El Nino lag looks likely to give us an Indian summer
Our weather-influencing El Nino climate pattern may have already peaked — but multiple regions are seeing their windiest, driest conditions now. What’s going on? Science reporter Jamie Morton explains
What’s going on with this dryness?
In meteorological terms, our summer ends in less than two weeks — but that doesn’t mean windy, warm, dry weather won’t linger for much of the country.
Restricted fire seasons have only just been declared across Auckland, Northland and Bay of Plenty — and regions including Northland, Wairarapa, Marlborough and Central Otago are running “dry” or “very dry” under the NZ Drought Index. While long-range forecasts indicate dryness carrying on in Wairarapa and the northeastern South Island, there’s fortunately little sign of full-blown drought over the next month. Northwesterly winds have meanwhile been blowing strong across the country, hampering efforts to fight major fires in Canterbury. “The dry conditions and warm temperatures we have been seeing about eastern regions tie directly into the stronger west to northwesterly winds which have been coming and going this summer,” MetService meteorologist Claire O’Connor said. “Regular southerly wind changes have offered respite to the temperatures, but the associated precipitation has not been enough to reverse the drying effect. So despite the winds easing, the fire danger remains a concern.”
Where does El Nino fit in?
While dryness might be a sign of a Kiwi summer doing its thing, this season has also been flavoured by an El Nino climate system that hasn’t entirely been running to script. Traditionally, the big driver forms as warmer water amasses in the eastern Pacific, while water in the western Pacific cools below average — and in New Zealand it typically spells dryness in the northeast, with plenty of strong westerly winds.
But this El Nino happened to develop amid warmth across the entire Pacific and a lingering “warm pool” in the west has helped deliver sporadic bursts of moisture and humidity to those normally parched places. Added to this unusual mix has been the emergence of local and regional marine heatwave conditions, which the North Island’s East Coast has continued to experience.
Niwa meteorologist Ben Noll said many of the classic characteristics of El Nino were just showing up now. “From an oceanic perspective, El Nino peaked back in December, but from an atmospheric perspective, we’re still seeing a lag,” he said. “This is associated with what we’ve seen over the last few weeks, with fires, dryness and winds and it’s something we’ll be watching into autumn.”
Niwa’s outlook until April gave El Nino a 100 per cent chance of persisting, with more northwesterly winds, and near- to below-normal rainfall across most of New Zealand.
So, what comes next?
The El Nino, and its constant northwesterlies, is expected to fade away by mid-year. What happens to climate patterns beyond our autumn is murky at best, given a notorious “predictability barrier” hampering models around this time of year. Still, the United States National Oceanic and Atmospheric Administration’s (NOAA) latest outlook gives a near-80 per cent chance of the present El Nino shifting to “Enso [El Nino-Southern Oscillation]-neutral” conditions over our autumn — and “increasing odds” of La Nina developing afterwards. The agency stated a 55 per cent probability of La Nina conditions forming over June to August, and a roughly three-in-four chance of them forming by the end of our spring. Australia’s Bureau of Meteorology and our Niwa each have different criteria for issuing formal La Nina “watch” advisories and haven’t yet done so.
While the prospect of a returning La Nina mightn’t be welcome news to those in the north and east of New Zealand — especially after the infamously soggy summer of 2022-23 — Noll said the odds of one this year was still a coin-toss.
And if a fourth La Nina in five years did eventuate, he said, there was nothing to suggest it’d mean our next summer would be as extreme as our last one.
“That was the culmination of a lot of extremely warm water building up over a three-year period — and if another La Nina developed, we’d really be back at square one.”