A small meteotsunami detected in several North Island locations was triggered by intense weather. File photo / Warren Buckland
A small meteotsunami detected in several North Island locations was triggered by intense weather. File photo / Warren Buckland
Gauges used to measure tsunamis suggest one hit Napier last week, without an earthquake to trigger it.
It was a headscratcher for scientists too, but they believe they’ve figured out the mystery.
Last week, as a powerful weather front swept across the upper North Island, GeoNet’s 24 hours a day,seven days a week eyes-on team at the National Geohazards Monitoring Centre noticed something unusual.
Tsunami gauges along the East Coast and in Lake Taupō started showing strange signals.
With no earthquake to blame, the team alerted GeoNet’s tsunami experts who pieced together the puzzle – the readings were likely the signature of a small meteotsunami triggered by the intense weather.
A meteotsunami, short for meteorological tsunami, is a series of long waves caused by sudden changes in air pressure from fast-moving weather systems, like thunderstorms.
These pressure changes push on the ocean surface, creating waves that travel toward the shore.
GeoNet says if the coastline has features like a shallow shelf, a bay, or an inlet, the waves can grow even bigger.
But the key to a meteotsunami is timing; when the speed of the pressure disturbance matches the speed of the wave in the water which depends on how deep the water is, the wave’s energy builds up rapidly.
This can make the waves much larger than usual, even without an earthquake.
Earth Sciences New Zealand forecasting and media principal scientist Chris Brandolino said from the afternoon of Tuesday December 2 to Wednesday December 3, Earth Sciences’ weather team had seen waves of active thunderstorms moving across the upper North Island.
“It’s almost certain that the minor tsunami activity detected nearby is related,” he said.
Top graph, de-tided water height at GeoNet's Napier tide Gauge showing meteotsunami activity beginning at 7am on December 3. Bottom graph, water height at GeoNet's Lake Taupō tide gauge showing meteotsunami activity about 4am. Composite image.
GeoNet instruments at Napier, Castlepoint, Gisborne and Lake Taupō, part of its tsunami gauge network, picked up these waves.
The largest observed wave was 25cm in Napier.
To detect a tsunami, GeoNet look at de-tided data.
This means it takes the original sea-level measurements and remove the regular tidal patterns scientists expect.
What’s left shows other changes, like storm surges, tsunami waves or gradual shifts in sea level.
Tsunamis aren’t just triggered by earthquakes or weather, they can also be caused by underwater landslides or volcanic eruptions.
When people hear the word tsunami, people often imagine towering waves crashing on to shorelines.
However, in a statement, GeoNet said tsunamis behave more like fast-flowing tides than regular ocean waves, carrying water and debris in and offshore and generating strong currents as they do so.
While a small tsunami is unlikely to sweep over people and structures, it still poses serious risks to people in the water or on boats.
GeoNet said it was a good reminder that tsunamis do happen in New Zealand, and it’s always important to be prepared.
