of the inter-tropical convergence zone, which is a narrow band of convection triggered where prevailing northeasterlies from the Northern Hemisphere converge with prevailing southeasterlies from the Southern Hemisphere. The SPCZ juts off from this band, generally along a line between the Solomon Islands in the west, extending past French Polynesia and into the South Pacific Ocean. During El Niño conditions, the SPCZ is pushed eastwards, along with its rainfall. And since the SPCZ is the main generation region for tropical cyclones in the Pacific, the risk of a tropical cyclone forming rises in the east, and drops in the west.

Additionally, since the surface of the Pacific Ocean is warmer during El Niño conditions, global temperatures increase slightly. Strong El Niño events, where SSTs in the eastern Pacific are at least 2°C warmer than normal, increase temperatures by around 0.2°C in the year following the event.

Footnotes

¹ The terms ‘El Niño’ and ‘La Niña’ refer only to the change in sea surface temperatures. The atmospheric component is called the ‘Southern Oscillation’. The whole phenomenon is known as the ‘El Niño–Southern Oscillation’, or ENSO.

Photo iStock.com/EduardoRibas