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La Nina Back But A Typical Summer Likely

La Nina Back But A Typical Summer Likely

The NIWA National Climate Centre’s outlook for early summer, November 2011 to January 2012, indicates that temperatures are likely to be near average across all of the North Island and average or above average in the South Island.

Seasonal rainfalls, soil moisture levels and river flows are all likely to be near normal in all regions of the country. The Centre notes that significant rain in October has improved soil moisture deficits in many regions, apart from coastal Wairarapa and the Gisborne region.

La Niña conditions have redeveloped in the tropical Pacific, and weak to moderate La Nina conditions are likely to continue through the summer, according to the NIWA National Climate Centre.

The outlook states that mean sea level pressures during the November-January period as a whole are likely to be near normal or above normal across New Zealand, with weaker westerlies over the country.

For the tropical cyclone season (November to 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 November-January period as a whole, air temperatures are likely to be near average over the North Island and average or above average over the South Island. Sea surface temperatures are likely to remain near normal around New Zealand.

Rainfall, soil moisture, and river flows:
The National Climate Centre says that early summer rainfalls, soil moisture levels and river flows are all likely to be near normal in all regions of the country.

Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:
Temperatures are likely to be near average for the time of year. Seasonal rainfall totals, 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 30% 35% 35% 30%
Near average 50% 45% 45% 40%
Below average 20% 20% 20% 30%

Central North Island, Taranaki, Wanganui, Manawatu and Wellington:
Temperatures in the early summer period are likely to be near average. Rainfall totals are likely to be in the normal range, as are soil moisture levels and river flows, for the three months 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 30% 30% 30% 35%
Near average 50% 50% 50% 45%
Below average 20% 20% 20% 20%

Gisborne, Hawke’s Bay, Wairarapa:
Temperatures are likely to be average for the time of year, and rainfall is likely to be in the normal range. Soil moisture levels and river flows are likely to be near 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% 30% 30% 30%
Near average 50% 50% 50% 45%
Below average 20% 20% 20% 25%

Nelson, Marlborough, Buller:
Temperatures are likely to be average or above average for the time of year, while rainfall is likely to be in the normal range. Soil moisture levels and river flows are also 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 River flows
Above average 40% 20% 20% 20%
Near average 40% 50% 50% 50%
Below average 20% 30% 30% 30%


West Coast, Alps and Foothills, Inland Otago, Southland:
Temperatures are equally likely to be near average or above average for the time of year. Seasonal rainfall totals, soil moisture levels and river flows are all likely to be in the normal range, over the November to January period 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 40% 20% 20% 20%
Near average 40% 50% 45% 45%
Below average 20% 30% 35% 35%

Coastal Canterbury, East Otago:
Temperatures are equally likely to be near average or above average, and rainfall is likely to be in the normal range, over the November to January period as a whole. Soil moisture levels and river flows are 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 40% 20% 30% 25%
Near average 40% 50% 45% 50%
Below average 20% 30% 25% 25%

Background

Following the La Niña event over July 2010 to April 2011, the tropical Pacific returned to neutral conditions. However, from late July, a transition back to La Niña conditions began. The Southern Oscillation Index has become consistently positive, sea surface temperature anomalies have become increasingly negative in the east-central equatorial Pacific, and the easterly trade winds have intensified near and west of the Date Line. The majority of global climate models which predict El Niño-Southern Oscillation conditions are forecasting the persistence of weak to moderate La Niña conditions through the summer of 2011/12.


© 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:
• 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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