The chances of big deluges like the torrential storm that hit Northland over the weekend could become more likely as La Niña continues to develop in the tropics to the north of New Zealand.
The ocean-made driver could also warm up the Tasman Sea towards the end of the year – the last La Niña in 2017-18 came with a freak marine heatwave that set the stage for New Zealand's hottest summer on record.
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 expect more north-easterly winds that bring rainy conditions to the North Island's north-east, and drier conditions to the south and south-west of the South Island.
The latest trends suggest La Niña conditions are now more likely, with a 55 per cent chance between September-December, than El Nino-Southern Oscillation (ENSO) neutral conditions.
Niwa's criteria for "La Niña watch" have now been met, on the back of ocean waters becoming unusually cool in the equatorial Pacific and trade winds increasing in strength.
Niwa has moved to the "watch" phase of monitoring for a La Nina, which currently has a 55 per cent chance of developing. Image / Niwa
"And even if we never make it to the official threshold, that doesn't mean our weather patterns won't be aligned with La Niña-like conditions," Niwa forecaster Ben Noll said.
"So, in a sense, whether we reach La Niña status is more of a formality. We've seen the patterns align so far and that will probably continue to be the case as we work our way into the back half of 2020."
For northern parts of the country particularly, the background effect of La Niña could make rain events even wetter – sometimes dramatically so.
"During La Niña events, you are more likely to see the type system like we had near the upper North Island late last week and into the weekend," he said.
"The dice are a bit more loaded toward wetness in the north and east of the country."
"And if you get a system that puts Auckland in its cross hairs, that can bring some intense moisture – or some heavy rainfall totals in a short period."
Temperature trends expected for the next three months. Image / Niwa
The drought-parched region had enjoyed some wet patches in recent weeks, he said, but not enough to bring dam levels up to where they should be.
While the Hunua and Waitakere ranges respectively received around 41.5mm and 35mm in the past seven days, total dam storage levels were sitting at about 58 per cent full, compared with a normal average of around 83 per cent for this time of year.
Noll said forecasters were also closely watching for temperature shifts in the Tasman Sea.
"Our climate guidance is hinting at the Tasman Sea potentially becoming pretty warm later in the year.
"Of course, last time we had a La Niña, over the summer of 2017-18, we also had a marine heatwave.
"It doesn't mean we are going to have one this time around but there are certainly signals for a warm Tasman later in 2020."
Rainfall trends expected for the next three months. Image / Niwa
Other possibilities for the long-term were a greater chance of extreme weather delivered by "atmospheric rivers" and a higher number of ex-tropical cyclones.
"If we remember in 2018, we had three ex-tropical cyclones come into the New Zealand region, rather than just the average of one. These are the types of conditions you can experience during La Niña."
That was in clear contrast to the flavour of 2020 to date, he said.
This season – and largely those before it this year – had been held in an ENSO-neutral pattern dominated by high pressure systems, making for settled weather.
"The other side to winter so far has been the semi-regular visitation of low pressure systems through the upper eastern side of the North Island," he said.
"Of course, the latest instance of that is what happened over Northland and Gisborne late last week and into the weekend, and both those systems happened to be La Niña-like in their nature."
In the shorter term, he expected to see more of these stormy interludes play out over the rest of winter.
"It won't be every single week, but I would not be surprised to see stretches where things are calm for five to seven days, and then we get a window where things are more typical for this time of year."
At the same time, he predicted windier weather for the coming weeks.
"I expect there is going to be more wind in general around the country, such as in Auckland on Wednesday. So far this season, we've had more north-easterly and easterly winds than the typical prevailing south-westerly.
"Now it looks like we are going to see a lot more westerly wind action over the next couple of weeks around the western South Island, which has actually enjoyed some of the best weather in the whole country to date this winter."
