Climate Outlook Sept-Nov '10: mild La Niña spring

NIWA NATIONAL CLIMATE CENTRE 31 August 2010

SEASONAL CLIMATE OUTLOOK: September – November 2010

A mild La Niña spring on the way

A moderate La Niña is well-established in the tropical Pacific, and may strengthen further through the rest of 2010, says the NIWA National Climate Centre. La Niña conditions are likely to continue through the summer of 2010–11.

Spring temperatures are likely to be above average across the whole country. However, short-term cold snaps typical of spring will still occur from time to time.

Rainfall is likely to be near normal in most regions, averaged over the three months of spring (September, October, November). The north and east of the North Island is likely to experience normal or above normal rainfalls, according to the Centre’s latest seasonal outlook.

Soil moisture levels and stream flows are likely to be near normal in all regions, on average through the spring.

The National Climate Centre’s seasonal outlook states that mean sea level pressures are likely to be above normal near New Zealand, for spring as a whole.

Overall Picture

Temperature:

Spring temperatures are likely to be above average in all regions. However, short-term cold snaps typical of spring will still occur from time to time. Sea surface temperatures are expected to be near average or above average around New Zealand over the September to November period.

Rainfall, soil moisture, and stream flows:

Rainfall is likely to be near normal in most places, but normal or above normal in the north and east of the North Island. 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:

Temperatures are likely to be in the above average range. Seasonal rainfall is equally likely to be near normal or above normal, while 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 / 50% / 40% / 25% / 35%
Near average / 30% / 40% / 45% / 40%
Below average / 20% / 20% / 30% / 25%

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

Temperatures are likely to be above average for spring. Seasonal rainfall totals, soil moisture levels, and stream flows are likely to be near normal overall.

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 / 50% / 20% / 20% / 20%
Near average / 30% / 45% / 45% / 45%
Below average / 20% / 35% / 35% / 35%

Gisborne, Hawke’s Bay, Wairarapa:

Temperatures are likely to be in the above average range. Seasonal rainfall is equally likely to be near normal or above normal. Soil moisture levels and stream flows are likely to be near normal overall.

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 / 50% / 40% / 30% / 30%
Near average / 30% / 40% / 45% / 45%
Below average / 20% / 20% / 25% / 25%

Nelson, Marlborough, Buller:

Temperatures are very likely to be in the above average range. Seasonal rainfall, soil moisture levels, and stream 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 / Stream flows
Above average / 60% / 35% / 25% / 25%
Near average / 30% / 45% / 50% / 50%
Below average / 10% / 20% / 25% / 25%

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

Temperatures are very likely to be in the above average range, for spring as a whole. Seasonal rainfall soil moisture levels, and stream 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 / Stream flows
Above average / 60% / 20% / 25% / 25%
Near average / 30% / 50% / 50% / 50%
Below average / 10% / 30% / 25% / 25%

Coastal Canterbury, East Otago:

Temperatures are very likely to be in the above average range. Seasonal rainfall, soil moisture levels, and stream flows are likely to be near normal for spring.

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 / 60% / 20% / 15% / 15%
Near average / 30% / 45% / 50% / 50%
Below average / 10% / 35% / 35% / 35%

Background

The tropical Pacific is in a moderate La Niña state, which is likely to continue and may strengthen further through the rest of 2010.

ENDS

© Copyright NIWA 2010. 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 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%
• / 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.