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Late summer as La Niña continues

NATIONAL CLIMATE CENTRE

January 2008

January - March 2008

Warm late summer as La Niña continues to strengthen

Temperatures over late summer (January - March) are very likely to be above average for the time of year in the west of the North Island and the entire South Island says NIWA’s National Climate Centre. According to the centre’s seasonal climate outlook there is only a 10–20% chance of below average temperatures in all regions.

The centre says a moderate to strong La Niña is now in progress across the entire equatorial and tropical Pacific, and is expected to persist through autumn 2008. As a result more easterly and north easterly winds than normal are expected over the North Island, and lighter than normal winds over the South Island.

Because La Niña is in full swing above normal rainfall is very likely in the upper North Island. In the north and east of the North Island there is only a 10–20% chance of below normal rainfall. Conversely below normal rainfall is very likely in the west, south and east of the South Island.

Soil moisture levels are below normal in the south west of the North Island and the east coast and inland areas of the South Island including Southland. Below normal soil moisture conditions and stream flows are likely to continue in the west of both islands, and the south and east of the South Island. The centre’s 3-month outlook cannot forecast the precise timing of sufficient rain to replenish soil moisture deficits. It is possible to get “normal” rain over a 3 month period but still have very dry soils because it takes 20–30 millimetres of rain over several days to soak into the soil and revive pasture.

For the tropical cyclone season (through to May 2008), there is a normal risk of an ex-tropical cyclone passing within 500 km of the country. Should an ex-tropical cyclone approach New Zealand, the regions most at risk are the north and northeast of the North Island.

[Reporters please note: Probabilities are assigned in THREE categories; above average, average, and below average. See end for more explanation.]

Overall Picture

Temperature:
Air temperatures are likely to be above average in all regions. Sea surface temperatures are expected to be above average around the entire country.

Rainfall, soil moisture, and stream flows:
Rainfall is expected to be above normal in the north of the North Island, below normal in the west, south and east of the South Island, and near normal elsewhere. Soil moisture and stream flows are likely to grade from above normal in northern New Zealand to below normal in the west of the North Island, and west, south and east of the South Island.

Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:
Above average temperatures are likely with above normal rainfall very likely. Soil moisture and stream flows are likely to be above normal.

Central North Island, Taranaki, Wanganui, Manawatu and Wellington:
Above average temperatures are very likely. Normal rainfall, with below normal soil moisture is likely. Below normal stream flows are very likely.

Gisborne, Hawkes Bay, Wairarapa:
Above average seasonal temperatures are likely. Normal rainfall, soil moisture levels and stream flows are likely overall.

Nelson, Marlborough, Buller:
Above average temperatures are very likely. Rainfall, soil moisture levels and stream flows are likely to be normal.

West Coast, Alps and Foothills, Inland Otago, Southland:
Above average temperatures are very likely. Below normal rainfall soil moisture and stream flows are very likely.

Coastal Canterbury, East Otago:
Above average temperatures are very likely. Below normal rainfall, soil moisture and stream flows are very likely.

Background

Climate and Oceans:
Over the January to March 2008 late summer period, mean sea level pressures are expected to be higher than normal over and south east of the South Island, and lower than normal to the north, with a tendency towards easterly or north easterly winds onto the North Island, and lighter than normal winds across the South Island.

During December La Niña conditions strengthened into a moderate – strong event, and La Niña conditions spread into the western tropical Pacific. All climate forecasting models indicate these conditions continuing into autumn 2008. The chances of El Niño re-emerging before autumn are at or below 10%.

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, 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%
 Average: 30%
 Below average: 10%
We therefore conclude 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.

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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