Why this Pacific Ocean cyclone season might be a 'late bloomer'

If a tropical cyclone did come close to New Zealand this season, it had equal probability of passing east or west of Auckland and the North Island. Photo / File

Despite the South Pacific just experiencing its first September tropical cyclone in more than 70 years, forecasters are predicting a typical but "late-blooming" season.

Analysis by forecasting centres across the southwest Pacific indicated seven to 11 named cyclones would form in the basin between next month and April.

Activity was expected to be lower than normal around the northern and eastern Coral Sea margin, but higher east of the International Date Line.

New Caledonia, Fiji, Vanuatu and Tonga might experience two or more cyclones during the season.

Three or four severe cyclones - reaching Category 3 or higher - were expected anywhere across the region during the season.

On average, at least one ex-tropical cyclone passes within 550km of New Zealand each year.

For the coming season, the risk for an ex-tropical cyclone reaching the country was considered near normal.

If one did come close to the country, it had equal probability of passing east or west of Auckland and the North Island, likely bringing significant rainfall, extreme winds, hazardous marine conditions and coastal damage.

Niwa forecaster Ben Noll said an El Nino event on the cards for next year was expected to be weaker than typical such systems – but could also peak late, in February or March.

"Because of this, the [cyclone] season could start off on the slower side but wind up more active in early 2019."

There was even potential for the season to linger longer into 2019, as happened in the 1986-87 season, when big storms hit as late as April and May.

"The late-blossoming El Nino may increase the odds for late season activity."

But this season might also be softened by a sea surface temperature gradient slicing between New Caledonia-Vanuatu and Fiji, driving atmospheric winds that were unfavourable for any tropical cyclones west of Fiji.

This season's relatively calm outlook stood in stark contrast with the three ex-tropical cyclones that hammered New Zealand in 2017-18.

Noll pointed out that season also co-incided with an unusually intense marine heatwave, which enabled tropical cyclones to better maintain their structure as they pushed southward.

"The marine heatwave, in conjunction with warmer than average sea surface temperatures in the Coral Sea, and La Nina, probably increased the chance for an impact from the west, compared to normal," he said.

"Current sea surface temperatures are not as anomalously warm to the west and north of New Zealand as they were last year nor are they forecast to be, although a gradual warming trend, to above average, is possible during the summer season.

"This may lead to cyclone tracks that have a tendency to dive east - not west - of New Zealand through the season."

One of the seven "analogue" years – or those with similar climactic conditions to this year - that forecasters used for building their outlook included two ex-tropical cyclones passing within 500km of Auckland, while the others had either one or none.