Seasonal Climate Outlook: July - September 2016

The latest remnants of the strong El Niño which peaked late in 2015 have now vanished in the tropical Pacific Ocean, and the Pacific is ENSO-neutral. Ocean temperatures along the equatorial Pacific are now near or slightly below normal, and recent developments in the ocean – atmosphere system are pointing to a possible transition to La Niña over the coming three months.

Atmospheric circulation along the equatorial Pacific has returned to near-normal, and the Southern Oscillation Index (SOI) is slightly positive. Cooler than normal sub-surface ocean waters are present in the central equatorial Pacific.

International guidance indicates that La Niña conditions are likely (57% chance) to appear over the next three month period (July – September 2016). The likelihood of La Niña conditions becoming established in the Pacific increases slightly later in the year, and reaches 65% in the first three months of 2017. However, models forecasts indicate that if La Niña indeed develops, it is likely to remain in the weak or moderate category.

For July – September 2016, weak anomalously high pressure is forecast to the north of New Zealand and expected to bring weak northerly quarter flow anomalies. Anomalously high ocean temperatures around the country mean warmer and more humid air masses are likely to affect most of New Zealand.

Seasonal Climate Outlook Video

In the video below NIWA Forecaster Chris Brandolino describes what New Zealanders can expect, in broad terms, in the way of rainfall and temperature over the coming winter season:

Summary

July – September 2016 temperatures are very likely (60-70% chance) to be above average in all regions of the country. Nevertheless, frosts and cold snaps will occur from time to time. Sea surface temperatures are forecast to remain above normal over the next three months, especially to the west of New Zealand.

July – September 2016 seasonal rainfall totals are about equally likely to be near normal (35-40% chance) or above normal (35-40% chance) for all regions of New Zealand. Warmer sea surface temperatures around the country will remain a major driving force for the New Zealand’s climate over the coming season. Consequently, there remains an elevated risk for significant rainfall events and severe storms.

July – September 2016 soil moisture levels and river flows are about equally likely to be in the near normal (35-40% chance) or above normal (35-40% chance) range in the north and west of both Islands. In the east of the North Island, soil moisture levels are most likely to be near normal (40% chance) and river flows are equally likely (35% chance) to be near or below normal. In the east of the South Island, soil moisture levels are about equally likely to be in the below normal (40% chance) or normal range (35% chance), while below normal river flows are most likely (50% chance).

Regional predictions for the July – September 2016 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 very likely to be above average (70% chance).
• Rainfall totals are about equally likely to be above normal (40% chance) or near normal (35% chance).
• Soil moisture levels are about equally likely to be in the above normal range (40%) or near normal range (35% chance).
• River flows are equally likely (35% chance) to be near or above normal.

The full probability breakdown is:

Central North Island, Taranaki, Wanganui, Manawatu, Wellington

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

• Temperatures are very likely to be above average (70% chance).
• Rainfall totals and river flows are equally likely (40% chance) to be normal or above normal.
• Soil moisture levels are about equally likely to be in the above normal (40% chance) or normal (35% chance) range.

The full probability breakdown is:

Gisborne, Hawke’s Bay, Wairarapa

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

• Temperatures are very likely to be above average (70% chance).
• Rainfall totals are about equally likely to be normal (40% chance) or above normal (35% chance).
• Soil moisture levels are most likely (40% chance) to be in the near normal range.
• River flows are equally likely (35% chance) to be in the near normal or below normal range.

The full probability breakdown is:

Nelson, Marlborough, Buller

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

• Temperatures are very likely to be above average (70% chance).
• Rainfall totals are about equally likely to be normal (40% chance) or above normal (35% chance).
• Soil moisture levels are equally likely (40% chance) to be in the near normal or above normal range.
• River flows are about equally likely to be in the above normal (40% chance) or near normal (35% chance) range.

The full probability breakdown is:

West Coast, Alps and foothills, inland Otago, Southland

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

• Temperatures are very likely to be above average (60% chance).
• Rainfall totals are about equally likely to be near normal (40% chance) or above normal (35% chance).
• Soil moisture levels and river flows are both equally likely (40% chance) to be in the above normal or normal range.

The full probability breakdown is:

Coastal Canterbury, east Otago

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

• Temperatures are very likely to be above average (65% chance).
• Rainfall totals are about equally likely to be normal (40% chance) or above normal (35% chance).
• Soil moisture levels are about equally likely to be below normal (40% chance) or near normal (35% chance).
• River flows are most likely to be in the below normal range (50% chance).

The full probability breakdown is:

Graphical representation of the regional probabilities

Background

The latest remnants of the strong El Niño which peaked late in 2015 have now vanished in the tropical Pacific Ocean. The latest sea surface temperatures (SSTs) along the equatorial Pacific are now near or slightly below normal, and recent developments in the ocean – atmosphere system are pointing to a possible transition to La Niña state over the coming three months.

Convective activity and rainfall anomalies have clearly veered away from the patterns usually associated with El Niño: while some regions of anomalously low convection and rainfall persist in the western Equatorial Pacific, large swaths of the Maritime Continent (e.g. Indonesia, Papua New Guinea) have recorded normal or above normal rainfall in June 2016, while the central and eastern Equatorial Pacific tends to be drier than normal. The Southern Oscillation Index (SOI) is currently slightly positive (estimated value for June 2016: +0.4) and slightly enhanced trade-winds are present in the western and central Pacific west of about 140^oW.

International guidance indicates that La Niña conditions are likely (57% chance) to appear over the next three month period (July – September 2016). The likelihood of La Niña conditions becoming established in the Pacific increases slightly later in the year, and reaches 65% in the first three months of 2017. However, models forecasts indicate that if La Niña indeed develops, it is likely to remain in the weak or moderate category.

Waters surrounding New Zealand remain exceptionally warm, and ocean models predict that these warm conditions will persist through July – September 2016. These warmer regional sea surface temperatures will remain a major driving force for New Zealand’s climate over the coming season, with warmer and more humid air masses likely to affect the country, and more severe storms than usual possible.

To find out more about normal conditions for this outlook period, refer to NIWA’s website, where daily updates on climate maps are available.

Contact

For comment, please contact:

Chris Brandolino, Principal Scientist – Forecasting, 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).
   1. 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
   2. 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.
   3. 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.

Where probabilities are within 5% of one another, the term “about equally” is used.

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