Seasonal Climate Outlook: July - September 2015
An El Niño event is under way in the tropical Pacific. Sea surface temperatures have continued to warm across many areas of the eastern and central Tropical Pacific. Cloudiness, rainfall and convection activity near, and to the east of, the International Date Line has also intensified. These oceanic and atmospheric features are indicative of consolidating El Niño conditions.
International guidance indicates that El Niño conditions will to continue through winter and spring, and most likely into summer 2015/16.
El Niño events are typically (but not always) associated with stronger and/or more frequent southerly winds during the winter in New Zealand. Such a circulation pattern typically leads to cooler conditions in most regions of the country. Despite the forecast for El Niño to continue over the next three months, the temperature outlook - as synthesized from various dynamical and statistical models - indicates that average temperatures are likely in most regions.
Waters surrounding New Zealand are currently near normal. Ocean models forecasts indicate that sea surface temperatures around the country are likely to be in the near average temperature range over the next three months
To find out more about normal conditions for this outlook period, view climate maps on NIWA’s website, where daily updates on climate maps are available.
For comment, please contact the following:
Chris Brandolino, NIWA forecaster, NIWA National Climate Centre
Tel (09) 375 6335, Mobile (027) 886 0014
Darren King, Senior Scientist, NIWA National Climate Centre
Tel (04) 386 0508, Mobile (027) 294 1169.
Notes to reporters and editors
- 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.
- 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.
- 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.
- 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.
- 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).
- 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
- 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.
- 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.