NIWA Outlook: September - November 2016
Overview
ENSO (El Niño – Southern Oscillation) neutral conditions continued in the tropical Pacific Ocean during August: sea
surface temperatures 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 (July 2016).
The Southern Oscillation Index (SOI) is currently positive but within the neutral range. Slightly enhanced trade-winds
are present in the central and eastern Pacific (110-140oW). 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 (55% chance) over the next three month period
(September – November 2016), however the probability of neutral conditions over the next 3 months is almost equally as
high (45% chance). The likelihood of La Niña conditions becoming established in the Pacific remains at a 55% chance for
December – February 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 if a La Niña event develops, it will be characterized by a
relatively short duration and weak amplitude.
For September - November 2016, lower than normal pressure is forecast to the north-east of New Zealand while higher than
normal pressure is expected to the south of the country. This airflow set-up is expected to produce more north-easterly
winds than normal. The continuation of warmer than normal sea surface temperatures around the country suggests that
warmer and more humid air masses are likely to affect New Zealand, especially the North Island.
Outlook Summary
September-November 2016 temperatures are likely (50% chance) to be above average in the north of the South Island and
very likely (60-70% chance) to be above average in the remaining regions of the country. Nevertheless, as we transition
into spring, frosts and cold snaps will occur from time to time, particularly in the first half of the season. Sea
surface temperatures are forecast to remain above normal over the next three months, especially to the north of New
Zealand.
September-November 2016 rainfall totals are about equally likely to be in the near normal range (40% chance) or above
normal range (35-40% chance) in the north and east of the North Island. Seasonal rainfall is most likely (45% chance) to
be in the near normal range in the west of the North Island and the north of the South Island. In the east and west of
the South Island, rainfall for the September-November 2016 period is about equally likely to be near normal (40% chance)
or below normal (35% chance).
September-November 2016 soil moisture levels and river flows are most likely to be in the near normal (40% chance) or
above normal (35% chance) range for the east of the North Island, while below normal (45-50% chance) soil moisture
levels and river flows are most likely for the east of the South Island. For the north of the South Island, near normal
(40-45% chance) soil moisture levels and river flows are most likely while near normal (40% chance) or below normal (40%
chance) soil moisture levels and river flows are equally likely in the west of the South Island. In the north and west
of the North Island near normal (45% chance) soil moisture levels are most likely while river flows are expected to be
in the normal (40% chance) or above normal (35-40%) range.
Download full PDF here: SCO_Sep2016_Final.pdf