Seasonal Climate Outlook: August - October 2015

El Niño conditions continued to strengthen during July 2015. Sea surface temperature anomalies in the eastern and central Equatorial Pacific have been exceeding +1oC for the past two months. The Southern Oscillation Index (SOI) was strongly negative (-1.3 for July 2015, value estimated on the 30th of July) and westerly wind anomalies (weaker trade-winds) dominated the central and western equatorial Pacific.

International guidance indicates that El Niño is virtually certain (97% chance) to continue over the next three months period (August – October 2015) and extremely likely (above 90% chance) to persist into the summer 2015 / 2016.

During August – October 2015, above normal pressure is forecast over and to the south of Australia, while below normal pressure is expected well to the northeast of New Zealand. This circulation pattern is likely to be accompanied with anomalous south-westerly winds, which is typical of an El Niño influence during winter-spring.

Sea surface temperatures for the coming three months are expected to be near average around the coasts of New Zealand.

Outlook Summary

August – October 2015 temperatures are about equally likely (35-40% chance) to be average or below average in all regions of New Zealand. Note that cold snaps and frosts are to be expected in some parts of the country from time to time.

August – October 2015 rainfall totals are about equally likely (40 to 45% chance) to be in the near normal or below normal range for all regions of New Zealand except for the west of the South Island, where near normal or above normal rainfall is about equally likely  (35-40%  che).

Soil moisture and river flow are most likely (45 to 65% chance) to be in the below normal range for the north of the North Island and the east of the South Island. In the west of the South Island, soil moisture levels and river flows for the August – October 2015 season as a whole are most likely (45 – 50% chance) to be in the above normal range. Soil moisture and river flows are equally likely (40% chance) to be near normal or below normal in the east of the North Island and the north of the South Island. In the west of the North Island, soil moisture levels are most likely (45% chance) to be in the near normal range, while river flows are equally likely (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 are about equally likely (40-45% chance) to be in the normal or below normal range.
• Soil moisture levels and river flows are 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	15	20	20
Near average	40	40	35	30
Below average	40	45	45	50

Central North Island, Taranaki, Wanganui, Manawatu, Wellington

Probabilities are assigned in three categories: above average, near average, and below average.

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

The full probability breakdown is:

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

Gisborne, Hawke’s Bay, Wairarapa

Probabilities are assigned in three categories: above average, near average, and below average.

• Temperatures are about equally likely (35-40% chance) to be average or below average.
• Rainfall totals are about equally likely (40-45% chance) to be in the below normal or near normal range.
• Soil moisture levels and river flows are equally likely (40% chance) to be in the near normal or below normal range.

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	25	15	20	20
Near average	35	40	40	40
Below average	40	45	40	40

Nelson, Marlborough, Buller

Probabilities are assigned in three categories: above average, near average, and below average.

• Temperatures are about equally likely (35-40% chance) to be near average or below average.
• Rainfall totals are about equally likely (40-45% chance) to be in the below normal or near normal range.
• Soil moisture levels and river flows are equally likely (40% chance) to be in the near normal or below normal range.

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	25	15	20	20
Near average	40	40	40	40
Below average	35	45	40	40

West Coast, Alps and foothills, inland Otago, Southland

Probabilities are assigned in three categories: above average, near average, and below average.

• Temperatures are about equally likely (35-40% chance) to be in the near average or below average range.
• Rainfall totals are about equally likely (35-40% chance) to be in the near normal or above normal range.
• Soil moisture levels and river flows are most likely (45-50% chance) to be in the above normal range.

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	25	40	45	50
Near average	35	35	35	30
Below average	40	25	20	20

Coastal Canterbury, east Otago

Probabilities are assigned in three categories: above average, near average, and below average.

• Temperatures are equally likely (40% chance) to be in the near average or below average range.
• Rainfall totals are about equally likely (40-45% chance) to be in the near normal or below normal range.
• Soil moisture levels and river flows are most likely (55-65% chance) to be in the below normal range.

