La Niña intensifies; warm late spring very likely

NIWA NATIONAL CLIMATE CENTRE 30 September 2010

SEASONAL CLIMATE OUTLOOK: October–December 2010
La Niña intensifies; warm late spring period very likely

The current La Niña has recently strengthened, says the NIWA National Climate Centre. A moderate to strong La Niña event is presently underway, with further intensification possible this year. La Niña conditions are likely to continue through to at least autumn of 2011.

Late spring (October¬–December) temperatures are likely to be above average, in all regions. Rainfall is likely to be near normal or below normal in the east of both islands and the southwest of the South Island, and near normal elsewhere, averaged over the three month period October–December.

Soil moisture levels and stream flows are likely to be near normal or below normal in most regions, except the southwest of the North Island and northern South Island where near normal levels are likely.

The National Climate Centre’s seasonal outlook states that mean sea level pressures are likely to be above normal over and south of New Zealand, for October–December as a whole, with weaker than normal westerly winds.

Overall Picture

Temperature:

Temperatures are likely to be above average in the east of both islands and in the north of the South Island, and very likely to be above average in other districts. Sea surface temperatures are presently near normal around New Zealand, but are expected to become warmer than normal around the North Island as the season progresses.

Rainfall, soil moisture, and stream flows:
Seasonal rainfall is likely to be near normal or below normal in the east of both islands and the southwest of the South Island, and near normal elsewhere. Soil moisture levels and stream flows are likely to be near normal or below normal in most regions, except in the southwest of the North Island and north of the South Island, where they are likely to be near normal.

Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:
Temperatures are very likely to be in the above average range. Near normal seasonal rainfall is likely, while soil moisture levels and stream 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:

/ Temperature / Rainfall / Soil moisture / Stream flows
Above average / 60% / 30% / 25% / 25%
Near average / 25% / 50% / 40% / 40%
Below average / 15% / 20% / 35% / 35%

Central North Island, Taranaki, Wanganui, Manawatu and Wellington:
Temperatures are very likely to be above average for the late spring period. Seasonal rainfall totals, 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 / 65% / 25% / 25% / 25%
Near average / 20% / 50% / 45% / 45%
Below average / 15% / 25% / 30% / 30%

Gisborne, Hawke’s Bay, Wairarapa:
Above average seasonal temperatures are likely. Seasonal rainfall totals, soil moisture levels and stream flows are likely to be near normal or 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 / 50% / 20% / 25% / 15%
Near average / 40% / 40% / 40% / 45%
Below average / 10% / 40% / 35% / 40%

Nelson, Marlborough, Buller:
Temperatures are likely to be above average. Seasonal rainfall, soil moisture, 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 / 50% / 35% / 25% / 20%
Near average / 40% / 50% / 50% / 50%
Below average / 10% / 15% / 25% / 30%

West Coast, Alps and Foothills, Inland Otago, Southland:
Seasonal temperatures are very likely to be above average. Seasonal rainfall totals, soil moisture and stream flows are equally likely to be in the near normal or below 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 / 65% / 20% / 20% / 20%
Near average / 25% / 40% / 40% / 40%
Below average / 10% / 40% / 40% / 40%

Coastal Canterbury, East Otago:
Temperatures are likely to be in the above average range. Late spring rainfall totals, soil moisture and stream 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:

/ Temperature / Rainfall / Soil moisture / Stream flows
Above average / 45% / 20% / 25% / 20%
Near average / 30% / 40% / 40% / 40%
Below average / 25% / 40% / 35% / 40%

Background

The tropical Pacific is now exhibiting moderate to strong La Niña conditions, as the current event continues to strengthen. The La Niña state is expected to continue to at least the autumn of 2011.

--

© 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.

The three categories (above average, near average, below average) are known as ‘terciles’.

A tercile with a probability of 60% or more is described as "very likely" to occur (e.g., 60:30:10, very likely to be above normal). Otherwise, the qualifier "likely" is applied to the tercile with the highest probability (e.g., 50:30:10, likely to be above normal). If two terciles have the same probability, or are within 5% of each other, then "likely" is applied to both terciles (e.g., 40:40:20, likely to be normal or above normal).

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

© Scoop Media