NIWA Outlook: September-November 2015
Overview
Strong El Niño conditions are present in the tropical Pacific ocean. Sea surface temperature anomalies in the central
and eastern Pacific intensified during August 2015 and are now close to +2oC. The Southern Oscillation Index (SOI) is
strongly negative (-2.0 for August 2015, value estimated on the 2nd of September) and westerly wind anomalies (weaker
trade-winds) dominate the central and western equatorial Pacific, indicating a strong coupling between the ocean and the
atmosphere.
International guidance indicates that El Niño is certain (100% chance) to continue over the next three months (September
– November 2015) and extremely likely (above 90% chance) to persist into summer 2015/2016. The current state of the
ocean-atmosphere in the Pacific and the international consensus forecast suggest that this event could then rank amongst
the 4 strongest El Niño events recorded (along with 1972/73, 1982/83 and 1997/98).
During September – November 2015, above normal pressure is forecast over and to the south of Australia, while below
normal pressure is expected to the east of New Zealand. This circulation pattern is likely to be accompanied with
anomalous southwesterly winds.
Sea surface temperatures are forecast to be normal or below normal in the Tasman west of New Zealand, while surface
water temperatures are expected to be in the below normal range to the east of the country.
Outlook Summary
September – November 2015 temperatures are about equally likely (40-45% chance) to be average or below average for the
North Island and in the north of the South Island. Mean temperatures for the season as a whole are most likely (50%
chance) to be in the below average range for the west and east of the South Island. Cold snaps and frosts are to be
expected in some parts of the country from time to time.
September – November 2015 rainfall totals are most likely (45-50% chance) to be in the below normal range for the north
and east of the North Island. Seasonal rainfall totals are about equally likely (35-40% chance) to be in the near normal
or below normal range in the west of the North Island and the north and east of the South Island. Near normal rainfall
is the most likely outcome (45% chance) for the west of South Island.
September – November 2015 soil moisture levels and river flows are most likely (45-50% chance) to be below normal in the
north and east of the North Island and the east of the South Island. In the western regions of both Islands, soil
moisture levels and river flows are about equally likely (35-40% chance) to be in the near normal or below normal range.
In the north of the South Island, soil moisture levels are most likely (45% chance) to be below normal, and river flows
are about equally likely (35-40% chance) to be in the near normal or below normal range.