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	20	15	15	10
Near average	40	45	30	25
Below average	40	40	55	65

Graphical representation of the regional probabilities

Background

Sea surface temperature (SST) anomalies have continued to increase in the eastern and central Equatorial Pacific and are consistent with the signature of a typical El Niño event. The Southern Oscillation Index (SOI) remained strongly negative and is at -1.3 for the month of July 2015 (value estimated on the 30^th of July). Westerly wind anomalies (i.e. a weakening of the easterly trade-winds with occasional westerly winds) dominated the central and western Pacific in July 2015.

Consistently with these large adjustments of the atmospheric circulation along the Equator, convection and rainfall were suppressed in the western Pacific and over the Maritime Continent (west of the International Dateline), while the Intertropical Convergence Zone was much more intense than normal in the eastern Pacific.

International guidance indicates that El Niño is virtually certain (97% chance) to continue over the next three months period (August – October 2015). The likelihood of El Niño conditions persisting or strengthening as we reach into summer is also extremely high (above 90%).

Note that El Niño events are typically (but not always) associated with stronger and/or more frequent south-westerly winds during spring in New Zealand. Such a circulation pattern typically leads to cooler conditions in most regions of the country and wetter conditions to the west of the Southern Alps. The circulation, temperatures and rainfall forecasts for the next three months (August – October 2015) broadly reflect these typical impacts.  

Waters surrounding New Zealand are currently close to normal. Ocean models forecasts indicate that SSTs are likely to remain in the near normal range around the country over the next three months.

Contact

For comment, please contact:

Chris Brandolino, NIWA forecaster, NIWA National Climate Centre Tel (09) 375 6335, Mobile (027) 886 0014

Dr Brett Mullan, Principal Scientist, NIWA National Climate Centre Tel (04) 386 0508, Mobile (027) 294 1169.

Notes to reporters and editors

1. NIWA’s outlooks indicate the likelihood of climate conditions being at, above, or below average for the season as a whole. They are not ‘weather forecasts’. It is not possible to forecast precise weather conditions three months ahead of time.
2. The outlooks are the result of the expert judgment of NIWA’s climate scientists. They take into account observations of atmospheric and ocean conditions and output from global and local climate models. The presence of El Niño or La Niña conditions and the sea surface temperatures around New Zealand can be a useful indicator of likely overall climate conditions for a season.
3. The outlooks state the probability for above average conditions, near average conditions, and below average conditions for rainfall, temperature, soil moisture, and river flows. For example, for winter (June–July–August) 2007, for all the North Island, we assigned the following probabilities for temperature: ·  Above average: 60 per cent ·  Near average: 30 per cent ·  Below average: 10 per cent We therefore concluded that above average temperatures were very likely.
4. This three-way probability means that a random choice would be correct only 33 per cent (or one-third) of the time. It would be like randomly throwing a dart at a board divided into three equal parts, or throwing a dice with three numbers on it. An analogy with coin tossing (a two-way probability) is not correct.
5. A 50 per cent ‘hit rate’ is substantially better than guesswork, and comparable with the skill level of the best overseas climate outlooks. See, for example, analysis of global outlooks issued by the International Research Institute for Climate and Society based in the US published in the Bulletin of the American Meteorological Society (Goddard, L., A. G. Barnston, and S. J. Mason, 2003: Evaluation of the IRI’s “net assessment” seasonal climate forecasts 1997–2001. Bull. Amer. Meteor. Soc., 84, 1761–1781)
6. Each month, NIWA publishes an analysis of how well its outlooks perform. This is available online and is sent to about 3500 recipients of NIWA’s newsletters, including many farmers. See www.niwa.co.nz/our-science/climate/publications/all/cu
7. All outlooks are for the three months as a whole. There will inevitably be wet and dry days, and hot and cold days, within a season. The exact range in temperature and rainfall within each of the three categories varies with location and season. However, as a guide, the “near average” or middle category for the temperature predictions includes deviations up to ±0.5°C for the long-term mean, whereas for rainfall the “near normal” category lies between approximately 80 per cent and 115 per cent of the long-term mean.
8. The seasonal climate outlooks are an output of a scientific research programme, supplemented by NIWA’s Capability Funding. NIWA does not have a government contract to produce these outlooks.