Weather: We're on track for another record warm winter. Have you noticed?

This winter has brought plenty of moisture across the country - but little of the crisp cold that's usually characterised the season. Photo / Michael Craig

Last week, Niwa reported that this winter - characterised by a series of tropical-charged deluges, between long spells of mild weather - could well end up our warmest ever for the second year in a row. What's caused it? How unusual is it? And are we getting used to these anomalies too quickly as our climate warms? Science reporter Jamie Morton talks to Niwa meteorologist Ben Noll.

The latest data shows New Zealand is on track to chalk up back-to-back, record-warm winters. Just how rare is this?

If we focus on June and July this year, it's been pretty unusual.

We've just seen our warmest June and July period since records began in 1909 - with a combined temperature sitting at 1.53C above the long-term average.

Before this, several warmer-than-average winters were bunched together between 1998 and 2000 on the back of one of the strongest El Nino events on record.

Source / Niwa

As for the actual difference from average, however, we were only talking 0.7C then, and with this go-round, we've seen much more significant anomalies - in the order of a degree and a half.

That's substantial.

When we compare this winter with last year's, what factors are different and which are the same?

Overall, I think there are more similarities than differences.

The tropical Pacific has been sitting in a cool Enso-neutral state and it was pretty similar this time last year.

We've also had more high pressure around - that's been a factor in the last couple of years - along with a lack of southerly winds.

One difference was that last year, we didn't have another climate driver in the Indian Ocean, called the Indian Ocean Dipole, or IOD, contributing.

Although Indian Ocean temperatures were warmer than average last year, the difference in temperatures between the west and east of the ocean weren't large enough qualify as an IOD.

I think there's been a perception on the ground that it's been wetter this winter than last winter, and that seems to have been skewing the view that it hasn't been as warm as last time.

Last winter was indeed pretty darn dry, and there was a good deal of sunshine along with those warmer temperatures.

This year, it might have been more that those warmer temperatures were happening at night with more cloud and moisture.

A car sits in floodwaters following a devastating deluge that swamped Westport last month. The season has seen several major storms driven by atmospheric rivers from the tropics. Photo / George Heard

That's something that links into that influence from the IOD, with more plumes of moisture coming across from the tropics.

When we look further south, another climate indicator called the Southern Annular Mode has been in a positive phase for 69 per cent of days between January through to July.

This has meant much of those low-pressure systems to our south have been hanging about in the Southern Ocean and closer to the pole, while up here in New Zealand we've had more frequent high-pressure systems.

As well, a stronger polar vortex this year has kept cold air locked up closer to the pole.

How does climate change fit into the picture?

It's front and centre.

You'd be hard pressed to get anomalies at this scale without that background influence of climate change.

Another major storm that swept into Auckland this month left this yacht aground on the eastern side of St Heliers Bay. Photo / Jason Oxenham

If we had the same combination of natural climate drivers that we have now but turned back the clock 50, 70 or 90 years, it probably wouldn't be as warm as it has been this winter.

The effect of a warming world makes up that difference.

Attribution studies can determine the exact amount that climate change has played a role.

We haven't yet done that in this case, but it's fair to say you wouldn't be hitting back-to-back warmest winters without climate change likely playing a significant role.

We know that climate change continues to bring more warm temperatures, while reducing the chance for those cold temperature extremes. We've seen it year after year.

It's also had an influence on another piece of the puzzle of this winter - that's warmer ocean temperatures, which have been running above average for years on end now.

Do you think people are starting to really feel this trend?

A really interesting thing we've noticed from responses from the public, and on social media, is this perception that this winter has been pretty cold, and that there's surprise it's ranking as high on the records as it currently is.

I'm no psychologist, but I'd be willing to bet that part of the reason is that we're quickly getting used to a "new normal" when it comes to temperatures.

"We're getting used to living in a warmer world. We're adapting subconsciously," Niwa meteorologist Ben Noll says. Photo / Michael Craig

We're getting used to living in a warmer world.

We're adapting subconsciously.

Because of this, when it does get quite cold for a couple of days, people say, wow, it's really chilly outside, and these events really stand out.

This effect aside, who do you think particularly would have noticed the unusual warmth we've had?

Parts of the country have had less snow and our temperatures have been warmer than normal, so skifield operators - particularly at lower elevations - would have felt that.

For your average Aucklander, they've likely noticed we've had less in the way of weather extremes.

Right across the North Island, in fact, people have probably found they've had to use their electric blankets less, or have only needed to wear a heavier jacket on the odd cold day here and there.

What's interesting is we've had comments from people in Queenstown, saying it's felt really cold there this winter.

🔥 Warmest June-July on record — this winter has been the warmest on record so far (+1.53˚C)...

The previous record was set just last year.

Climatic causes:

🌤️ High pressure
🌬️ Sub-tropical winds
🌊 Warm seas
🏔️ Less snowfall
📈 Climate change tailwindhttps://t.co/EtXEd8tGII pic.twitter.com/ad7eRsbIaW

— NIWA Weather (@NiwaWeather) August 5, 2021

When I look at the data, their average minimum temperature in July is around -1.6C - but for this July, the average has been -0.2C.

So for those night-time minimum temperatures there, July's been running at 1.4C above average.

But because either -1.6C or -0.2C are both still quite cold, it's sometimes hard to see the bigger picture.

A tornado that ripped through Papatoetoe caused widespread damage. Auckland hasn't been immune to some of this winter's wild weather. Photo / Alex Burton

Moving away from your own backyard, that bigger picture reveals that these seemingly small deviations cause rising ocean temperatures, can impact marine life and sea ice, increase the risk for drought and floods, and make conditions more favourable for wildfires, which we've been seeing in the Northern Hemisphere.

Fast forward 100 years and the winters of 2020 and 2021 would probably be considered colder than average to a person living in the year 2121.

What's in store for the rest of 2021? Could we see the return of La Nina?

The chances of another La Nina forming later this year have now increased to around 45 or 50 per cent, so there's a distinct possibility that could happen.

It also seems pretty unlikely that we'll have a totally different flavour, over the next three to six months, to what we had last year - and we're already seeing a lot of similarities.

So people may be able to draw on their experiences from last spring to give them a sense of the climatic conditions we can expect going forward.

And as we work toward summer, it's not out of the question, if you're in the eastern or northern North Island or South Island, that we could be headed for a third straight summer with drier-than-normal conditions prevalent.

The details on that are still a bit fuzzy, but it's definitely something we're watching for.