MetService National Weather: September 8th - 10th.
New Zealand's coastal waters are running unusually warm - but meteorologists say it's still too early to tell whether the country will be engulfed in another "marine heatwave" this summer.
Niwa forecaster Ben Noll said coastal seas over August ranged from 0.6C above average off the east coast of the North Island, to 1.01C above average to the west of the South Island - and this picture hadn't changed.
That was warmer than where temperatures were sitting the same month last year, and also in 2018, which saw the second of back-to-back marine heatwaves.
Noll said readings like the current ones often proved a "starting point" for warmer air temperatures to build over land - which was why forecasters were confident this spring would be warmer than average.
"For later in the year, we don't know exactly how unusual sea surface temperatures are going to be - and whether it will be quite to the level we had in 2017 and 2018 remains to be seen," he said.
"But I think for folks who were impacted by that event, they may want to draw on their experiences from it and use them for forward planning as we work our way through the rest of spring and into summer."
What was clear, he said, was the early presence of some of the same calling cards from previous events.
Over New Zealand's record summer of 2017-18, sea surface temperatures were pushed to 1.5C above average, and as high as 6C above normal in some spots off the West Coast.
New Zealand's most severe marine heatwave as observed in late November, 2017. Image / Ben Noll, NIWA
While its effects made for balmy surf at our favourite beaches, it also drove the biggest melt ever seen on the Southern Alps, pushed warm-water fish south and had a big effect on growth in orchards and vineyards.
It resulted from a combination of persistent highs, a La Niña climate pattern in the tropics and a positive Southern Annular Mode pattern to our south, all set against a background of climate change.
Right now, the probability of a La Niña developing in the Pacific over the next three months was 57 per cent - and most of Niwa's criteria for "La Niña Alert" had now been met.
Even without a system completely in place, Noll said New Zealand could still be influenced by "La Niña-like" conditions.
"We've also got warm temperatures up in the Coral Sea right now, and when weather systems track in from the north, that warmth can be dragged down toward New Zealand."
That left meteorologists closely watching the Southern Annular Mode, or SAM.
The indicator measured the north–south movement of the westerly wind belt that circled Antarctica, dominating the middle to higher latitudes of the southern hemisphere.
It was the changing position of that westerly wind belt which influenced the strength and position of cold fronts and mid-latitude storm systems over New Zealand.
While the 2017-18 summer's marine heatwave made for balmy surf, it also melted ice caps and had a big effect on growing conditions in orchards and vineyards. Photo / Alan Gibson
A positive SAM had meant there were weaker westerly winds than normal over the South Island with higher pressures - and less cold fronts crossing New Zealand with bursts of cold air.
In a positive phase, it also blocked highs to the east of the country, and sometimes bulging back over New Zealand, with mild northerly airflows across New Zealand.
Noll said the SAM had been in its positive phase slightly more than its negative one this year and it could track in its positive phase more frequently during October or November.
"In any case, the SAM is not always the best metric to consider on its own - it's rather just one in a much wider climate-driver pie," he said.
"So I think you could say, based on what's happened over the last several years, we have a heightened level of awareness and alert right now.
"What happened in 2017/18 was unprecedented - we haven't seen anything like that to date, it was just off the charts.
"Whether 2020/21 goes down that track remains to be seen, but certainly, some of those same climate driver boxes have already been ticked."
It comes after New Zealand just saw its warmest winter, which Niwa put down to sub-tropical winds, warmer-than-average seas, more sunshine, and climate change.
The countrywide average for 2020 was 9.6C - 1.14C above the 1981-2010 average, taken from Niwa's seven station temperature series which begins in 1909, and pipping the previous record of 2013 by just 0.06C.
Source / Niwa
For the next three months, Niwa was predicting warmer air temperatures over the entire country, along with higher air pressure to the east and lower than normal to the northwest of New Zealand.
