La Niña set to fade with a near-normal lead-in to autumn.
The NIWA National Climate Centre’s outlook for early Autumn, February to April 2012, indicates that seasonal rainfalls and temperatures are likely to be near normal for almost all regions, apart from the west and south of the South Island where rainfall is likely to be normal or below normal and temperatures are likely to be average or above average.
Soil moisture levels and river flows are both predicted to follow the same regional pattern to rainfall: likely to be near normal for almost all regions, but normal or below normal for the west and south of the South Island.
A moderate La Niña is in place in the tropical Pacific and should persist into early autumn 2012, according to the NIWA National Climate Centre, before fading back to neutral conditions.
For the February to April season, mean sea level pressures for early autumn (Feb-Apr) are likely to be above average south and southeast of the country, but below average to the north of New Zealand.
For the remainder of the tropical cyclone season through to May, around the normal number of cyclones is expected overall (January to March is typically the most active part of the cyclone season). On average, at least one ex-tropical cyclone passes within 500km of New Zealand in 9 out of 10 cyclone seasons.
Overall Picture
Temperature:
For the February-April period as a whole, air temperatures are likely to be near average in most regions, but near average or above average in the west and south of the South Island. Sea surface temperatures in the New Zealand region are likely to be near average.
Rainfall, soil moisture, and river flows:
The National Climate Centre projects that early autumn rainfall totals, soil moisture levels, and river flows are all likely to be near normal throughout most of the country. The exception is the west and south of the South Island where normal or below normal conditions are likely.
Regional predictions for the next three months:
Northland, Auckland, Waikato, Bay of Plenty:
Early autumn temperatures are likely to be near average. Seasonal rainfall totals, soil moisture levels, and river flows are all likely to be near 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 |
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
Central North Island, Taranaki, Wanganui, Manawatu and Wellington:
Seasonal temperatures are likely to be near average. Early autumn rainfall totals, soil moisture levels, and river flows are likely to be near normal, for the three month season as a whole.
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% |
20% |
30% |
30% |
|
Near average |
50% |
50% |
40% |
40% |
|
Below average |
20% |
30% |
30% |
30% |
Click these links to view the historic rainfall and temperature ranges for this region
Gisborne, Hawke’s Bay, Wairarapa:
Late summer temperatures are likely to be near average. Seasonal rainfall totals, soil moisture levels, and river flows are all likely to be near 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 |
20% |
30% |
35% |
35% |
|
Near average |
50% |
50% |
45% |
45% |
|
Below average |
30% |
20% |
20% |
20% |
Click these links to view the historic rainfall and temperature ranges for this region
Nelson, Marlborough, Buller:
Temperatures over the early autumn period are likely to be near average. Seasonal rainfall totals, soil moisture levels and river flows are likely to be near normal for the time of year.
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% |
30% |
35% |
30% |
|
Near average |
50% |
50% |
45% |
50% |
|
Below average |
25% |
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:
Early autumn temperatures are equally likely to be near average or above average. Seasonal rainfall totals, soil moisture levels and river flows are all equally likely to be near normal or 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 |
40% |
20% |
20% |
20% |
|
Near average |
40% |
40% |
40% |
40% |
|
Below average |
20% |
40% |
40% |
40% |
Click these links to view the historic rainfall and temperature ranges for this region
Coastal Canterbury, East Otago:
Early autumn temperatures are likely to be average. Seasonal rainfall totals, soil moisture levels and river flows are likely to be near 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 |
20% |
30% |
25% |
20% |
|
Near average |
50% |
50% |
50% |
50% |
|
Below average |
30% |
20% |
25% |
30% |
Click these links to view the historic rainfall and temperature ranges for this region
Background
The tropical Pacific is currently in moderate La Niña conditions, and the ocean now appears to be fully coupled with the tropical atmosphere. Global climate models predict a continuation of La Niña conditions through March 2012, followed by an easing to neutral conditions by early winter.
For comment, please contact:
Dr James Renwick, NIWA Principal Scientist, Climate Variability & Change
Mobile (021) 178 5550
Dr Brett Mullan, NIWA Principal Scientist, Climate Variability & Change
Tel (04) 386 0508 (office DDI), Mobile (027) 294 1169
© Copyright NIWA 2011. All rights reserved. Acknowledgement of NIWA as the source is required.
Notes to reporters & 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%
- Near average: 30%
- Below average: 10%
We therefore concluded that above average temperatures were very likely.
4. 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.
5. 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).
6. 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: www.niwascience.co.nz/ncc
7. 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 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 from the long-term mean, whereas for rainfall the “near normal” category lies approximately between 80% and 115% 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.