Seasonal Climate Outlook: October - December 2011

Dry conditions likely as La Niña returns.

The NIWA National Climate Centre's outlook for late spring and early summer, October to December 2011, indicates that seasonal rainfall is likely to be normal or below normal in all regions. Soil moisture levels and river flows are likely to be below normal in all regions of the country, except for the west and south of the South Island where normal or below normal soil moisture levels are likely. The Centre notes that soils are already drier than normal for this time of year in north Canterbury, MacKenzie country and central Otago, as well as parts of the North Island.

Temperatures for October-December are likely to be average or above average in the North Island and northern South Island, and near average in the rest of the South Island.

La Niña conditions are redeveloping in the tropical Pacific, and the event is expected to build through spring and continue over the summer season, according to the NIWA National Climate Centre.

The outlook states that mean sea level pressures during the October-December period as a whole are likely to be above normal across New Zealand, with weaker westerlies over the country.

Overall Picture

Temperature:

For the October – December period as a whole, air temperatures are likely to be average or above average in the North Island and in Nelson-Marlborough. For the remainder of the South Island, temperatures are likely to be near average. Sea surface temperatures near New Zealand are expected to be close to normal or slightly above normal through the period.

Rainfall, soil moisture, and river flows:

The National Climate Centre says that late spring-early summer rainfall is likely to be normal or below normal for all regions of New Zealand. Soil moisture levels and river flows are likely to be below normal everywhere, except for the west and south of the South Island where normal or below normal soil moisture levels are likely. Soils are already drier than normal for this time of year in north Canterbury, MacKenzie country and central Otago, as well as parts of the North Island. 

Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:

Temperatures are likely to be average or above average for the time of year.  October to December rainfall totals are equally likely to be in the normal or below normal range.  Soil moisture levels and river flows 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	40%	20%	15%	15%
Near average	40%	40%	35%	35%
Below average	20%	40%	50%	50%

Click these links to view the historic rainfall and temperature ranges for this region 

Central North Island, Taranaki, Wanganui, Manawatu and Wellington: 

Temperatures in the late spring and early summer period are likely to be average or above average.  Rainfall totals are equally likely to be in the normal or below normal range, while soil moisture levels and river flows 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	40%	20%	10%	15%
Near average	40%	40%	40%	35%
Below average	20%	40%	50%	50%

Click these links to view the historic rainfall and temperature ranges for this region 

Gisborne, Hawke’s Bay, Wairarapa:

Temperatures are likely to be average or above average for the time of year, while rainfall is equally likely to be in the normal or below normal range. Soil moisture levels and river flows are likely to be below normal.

Probabilities are assigned in three categories; above normal, near normal, and below normal. The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	40%	20%	15%	15%
Near average	40%	40%	35%	35%
Below average	20%	40%	50%	50%

Click these links to view the historic rainfall and temperature ranges for this region 

Nelson, Marlborough, Buller:

Temperatures are likely to be average or above average for the time of year, while rainfall is equally likely to be in the normal or below normal range. Soil moisture levels and river flows 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	40%	20%	15%	15%
Near average	40%	40%	35%	35%
Below average	20%	40%	50%	50

Click these links to view the historic rainfall and temperature ranges for this region 

West Coast, Alps and Foothills, Inland Otago, Southland:

Temperatures are likely to be near average for the time of year.  Seasonal rainfall totals and soil moisture levels are equally likely to be in the normal or below normal range, over the October to December period as a whole. River flows 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	30%	20%	20%	20%
Near average	50%	40%	40%	35%
Below average	20%	40%	40%	45%

Click these links to view the historic rainfall and temperature ranges for this region 

Coastal Canterbury, East Otago:

Temperatures are likely to be near average, and rainfall is equally likely to be in the normal or below normal range, over the October to December period as a whole. Soil moisture levels and river flows 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	30%	20%	15%	15%
Near average	50%	40%	35%	30%
Below average	20%	40%	50%	55

Click these links to view the historic rainfall and temperature ranges for this region 

Background

Following the La Niña event over July 2010 to April 2011, the tropical Pacific returned to neutral conditions. However, over the past 4-6 weeks there is mounting evidence of a transition back to La Niña conditions.  The Southern Oscillation Index has become more positive, sea surface temperature anomalies have become increasingly negative in the east-central equatorial Pacific, and the easterly trade winds have intensified near the Date Line. There has been a major shift in the consensus from global climate models which predict El Niño-Southern Oscillation conditions. The majority of these models are now forecasting further development of La Niña patterns over spring and a continuation through the summer of 2011/12. At this early stage, we cannot be confident about the intensity of the event. 

For comment, please contact:

Dr Brett Mullan, NIWA Principal Scientist, Climate Variability & Change

Tel (04) 386 0508 (office DDI), Mobile (027) 294 1169

Dr James Renwick, NIWA Principal Scientist, Climate Variability & Change

Mobile (021) 178 5550, Tel (04) 386 0343 (office DDI)

© 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.