Weather: why New Zealand just saw two-thirds of its annual lightning strikes
Watch: Lightning strike over Wellington. Video/Nick James/Supplied
If you think the spectacular light show that has unfolded across our stormy skies over the past week is a bit unusual, you’re not mistaken.
In just a few days, meteorologists recorded nearly 112,000 strikes over New Zealand – a country which, on average, receives around 187,000 strikes on land and sea each year.
In just 12 hours yesterday, 12,500 strikes were observed.
“Some places like Wellington and the Kapiti Coast have had consecutive days of thunderstorm activity which is truly remarkable,” Niwa forecaster Ben Noll said.
“Having tens of thousands of lightning strikes in a week – you’re talking a big chunk of New Zealand’s annual lightning normal.”
What is lightning anyway?
Before we look at what brought all this lightning here, let’s explain what lightning is.
They are the most dramatic feature of thunderstorms, which are powered by the up and down motions we associate with convection – think boiling water bubbling over the stove.
Thunderstorms typically feed on the warm, moist air below them – and when this air reaches the base of the cloud, the water vapor in the air condenses and collects on it.
The action of rising and falling air in the thunderstorm separates the positive and negative charges, while the water and ice particles in the cloud also affect the electrical charge distribution.
Eventually, the buildup and discharge of electrical energy between positively and negatively charged areas manifests as lightning – most occurring within the cloud, or between the cloud and the ground.
The average flash from one of these bolts is strong enough to light a 100 watt light bulb for over three months.
The air near a lightning strike, meanwhile, is heated to 27,760°C – hotter than the surface of the sun – and the rapid heating and cooling of the air near the lightning channel causes a shock wave that causes thunder.
While lightning kills an estimated 6,000 to 24,000 people worldwide each year, deaths and injuries caused by lightning strikes are incredibly rare in New Zealand – with only a few dozen ACC claims over the past two decades.
It should also be noted that thunderstorms in New Zealand rarely measure more than
one kilometer wide, unlike in the United States, where they can extend to 20 km.
Although thunderstorms are most often single cell convective, they sometimes strengthen into the well-known “supercell” thunderstorms.
Other times they expanded to create what are called squall lines – which are exactly what the country has seen in recent days and which are sometimes accompanied by tornadoes.
What caused all these flashes?
“There are several things going on here,” Noll said.
“The first factor is a pocket of really, really cold air aloft that emerged from the Southern Ocean and is basically underpinned by much warmer and wetter conditions at the surface.”
If you’ve heard Noll and other meteorologists discussing that local sea surface temperatures have been abnormally warm – between 1°C and 3°C above average like last week – these thunderstorms are influenced by this extra heat.
“This marine heatwave pattern likely enhanced the potential for convective activity – and that likely means we ended up with more instability than we otherwise would have.”
Noll added that warmer ocean waters weren’t just isolated to the immediate vicinity of New Zealand at the moment – with warm sea temperatures across our region and beyond.
While stronger westerly winds and more frequent low pressure forecast for this month could take over, Niwa still expected the seas to remain warm through the winter and beyond.
As for what had warmed our seas in the first place, the pilots were complex.
But the factors included a strong and persistent La Niña weather system; a southern annular mode being mostly positive for months and blocking southern low-pressure systems that would normally help churning and cooling surface waters; and the background effect of global climate change.
Noll said it would be interesting to model how this week’s storm might have played out if local sea temperatures had reached their colder historical average for this time of year.
Is this La Niña storm in action?
Aside from that warmer oceanic influence — not really, Noll said.
“The weather pattern we find ourselves in right now is something we would characterize as destructive interference against La Niña – and is actually more like El Niño than La Niña.”
During a La Niña event, ocean water from the coasts of South America to the central tropical Pacific cools below average – due to stronger than normal easterly trade winds , which brew cooler, deeper seawater up to the ocean surface.
This unusually cool water in the eastern Pacific then suppresses clouds, rain and thunderstorms, as sea temperatures in the far western ocean warm to above average temperatures.
Here in New Zealand, you can generally expect more northeasterly winds which bring wet conditions to the northeast of the North Island and drier conditions to the south and southeast of the South Island.
Thanks to northeasterly winds, warmer temperatures also tended to occur over much of the country during La Niña, although there are always regional and seasonal exceptions.
El Niño brings the opposite pattern – but the last time New Zealand had one was in 2015-2016, as most of the past 10 years have tended to move towards neutral La Nina or ENSO.
“So it’s a bit of a curveball, and since the beginning of June the atmosphere has been behaving in a way that’s inconsistent with La Niña – and even more so with what El Niño winters are.”
Noll said this was due to the complex atmospheric processes taking place in the tropical Pacific northeast – but La Niña-flavored wintry weather is expected to return within weeks.
That meant warmer – but also wetter – conditions, with much of that activity coming from the north and west rather than the south.
“It is possible that what we have just observed represents one of the most wintry periods that we will have here this season overall,” he said.
“That doesn’t mean winter is over, but these types of weather periods will likely be few.”
For people spooked by thunder and lightning, fewer cooler air particles interacting with warmer seas might be welcome.
But – in the context of global climate change in particular – the exceptionally mild oceans were certainly not something to celebrate.