Warmer sea temperatures south-west of the Pacific Islands, coupled with atmospheric conditions driven by La Niña has led to the formation of cyclones. Video / NZ Herald
After three years of meddling with New Zealand’s weather, La Niña is finally shuffling off the stage - but not without leaving our drenched northeast a damp after-taste.
The ocean-driven system has been influencing our climate patterns since the start of the decade, contributing to the northeast’s warm, wet and wild summer, while helping push parts of the South Island into meteorological drought.
In its just-issued outlook for the next three months, Niwa reported that La Niña would finally fade to “Enso-neutral” conditions this month – meaning there’d be no dominant climate driver shaping our weather until the expected arrival of counterpart El Niño later in 2023.
“For Aotearoa New Zealand, this transition is expected to result in more variable airflow, temperature, and rainfall patterns during autumn,” Niwa meteorologist Ben Noll said.
But a “teleconnection” lag between ocean and atmosphere meant La Niña’s exit wouldn’t prove an immediate shift into a different regime for our soaked north, but rather a gradual one.
Source / Niwa
Out to May, rainfall was equally likely to be near or above normal in the east of North Island, and most likely near normal everywhere else.
“This represents a drier signal compared to previous outlooks, but the remnant effect of La Niña could still produce some sub-tropical or tropical low-pressure systems, occasionally increasing the risk for heavy rain in the North Island.”
Noll said we could expect more low pressure over the Tasman Sea and the South Island this month, leading to spells of westerly winds – a distinctive change from summer.
“These westerly winds, while not dominant, will bring some more typical cold fronts,” Noll said.
“Low-pressure systems to the western side of both islands are expected to produce rainfall that will gradually ease long-term rainfall deficits in the west and south of the South Island.”
Source / Niwa
While autumn temperatures were most likely to be above average in the west of the South Island - and about equally likely to be near average or above average in all other regions – Noll said cold spells would become more common.
“The seasonal transition to autumn will also be accompanied by an increasing risk for frosts.”
Away from land, Noll said coastal sea surface temperatures continued to run warm last month, ranging from 0.4C to 2.9C above average.
“February sea surface temperatures were the warmest on record in the west and east of the South Island, and second warmest on record in the north of the South Island.”
Source / Niwa
Those, of course, weren’t the only records broken in what’s been one of New Zealand’s most memorable – and extreme – summers ever experienced.
“We’ve had 200 to 400 per cent of normal rainfall, so two to four summers’ worth of rain for many regions,” Noll told the Herald this week.
“And that puts us up to around 400 to 800mm of rain this summer. As of last week, before the big rain on Friday, Auckland had received 56 per cent of its normal annual rainfall in just the first two months of 2023.
“That puts some perspective on just how extreme the summer rainfall has been.”
