An obscure driver in our atmosphere is increasingly pointing toward a La Nina climate system dampening down New Zealand's drought-parched north this year.
Currently, there's no big influencer at the steeringwheel of our winter climate, with waters in the central Pacific sitting in a state called El Nino-Southern Oscillation (ENSO) neutral.
These conditions, which have formed the backdrop to New Zealand's settled weather over the past few months, are likely to persist until the end of winter, forcing meteorologists to dig a little bit deeper to uncover forecast trends for the medium-term.
There's a much stronger flavour when our climate is sitting either in an El Nino state – bringing warm westerlies in summer, cold southerlies in winter, and south-westerlies the rest of the time – or La Nina.
These maps show the differences in summer rainfall between El Niño and La Niña systems. Image / Niwa
During a La Nina event, ocean water from off the coast of South America to the central tropical Pacific cools to below average - a result of stronger than normal easterly trade winds, which churns cooler, deeper sea water up to the ocean's surface.
This unusually cool water in the eastern Pacific then suppresses cloud, rain, and thunderstorms, as sea temperatures in the far west of the ocean warm to above average temperatures.
Here in New Zealand, we can expect more north-easterly winds that bring rainy conditions to North Island's north-east, and drier conditions to the south and south-east of the South Island.
Thanks to the north-easterly winds, warmer temperatures also tended to play out over much of the country during La Nina, although there are always regional and seasonal exceptions.
Niwa scientists have been closely watching for early pointers to La Nina conditions, which could be in place by spring, bringing some much-needed moisture to the drought-hit north and east.
And one sign in particular – a little-discussed climate pattern called atmospheric angular momentum, or AAM - has caught the interest of Niwa forecaster Ben Noll.
It's a measure of how fast our atmosphere is spinning relative to the Earth's rotation – but this complex variable can also reveal what broader trends are under way.
Noll explained that, in our part of the world, when the AAM is positive, it typically comes with El Nino-like patterns – and La Nina-like patterns when it's tracking negative.
"Lately, we've been seeing the AAM take a bit of a nosedive toward what you'd consider the La Nina side of the spectrum," he said.
"Why is that significant? Sea surface temperatures in the Pacific have cooled down and now the AAM has started to drop – in a general sense, it tells us that the atmosphere is beginning to react to those cooler seas.
"It paints a picture that, although strictly speaking, the atmosphere is in an ENSO-neutral phase now, we could see a transition toward La Nina as we go through the upcoming months."
International models are currently picking a 46 per cent chance for La Nina conditions during the spring season - and 44 per cent for summer.
La Nina conditions played a hand in New Zealand's hottest summer, over 2017 and 2018, but it's still not clear how strong the system waiting in the wings might be.
"We can think of this move to La Nina not as a light switch, but a dimmer. Right now, you might say the room is dimly lit – but when we're sitting here in August or September, we'll be able to make things out a lot more clearly."
At the same time, Niwa has pointed to the potential of another rain-making system.
That's what's called an Indian Ocean Dipole, or IOD – a seesaw-like system that can set up a temperature difference across the tropical Indian Ocean.
It was an intensely positive IOD that primed Australia for its unprecedented and catastrophic bushfires over summer, but this year, climatologists were looking at a negative one – bringing further prospects of rain for New Zealand's north.
In the coming weeks, however, Noll said we could expect more periods of high-pressure – like one that plunged temperatures in the south to icy extremes over the weekend, but not before a stormy interlude later this week.
The week ahead...
🌤️ Some dry 🌧️ Some wet 🌡️ Rather warm
On Thursday, a disturbance will get caught amid the sea of high pressure, bringing rain to northern areas still dealing with water shortages 💧 pic.twitter.com/9vITIk4FGe
Over the rest of winter, air pressure was forecast to be higher to the north of New Zealand, and sometimes over the country, spelling cold, frosty mornings in typically colder places.
"So those mornings that we experienced over the weekend may be likely to occur again."
