A drought affecting parched pockets of the country is expected to widen across the northern North Island over the next week, and there is no decent rain in sight. Science reporter Jamie Morton looks at some of the big questions.

What's the current picture?

Earlier this month, the Herald reported that dry patches of the North Island were on the verge of meteorological drought.

"That's now what's happened," Niwa forecaster Ben Noll said.

"In eastern Northland, eastern Auckland, Great Barrier Island, Waiheke Island and northern Waikato, we are now seeing a few patches of meteorological drought on the map.

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"Generally, we're looking at widespread extreme dryness from northern Waikato up to Northland."

How did we get here?

Put simply, too many dry westerly winds, and not enough rain.

Noll said a large subtropical ridge of high pressure that had been parked northwest of the North Island since the start of the summer had blunted fronts that otherwise would have brought moisture to the north.

Further in the background, he pointed to a climate driver called the Indian Ocean Dipole (IOD), which had also helped plunge Australia into its bush fire crisis.

The IOD, a seesaw-like system, had been in a strong positive phase for the past few months and created a temperature difference across the tropical Indian Ocean, which had implications for the climate in our part of the world.

Another big driver – a negative Southern Annular Mode – had also had an influence on our weather for months, although both that and the IOD were expected to switch back to neutral over the next few weeks.

What's the upshot for the short term?

"We have an outlook of dry weather over the next seven days, and that's like to worsen the situation," Noll said.

Those droughty patches would creep outwards to cover larger areas of land.

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"With the amount of soil moisture that we're losing from the ground on a weekly basis, we need a solid 20mm to 25mm a week to maintain the balance – and we definitely don't see that type of rain coming anytime soon.

"There is also likely to be an increase in windiness as we go from this weekend into next week. That doesn't bode well for other parts of the country, like eastern parts of the North and South islands, where dryness is not as severe as it is further north."

Noll said a predicted increase in westerlies over the next two weeks would bring similar weather conditions to the warm run in December.

"So it looks like that positive IOD is going to have a last gasp of sorts."

While there was some rain forecast over the next seven days – a deep trough of low pressure in the Tasman Sea, expected to move up the country around January 30 – that likely wouldn't be enough to change the picture.

"It'll bring some rain, but not to those places that are the driest. The west and lower parts of the South Island will be where it's most likely to occur.

"We may have some pushing into Waikato, Auckland and Northland, but it doesn't look like it will bring that much rain. In saying that, any would be beneficial to those places."

What does it mean for farmers?

The sector-led Rural Support Group was closely assessing the situation – although the Ministry for Primary Industries had yet to declare an adverse event.

"We were all probably thinking about drought a week ago, and now we're starting to talk about it," Federated Farmers Northland president John Blackwell said.

Source / Niwa
Source / Niwa

"It's definitely browning off each day. The feed is just not growing, and with farming, it's a long-term game – what happened a month ago relates to what is happening today."

The region was already well behind on its average rainfall for the past 12 months, which had affected groundwater levels.

Farmers would really feel the pinch if the dry trend carried on into April and May – and that could come to pass.

Niwa's latest seasonal outlook, covering the next three months, picked temperatures to be near or above average for all of the North Island – and rainfall to be near or below normal for the north of the North Island.

Is there a climate change aspect here?

Projections show that, under climate change, drought will become a more frequent occurrence in New Zealand – particularly around the north and east of the North Island and the east of the South Island.

But Victoria University climate scientist Professor James Renwick said the current situation in the north was largely a result of natural variability.

"For whatever reason, we've had this tendency for the highs to sit up over north and northwest of the North Island. So the westerlies have been a lot stronger average but most of the time they've been quite dry, and not wet ones.

"And the weather patterns have been quite typical of an El Nino summer where you do get it dry in north and the east. The only catch is, there's no El Nino."

The influence of climate change was more clearly seen across the Tasman, in Australia's bush fire crisis, he said.