Weak El Niño not likely to greatly influence New Zealand rainfall in the coming season.
A weak El Niño in the equatorial Pacific Ocean is not expected to unduly affect New Zealand rainfall in the coming 3 months, according to the latest outlook from NIWA’s National Climate Centre.
After a recent dry period in eastern parts of the South Island, the Centre says rainfall is likely to be in the normal range in all regions for the October-November-December season overall.
Temperatures for the coming 3-month period as a whole (October, November and December combined) are expected to be near average for the North Island, but are equally likely to be in the average or below average ranges for the South Island.
In line with the rainfall outlook, seasonal soil moisture levels and stream flows are likely to be in the normal range throughout the country.
Overall Picture
Temperature:
Seasonal air temperatures are likely to be in the average range for the North Island, and in the average or below average ranges for the South Island. Sea surface temperatures near New Zealand are expected to continue below average through October-December.
Rainfall, soil moisture, and stream flows:
Rainfall, soil moisture levels, and stream flows for October-December are projected to be in the normal range for all regions.
Regional predictions for the next three months:
Northland, Auckland, Waikato, Bay of Plenty:
Temperatures are likely to be in the near average category. Seasonal (3-month) rainfall totals, soil moisture levels and stream flows are expected to be in the near normal range.
Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:
| Temperature | Rainfall | Soil moisture | Stream flows | |
|---|---|---|---|---|
| Above average | 20% | 20% | 25% | 25% |
| Near average | 50% | 50% | 50% | 50% |
| Below average | 30% | 30% | 25% | 25% |
Central North Island, Taranaki, Wanganui, Manawatu and Wellington
Seasonal temperatures are projected to be in the near average category. Three-month rainfall totals, soil moisture levels and stream flows, are expected to be in the near normal range.
Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:
| Temperature | Rainfall | Soil moisture | Stream flows | |
|---|---|---|---|---|
| Above average | 20% | 20% | 20% | 20% |
| Near average | 50% | 50% | 50% | 50% |
| Below average | 30% | 30% | 30% | 30% |
Gisborne, Hawke’s Bay, Wairarapa:
Three-month temperatures are likely to be in the near average category. Seasonal rainfall totals, soil moisture levels, and stream flows are also expected to be in the near normal range.
Probabilities are assigned in three categories; above normal, near normal, and below normal.The full probability breakdown is:
| Temperature | Rainfall | Soil moisture | Stream flows | |
|---|---|---|---|---|
| Above average | 20% | 15% | 20% | 20% |
| Near average | 50% | 50% | 50% | 50% |
| Below average | 30% | 35% | 30% | 30% |
Nelson, Marlborough, Buller:
Temperatures are equally likely to be in the near average or below average categories. October-December rainfall, soil moisture levels and stream flows are likely to be in the normal range.
Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:
| Temperature | Rainfall | Soil moisture | Stream flows | |
|---|---|---|---|---|
| Above average | 20% | 25% | 20% | 20% |
| Near average | 40% | 50% | 50% | 50% |
| Below average | 40% | 25% | 30% | 30% |
West Coast, Alps and Foothills, Inland Otago, Southland:
Temperatures are equally likely to be in the near average or below average categories. Seasonal rainfall, soil moisture levels and stream flows are projected to be in the normal range.
Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:
| Temperature | Rainfall | Soil moisture | Stream flows | |
|---|---|---|---|---|
| Above average | 20% | 30% | 30% | 30% |
| Near average | 40% | 50% | 50% | 50% |
| Below average | 40% | 20% | 20% | 20% |
Coastal Canterbury, East Otago:
Temperatures are equally likely to be in the near average or below average categories. Seasonal rainfall, soil moisture levels and stream flows are expected to be in the normal range.
Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:
| Temperature | Rainfall | Soil moisture | Stream flows | |
|---|---|---|---|---|
| Above average | 20% | 30% | 20% | 20% |
| Near average | 40% | 50% | 50% | 50% |
| Below average | 40% | 20% | 30% | 30% |
Background
A weak El Niño is present in the equatorial Pacific Ocean, but atmospheric circulation is still not well-coupled with the ocean. The Southern Oscillation Index (SOI), which is an indicator of the strength of the atmospheric component of the El Nino, was near zero when averaged over the last three months. During strong El Niño events, the SOI would be negative (at least -1). Most of the El Niño forecast models predict El Niño persisting to the end of summer, with some strengthening between now and the end of the year.
El Niño events can lead to dry conditions in eastern parts of New Zealand over the spring and summer seasons. However, the latest guidance suggests that October-December 2009 rainfall is likely to remain in the normal range.
For comment, please contact:
Georgina Griffiths, Climate Scientist
Tel (09) 375 4506 (office DDI), mobile (027) 293 6545
Dr Brett Mullan, NIWA Principal Scientist, Climate Variability & Change
Tel (04) 386 0508 (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.