Sesonal Climate Outlook: A typical La Niña summer likely

NIWA NATIONAL CLIMATE CENTRE 1 December 2010

SEASONAL CLIMATE OUTLOOK: December 2010 – February 2011

A typical La Niña summer likely

A moderate to strong La Niña in the tropical Pacific is expected to persist through the summer of 2010/11, says the NIWA National Climate Centre. La Niña conditions are likely to continue through to autumn of 2011 and then to ease.

Summer (December to February) temperatures are likely to be average or above average for the time of year, for all districts of New Zealand.

Seasonal rainfall is likely to be below normal in the western South Island, normal or below normal in the north of the South Island, normal or above normal in the north and east of the North Island, and normal elsewhere. On average, La Niña summers tend to exhibit a gradient in rainfall in the north and east of the North Island, tending to be wetter in eastern Northland, coastal Bay of Plenty and Gisborne, and drier farther south and inland. Although rainfall is likely to be normal or above normal in the north and east of the North Island, summer soil moisture levels and river flows are likely to be normal or below normal in those regions, because of the already dry conditions. River flows and soil moisture levels are very likely to be below normal in the west and south of the South Island, and are likely to be near normal or below normal in all other regions, according to the National Climate Centre’s latest seasonal outlook.

The seasonal outlook states that mean sea level pressures are likely to be above normal over New Zealand and to the south of the country, for the three months as a whole.

Tropical cyclone activity is likely to be near- or above-normal this season (through to May 2011). The risk of an ex-tropical cyclone passing close to New Zealand is slightly above 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:

On average for summer (December-February), temperatures are likely to be near or above average in all districts. Sea surface temperatures are presently below normal on average around New Zealand, but are expected to warm to near normal over the summer.

Rainfall, soil moisture, and river flows:

Seasonal rainfall is likely to be below normal in the western South Island, normal or below normal in the north of the South Island, normal or above normal in the north and east of the North Island, and normal elsewhere. Summer soil moisture levels and river flows are very likely to be below normal in the west and south of the South Island, and are likely to be near normal or below normal in all other regions, according to the National Climate Centre’s latest seasonal outlook.

Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:

Temperatures are equally likely to be near average or above average. Seasonal rainfall is equally likely to be near normal or above normal, while soil moisture levels and river flows are equally likely to be below normal or near 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 / 40% / 40% / 25% / 25%
Near average / 40% / 40% / 35% / 35%
Below average / 20% / 20% / 40% / 40%

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

Temperatures are equally likely to be near average or above average over summer. Seasonal rainfall totals are likely to be near normal. Seasonal soil moisture levels and river flows are 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 / River flows
Above average / 40% / 20% / 15% / 20%
Near average / 40% / 50% / 45% / 40%
Below average / 20% / 30% / 40% / 40%

Gisborne, Hawke’s Bay, Wairarapa:

Temperatures are likely to be near average. Seasonal rainfall totals are equally likely to be in the near normal or above normal range, while soil moisture levels and river flows are likely to be the reverse (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 / River flows
Above average / 30% / 40% / 25% / 20%
Near average / 50% / 40% / 40% / 40%
Below average / 20% / 20% / 35% / 40%

Nelson, Marlborough, Buller:

Temperatures are equally likely to be in the near average or above average range. Seasonal rainfall, soil moisture levels, and river flows are likely to be in the below normal or near 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 / 40% / 20% / 15% / 15%
Near average / 40% / 40% / 40% / 40%
Below average / 20% / 40% / 45% / 45%

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

Temperatures are likely to be above average, for the three months as a whole. Seasonal rainfall is likely to be below normal. Soil moisture levels and river flows are very 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 / River flows
Above average / 50% / 20% / 10% / 10%
Near average / 30% / 30% / 30% / 30%
Below average / 20% / 50% / 60% / 60%

Coastal Canterbury, East Otago:

Temperatures are equally likely to be in the near average or above average range. Seasonal rainfall is likely to be near normal. Soil moisture levels and river flows are equally likely to be in the 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 / River flows
Above average / 40% / 30% / 20% / 20%
Near average / 40% / 50% / 40% / 40%
Below average / 20% / 20% / 40% / 40%

Background

The tropical Pacific is in a moderate to strong La Niña state, which is likely to continue through the summer of 2010/11.

ENDS

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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