A climate phenomenon unfolding thousands of kilometres away in the Indian Ocean may spell a soggy end to winter – with a taste to come this weekend.
Meteorologists are closely watching what could be a strongly negative phase of what's called the Indian Ocean Dipole (IOD) - a gauge of relative sea-surface temperatures on the west and east of the ocean's basin.
Niwa forecaster Ben Noll described the IOD as a seesaw-like system that typically had three phases: neutral, positive and negative.
Its strongly-positive state in 2019 – which created cooler waters in the ocean's east, and warmer seas in its west – fuelled dry conditions across Australia and worsened its devastating bushfires.
The current picture, however, was much the opposite: and eastern Australia was bracing for more deluges to follow a slew of record-breaking floods.
While New Zealand was less affected than its neighbour by swings in the IOD, its influence could be enough to give parts of our country a wetter flavour over the back half of the year, Noll said.
"New Zealand is kind of on the periphery of this teleconnection – but it may affect our climate, especially in late winter, throughout spring, and perhaps in early summer."
But we could get a glimpse of it as early as this weekend and early next week, with a bout of rain forecast to fall in many regions.
Right now, Noll said two pulses of convective activity in the tropics – one representing a fluctuation called the Madden-Julian Oscillation (MJO) and the other an atmospheric Kelvin wave – were combining to drive cyclones causing an equatorial "westerly wind burst" in the eastern Indian Ocean.
"That westerly wind burst is important as it's the first inkling of a possible IOD event in the coming winter and spring, transporting warmer water from the central and western part of the Indian Ocean eastward," he said.
"It's also limiting the normal prevailing trade winds in that part of the Indian Ocean, which can lead to a warming of ocean water northwest of Australia.
"But the short-term picture for us here in New Zealand is that some of the moisture from one of those cyclones will get tied up into the South Pacific jet stream at the weekend – and perhaps arrive here."
Noll added that an accompanying change to gusty, westerly winds would also mark a brief shift away from the fine, settled conditions Kiwis have been enjoying over what's been an atypically warmer autumn.
Into winter, Noll said the IOD's influence could also manifest with heavy downpours – particularly with a lingering La Niña climate system keeping waters across the southwest Pacific warm.
"Between those two drivers, the odds increase for heavier events for New Zealand throughout winter and spring – and those places that have been dealing with dryness and drought may well be seeing a much wetter flavour within a few months' time," he said.
"A series of fronts this weekend into next week will really be the first taste of it."
Even further down the track, Noll said there was a possibility of a third consecutive La Niña forming – making for a rare "triple dip".
La Niña systems have helped drive a series of marine heatwaves around the country over recent years – as late as last month, some of our coastal waters were running nearly 3C above average – but there was still nothing to suggest global climate change was creating a trend toward them.
Niwa's wider outlook for the May to July period picked above-average temperatures in the north and either near or above average in most of the south.
It also predicted fewer southerly winds, which could reduce chances of snowfall, while rain around the country could continue to be "irregular", with longer dry spells interspersed with heavy downpours and possibly flooding.
The north and east of the North Island were most likely to get above-average rainfall – with a 35 per cent chance - while the rest of New Zealand can expect rainfall to be normal or below normal.