Southland and inland parts of Otago are being warned to expect a wet winter this year, as a "formidable" El Niño weather cycle looks set to hit New Zealand.
The good news is the same phenomenon will bring drier weather to eastern parts of Otago, including Dunedin.
Earth Sciences New Zealand (ESNZ) has been monitoring signs of a developing El Niño in the tropical Pacific Ocean for the past six months, and now global climate models are showing a clear consensus for an El Niño event to arrive this winter.
ESNZ and MetService meteorologist Jon Tunster said there was a greater than 60% probability the "formidable"event would be classified as "strong" by spring, with the potential to intensify further later in the year.
El Niño and La Niña are the two phases of the El Niño -Southern Oscillation (Enso), a natural climate cycle driven by ocean temperatures and atmospheric pressure across the tropical Pacific.
During El Niño , warmer-than-average ocean temperatures develop in the central and eastern Pacific, weakening the trade winds and shifting tropical rainfall towards the central and eastern Pacific.
Mr Tunster said El Niño ’s impacts would extend downstream of the circulation changes in the tropics, and would shape weather in New Zealand.
While Enso only accounted for some of New Zealand’s weather variability, it played a key part in influencing where areas of high pressure tended to sit over the country.
Those pressure systems during El Niño tended to cause a cooler southwesterly flow over New Zealand, which usually brought increased rainfall to Southland, parts of Otago and western-facing areas of the South Island.
However, it also brought reduced rainfall for much of the remainder of the country, including eastern parts of Otago, where there would be an increase in frosty weather, he said.
"Dryness could become an issue as spring approaches for areas that see a reduction in their normal winter rainfall — such as the east of both islands, or the upper North Island."
If La Niña does persist into spring and summer, there is an increased potential for drought conditions.
Mr Tunster said southwesterly patterns were also often associated with cold winters.
But at this stage, he believed it was unlikely this winter would be colder than usual.
"We could see occasional cold snaps, lasting a few days at a time — except in inland parts of the South Island, where the cold air can linger for longer periods.
"Or we could see a colder-than-average month within an otherwise near-average season."
