Biggest waves recorded around New Zealand this year include one four storeys high

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High-tech buoys deployed in our ocean waters have registered some monster waves this year - including a "one-in-3000-year" wave near Napier and a four-storey-high giant further east of the country.

The buoys – found around our coasts, but also hundreds of kilometres south of the country in some of the planet's most perilous waters – are now helping scientists refine models and forecasts.

This year, amid a slew of extreme weather events, they’ve returned some remarkable readings.

Amid February's destructive ex-tropical Cyclone Gabrielle, wave buoys stationed off the coast of Whangārei showed maximum wave heights close to 10m high – and significant wave heights of just over 6m.

Maximum wave height measured the largest single wave recorded, from peak to trough, while significant wave height was the average measurement of the largest third of waves – something that corresponded well to our own visual estimates.

Amid Gabrielle, the Port of Napier's Wave Ride Buoy picked up a wave that reached a significant wave height of 6m, shortly before it snapped free of its moorings.

"Our analysis suggests that a sea state with wave heights of six metres would occur on average only once every 3000 years at the Wave Rider location," MetOcean Solutions oceanographer Dr Séverin Thiébaut said.

"Although these events tend to occur more frequently due to climate change, they remain rare and are not necessarily accounted for in hindcast models."

Further afield, a drifting buoy east of the Chatham Islands recorded a huge wave on April 29 with a significant wave height of 13.6m: roughly the equivalent height of a four-storey building.

The same drifting buoy picked up similarly high waves on April 16 and March 8.

Yet that was still much smaller than the largest maximum wave height ever measured in the Southern Ocean: a towering 23.8m wave near Campbell Island that formed in the thick of a huge storm in May 2018.

During the depths of winter, these waves were enormous, averaging more than 5m, regularly exceeding 10m - and sometimes likely reaching more than 25m.

Anything more than 20m high was highly hazardous to vessels.

In 2014, waves that climbed to 14m forced the HMNZS Wellington to turn around partway to the subantarctic islands - and ships tended to negotiate such heavy seas by sailing head-on into the direction the waves were coming from.

MetOcean Solutions general manager Dr Brett Beamsley said it was possible waves this size were occurring frequently in the Southern Ocean, but there was no way to measure them directly.

"We have access to global satellite altimeter data which measures the altitude of the sea surface which can provide information on the wave height, but we are constrained by their coverage and these observations rarely coincide with the peaks of storm events."

For waves to grow that big, three factors were needed: a high wind speed, a large area of water for winds to blow across – or what's called fetch – and a persistent wind direction.

"The greater all of these factors are, the larger the waves will be," Beamsley said.

"Ultimately, the wind drives the waves."

The depths of the Southern Ocean – below latitude 40 south, into the "roaring 40s", "furious 50s" and "screaming 60s" – happened to be home to the strongest average winds on the planet, and its wave-making potential was only increasing under climate change.

One study found extreme waves in the ocean had grown by 30cm – or 5 per cent – in the past three decades, all while the region had grown stormier, and even gustier, with extreme winds strengthening by 1.5m a second.

Another 2020 study found a warming planet would cause stronger storm winds triggering larger and more frequent extreme waves over the next 80 years – with the largest increases occurring in the Southern Ocean.

That wasn't to say huge waves couldn't also be found closer to our shores – especially when deep ex-tropical cyclones or subtropical lows blew through with extreme wind speeds.

"Significant waves of around five to seven metres are often measured around New Zealand at the sparsely distributed moored wave buoys, with maximum individual waves likely to be almost twice as big," Beamsley said.

At their simplest, the moored buoys like these measure wave heights, periods and directions.

Using vertical accelerometers to estimate the heave of the buoy, along with tilt sensors to infer direction, they constantly feed data to satellite or cellular networks.

Beamsley said this data was critical for understanding the current state of the marine environment and ensuring the accuracy of forecast models.

"We also use the observations to verify and validate ocean models, providing opportunities to tune and improve the models."

More generally, the data was helping scientists study the vulnerability of our coasts from large waves and swells – namely with storm surge, inundation and erosion.

"In the longer term this will only become worse with sea level rise and coastal erosion."

Unlike many other countries, New Zealand didn't have a single, centrally operated wave buoy network focused on better understanding the maritime environment.

"Around New Zealand, without funding, buoy deployments are piecemeal and resourced by ports or regional councils."

In deploying its own wave buoys in the Southern Ocean, MetOcean Solutions – the oceanographic branch of MetService – has previously partnered with the New Zealand Defence Force (NZDF).

MetService itself contributed to the Global Drifter Programme by deploying drifting buoys provided by the US National Oceanic and Atmospheric Administration in the Tasman Sea and the Southern Ocean, using cruise liners and container ships.

After disruptions caused by the pandemic, the number of active, drifting buoys surrounding New Zealand dropped to just eight, but with NZDF assistance, more than 40 have been deployed since late 2021.

"Unfortunately, in the Southern Ocean, it's becoming more difficult to justify deploying fixed wave buoys as the adverse conditions mean they ultimately break moorings and drift," Beamsley said.

"Last year, we actually got one back from Chile over one year after its mooring broke at Campbell Island and the batteries failed due to low light.

"It was in an okay state, and we recovered the data, but it highlights the challenges."

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