During a La Niña 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 usually 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 Niña, although there are always regional and seasonal exceptions.
One such exception was last year, when an odd-ball La Niña event delivered a somewhat unexpected flavour to New Zealand – and became among four of 17 La Niña events measured since 1972 that failed to bring near or above normal rainfall for Auckland.
It also proved a much drier summer elsewhere in the country, especially in parts of Waikato, Bay of Plenty, Hawke's Bay, Wairarapa and Marlborough, where rainfall totals finished up at less than half of normal.
Among other factors, meteorologists put that non-traditional summer set-up down to the system's energy being focused over the Indian Ocean, and west of the West Pacific "warm pool" where its thunderstorms and cyclones were normally centred.
"Essentially, the ocean was in a La Niña state, but the ocean didn't catch up to it until later on," climate scientist Professor Jim Salinger said.
This month, forecasters at the Climate Prediction Centre, with the US National Oceanic and Atmospheric Administration (NOAA), issued a "La Niña Watch".
That meant there could be favourable conditions for one to develop during the September-November period and lasting through winter, or New Zealand's summer.
Niwa's own outlook for the next three months pointed to a slight cooling trend in the central part of the Pacific basin – something that was worth watching given some models' predictions for the development of another La Niña event.
Warmer-than-average ocean temperatures in the central and eastern equatorial Atlantic had also culminated in the formation of something called an "Atlantic Niño".
When this pattern was active in austral winter, there was a tendency for La Niña to become active during the following summer.
Salinger highlighted another potential ingredient.
That was the interdecadal Pacific oscillation, or IPO – an indicator of cyclical change in the Pacific ocean-atmosphere system – being in a negative phase.
Along with favouring more easterlies and northeasterlies, a negative IPO also came with a higher propensity for La Niña conditions, he said.
Niwa forecaster Nava Fedaeff said there was another much simpler trend to consider.
Unlike El Niño, it was historically common to see back-to-back La Niña events.
Right now, the consensus of international models gave a 72 per cent probability of ENSO-neutral conditions remaining until September.
But over the last three months of 2021, that chance dropped to 50 per cent, while the probability of La Niña re-emerging increased to 34 per cent.
"Basically, we are seeing a greater proportion of models showing there's going to a La Niña year," she said.
"That doesn't mean it's going to happen, but a lot of models are picking up on potential developments."
She said one big reason why these predictions had to be treated with caution was there was currently a higher level of uncertainty with ENSO forecasts – a period climate modellers called the "spring predictability barrier".
"The models aren't doing their best job at the moment, so when we come out of the spring predictability barrier, in a few months' time, that's when we start to put a little bit more stock in them."
If a La Niña did happen to form, she said it was too early to say what shape it might take.
"There are a few little things in the chain that could change, and then our impacts in New Zealand might be quite different," she said.
"Everyone always asks, what's the summer going to be like? It's a burning question for people, and we're maybe starting to get some glimpses here.
"But it's going to become more apparent once the models really get a handle on what's happening.
"Right now, we're just keeping our eyes open."
