Spring could be wet for some regions - NIWA

LEILANI HATCH/Manawatu Standard

SPRING OUTLOOK: The moderate La Nina weather pattern in the tropical Pacific is raising the likelihood of wetter than normal conditions in the north and east of the North Island.

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La Nina could bring a wetter than usual spring for some regions, says the National Institute of Water and Atmospheric Research (NIWA).

Spring arrived today with the moderate La Nina weather pattern in the tropical Pacific raising the likelihood of wetter than normal conditions in the north and east of the North Island.

Mild conditions and near-normal rainfall in most regions are predicted in the run-up to the summer school holidays, just over three months away, but this La Nina weather pattern may extend into the New Year, says NIWA.

"We have no outlook for summer as yet, but the spring outlook may well be a good starting point for summer as well," said senior climate scientist Dr James Renwick, the NIWA principal scientist working on climate variability.

Dr Renwick said the west coast of the South Island had a wet summer in 6 out of 10 El Ninos, but it had a dry summer in 7 or 8 out of 10 La Nina weather patterns.

The typical sequence in a La Nina was a spring with more frequent northerly winds than normal, with above-normal rainfall likely in many regions, plus mild temperatures.

If the effect carried over to summer, there tended to be more frequent northeasterly winds than normal, with above normal rainfall in the north and east of the North Island, and below normal rainfall in the west and south of the South Island - including the Southern Alps.

Both spring and summer tended to be warmer than average in many places during a La Nina pattern.

Some La Nina events coincided with the positive phase of a separate weather pattern, the Southern Annular Mode (SAM), which can change rapidly and control how strongly the westerly winds blow over the Southern Ocean.

In its positive phase, the SAM can give more anticyclones over the South Island and more settled weather and if this coincides with a La Nina summer, it which can bring very settled and dry conditions to much of the country. When this happened in the summer of 2007-2008, much of the North Island experienced drought conditions.

"The SAM has been very positive indeed lately, but it's unclear whether this will continue through the rest of the year," said Dr Renwick.

In a La Nina big high pressure systems tended to sit east of NZ, meaning storms coming from south of Australia migrated south of New Zealand, around the southern side of the highs, but the country was open to more storm activity from the Tasman Sea.

But a La Nina tended to bring a little less tropical cyclone activity overall, and the cyclones that form tend to stay in the west and do not often come close to NZ.

Dr Renwick said the timing and magnitude of the changeover from El Nino to La Nina was roughly the same as in 1998, when a very strong 1997-98 El Nino changed to a moderate 1998-99 La Nina.

"This time around, the preceding El Nino was only moderate, while the present La Nina has become more intense more quickly than the 1998-99 event," he said.

- NZPA