NIWA Outlook: April – June 2018
Overview
Weak La Niña conditions continued in the tropical Pacific during March 2018, but trends in low-level winds and in
sub-surface ocean temperatures during the month indicate that the event is coming to an end. The Southern Oscillation
Index (SOI) is currently positive at about +1.0 for March. Sea surface temperatures (SSTs) remained colder than average
in the eastern equatorial Pacific Ocean but have returned to near normal around the Dateline. However, the layer of
colder than average SSTs is now very shallow (75 metres or less, except close to the South American coast), with warmer
than average sub-surface water spreading eastwards during March as far as 120°W.
The consensus from international models is for the tropical Pacific to transition to an ENSO-neutral state over the next
3 month period (75% chance over April – June 2018). ENSO-neutral remains the most likely outcome over the late-winter
season (July – September 2018). The forecast models predict about an equal chance of the Pacific remaining neutral or
transitioning towards El Niño over the spring (September – November 2018).
Even though La Niña’s influence will wane over the next three-month period, New Zealand’s regional climate over April –
June 2018 is expected to be driven by persistence of more northeasterly airflow than normal, and by the persistence of
warm ocean waters that are present around the country. The northern Tasman Sea will remain unsettled, with lower
pressures than normal, bringing the likelihood of significant rainfall events to the North Island and the upper South
Island.
Outlook Summary
April – June 2018 temperatures are forecast to be above average for all regions of New Zealand with high confidence (60%
chance). The warm ocean waters around the country are forecast to persist through the next three months, but by the end
of winter are expected to return to near normal west of the country. Even though seasonal-average temperatures are very
likely to be above normal, frosts will occur in cooler locations as late autumn progresses.
For April – June 2018, rainfall totals are forecast to be above normal in the north and east of the North Island (45%
chance), near normal in the west of the South Island (45% chance), and near normal (40% chance) or above normal (35 to
40% chance) in all other regions.
April – June 2018 soil moisture levels and river flows are forecast to be above normal in the north and east of the
North Island and in the north of the South Island (45% chance), near normal (40% chance) or above normal (40% chance) in
the west of the North Island, and near normal (45% chance) in the west and east of the South Island.
Full release available at: www.niwa.co.nz/news-publications/media-centre