NIWA Outlook: August - October 2016
Overview
ENSO (El Niño – Southern Oscillation) neutral conditions are currently present in the tropical Pacific: Sea surface
temperatures (SSTs) along the eastern equatorial Pacific are near or slightly below normal, and the atmospheric
conditions over the tropical Pacific are generally consistent with an ENSO-neutral state. As a whole the tropical
ocean-atmosphere system still shows a leaning towards La Niña, but with a slight weakening of the signals that were
observed last month (June 2016).
The Southern Oscillation Index (SOI) is currently slightly positive but well within the neutral range. Slightly enhanced
trade-winds are present in the western and central Pacific west of about 150oW. The subsurface ocean remains cooler than
normal in the central and eastern Pacific, however these anomalies have weakened compared to last month.
International guidance still favours development of La Niña conditions (50% chance) over the next three month period
(August – October 2016), but note that this probability is lower than this time last month. The likelihood of La Niña
conditions becoming established in the Pacific increases slightly later on, reaching 55% in November – January
2016/2017. In summary, both the current state and recent evolution of the ocean-atmosphere system in the Pacific, as
well as the models’ forecasts, suggest that this La Niña event, if it develops, will be characterized by a relatively
late onset, short duration and weak amplitude.
For August - October 2016, there is no clear guidance or indication on seasonal airflow anomaly. However, weak
anomalously low pressures are forecast around New Zealand and are likely to be accompanied with unsettled conditions.
Anomalously high ocean temperatures around the country mean warmer and more humid air masses are likely to affect New
Zealand, especially the North Island. Consequently, there remains an elevated risk for significant rainfall events and
severe storms.
Outlook Summary
August – October 2016 temperatures are very likely (65-70% chance) to be above average in all regions of the country.
Nevertheless, frosts and cold snaps will occur from time to time in cooler locations. Sea surface temperatures are
forecast to remain above normal over the next three months, especially to the west of New Zealand.
August – October 2016 rainfall totals are about equally likely to be in the near normal range (40% chance) or above
normal (35-40% chance) range in the north and west of the North Island and the west of the South Island. Seasonal
rainfall is most likely (40% chance) to be in the near normal range in the east of the North Island and the north of the
South Island. In the east of the South Island, rainfall for the August – October 2016 period is about equally likely to
be near normal (40% chance) or below normal (35% chance).
August - October 2016 soil moisture levels and river flows are most likely (45% chance) to be below normal in the east
of the South Island. In the north of the North Island, soil moisture levels and river flows are about equally likely to
be in the near normal range (35-40% chance) or above normal range (35% chance). In the west of the North Island, soil
moisture levels are about equally likely to be near normal (40% chance) or above normal (35% chance), and river flows
are most likely to be in the near normal range (40% chance). Seasonal soil moisture levels and river flows are about
equally likely to be near normal (35-40% chance) or below normal (35% chance) in the east of the North Island and the
north of the South Island. In the west of the South Island, soil moisture levels are most likely to be near normal (40%
chance) while river flows are about equally likely to be near normal (40% chance) or above normal (35% chance).
Full outlook: SCO_Aug2016_Final.pdf