Seasonal Climate Outlook: September - November 2012
Weak El Niño forecast for spring and summer.
Borderline El Niño conditions are present in the tropical Pacific, and a weak short-lived El Niño is predicted for the spring and summer periods. However, the seasonal forecast models do not yet show any signs of the enhanced south-westerly and westerly airflow over New Zealand that is usually associated with El Niño events. Over the spring period (September-November), higher than normal pressures are expected in the south Tasman and over southern New Zealand, along with lower pressures to the north of the North Island, resulting in weaker than normal westerlies over the country.
Spring air temperatures are likely to be near average in all regions. Sea temperatures around New Zealand are also likely to be near average for the season as a whole.
Spring rainfall, soil moisture and river flows are likely to be above normal or near normal in the north and east of the North Island, near normal in the west of the North Island and in Nelson-Marlborough, and near or below normal in remaining South Island regions.
Overall picture
Temperature
Spring air temperatures are likely to be near average in all regions of the country. The occurrence of frosts and spring snowfalls cannot be ruled out. Sea temperatures around New Zealand are likely to be near average for the season as a whole.
Rainfall, soil moisture and river flows
September-November rainfall, soil moisture levels and river flows are all likely to be above normal or near normal in the north and east of the North Island, and near normal in the southwest of the North Island and in Nelson-Marlborough.
In the west, south and east of the South Island, rainfall and river flows are likely to be near normal or below normal. Seasonal soil moisture levels are likely to be below normal in the west and south of the South Island, and near normal or below normal in the east of the South island.
Regional predictions for the next three months
Northland, Auckland, Waikato, Bay of Plenty
September to November temperatures are likely to be in the near average range. Seasonal rainfall totals, soil moisture levels, and river flows are likely to be in the above normal or 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 |
River flows |
|
Above average |
30 |
40 |
45 |
45 |
|
Near average |
50 |
40 |
40 |
40 |
|
Below average |
20 |
20 |
15 |
15 |
Click these links to view the historic rainfall and temperature ranges for this region
Central North Island, Taranaki, Wanganui, Manawatu, Wellington
Seasonal temperatures are likely to be near average. Near normal spring rainfall, soil moisture levels and river flows are likely.
Probabilities are assigned in three categories: above average, near average, and below average. The full probability breakdown is:
|
|
Temperature |
Rainfall |
Soil moisture |
River flows |
|
Above average |
30 |
30 |
35 |
35 |
|
Near average |
50 |
50 |
45 |
45 |
|
Below average |
20 |
20 |
20 |
20 |
Click these links to view the historic rainfall and temperature ranges for this region
Gisborne, Hawke’s Bay, Wairarapa
Seasonal temperatures are likely to be in the near average range. September-November rainfall totals, soil moisture levels and river flows are all likely to be in the above normal or 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 |
River flows |
|
Above average |
30 |
40 |
45 |
45 |
|
Near average |
50 |
40 |
40 |
40 |
|
Below average |
20 |
20 |
15 |
15 |
Click these links to view the historic rainfall and temperature ranges for this region
Nelson, Marlborough, Buller
September to November temperatures are likely to be near average. Near normal seasonal rainfall totals, soil moisture levels, and river flows are likely.
Probabilities are assigned in three categories: above average, near average, and below average. The full probability breakdown is:
|
|
Temperature |
Rainfall |
Soil moisture |
River flows |
|
Above average |
35 |
30 |
35 |
35 |
|
Near average |
45 |
50 |
45 |
45 |
|
Below average |
20 |
20 |
20 |
20 |
Click these links to view the historic rainfall and temperature ranges for this region
West Coast, Alps and foothills, inland Otago, Southland
Seasonal temperatures are likely to be in the near average range. Spring rainfall and river flows are likely to be in the below normal or near normal range. Soil moisture levels are likely to be below normal.
Probabilities are assigned in three categories: above average, near average, and below average. The full probability breakdown is:
|
|
Temperature |
Rainfall |
Soil moisture |
River flows |
|
Above average |
35 |
20 |
15 |
15 |
|
Near average |
45 |
40 |
35 |
40 |
|
Below average |
20 |
40 |
50 |
45 |
Click these links to view the historic rainfall and temperature ranges for this region
Coastal Canterbury, east Otago
September to November temperatures are likely to be in the near average range. Near normal or below normal seasonal rainfall totals, soil moisture levels, and river flows are likely.
Probabilities are assigned in three categories: above average, near average, and below average. The full probability breakdown is:
|
|
Temperature |
Rainfall |
Soil moisture |
River flows |
|
Above average |
25 |
20 |
25 |
25 |
|
Near average |
45 |
40 |
40 |
40 |
|
Below average |
30 |
40 |
35 |
35 |
Click these links to view the historic rainfall and temperature ranges for this region
Background
Sea surface temperatures have continued to warm in the central equatorial Pacific, and currently exceed the accepted El Niño threshold. However, other important El Niño indicators, such as the strength of the trade winds and the location of convection along the equator, still remain fairly close to normal. Thus, international forecasting institutions are being cautious about claiming an El Niño is in place. Almost all climate models which NIWA monitors predict the presence of El Niño during the remaining months of 2012, but the amplitude is weak to moderate, and the event is predicted to decay in the first quarter of 2013. The Southern Oscillation is negative (-0.7), but not strong.
For comment, please contact
Dr Brett Mullan, NIWA Principal Scientist, Climate Variability & Change
Tel (04) 386 0508, Mobile (027) 294 1169
Georgina Griffiths, Senior Climate Scientist
Tel (09) 375 4506, Mobile (027) 293 6545
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.
