Balmy weather to roll on through spring, as La Niña lurks

Forecasters are picking a balmy spring - bringing "unseasonably" warm temperatures at times. Photo / Hayden Woodward

Forecasters are picking a balmy spring - bringing unseasonably warm temperatures at times - along with a coin-toss chance of a second consecutive La Niña meddling with our weather.

Niwa's just-issued seasonal outlook, covering spring, found winter's trend of warmer-than-average temperatures was likely to roll on for the next three months.

That could mean stints of especially warm conditions - particularly on the east of both islands - although cold spells and frosts could still occur.

With air pressure expected to be higher than normal over and to the east of New Zealand, frequent highs could contribute to "lengthy" dry spells for the time of year, Niwa reported.

Rainfall across the season was likely to be below normal in the east of the North Island, near normal in the west of the South Island, and about equally likely to be near normal or below normal everywhere else.

Source / Niwa

Soil moisture levels were predicted to be either near or below normal in the north and west of the North Island, near normal in the east of the South Island, and near or above normal in the west of the South Island.

Further in the background, climate scientists were watching the potential arrival of another La Niña, with models indicating that ocean and atmosphere conditions in the Pacific were trending towards a repeat.

There's no big influencer at the steering wheel of our mid-year climate, with waters in the central Pacific sitting in a state called El Nino-Southern Oscillation (ENSO) neutral.

Instead, local conditions have been coloured by a mix of other background drivers, such as large periods of high pressure, fewer southerly winds, the balmy influence of a phenomenon called the Indian Ocean Dipole (IOD), and warmer coastal sea surface temperatures.

These factors, which, save for the odd big deluge such as last weekend's, have formed the backdrop to New Zealand's settled weather over the past few months.

Source / Niwa

With the background effect of climate change, they also helped produce what could be New Zealand's warmest winter - a record that was reached only last year.

There's a much stronger flavour when our climate is sitting either in an El Niño state – bringing warm westerlies in summer, cold southerlies in winter, and southwesterlies the rest of the time – or La Niña.

During La Niña, ocean water from off the coast of South America to the central tropical Pacific cools to below average - a result of stronger than normal easterly trade winds, which churn cooler, deeper sea water up to the ocean's surface.

This unusually cool water in the eastern Pacific then suppresses cloud, rain and thunderstorms, as sea temperatures in the far west of the ocean warm to above average temperatures.

Here in New Zealand, we can usually expect more northeasterly winds that bring rainy conditions to North Island's northeast, and drier conditions to the south and southeast of the South Island.

Thanks to the northeasterly winds, warmer temperatures also tended to play out over much of the country during La Niña, although there are always regional and seasonal exceptions.

Source / Niwa

One such exception was last year, when an odd-ball La Niña delivered a somewhat unexpected flavour to New Zealand – and became one of four of 17 La Niña events measured since 1972 that failed to bring near or above normal rainfall for Auckland.

The latest guidance suggested that either ENSO "cool" neutral, or La Niña, conditions were equally likely to play out over spring - before the chance for La Niña peaked at 50 to 60 per cent during summer.