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Seasonal Climate Outlook: September - November 2011

31 August 2011

Seasonal Climate Outlook: September - November 2011

COOL START, MILD AND DRY LATER

The NIWA National Climate Centre’s spring outlook for New Zealand, for September – November as a whole, indicates that temperatures are likely to be above average in the North Island as well as for the northern South Island, and near average or above average in other regions. Cool spells typical of spring will occur from time to time through the period, especially during September. Seasonal rainfall, soil moistures, and river flows are all likely to be normal or below normal for the North Island as well as the western South Island, and near normal in other regions.

The tropical Pacific is currently in the neutral range (neither La Niña nor El Niño), and is expected to remain neutral through the spring, according to the NIWA National Climate Centre.

The outlook states that mean sea level pressures are likely to be above normal across much of New Zealand, with weaker westerly circulation than normal over the country, for the season as a whole.

Overall Picture

Temperature:
For the September - November period as a whole, air temperatures are likely to be above average in the North Island as well as for the northern South Island, and near average or above average in other regions. Cool spells typical of spring will occur from time to time through the period, especially during September. Sea surface temperatures near New Zealand are expected to be close to normal or slightly below normal through the period.

Rainfall, soil moisture, and river flows:
The National Climate Centre says that spring rainfall is likely to be normal or below normal for all of the North Island as well as the west of the South Island, and near normal in other regions. Soil moisture levels and river flows are also likely to be normal or below normal in all of the North Island regions and the west of the South Island, and near normal in other regions.

Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:
Temperatures are likely to be above average for the time of year. Spring rainfall totals are equally likely to be in the normal or below normal range, as are soil moisture levels and river flows.
Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

TemperatureRainfallSoil moistureRiver flows
Above average50%20%20%20%
Near average30%40%40%40%
Below average20%40%40%40%

Central North Island, Taranaki, Wanganui, Manawatu and Wellington:
Spring temperatures are likely to be above average. Rainfall totals, soil moisture levels and river flows for September – November as a whole are equally likely to be in the normal or below normal range.
Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

TemperatureRainfallSoil moistureRiver flows
Above average50%20%20%15%
Near average30%40%40%45%
Below average20%40%40%40%

Gisborne, Hawke’s Bay, Wairarapa:
Temperatures are likely to be above average for the time of year. Spring rainfall totals are equally likely to be in the normal or below normal range, as are soil moisture levels and river flows.
Probabilities are assigned in three categories; above normal, near normal, and below normal. The full probability breakdown is:

TemperatureRainfallSoil moistureRiver flows
Above average50%20%20%20%
Near average30%40%40%40%
Below average20%40%40%40%

Nelson, Marlborough, Buller:
Temperatures over the September - November period are likely to be above average. Rainfall totals, soil moisture levels and river flows for spring as a whole 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:

TemperatureRainfallSoil moistureRiver flows
Above average50%25%25%20%
Near average30%50%50%50%
Below average20%25%25%30%

West Coast, Alps and Foothills, Inland Otago, Southland:
Temperatures over the September - November period are equally likely to be above average or near average. Rainfall totals for spring as a whole are equally likely to be in the normal or below normal range, as are soil moisture levels and river flows.
Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

TemperatureRainfallSoil moistureRiver flows
Above average40%20%20%15%
Near average40%40%40%45%
Below average20%40%40%40%

Coastal Canterbury, East Otago:
Spring temperatures are equally likely to be above average or near average. Rainfall totals, soil moisture levels and river flows for spring as a whole 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:

TemperatureRainfallSoil moistureRiver flows
Above average40%25%20%20%
Near average40%50%50%45%
Below average20%25%30%35%

Background

The tropical Pacific remains in an ENSO-neutral state, with most signs of the preceding La Niña event having dissipated. However, recent developments suggest a possible resurgence of La Niña conditions, at least temporarily. However, global models which predict ENSO (El Niño-Southern Oscillation) are all indicating ENSO-neutral conditions for spring as a whole.

Sea temperatures in the Tasman Sea are now slightly below average, although the oceans to the northeast of the North Island continue relatively warm for the time of year.

Notes

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.

ENDS

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