El Niño conditions have been reached in the tropical Pacific, Earth Sciences NZ says – and it has the "potential to exceed" the five strongest events on record.
The naturally occurring climate phenomenon, originating with the warming of ocean temperatures in the eastern and central equatorial Pacific, affects weather patterns globally.
Historically, El Niño conditions have led to New Zealand spring having drier than usual conditions for eastern and northern areas of both islands and wetter than usual conditions for the western and lower South Island, according to Earth Sciences.
"Observations of sea surface temperatures in the east-central tropical Pacific show the hallmarks of a historically strong event, which is also echoed by the modelling," Earth Sciences chief scientist weather and seasonal forecasting Chris Brandolino said.
A sea surface temperature difference from average, known as an "anomaly", of 1.5C is a marker of a "strong" El Niño. If the anomaly reaches at least 2.0C, it’s considered very strong, sometimes termed a "super" El Niño.
The top five strongest El Niño events (1972, 1982, 1991, 1997, 2015) all had sea surface temperature anomalies near or slightly above 2.5C. The anomaly recorded for this event had already exceeded 1.0C and would continue to increase.
"At the very least, we’re expecting this event to rival the intensity of these historical top five events, but it has considerable potential to exceed them all," Brandolino said.
El Niño to influence NZ weather from September
Earth Sciences and MetService meteorologist Jon Tunster said it was important to recognise New Zealand’s weather was influenced by many factors, including the Indian Ocean Dipole, also known as shifting ocean temperatures in the Indian Ocean; regional sea surface temperatures; occasional influences from Antarctica; and climate change.
"There’s more to it than El Niño, or even Super El Niño, but this El Niño event is shaping up to have a major influence, potentially from September onwards," Tunster said.
"As we’ve seen with the previous El Niño events, the overwhelming majority of long-range models are favouring drier than usual conditions for northern and eastern areas of New Zealand, while western regions – especially of the South Island – are likely to see increased rainfall."
Tunster said the combination of wind and reduced rainfall would dry soils, leading to "periods of increased wildfire danger" – particularly in eastern and potentially northern parts of the country.
Lower chances of rainfall were also expected to create challenges for sectors reliant on water, he added.
Climate outlook
Earth Sciences' seasonal climate outlook for July to September is forecasting an "eventual lean" towards drier than usual conditions for Canterbury, Marlborough, eastern Otago, Wairarapa, and from Hawke’s Bay to Gisborne.
Drier than normal conditions were also expected for Taranaki to Bay of Plenty through to Auckland and Northland.
Wetter than usual conditions, meanwhile, were expected for Westland through to Southland, perhaps extending into parts of Central Otago.
Windier than usual conditions were also expected for much of the country from late winter into spring.
'No El Niño is average'
When comparing the top five historical El Niño events, dry conditions were "particularly pronounced" in parts of Canterbury – with only two-thirds of normal rainfall from July to September.
However, Brandolino said it was important to bear in mind that "while we know the average outcome of El Niño because of historical data, no El Niño is average".
"Each comes with a unique set of climate characteristics and therefore can be expected to influence the weather differently."
Tunster added that subtle differences in wind direction could affect temperature and strongly inform rainfall patterns, and there were examples of notable variation across the five historical events.
"In 2015, springtime weather patterns favoured an increase in southwesterly and southerly flows, whereas 1972 saw more high-pressure systems near the North Island, bringing increased northwesterlies across the South Island. As a result, spring of 2015 was almost a full degree cooler than 1972, which is especially notable given that the background climate has warmed over that same period," he said.
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