NIWA Outlook: January – March 2018
Weak La Niña conditions persisted in the tropical Pacific during December 2017. Below average sea surface temperatures
(SSTs) are present across the central and eastern Equatorial Pacific Ocean, and have cooled further compared to November
2017, with the strongest anomalies currently measured off the coast of Ecuador and Peru. Subsurface temperature
anomalies are also consistent with weak La Niña conditions, however atmospheric signals have become less consistent over
the course of December 2017: the enhanced trade winds that were present in the western Pacific Ocean earlier on have
weakened, and in December 2017 the Southern Oscillation Index (SOI) weakened and is currently in the neutral range.
The consensus from international models is that weak La Niña conditions are likely (72% chance) to continue over the
next 3 months (January – March 2018). Thereafter, the models agree,, that La Niña will decay rapidly and a return to
ENSO-neutral conditions is most likely (74% chance) over the April – June 2018 period.
Aside from these weak La Niña conditions, New Zealand’s regional climate over the next three month period is anticipated
to be dominated by the very warm ocean waters present around the country and in the Tasman sea, which will influence
surface air temperatures and the likelihood of significant rainfall events.
For January – March 2018, the atmospheric circulation around New Zealand is forecast to be characterized by higher
pressures than normal east and south of the country, while lower pressure than normal is forecast over the Tasman Sea
area, extending over the country. This pressure pattern is expected to be associated with unsettled conditions with
periods of north-easterly quarter flow anomalies.
For the current tropical cyclone season (November 2017 to April 2018), NIWA's Southwest Pacific Tropical Cyclone Outlook
indicates that the risk for New Zealand is normal or above normal. On average, at least one ex-tropical cyclone passes
within 550km of New Zealand each year. If an extropical cyclone comes close to the country, it has equal probability of
passing east or west of Auckland and the North Island. Significant rainfall, damaging winds and coastal damage can occur
leading up to and during these events.