Unusually balmy sea conditions predicted to delay our winter by a month follow one of the wider New Zealand region's warmest seasons on record, a climate scientist says.
Niwa is forecasting another winter of unusually warmer temperatures – they're predicted to be above-average in the north and either near or above average in most of the south – as a lingering La Niña climate system continues to influence our weather.
As well, coastal waters were running abnormally hot – sea temperatures ranged as high as 2.7C above average over April – which in turn were driving up the mercury on land.
"Relatively speaking, we're surrounded by this boiling pot of water – so it's pretty tough to cool down when you have that going on," Niwa meteorologist Ben Noll said.
Warmed-up waters happened to play a big role in our summer temperatures, which went down as the fifth-highest on average in more than a century of records.
That was according to Niwa's seven-station temperature series, pioneered by veteran climate scientist Professor Jim Salinger.
Salinger told the Herald that, when accounting for more climate stations and ocean temperatures around the wider New Zealand region, the November to March period happened to be our third-warmest – registering at 1.19C above the 1981-2010 average.
His 22-station series showed that three summers in the past five summers were among the hottest recorded – 2017-18's reading of 1.72C above average topping the index – underscoring a clear pattern of global warming.
He also singled out the dominant role of La Niña over summer, along with a prevailing wind direction that was the most easterly he'd observed.
"Of all the summers we've analysed, this one also had among the most blocking anti-cyclones, which have basically been to the south and south-east of New Zealand and partly explains why lake levels have been running so low."
Noll singled out another indicator that reflected the relative warmth about the country.
What's called the Southern Oscillation Index – one of the biggest measures of strength for La Niña events – was tracking to hit the third-highest April value since at least 1876.
"That kind of paints the picture of what rare territory we're in, for conditions at this time of year," he said.
"And we're still seeing pretty much every region aside from the eastern North Island reporting marine heatwave conditions, with localised sea temperature anomalies in central parts of New Zealand as high as 2C to 3C," Noll said.
"These are probably closer to what would be values in early autumn – and show that our seasonal transition to cooler temperatures is solidly delayed by a month or two."
It remained to be seen whether New Zealand would eventually chalk up its third-consecutive record-hot winter.
Elsewhere in its winter outlook, Niwa predicted fewer southerly winds, which could reduce chances of snowfall, while rain around the country could continue to be "irregular", with longer dry spells interspersed with heavy downpours and possibly flooding.
The north and east of the North Island were most likely to get above-average rainfall – with a 35 per cent chance - while the rest of New Zealand can expect rainfall to be normal or below normal.
The lower North Island, as well as the east and north of the South Island, have a 35 per cent chance of below-average rainfall.