A resident crosses the waterlogged Latham St in Marewa, Napier, after the city's second wettest day on record. Photo / Paul Taylor
A resident crosses the waterlogged Latham St in Marewa, Napier, after the city's second wettest day on record. Photo / Paul Taylor
A moisture-packed plume that yesterday dumped 463 per cent of Napier's monthly rainfall over the city in just a few hours was a glance at extreme events New Zealand can expect under climate change.
But it was also "unfortunate" that a collision of systems that created the monster deluge happened directly over Napier, and not 20km or 30km away out over the ocean, Niwa meteorologist Ben Noll says.
What was Napier's second wettest day on record in 150 years – 242mm of rain dropped - caused widespread flooding, forced evacuations around the city and left more than 3000 homes without power.
Noll said the set-up first came with a broad area of low pressure parked over the North Island.
"It was a low that was stacked up [with pressure] - with a low at the surface, as well as at the mid and upper levels of the atmosphere," he said.
"That's your forcing for ascent, or lift, which can lead to the development of clouds and rain."
Over Napier yesterday, it came head to head with another air mass, as opposing winds converged late in the afternoon.
"At the surface, we had southeasterly winds converging with northeasterly winds - and this point in the convergence took place pretty much smack over Napier."
Noll said one of the two air masses involved, which arrived from the northeast, carried a plume of moisture that extended from the east of the North Island - and out to Vanuatu and could be traced back as far as Papua New Guinea.
What was Napier's second wettest day on record in 150 years - 242mm of rain dropped - caused widespread flooding and forced evacuations around the city. Photo / Warren Buckland
"That was the trajectory of the moisture that has been affecting the North Island over the last couple of days.
"And while it has pushed to the east, this area of low pressure that was sitting over the North Island on Monday helped to actually steer it back and drag it toward the eastern side of the North Island, pretty much landing right there on Napier.
🌧️ Record breaking rainfall in Napier 🌧️
💧 2nd wettest day on record (since 1870): 237 mm (and counting) since 9am
"If that plume had been moved 20km north or 20km south to a much less densely populated area, or over the ranges, the impact could have been much different and we probably wouldn't be having this conversation right now.
"In total, we had 242.4mm falling between 9am on Monday and 9am on Tuesday, which will go down in our books as Napier's second wettest day on record, and 463 per cent of normal in a single day - that's pretty significant."
The record remained a deluge that dumped 297mm over one day in 1963, while the third highest figure was 204mm, recorded back in 1924.
"Not to be outshone is the 54mm that occurred in one single hour between 5pm and 6pm yesterday - which was just less than Napier's normal November monthly rainfall."
Sitting in the background was another factor: climate change.
"We know that a warmer atmosphere can hold more water vapour, and can produce heavier rainfall - so that's kind of expected with climate change," he said.
"With this summer in general, we're expecting a La Nina [climate system] to bring down weather systems from the tropics and subtropics of this nature.
"In a warmer world, we'd expect those systems in 2020 to have the chance to produce more moisture than would have been in the case back 50 years.
Rain sweeps over the seaside suburb of Ahuriri, Napier, yesterday afternoon. Photo / Warren Buckland
"So, while you can't obviously pin climate change on this one event, you can say this is what we expect to see more of."
Some regions in New Zealand have already been getting drier under climate change - and others wetter.
A third of places, many of them in the northern North Island, have recorded less rain, while spots in the south and west of the South Island have seen more.
For instance, annual rainfall has shrunk by an average 4.3 per cent per decade in Whangarei, and by 3.2 per cent in Tauranga.
But it's been increasing by 2.8 per cent per decade in Whanganui, by 2.1 per cent in Milford Sound, and 1.3 per cent per decade in Hokitika.
Changes in rainfall haven't happened evenly across the year and it's been over spring, especially, that most of the sites have recorded shifts.
Most places with increasing rainfall have also had more "intense" rainfall, where it's fallen in a shorter period of time rather than being spread out over the year.
But as for "extreme" rainfall like yesterday, scientists say a longer record of data is needed to more clearly pick out trends.
Flooding closed Napier's Emerson St yesterday. Photo / Warren Buckland
Looking to weather over the rest of the month, Noll picked out another wider influence playing out now.
That was the Madden-Julian oscillation, or MJO - the largest element of the intra-seasonal variability in the tropical atmosphere.
It was often responsible for driving ridges and troughs, and pulses of thunderstorms and cloud around the equator.
"So rainfall and thunderstorms in the tropics that were moving across the western and central Pacific - along our line of longitude - that's moving out of the basin now," he said.
"And the pattern that we're about to go into through mid-November will be a little bit more reminiscent of spring, which isn't so wet in the eastern areas, but it can provide fronts to the western parts of both islands.
"So those eastern areas that were just hammered the last couple days and likely will be today and tomorrow to some degree, I think they'll get a bit of a reprieve as we go forward."
