Scientists call it "season creep" and, despite a couple of bracing southerly storms, the second half of this winter has resembled a very early spring.
On the back of unseasonably warm days in the past two months, and the past two hot, dry summers, WeatherWatch head analyst Philip Duncan says there is nothing to suggest the country won't be in for another long, hot summer.
In late July the private forecaster declared spring had already sprung.
"It hasn't been much of a winter, both here and in Australia."
Winter had been a balmy affair not because of climate change but because of a lack of southerlies, which was just good fortune or bad luck, depending on your point of view, Mr Duncan said.
But the climate change argument did come into play when the past decade's trend towards warmer winters was taken into account.
Using a motoring analogy, Mr Duncan tentatively predicts spring and summer will be warm and dry.
"It's like winter is a 50kmh zone and we've had spring temperatures in August, and that's like going into the 70kmh zone early but doesn't necessarily mean we'll get to the summer 100kmh zone early. But I think spring is going to be warm and will dry out once the westerlies pick up. Because we're in a neutral pattern, summer is no different - just like the last two summers, it could be very dry again."
The main harbinger for summer hinges on the equinoxal winds that typically cause a breezy chaos in mid-to-late September. If these are extreme and buoyed by temperatures in the 20-plus range, we could be in for another scorching summer, which brings with it the possibility of another big dry and more water restrictions.
Despite farmers being generally well set up by a relatively lush winter, Mr Duncan urged anyone who relied on water for their livelihood to "keep a close eye on the discussion about what summer's going to look like".
Earlier this month, climate scientist Jim Salinger said New Zealand had experienced the mildest winter since records began in the 1860s, with the entire country between 1 and 2 degrees warmer than average.
Wellington had its warmest winter on record, and capital dwellers should get used to less wind and milder weather, he said, with records showing a "warming trend" over time. "The clearest climate warming signal is seen in winter, where temperatures are now 1.1 degrees warmer than they were around 1870," he said.
Although it was too early to get a read on summer, Dr Salinger said above-average sea surface temperatures around the country were part of a largely neutral El Nino/La Nina pattern heralding a warmer, calmer, drier-than-usual spring. Based on his interpretation of British Met Office and US meteorological services data on Australasia, the lower North Island and South Island would be the main benefactors of the fairweather spring.
More highs to the east of the South Island suggested a more arid spring, while the capital also looked likely to benefit from the easing of the traditional, often maddening, onslaught of westerly winds.
"That will be good for Wellington - there'll be less of those roaring spring gales," Dr Salinger said.
OZONE HOLE SLOWLY HEALING UP
Niwa chemistry-climate modeller Olaf Morgenstern is conducting research on how the Antarctic ozone hole affects New Zealand's weather.
Over recent years, the hole has begun slowly healing due to a reduction of ozone- destroying chlorofluorocarbons and other halogenated compounds, which have been mostly banned worldwide.
The record for ozone depletion was set in 2006, but this year, Dr Morgenstern said, the hole was likely to be larger than it had been over some recent years, which were characterised by relatively weak ozone depletion.
The ozone season runs between about late August and November or December so it was hard to give a definitive forecast.
However, low to average heat flux coming from mid- latitudes during winter was "conducive to above-average ozone depletion", he said.
Typically, the ozone hole will break up in November or December. After the break-up, ozone- depleted air will be mixed with mid-latitude air. It is in this late-spring/early-summer season that New Zealand can expect episodes of relatively low ozone and correspondingly elevated UV, which particularly fair-skinned people need to be wary of.
- © Fairfax NZ News
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