Seasonal Climate Outlook March - May 10

NIWA NATIONAL CLIMATE CENTRE 26 February 2010

SEASONAL CLIMATE OUTLOOK: March – May 2010

El Niño to exit by the end of Autumn

The current significant El Niño continues in the equatorial Pacific, but is likely to weaken towards neutral conditions by the end of autumn. The NIWA National Climate Centre says that means temperatures are likely to be near the long-term average in many areas, with near-normal rainfalls likely in most places.

The centre’s latest outlook, for autumn 2010, states that mean sea level pressures are likely to be higher than normal to the north of the country, associated with slightly stronger than normal westerlies over New Zealand on average for March-May.

Current dry soil conditions are likely to continue in the north of the North Island and the east of the South Island, where below normal stream flows and soil moisture levels are likely through to May.

The centre says autumn rainfall totals are likely to be in the normal range in the North Island and the north of the South Island, normal or above normal in the western South Island, and normal or below normal in Canterbury and Otago.

Temperatures are likely to be near average in the North Island and in the average or above average range in the South Island, over the three months of March to May as a whole.

Below normal soil moisture levels and streamflows are likely in the northern North Island and in the east of the South Island. Near normal or below normal soil moisture levels and streamflows are likely in the rest of the North Island and the north of the South Island, and normal or above normal conditions are likely in the west and south of the South Island.

Overall Picture

Temperature:

Temperatures are expected to be near average in the North Island, but average or above average in the South Island. Sea surface temperatures are expected to near average or above average around the coasts.

Rainfall, soil moisture, and stream flows:

Rainfall is likely to be near normal in the North Island and northern South Island, normal or above normal in the western South Island, and normal or below normal in the eastern South Island. Below normal soil moisture levels and streamflows are likely in the northern North Island and in the east of the South Island. Near normal or below normal soil moisture levels and streamflows are likely in the rest of the North Island and the north of the South Island, and normal or above normal conditions are likely in the west and south of the South Island.

Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:

Temperatures are likely to be in the near average category. Seasonal rainfall totals are likely to be near normal, while river flows and soil moisture levels are likely to be in the 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 Stream flows
Above average 30% 20% 20% 20%
Near average 50% 50% 30% 30%
Below average 20% 30% 50% 50%

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

Seasonal temperatures are likely to be in the average category. Rainfall totals are likely to be near normal, while stream flows and soil moisture levels are likely to be near normal or below normal, 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 Stream flows
Above average 30% 25% 20% 20%
Near average 50% 50% 40% 40%
Below average 20% 25% 40% 40%

Gisborne, Hawke’s Bay, Wairarapa:

Temperatures averaged over the three months are likely to be in the near average category. Seasonal rainfall totals, stream flows and soil moisture levels 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 Stream flows
Above average 30% 20% 20% 20%
Near average 50% 50% 50% 50%
Below average 20% 30% 30% 30%

Nelson, Marlborough, Buller:

Seasonal temperatures are equally likely to be in the near average or above average range. Rainfalls are likely to be near normal over all, while stream flows and soil moisture levels are equally likely to be near normal or below normal.

Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

Temperature Rainfall Soil moisture Stream flows
Above average 40% 25% 20% 20%
Near average 40% 50% 40% 40%
Below average 20% 25% 40% 40%

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

Temperatures are equally likely to be in the average or above average category. Seasonal rainfall, stream flows and soil moisture levels are equally likely to be near normal or above normal.

Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

Temperature Rainfall Soil moisture Stream flows
Above average 40% 40% 40% 40%
Near average 40% 40% 40% 40%
Below average 20% 20% 20% 20%

Coastal Canterbury, East Otago:

Temperatures are equally likely to be in the near average or above average category, on the whole during March-May. Seasonal rainfall totals are all likely to be in the normal or the below normal range, while soil moisture levels and stream flows are 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 Stream flows
Above average 40% 20% 20% 20%
Near average 40% 40% 30% 30%
Below average 20% 40% 50% 50%

Background

A significant El Niño continues in the Tropical Pacific, and appears to have matured. As usually occurs, the El Niño is expected to weaken towards neutral conditions through the autumn.

El Niño events can lead to dry conditions in parts of New Zealand over the autumn season. Soil moistures are very low in Northland, and in some parts of eastern south Canterbury and Otago. The latest outlook suggests that below normal stream flows and soil moisture levels are likely in the northern North Island and the east of the South Island on average through to May.

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© Copyright NIWA 2009. All rights reserved. Acknowledgement of NIWA as the source is required.

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 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%
• 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.