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Seasonal Climate Outlook Dec 09 - Feb 10

NIWA NATIONAL CLIMATE CENTRE 01 December 2009


SEASONAL CLIMATE OUTLOOK: December 2009 – February 2010
El Niño strengthens, with conditions likely to be cooler in south and drier in northeast
The El Niño in the equatorial Pacific has strengthened during October and November, and is expected to persist at moderate intensity through the summer of 2010 before weakening during the autumn. According to the latest outlook from NIWA’s National Climate Centre, mean sea level pressures are likely to be higher than normal to the north of the country and lower than normal to the south, resulting in stronger than normal south-westerlies over New Zealand.
The Centre says summer rainfall totals are likely to be in the normal or below normal range in the north and east of the North Island, and in Nelson-Marlborough, but near normal in other regions.

Temperatures for the coming 3-month period (December, January and February combined) are likely to be below average or average in all regions.

Rivers flows and soil moistures are likely to be in the below normal or normal ranges in all regions.

Overall Picture

Temperature:

Air temperatures are likely to be below average or average in all South Island regions and in the west of the North Island, and near average in other North Island regions. Sea surface temperatures near New Zealand are expected to be below average during December-February.

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Rainfall, soil moisture, and stream flows:

Rainfall is likely to be normal or below normal in the north and east of the North Island, and in Nelson-Marlborough, and near normal elsewhere. River flows are likely to be below normal in the north and east of both Islands, with soil moistures also likely to be below normal in the east of the South Island. In all other regions, river flows and soil moistures are expected to be normal or below normal for the summer season as a whole.
Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:
Temperatures are likely to be in the near average category. Seasonal (3-month) rainfall totals and soil moisture levels are expected to be in the normal or below normal range, with stream flows below 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 / 20% / 20% / 20% / 10%
Near average / 50% / 40% / 40% / 40%
Below average / 30% / 40% / 40% / 50%

Central North Island, Taranaki, Wanganui, Manawatu and Wellington:
Seasonal temperatures are projected to be most likely in the below average category. Three-month rainfall totals and soil moisture levels are expected to be near normal, with stream flows 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:

/ Temperature / Rainfall / Soil moisture / Stream flows
Above average / 10% / 20% / 20% / 20%
Near average / 40% / 50% / 50% / 40%
Below average / 50% / 30% / 30% / 40%

Gisborne, Hawke’s Bay, Wairarapa:
Three-month temperatures are likely to be in the near average category. Seasonal rainfall totals and soil moisture levels are expected to be in the normal or below normal range, and stream flows are most 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 / Stream flows
Above average / 20% / 20% / 20% / 10%
Near average / 50% / 40% / 40% / 40%
Below average / 30% / 40% / 40% / 50%

Nelson, Marlborough, Buller:
Temperatures are most likely to be in the below average category. December-February rainfall and soil moisture levels are likely to be normal or below normal. Seasonal stream 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 / Stream flows
Above average / 10% / 20% / 20% / 20%
Near average / 40% / 40% / 40% / 30%
Below average / 50% / 40% / 40% / 50%

West Coast, Alps and Foothills, Inland Otago, Southland:
Temperatures are most likely to be in the below average category. Seasonal rainfall and soil moisture levels are expected to be near normal, while stream 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:

/ Temperature / Rainfall / Soil moisture / Stream flows
Above average / 10% / 20% / 20% / 20%
Near average / 40% / 50% / 50% / 40%
Below average / 50% / 30% / 30% / 40%

Coastal Canterbury, East Otago:
Temperatures are equally likely to be in the near average or below average categories. Seasonal rainfall is expected to be in the normal range, while soil moisture levels and stream flows are most likely to be below 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 / 20% / 20% / 10% / 10%
Near average / 40% / 50% / 40% / 40%
Below average / 40% / 30% / 50% / 50%

Background

The El Niño has strengthened over the past two months in terms of increased surface and sub-surface equatorial ocean temperatures, although the Southern Oscillation Index weakened slightly in November from the October value of -1.6. Most of the El Niño forecast models predict El Niño to reach its maximum intensity over the summer before declining in autumn.

El Niño events can lead to dry conditions in eastern parts of New Zealand over the spring and summer seasons, and soil moistures are already very low at the end of November in Northland, Bay of Plenty and central Otago. The latest guidance suggests that drier conditions in the north and east of both islands are likely over the December 2009-February 2010 period. El Niño events are also often associated with colder than average conditions in the New Zealand region, and this is consistent with this latest outlook.

--

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.


ENDS

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