NIWA Outlook: August – October 2017
Overview
ENSO (El Niño – Southern Oscillation) neutral conditions (neither El Niño nor La Niña) continued in the tropical Pacific
during July 2017, but this month mixed signals were again present. In particular some atmospheric patterns have been
recently leaning more towards weak La Niña conditions. Seasurface temperatures (SSTs) in the equatorial Pacific Ocean
are slightly above normal (i.e. on the El Niño side of neutral), but these positive anomalies weakened over the course
of the month. On the other hand, the Southern Oscillation Index is currently positive (+0.8, i.e. on the La Niña side of
neutral) and large-scale circulation anomalies along the Equator as well as rainfall anomalies in the western Pacific
are consistent with patterns usually associated with a La Niña state.
International guidance favours a persistence of ENSO-neutral conditions over the next three months period (59% chance
for August – October 2017), a likelihood comparable to the one indicated last month. ENSO-neutral remains the most
likely outcome throughout the first quarter of 2018. Note however that the models used to build this consensus forecast
have not picked up yet on the most recent changes shown by the La Niña-leaning atmospheric indicators.
For August – October 2017 as a whole, the atmospheric circulation around New Zealand is forecast to be characterised by
lower pressure than normal west of the country, leading to weak northwesterly flow anomalies over New Zealand. La
Niña-like atmospheric signals currently present in the Pacific might however be associated with intermittent periods of
anomalous easterly or northeasterly flows. Significant low pressure systems, occasionally with a deep sub-tropical
moisture connection, are also expected to sweep the country from time to time during the August – October 2017 period,
and may be associated with intense rainfall.
Outlook Summary
August – October 2017 temperatures are forecast to be above average for all regions of New Zealand, with high confidence
(55% to 70% chance for above average temperatures). Nevertheless, frosts and cold snaps will occur during the remainder
of winter and in early spring. Coastal water temperatures around New Zealand are forecast to remain above average over
the next three month period.
August – October 2017 rainfall totals are most likely to be near normal (45% chance) for the east of the South Island,
and about equally likely to be near normal (35-40% chance) or above normal (35- 40% chance) for all remaining regions of
New Zealand.
August – October 2017 soil moisture levels and river flows are most likely to be near normal (40-45% chance) in the west
of the North Island and the west and east of the South Island. In the north and east of the North Island, soil moisture
levels and river flows are about equally likely to be near normal (35-40% chance) or above normal (35-40% chance). In
the north of the South Island, soil moisture levels are about equally likely to be near normal (40% chance) or above
normal (35% chance), and river flows most likely to be in the near normal range (45% chance).