Seasonal Climate Outlook: August – October 2011

NIWA NATIONAL CLIMATE CENTRE / 01 August 2011

Seasonal Climate Outlook: August – October 2011

Mild conditions with near average rainfall likely in most places

The tropical Pacific are now in the neutral range (neither La Niña nor El Niño), and is expected to remain neutral over at least the next season, according to the NIWA National Climate Centre.

The Centre’s latest outlook for New Zealand, for early spring (August to October), indicates that temperatures are likely to be near average or above average in all regions, except for the east of the South Island where near average temperatures are likely. Cold snaps typical of winter will occur from time to time through the period.

Seasonal rainfall is likely to be normal or below normal in the east of the South Island, and near normal in all other regions. Soil moisture levels are likely to be below normal in the eastern South Island, and near normal in all other regions. River flows are likely to be below normal in eastern South Island, near normal or below normal in the north of the South Island, and normal in all other regions.

The outlook states that mean sea level pressures are likely to be above normal to the south and southeast of New Zealand, with weaker westerlies over the country, for the season as a whole. However, the month of August is expected to be rather different, with a continuation of the recent disturbed south-westerly flow.

Overall Picture

Temperature:

For the August-September-October period as a whole, air temperatures are likely to be near average or above average in all regions, except for the east of the South Island where near average temperatures are likely. Cold snaps typical of winter will occur from time to time through the period. Sea surface temperatures near New Zealand are expected to be near normal or above normal through the outlook period.

Rainfall, soil moisture, and river flows:

The National Climate Centre says that seasonal rainfall and soil moisture levels are likely to be near normal in all regions of the country, except for the east of the South Island where normal or below rainfall and soil moisture is likely. River flows are likely to be below normal in eastern South Island, near normal or below normal in the north of the South Island, and normal in all other regions.

Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:

Temperatures are equally likely to be near average or above average for the time of year. Rainfall totals, soil moisture levels, and river flows over August-October 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:

/ Temperature / Rainfall / Soil moisture / River flows

Above average / 40% / 25% / 30% / 25%

Near average / 40% / 50% / 45% / 45%

Below average / 20% / 25% / 25% / 30%

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

Temperatures are equally likely to be near average or above average for the time of year. Rainfall totals, soil moisture levels, and river flows 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:

/ Temperature / Rainfall / Soil moisture / River flows

Above average / 40% / 30% / 30% / 30%

Near average / 40% / 40% / 40% / 40%

Below average / 20% / 30% / 30% / 30%

Gisborne, Hawke’s Bay, Wairarapa:

Temperatures are equally likely to be near average or above average for the time of year. Seasonal rainfall totals, soil moisture levels, and river flows are all likely to be in the normal range.

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

/ Temperature / Rainfall / Soil moisture / River flows

Above average / 40% / 30% / 30% / 25%

Near average / 40% / 40% / 40% / 45%

Below average / 20% / 30% / 30% / 30%

Nelson, Marlborough, Buller:

Temperatures over the August-October period are equally likely to be near average or above average, with temperatures very unlikely to be in the below average range. Rainfall totals and soil moisture levels are likely to be in the normal range, whereas river flows are likely to be either in the near normal or below normal ranges.

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

/ Temperature / Rainfall / Soil moisture / River flows

Above average / 45% / 20% / 20% / 25%

Near average / 45% / 45% / 45% / 40%

Below average / 10% / 35% / 35% / 35%

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

Temperatures are likely to be near average or above average. Rainfall totals, soil moisture levels, and river flows 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:

/ Temperature / Rainfall / Soil moisture / River flows

Above average / 40% / 20% / 25% / 25%

Near average / 40% / 45% / 45% / 45%

Below average / 20% / 35% / 30% / 30%

Coastal Canterbury, East Otago:

Temperatures are likely to be near average for the time of year, over the season as a whole. Seasonal rainfall totals are likely to be in the below normal or normal ranges, whereas soil moisture levels and river flows 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 / River flows

Above average / 30% / 25% / 20% / 20%

Near average / 50% / 35% / 35% / 35%

Below average / 20% / 40% / 45% / 45%

Background

The tropical Pacific is now in a neutral state, with the previously strong La Niña event having dissipated in May. In the longer term through spring of 2011, a continuation of the neutral state is considered the most likely eventuality. Sea temperatures in the near-surface waters of the tropical Pacific Ocean were up to 2 degrees Celsius below average during the peak of the La Niña event, but these temperatures have increased and now are slightly above average. However, this warming trend has abated somewhat during July. Thus, which continuing neutral conditions are the most likely, we cannot rule out the possibility of either an El Niño development, or a return to La Niña conditions, by the end of 2011.

Sea temperatures around New Zealand are still about half a degree above average for this time of year, and have an influence on the seasonal forecast of average or above average air temperatures. With neutral conditions prevailing in the tropical Pacific, there is little else forcing New Zealand climate to differ from near normal: this is reflected in many regions having forecasts of rainfall and soil moisture in the near normal range. The similar tercile probabilities for above average, average and below average (e.g., 30:40:30) also indicate relatively low confidence in the rainfall and soil moisture outlooks for some regions.

ENDS

© Copyright NIWA 2011. 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, 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.

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