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Seasonal Climate Outlook:June–August 2008

29 May 2008

Seasonal Climate Outlook: June–August 2008

La Niña over, but little sign of reprieve for hydro catchments

The La Niña weather pattern which has contributed to low hydro lake levels is over, but there is little prospect that rain will fill the lakes over winter, based on analysis by NIWA’s National Climate Centre.

The centre’s seasonal climate outlook for winter (June – August) says normal rainfall is likely in most areas. For the west and south of the South Island, NIWA climate scientists put the probability of normal precipitation [rain & snow combined] at 50%, with a 30% probability of below normal precipitation, and a 20% probability of above normal precipitation.

Despite this, normal (50% probability) or below normal (40% probability) river flows are likely continue in Westland, Fiordland, and Alpine areas of the South Island and Southland. This is because of persisting drier than normal conditions. Winter is also the time of minimum seasonal inflows in the South Island catchments because precipitation that now falls in the higher areas of the river catchments is in the form of snow rather than rain, and therefore unavailable for flow into the lakes and rivers.

Overall Picture
Temperature:
Air temperatures are likely to be above average in the North Island, average or above average in the north of the South Island and near average over the remainder of the South Island. Despite the overall temperature expectation, cold outbreaks typical of winter will nevertheless occur from time to time. Sea surface temperatures around New Zealand are expected to remain above normal.

Rainfall, soil moisture, and stream flows:
Rainfall is likely to be normal or above normal in the north and east of the North Island, and normal in the west of the North Island and most of the South Island, except for the east of the South Island where rainfall is likely to be normal or below normal. Soil moisture levels and stream flows are expected to be normal or above normal in the north of the North Island, but normal or below normal in the southwest of the North Island. Stream flows are also expected to be normal or above normal in the east of the North Island. Normal moisture levels and stream flows are expected in other regions.

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

Regional predictions for the next three months:
Northland, Auckland, Waikato, Bay of Plenty:
Above average temperatures are likely. Rainfall, soil moisture and stream flows are likely to normal or above normal for the season as a whole.

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

Gisborne, Hawkes Bay, Wairarapa:
Above average seasonal temperatures are likely. Normal or above normal rainfall and stream flows are likely, with soil moisture levels likely to be normal.

Nelson, Marlborough, Buller:
Average or above average temperatures are likely. Normal rainfall, soil moisture and river flows are likely.

West Coast, Alps and Foothills, Inland Otago, Southland:
Average temperatures are likely with normal rainfall. Normal soil moisture and stream flows are likely.

Coastal Canterbury, East Otago:
Average temperatures are likely. Normal or below normal rainfall is likely, with normal soil moisture and stream flows likely.

Background
The effect of El Niño & La Niña on hydro lakes
During El Niño conditions, more westerly and southwesterly wind than usual occurs over the country, typically bringing rain into western and southern parts of the country. Conversely, La Niña conditions, which prevailed over the summer of 2007–2008, bring more northeasterly winds than usual. These bring moisture into the north and east of the North Island, but relatively little rain into the main hydroelectric power catchments.

Summer inflows (December–February) as MegaWatts (MW):

median
El Niño 3229
Neutral 3394
La Niña 2830

The summer (December–February) inflow for 2007–2008 was 2756 MW. This was close to the median for La Niña summers (2830 MW).

Climate and Oceans:
In the New Zealand region, mean sea level pressures are expected to remain higher than normal to the south of the South Island and lower than normal to the northwest of New Zealand, with more winds from the northeast than normal over the country.

La Niña has effectively ended, with conditions near neutral in the tropical Pacific. At the ocean surface, below normal temperature anomalies have disappeared across much of the Equatorial Pacific. The Southern Oscillation Index is –0.3 and continuing to fall. Most climate forecasting models indicate conditions in the neutral range during June to August.

© Copyright NIWA 2008. 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, 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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