Seasonal Climate Outlook August - October 2009
A pause before El Niño
The climate over the next three month period is expected to be typical of early spring right across New Zealand, according to the latest outlook from NIWA’s National Climate Centre.
Following the third cold month in a row, the centre says temperatures for the coming season (August, September, and October combined) are likely to be about average. Rainfalls are also likely to be near normal in all regions.
Going along with the rainfall expectation, the centre says that near normal soil moisture levels and stream flows are also likely in all regions.
In the New Zealand region over the coming three months, mean sea level pressures are likely to transition towards El Niño-like conditions, with enhanced westerly winds at times.
In this example below, the climate models suggest that below normal conditions are likely (50% chance), but, given the variable nature of the climate, the chance of normal or above normal conditions is also shown (30% and 20% respectively).
Overall Picture
Temperature:
Air temperatures are expected to be near average in all regions. Sea surface temperatures near New Zealand are likely to be somewhat below average through August-October.
Rainfall, soil moisture, and stream flows:
Rainfall, soil moisture levels and stream flows are likely to be near normal in all regions.
Regional predictions for the next three months:
Northland, Auckland, Waikato, Bay of Plenty:
Average temperatures are likely. Normal rainfall, soil moisture and stream flows are likely for the season as a whole.
Probabilities are assigned in three categories; above average, average, and below average. The full probability breakdown is:
| Temperature | Rainfall | Soil moisture | Stream flows | |
|---|---|---|---|---|
| Above normal | 20 % | 30 % | 30 % | 30 % |
| Normal | 50 % | 50 % | 50 % | 50 % |
| Below normal | 30 % | 20 % | 20 % | 20 % |
North Island, Taranaki, Wan ganui, Manawatu and Wellington
Average temperatures are likely. Normal rainfall, soil moisture and stream flows are likely, averaged over August-October.
Probabilities are assigned in three categories; above average, average, and below average. The full probability breakdown is:
| Temperature | Rainfall | Soil moisture | Stream flows | |
|---|---|---|---|---|
| Above normal | 20 % | 20 % | 20 % | 20 % |
| Normal | 50 % | 50 % | 50 % | 50 % |
| Below normal | 30% | 30 % | 30 % | 30 % |
Gisborne, Hawke’s Bay, Wairarapa:
Average temperatures are likely. Normal rainfall, soil moisture and stream flows are likely.
Probabilities are assigned in three categories; above average, average, and below average. The full probability breakdown is:
| Temperature | Rainfall | Soil moisture | Stream flows | |
|---|---|---|---|---|
| Above normal | 30 % | 20 % | 20 % | 20 % |
| Normal | 50 % | 50 % | 50 % | 50 % |
| Below normal | 20 % | 30 % | 30 % | 30 % |
Nelson, Marlborough, Buller:
Average temperatures are likely. Normal rainfall, soil moisture and stream flows are likely for the season.
Probabilities are assigned in three categories; above average, average, and below average. The full probability breakdown is:
| Temperature | Rainfall | Soil moisture | Stream flows | |
|---|---|---|---|---|
| Above normal | 25 % | 20 % | 20 % | 20 % |
| Normal | 50 % | 50 % | 50 % | 50 % |
| Below normal | 25 % | 30 % | 30 % | 30 % |
West Coast, Alps and Foothills, Inland Otago, Southland:
Average temperatures are likely. Normal rainfall, soil moisture and stream flows are likely.
Probabilities are assigned in three categories; above average, average, and below average. The full probability breakdown is:
| Temperature | Rainfall | Soil moisture | Stream flows | |
|---|---|---|---|---|
| Above normal | 20 % | 30 % | 30 % | 25 % |
| Normal | 50 % | 50 % | 50 % | 50 % |
| Below normal | 30 % | 20 % | 20 % | 25 % |
Coastal Canterbury, East Otago:
Average temperatures are likely. Normal rainfall, soil moisture and stream flows are likely.
Probabilities are assigned in three categories; above average, average, and below average. The full probability breakdown is:
| Temperature | Rainfall | Soil moisture | Stream flows | |
|---|---|---|---|---|
| Above normal | 30 % | 25 % | 20 % | 20 % |
| Normal | 50 % | 50 % | 50 % | 50 % |
| Below normal | 20 % | 25 % | 30 % | 30 % |
The equatorial Pacific ocean has moved into an El Niño state (with the atmosphere still to adjust), and El Niño conditions are likely to persist through the rest of 2009. Only weak impacts are expected over New Zealand in the coming three month season.
As the circulation near New Zealand begins to respond to El Niño conditions in the tropical Pacific, we would expect to see a shift to more frequent south-westerlies in the spring. Depending on the intensity of the El Niño, this would suggest a heightened risk of drought in northern and eastern regions over late spring and summer, following a good start to spring (with soil moisture levels likely to be near normal in all regions).
For comment, please contact:
Dr James Renwick, NIWA Principal Scientist, Climate Variability & Change
Tel (04) 386 0343 (office DDI), mobile (021) 178 5550
Dr Brett Mullan, NIWA Principal Scientist, Climate Variability & Change
Tel (04) 3860508 (office DDI)
© Copyright NIWA 2009. All rights reserved. Acknowledgement of NIWA as the source is required.
Notes to reporters & 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, average conditions, and below average
conditions for rainfall, temperature, soil moisture, and stream
flows. For example, for winter (June-July-August) 2007, for all
the North Island, we assigned the following probabilities for
temperature:
- Above average: 60%
- Average: 30%
- Below average: 10%
- This three-way probability means that a random choice would only be correct 33% (or one-third) of the time. It would be like randomly throwing a dart at a board divided into 3 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% ‘hit rate’ is substantially better than guess-work, 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 U.S. (http://iri.ldeo.columbia.edu/) 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 on-line and is sent to about 3,500 recipients of NIWA’s newsletters, including many farmers. See The Climate Update
- All outlooks are for the three months as a whole. There will inevitably be wet and dry days, hot and cold days, within a season.
- 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.