Watch: Lightning strike over Wellington. Video / Nick James / Supplied
If you think the spectacular lightshow that's been playing out in our stormy skies over the past week is a bit unusual, you're not wrong.
In just a few days, meteorologists have recorded nearly 112,000 strikes over New Zealand – a country that, on average, annually receives around 187,000 strikes over land and sea.
In just 12 hours yesterday, 12,500 strikes were observed.
"Some places like Wellington and the Kapiti coast have had several consecutive days of thunderstorm activity, which is really quite noteworthy," Niwa forecaster Ben Noll said.
"To have tens of thousands of lightning strikes over the course of a week – you're talking about a significant chunk of New Zealand's annual lightning normal."
What actually is lightning, anyway?
Before we look at what's brought all of this lightning here, let's explain what lightning is.
They're the most dramatic feature of thunderstorms, which are fuelled by the up-and-down motions that we associate with convection – think of boiling water bubbling away on the stove.
Thunderstorms typically feed off the warm, moist air below them – and when this air reaches the base of the cloud, water vapour within the air condenses and builds onto it.
The action of air rising and falling within the thunderstorm separates positive and negative charges, while water and ice particles within the cloud also affect the distribution of electrical charge.
Lightning strikes over the Toitoi Hawke's Bay Arts and Events Centre in Hastings on November 18, 2020. Photo / Paul Taylor
Eventually, the build-up and discharge of electrical energy between positively and negatively charged areas manifests as lightning bolts – most occurring within the cloud, or between cloud and ground.
The average flash of one of these bolts is powerful enough to light a 100-watt light bulb for more than three months.
The air near a lightning strike, meanwhile, is heated to 27,760C - hotter than the surface of the sun – and the rapid heating and cooling of air near the lightning channel causes a shock wave that results in thunder.
While lightning strikes kill an estimated 6000 to 24,000 people around the world each year, lightning fatalities and injuries are incredibly rare in New Zealand – with just a few dozen claims to ACC over the past two decades.
It's also worth noting that New Zealand's thunderstorms rarely measure more than a kilometre wide, unlike in the US, where they can span 20km across.
Although thunderstorms are more commonly single convective cells, sometimes these strengthen into well-known "supercell" thunderstorms.
"There's a couple of things going on here," Noll said.
"The first factor is a really, deep cold pocket air aloft that's emerged from the Southern Ocean and is basically sitting over top much warmer and more humid conditions at the surface."
"This marine heatwave set-up has likely enhanced the potential for convective activity – and it's probably meant we've wound up with more instability than we otherwise would have."
Noll added that warmer ocean waters weren't just isolated to New Zealand's immediate neighbourhood at the moment – with sea temperatures measuring hot across our wider region and beyond.
While stronger westerly winds and more frequent lows forecast for this month might take the edge off, Niwa nonetheless expected seas to keep running warm throughout winter and beyond.
Has it felt particularly stormy the last week? 🌩🌩🌩
Here's why:
⚡ 111,621 strikes from midnight last monday to 7am this morning!
As for what had warmed up our seas in the first place, the drivers were complex.
But factors included a strong and lingering La Niña climate system; a Southern Annular Mode being dominantly positive for months and blocking low-pressure systems from the south that'd normally help churn and cool surface water; 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 had local sea temperatures been running at their colder, historic average for this time of year.
Is this storm La Niña in action?
Aside from that warmer ocean influence – not really, Noll said.
"The weather pattern that we're in right now is something that we'd classify as destructive interference against La Niña – and is actually more like El Niño than La Niña."
During a La Niña event, ocean water from off the coast of South America to the central tropical Pacific cools to below average - a result of stronger than normal easterly trade winds, which churns cooler, deeper sea water up to the ocean's surface.
This unusually cool water in the eastern Pacific then suppresses cloud, rain, and thunderstorms, as sea temperatures in the far west of the ocean warm to above average temperatures.
Here in New Zealand, we can usually expect more north-easterly winds that bring rainy conditions to North Island's north-east, and drier conditions to the south and south-east of the South Island.
Thanks to the north-easterly winds, warmer temperatures also tended to play out over much of the country during La Niña, although there are always regional and seasonal exceptions.
El Niño brings the opposite set-up – but the last time New Zealand experienced one was back in 2015-16, as most of the last 10 years has trended toward either La Nina or ENSO-neutral conditions.
"So this is a bit of a curveball, and since the start of June the atmosphere has been behaving in a way that's inconsistent with La Niña – and more of what El Niño winters are like."
Noll said this owed to complex atmospheric processes playing out across the northeast tropical Pacific – but La Niña-flavoured winter weather was 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.
"There's a possibility that what we've just observed represents one of the more wintry periods we'll get here in this season overall," he said.
