A surge in La Nina conditions expected by summer is expected to increase the risk of cyclones across the western Pacific and near New Zealand, say NIWA scientists.
At present, sea surface temperature anomalies across the central and eastern equatorial Pacific Ocean and atmospheric circulation patterns over French Polynesia and northern Australia, indicate conditions are El Nino-Southern Oscillation neutral, but leaning toward La Niña.
If La Niña conditions develop, they're likely to result in a significant change from normal tropical cyclone activity in many Pacific Islands.
NIWA principal scientist forecasting, Chris Brandolino says New Zealand should also be vigilant.
During some analogue seasons used in the preparation of the current NIWA outlook, multiple ex-tropical cyclones passed within 550 km of the country bringing significant wind, waves and rainfall. Their effects can be spread over a larger area, particularly if the ex-tropical cyclone interacts with separate weather systems.
Tropical cyclones are categorised in strength from 1 to 5, with 5 being the most intense.
For the coming season, about four storms are anticipated to reach at least Category 3, with mean wind speeds of at least 118 km/h, says Chris.
La Nina conditions are changing the tropical cyclone risk. Photo: Supplied
Two of these may increase to at least Category 4 strength, with mean wind speeds of 159 km/h. Category 5 strength cylcones where winds are greater than 196 km/h, have occurred during seasons with similar antecedent conditions to 2017/18.
For some countries the stream of cooler water crossing eh Pacific under La Nina conditions is expected to reduce cyclone activity, especially islands east of 160°W longitude; the Cook Islands, the Marquesas and French Polynesia.
As with most years, activity is expected to increase during the second half of the season, from February-April, says Chris.
But islands on the fringe of the north Coral Sea, including Papua New Guinea, the Solomon Islands, Vanuatu, New Caledonia and Tonga may experience slightly increased activity.
Chris says the La Nina situation will be revisited after the New Year and up dated if there is new information.