'Lockdown weather': Is it really sunnier under level 4?

Walkers stroll along Auckland's Bethells Beach. Niwa forecasters have drawn parallels between the settled weather under this lockdown and last year's Level 4 period. Photo / Michael Craig

Forecasters have joked about the irony of weather seemingly being better while we're all locking down at home – but is it really true?

As Akaroa registered a record-equalling temperature of 23C on Tuesday, Niwa meteorologist Ben Noll felt a sense of déjà vu – with last year's nationwide lockdown also coinciding with widespread sunny and settled conditions.

So, he recruited the help of Niwa data scientist Neelesh Rampal to find that, sure enough, the number of dry days and the temperatures were higher in level 4 lockdown for March and April last year, and now for August this year.

While the analysis could be said to be more coincidental than scientifically remarkable, Noll said that, on average, lockdown days had featured more high pressure around New Zealand - and particularly west of the country, encouraging more settled conditions.

"The sample size of about 40 days isn't nearly long enough to say anything specific about trends, but it represents an interesting data point that might be more noticeable since we're all spending more time indoors."

As for where this lockdown phenomenon is being felt the most, warmer than average days have been happening across the North Island, the top of the South Island and about Otago.

Dry days - or days with less than 1mm of rain - have been common during level 4s past and present, especially in the North Island and eastern South Island.

Source / Niwa

This includes Otago, Canterbury, eastern Marlborough, Nelson, Wairarapa, Hawke's Bay, Gisborne, Bay of Plenty, eastern Waikato, and parts of Auckland and Northland.

Looking ahead, Noll said that there will be a spell of wet weather from tomorrow into early next week.

Beyond that, he saw a return to tranquillity.

"During September, patterns of high pressure may again dominate, bringing increased sunshine and below normal rainfall to swathes of the country.

"Temperatures in eastern areas could reach 20C to 25C at times."

Superstition aside, there were obvious reasons for climactic flavour – namely a vast swathe of higher-than-normal air pressure in the West Pacific.

Never fails 🙃 pic.twitter.com/ieGPLI2Vlp

— NIWA Weather (@NiwaWeather) August 20, 2021

Similar to this time last year, a La Nina climate system was also currently developing in the tropical Pacific.

The negative phase of another major climate driver, called the Indian Ocean Dipole, was also unfolding northwest of Australia.

"This feature is lowering the pressures in the north of Australia, which is pumping up a ridge of high pressure in the mid-latitudes to the south, where New Zealand is," Noll said earlier this week.

Added to that, the Southern Annular Mode (SAM) - a ring of climate variability that encircles the South Pole, but stretches far out to our own latitudes - had been positive for around three-quarters of 2021 so far.

"A positive SAM tends to be associated with more settled weather around New Zealand," Noll said.

"The SAM has been trending more positive in recent decades, explained by changes in hemispheric ozone levels as well as climate change.

"There will of course be some week-to-week variability, so predictions like this are only meant to provide a high-level overview for the coming weeks rather than a specific forecast for a town on a certain day."