Monday, April 22, 2024

La Niña offers summer soaking potential

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La niña conditions forecast this summer may be greeted with contrasting emotions by campers and farmers along the North Island’s east coast.
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WeatherWatch’s modelling indicates a swathe of NZ’s North Island east coast, from Cape Reinga to Kaikoura, could experience more rainfall than usual for the November-January period.

La Niña conditions forecast this summer may be greeted with contrasting emotions by campers and farmers along the North Island’s east coast.

WeatherWatch lead forecaster Phil Duncan’s latest three-month climate forecast has him forecasting a strong likelihood New Zealand will experience La Niña conditions this summer, with wetter, cloudier weather along the northern east coast and coastal Marlborough, but drier conditions to much of the rest of NZ.

“The Bureau of Meteorology (Australia) is into the ‘alert zone’ for November; it is not technically a La Niña yet, but the Bureau scientists are saying it is on the cards,” Duncan said.

A ‘model of models’ has the La Niña indicator shifting further into positive territory in January, pulling back to a more neutral position come March.

“So, it does look like we have a moderate La Niña coming,” he said.

A La Niña weather pattern is created when the warmer water on the ocean surface gets pushed all the way towards the western Pacific by nor’easterly winds.

“We are seeing on the sea surface temperature anomaly map that is indicating cooler than average temperatures across the eastern Pacific and warmer than average water temperatures down here by NZ,” he said.

“Warmer sea surface temperatures mean the higher the chances of cloud, rain and maybe tropical cyclones.”

Duncan’s modelling indicates a swathe of NZ’s North Island east coast from Cape Reinga to Kaikoura could experience more rainfall than usual for the November-January period.

However, he cautions that departure from normal may not be excessive, in some places possibly only a few millimetres above average.

“The risk with El Niño forecasting is that we need to consider that El Niño-type conditions will not necessarily occur the way they may in places closer to the equator, like Fiji for example,” he said.

“Further south here in New Zealand, it can be a real traffic light system, where large highs from Australia can block those easterly patterns, or low-pressure systems from the southern ocean can also have an effect.”

Any additional rainfall above-average over the summer period will, however, be welcome by districts along the North Island’s east coast.

Latest MetService rainfall data indicates by the end of October, Napier has only had two-thirds of its average year-to-date rainfall, while Tauranga and Auckland had only 80%.

All three regions have experienced less than 70% of their average annual rainfall for the past two and a half years, with significant drops in aquifer levels being reported in all regions.

How much rain comes to the north’s east coast is strongly influenced by how high pressure systems migrating across the Tasman sit across NZ.

“We are seeing these high pressure systems tending to sit further south now than they have in the past, meaning they are not as inclined to block low pressure systems coming from the north-east. This is compared to the past couple of years where they have sat higher and the low pressure systems have ‘bounced’ off the highs,” he said.

Should La Niña continue through summer, drier than average conditions could be expected from western Waikato down to Taranaki in the North Island and for almost the entire South Island, with the exception of Marlborough down to Kaikoura.

He cautions that La Niña may not necessarily mean NZ is more vulnerable to cyclones over the November- April season, although there may be more storms spun out of the warmer water temperatures experienced north of NZ over this time.

Across the Tasman, Australia is fast shedding its iconic dry image with the previous 18 months of regular rainfall across much of the continent forecast to continue under the La Niña conditions.

The entire continent, with the exception of western Tasmania, is expected to be wetter than average from November to January. This would make it the second consecutive summer after the devastating bushfire season of 2019-2020.

Bureau of Meteorology data indicates that in the six months to June this year, vast tracts of Australia were reporting above-average rainfall particularly in some districts of New South Wales, Western Australia and northern Queensland.

Duncan estimates the most eastern parts of the North Island may experience temperatures 0.6degC above-average this summer, while the South Island can experience 1degC above-average.

“The lower temperatures on the east coast, particularly around Bay of Plenty, reflect that higher cloud cover and rainfall expected,” he said.

More: The Weather Watch ClimateWatch report can be viewed here.

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