Seasonal Climate Outlook September - November 2009

A windy spring ahead with drier than normal conditions in many regions.

El Niño conditions now present in the equatorial Pacific are expected to result in more frequent south-westerly winds over New Zealand in the coming spring season, according to the latest outlook from NIWA’s National Climate Centre.

A warm August has followed two cold months at the start of winter. While September is also expected to be warmer than average, a change is likely later in the season. The centre says temperatures for the coming 3-month period as a whole (September, October, and November combined) are likely to be in the average range over most of the country, except for the north and east of the North Island where temperatures are equally likely to be in the near average or below average ranges.

Rainfall is likely to be in the near normal or below normal ranges in all regions, except in the south and west of the South Island where near normal rainfall is the most likely outcome.

Going along with the rainfall expectation, the centre says that near normal or below normal soil moisture levels and streamflows are likely everywhere apart from the west & south of the South Island where soil moisture levels and streamflows are likely to be in the near normal or above normal ranges.

 

Overall Picture

Temperature:

Air temperatures are likely to be in the near average or below average ranges in the north and east of the North Island, but near average in other regions. Sea surface temperatures near New Zealand are expected to be somewhat below average through September-November, especially east of the country.

Rainfall, soil moisture, and stream flows:

Rainfall, soil moisture levels and streamflows are likely to be in the near normal or below normal ranges in all regions, except in the south and west of the South Island where rainfall is likely to be near normal, and soil moisture levels and streamflows are likely to be in the near normal or above normal ranges.

Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:

Temperatures are equally likely to be in the near average or below average categories. Three-month rainfall totals are equally likely to be in the near normal or below normal ranges, as are soil moisture levels and streamflows over the season.

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 20% 20% 20% 20%
Near average 40% 40% 40% 40%
Below average 40% 40% 40% 40%

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

Temperatures are most likely to be in the near average category. Three-month rainfall totals and streamflows are equally likely to be in the near normal or below normal ranges. Soil moisture levels are likely to be in the near average category.

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 20% 20% 20% 20%
Near average 50% 40% 45% 40%
Below average 30% 40% 35% 40%

Gisborne, Hawke’s Bay, Wairarapa:

Temperatures are equally likely to be in the near average or below average categories. Three-month rainfall totals, soil moisture levels and streamflows are all likely to be in the near normal or below normal ranges.

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 20% 20% 20% 20%
Near average 40% 40% 40% 40%
Below average 40% 40% 40% 40%

Nelson, Marlborough, Buller:

Temperatures are most likely to be in the near average category. Rainfall, soil moisture levels and streamflows are likely to be in the below normal or near normal categories; it is very unlikely that any of these will be in the above normal range over the season 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 25% 15% 15% 10%
Near average 50% 40% 40% 40%
Below average 25% 45% 45% 50%

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

Temperatures are most likely to be in the near average category. Rainfall is most likely to be in the near normal range. Soil moisture levels and streamflows are likely to be in the near normal or above 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 20% 30% 40% 40%
Near average 50% 50% 40% 40%
Below average 30% 20% 20% 20%

Coastal Canterbury, East Otago:

Temperatures are most likely to be in the near average category. Three-month rainfall totals are equally likely to be in the near normal or below normal ranges. Soil moisture levels and streamflows are likely to be in the below normal range, with above normal values considered very unlikely.

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 20% 20% 10% 10%
Near average 50% 40% 40% 40%
Below average 30% 40% 50% 50%

Background

A weak El Niño is present in the equatorial Pacific Ocean, and is predicted to persist through to the autumn of 2010. However, at this stage the atmosphere in the tropical Pacific still displays only weak El Niño conditions. Warmer than average conditions are seen in the top 100 metres throughout the equatorial Pacific Ocean, but temperatures appear to have stabilised with little change between July and August anomalies. Nevertheless, El Niño forecast models suggest some further strengthening is likely as spring progresses.

Forecasts of circulation trends in the New Zealand region show a shift to more frequent south-westerlies in the spring, as might be expected with a developing El Niño. Drier than normal conditions are suggested in many regions, indicating a heightened risk of droughts developing in parts of northern and eastern regions of both Islands over late spring and summer.

For comment, please contact:

Dr James Renwick, NIWA Principal Scientist, Climate Variability & Change

Tel (04) 386 0343 (office DDI), mobile (021) 178 5550

Dr Brett Mullan, NIWA Principal Scientist, Climate Variability & Change

Tel (04) 386 0508 (office DDI)

© Copyright NIWA 2009. 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
  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.