Mild La Nina Summer Likely

NIWA NATIONAL CLIMATE CENTRE 30 November 2011

SEASONAL CLIMATE OUTLOOK: December 2011 - February 2012

Mild La Nina Summer Likely

The NIWA National Climate Centre’s outlook for summer, December 2011 to February 2012, indicates that seasonal temperatures are likely to be above average on the West Coast of the South Island, and near average or above average across the remainder of the country.

Summer rainfall totals are likely to be below normal on the West Coast, and normal or below normal across the remainder of the South Island, as well as in the southwest of the North Island. Near normal seasonal rainfall is likely for the eastern North Island. For the northern North Island, summer rainfall is equally likely to be normal or above normal. Soil moisture levels and river flows are likely to be near normal in eastern regions of both Islands, and normal or below normal elsewhere.

La Niña conditions are likely to continue through the summer, according to the NIWA National Climate Centre, although it is unlikely that the current La Niña event will be as strong as the previous La Niña event of 2010-11.

The outlook states that mean sea level pressures during the summer are likely to be above normal across the South Island, with weaker than normal westerlies over the North Island.

For the tropical cyclone season (through until May), the chance of an ex-tropical cyclone passing close to New Zealand is below the long-term average. 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 December-February period as a whole, air temperatures are likely to be above average on the West Coast of the South Island, and near average or above average across the remainder of the country. Sea surface temperatures are likely to remain above average to the northeast of the North Island during summer, but near average elsewhere about the New Zealand region.

Rainfall, soil moisture, and river flows:

The National Climate Centre projects that summer rainfall totals are likely to be below normal on the West Coast, and normal or below normal across the remainder of the South Island, as well as in the southwest of the North Island. Near normal seasonal rainfall is likely for the eastern North Island. For the northern North Island, summer rainfall is equally likely to be normal or above normal. Soil moisture levels and river flows are likely to be near normal in eastern regions of both Islands, and normal or below normal elsewhere.

Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:

Summer temperatures are equally likely to be near average or above average. Seasonal rainfall totals are equally likely to be near normal or above normal. Soil moisture levels are likely to be in the normal range for the season as a whole, while river flows are likely to be normal or below normal.

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

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

Seasonal temperatures are equally likely to be near average or above average. Summer rainfall totals, soil moisture levels, and river flows are likely to be near normal or below 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:

Gisborne, Hawke’s Bay, Wairarapa:

Summer temperatures are equally likely to be near average or above average. Seasonal rainfall totals, soil moisture levels, and river flows are all likely 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:

Nelson, Marlborough, Buller:

Temperatures over the summer period are equally likely to be near average or above average. Seasonal rainfall totals, soil moisture levels, and river flows are 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:

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

Above average summer temperatures are likely. Seasonal rainfall totals are likely to be below normal. Soil moisture levels and river flows are likely to be near normal or below normal, for the summer period as a whole.

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

Coastal Canterbury, East Otago:

Summer temperatures are likely to be near average or above average. Seasonal rainfall totals are equally likely to be near normal or below normal. Soil moisture levels and river flows are likely to be in the normal range, 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:

Background

The tropical Pacific is currently showing weak to moderate La Niña conditions. The Southern Oscillation Index has been consistently positive for the last 3 months, and sea surface temperature anomalies have become increasingly negative in the east-central equatorial Pacific. The majority of global climate models which predict El Niño-Southern Oscillation conditions are forecasting the continuation of weak to moderate La Niña conditions into early 2012. It is unlikely that the current La Niña will be as strong as La Niña event of 2010-11.

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© 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:
4.
• Above average: 60%

• Near average: 30%

• Below average: 10%

We therefore concluded that above average temperatures were very likely.

5. 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.

6. 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).

7. 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

8. 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.

9. 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.

ENDS

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