Seasonal Climate Outlook: September - November 2015

3 September 2015

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 +2^oC. The Southern Oscillation Index (SOI) is strongly negative (-2.0 for August 2015, value estimated on the 2^nd 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.

Regional predictions for the August to October season

Northland, Auckland, Waikato, Bay of Plenty

The table below shows the probabilities (or percent chances) for each of three categories: above average, near average, and below average. In the absence of any forecast guidance there would be an equal likelihood (33% chance) of the outcome being in any one of the three categories. Forecast information from local and global guidance models is used to indicate the deviation from equal chance expected for the coming three month period, with the following outcomes the most likely (but not certain) for this region:

Temperatures are equally likely (40% chance) to be near average or below average.
Rainfall totals, soil moisture levels and river flows are all most likely (45 - 50% chance) to be in the below normal range.

Other outcomes cannot be excluded. The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	20	20	20	20
Near average	40	35	35	30
Below average	40	45	45	50