Seasonal Climate Outlook: March – May 2011

NIWA NATIONAL CLIMATE CENTRE 1 March 2011

Seasonal Climate Outlook: March – May 2011

A mild autumn with La Niña easing
A strong La Niña event continues in the tropical Pacific, but is showing signs of easing, says the NIWA National Climate Centre. Neutral conditions are likely in the tropical Pacific by winter.

Autumn (March to May) temperatures are very likely to be above average in all North Island regions, and are likely to be above average in all South Island regions.

Seasonal rainfall is likely to be above normal in the northern North Island, normal or above normal in the remainder of the North Island and the northern South Island, and near normal elsewhere. Soil moisture levels and river flows are likely to be above normal in the North Island and the north of the South Island, and are likely to be near normal in the rest of the South Island.
The seasonal outlook states that mean sea level pressures are likely to be below normal to the north of New Zealand and above normal to the east and south of the country, with weaker westerlies across New Zealand.

Tropical cyclone activity is likely to be near- or above-normal for the rest of this cyclone season (through to May).

Overall Picture

Temperature:
On average for autumn, temperatures are very likely to be above average in all North Island regions, and are likely to be above average in all South Island regions. Sea surface temperatures are presently above normal especially west of New Zealand and north of the North Island, and are expected to remain above normal for the time of year through autumn.

Rainfall, soil moisture, and river flows:
The National Climate Centre says that seasonal rainfall is likely to be above normal in the northern North Island, normal or above normal in the remainder of the North Island and the northern South Island, and near normal elsewhere. Soil moisture levels and river flows are likely to be above normal in the North Island and the north of the South Island, and are likely to be near normal in the rest of the South Island.

Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:
Temperatures are very likely to be above average for the time of year. Seasonal rainfall totals, soil moisture levels, and river flows over autumn are likely to be above 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 60% 50% 50% 50%
Near average 30% 30% 30% 30%
Below average 10% 20% 20% 20%

Central North Island, Taranaki, Wanganui, Manawatu and Wellington:
Temperatures are very likely to be above average. Seasonal rainfall totals over autumn are equally likely to be either near normal or above normal, while soil moisture levels and river flows are likely to be in the above normal range.
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 60% 40% 45% 45%
Near average 30% 40% 35% 35%
Below average 10% 20% 20% 20%

Gisborne, Hawke’s Bay, Wairarapa:

Temperatures are very likely to be above average. Seasonal rainfall totals are equally likely to be in the near normal or above normal range. Soil moisture levels and river flows are likely to be above 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 60% 40% 45% 45%
Near average 30% 40% 35% 35%
Below average 10% 20% 20% 20%

Nelson, Marlborough, Buller:
Temperatures are likely to be in the above average range. Seasonal rainfall is equally likely to be in the near normal or above normal range, while soil moistures and river flows are likely to be above 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 50% 40% 40% 40%
Near average 30% 40% 35% 35%
Below average 20% 20% 25% 25%


West Coast, Alps and Foothills, Inland Otago, Southland:
Temperatures are likely to be above average, for the three months as a whole. Seasonal rainfall, soil moisture levels, and river flows are all 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 River flows
Above average 50% 30% 35% 35%
Near average 30% 45% 40% 40%
Below average 20% 25% 25% 25%

Coastal Canterbury, East Otago:
Temperatures are likely to be in the near average range. Seasonal rainfall, soil moisture levels, and river flows are all likely to be in the normal range, for the autumn as a whole.

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 50% 30% 35% 30%
Near average 30% 45% 40% 45%
Below average 20% 25% 25% 25%


Background

The tropical Pacific is in a strong La Niña state, which is showing signs of easing. Neutral conditions are likely in the tropical Pacific by winter. The present event has persisted since August 2010, and atmospheric indicators show this episode to be one of the strongest of the last 100 years. Previous very intense La Niña events occurred in 1975/76, 1971, and in 1917 (probably the strongest historically according to atmospheric indicators in the tropical Pacific).

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.

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