Weather/climate explained: The weird La Nina flavouring your summer

People across the North Island may have to endure this summer’s sticky humidity into autumn, because of an unusual La Nina climate system that isn’t moving away any time soon.

The naturally occurring, ocean-driven phenomenon – the strongest in nearly a decade – has been meddling with our weather since it formed over spring.

And while some of its influences have been typical – bringing heavy servings of heat and moisture – others have been starkly different to the big driver’s famous flavours.

A La Nina apart

La Nina has been a feature of our planet’s climate for millions of years.

It forms part of what’s called El Nino Southern Oscillation, or ENSO – that’s an irregular, periodic shift in trade winds, along with sea and air temperatures, in the equatorial Pacific.

Its warming phase – El Nino – and its cooling phase – La Nina – both have enormous influence on weather and climate across the globe.

Here, Kiwis have come to expect some familiar La Nina calling cards: notably, widespread warmth, north-easterly storms, rain about the north and east, dryness about the south and southwest, and long stretches of punishing mugginess.

But a quick glance at New Zealand’s current drought index – showing abnormally dry conditions again setting in across the north of the North Island – told us this system was dramatically different.

Niwa meteorologist Ben Noll said the answer lay not just in the Pacific – which usually formed the engine room of La Nina – but in waters further afield.

“During the normal life cycle of an ENSO event, whether it’s El Nino or La Nina, we typically see a peak around December or January,” he said.

“It’s around that time that temperature anomalies in the ocean are having their most profound impact on the atmosphere that sits above them.”

This manifested as what was known as the Walker Circulation – a rising and sinking convective cycle extending across the world’s tropical oceans.

“You can have rising air over top of warmer-than-average ocean waters – and you can have sinking air over cooler than average ocean waters,” Noll explained.

“Ultimately, the Walker Circulation is modulated by where those pockets of cooler and warmer seas are, relative to the norm in the global tropics.”

What did that have to do with La Nina?

Over recent months, the Indian Ocean – especially its central and western expanses – have been running persistently warm.

“We’ve seen this happening near Madagascar, to its east toward India, and more recently, to the west of Australia, in the eastern Indian Ocean,” he said.

“That’s meant the Walker circulation has kind of focused its energies more over the Indian Ocean.”

The shift had seen activity like thunderstorms and cyclones also based further west of the West Pacific “warm pool”, where La Nina traditionally delivered its most stormy weather.

“If you hop on over to the Pacific, meanwhile, what you have are really cool temperatures in the central part of the ocean basin, but less so further east,” he said.

“Put all of that together, and you are seeing rising motion in the Indian Ocean and a sinking cell in the Pacific that is displaced a bit west of what is typical.”

“This subtle abnormality in the Indian Ocean and in the Central Pacific are working hand in hand to produce a bit of a different flavour of La Nina in New Zealand.”

The gradual effects of climate change were adding their own ingredients to the mix – bringing more background warmth, but also moisture.

A scrambled menu

Broadly, this set-up favours high pressure over New Zealand with brief spates of unsettled weather, as summer holidaymakers will have noticed.

Instead of the classic regional differences, people in the north have seen plenty of hot and fine weather so far this season – hence the unfolding dry – while the far south hasn’t received its typical La Nina spread of clear weather.

Noll said the same items on the traditional La Nina menu were still there – they’d just been rearranged.

“In a normal La Nina, we’d see heavy rain-makers further up this menu – meaning they were more likely to happen – along with big southern highs causing hot weather over Otago and Southland.

“During this La Nina, those two things have been pushed down the menu. However, it looks like, as we go toward the end of January and into early February, we may see a pulse in the atmosphere that causes a response more typical of La Nina.

“We saw this in early December as two cyclones formed in the southwest Pacific and threatened us here in New Zealand for a short time.

“In the meantime, we’re still being influenced by this atypical flavour that’s shuffled the deck, somewhat.”

Between now and the end of March, Niwa predicts above-average temperatures nationwide, along with near-to-above normal chances of rainfall everywhere except the west of the South Island.

Just how out of step is that with New Zealand’s La Nina records?

Noll said all but three of 17 La Nina events measured since 1972 had delivered either near or above normal rainfall for Auckland.

“So if we have a December, January and February that goes down as being drier than normal in the upper North Island, that will be quite unusual,” he said.

“You’re talking about being within that 20 per cent outlier. But there’s a lot of game still to be played this summer, and when we close the books at the end of February I’d expect there will have been enough rain to get overall totals to at least near-normal.”

He added that, although the chances of an ex-tropical cyclone visiting New Zealand over the next few weeks were low, Niwa was still projecting elevated activity through to the end of the cyclone season in April.

And La Nina’s quirky character still comes with plenty of periods of sticky heat for the north, where levels of relative humidity could reach oppressive levels.

Unfortunately, those muggy days could drag on past the end of summer.

Niwa predicts this La Nina is likely to continue for at least the next three months – and international models similarly give a 92 per cent chance.

The probability of a continued La Nina falls to 32 per cent between April and June, when the ENSO pendulum is expected to move back to neutral.

Source: Read Full Article