Seasonal Climate Outlook: August - October 2012

On the brink of El Niño. 

Conditions in the tropical Pacific are currently on the brink of El Niño, and it is likely El Niño will develop during the early spring period.  In the New Zealand region, however, lower pressures to the north of the country, and higher pressures to the south, are likely to dominate for the August-October period.  Thus, the stronger than normal spring westerlies often associated with El Nino periods are not expected to be very prominent in the coming August-October period.

Early spring air temperatures are likely to be near average or above average for all regions of the country.  Nevertheless, cold snaps, frosts and snowfalls typical of early spring may still occur from time to time.  Sea temperatures around New Zealand are likely to be near normal for the season as a whole. 

August-October rainfall totals, soil moisture levels, and river flows are all likely to be near normal or above normal in the north of the North Island, below normal for the eastern South Island, and near normal in all other regions. 

Overall picture

Temperature

Early spring air temperatures are likely to be near average or above average for all regions of the country.  Nevertheless, cold snaps, frosts and snowfalls typical of early spring may still occur from time to time.   Sea temperatures around New Zealand are likely to be near normal for the season as a whole. 

Rainfall, soil moisture and river flows

August-October rainfall is likely to be near normal or above normal in the north of the North Island, below normal for the eastern South Island, and near normal in all other regions. 

Seasonal soil moisture levels and river flows are likely to be near normal or above normal in the north of the North Island, below normal for the eastern South Island, and near normal in all other regions. 

Regional predictions for the next three months

Northland, Auckland, Waikato, Bay of Plenty

August to October temperatures are equally likely to be in the near average or above average range.  Seasonal rainfall totals, soil moisture levels, and river flows 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

River flows

Above average

40

40

35

35

Near average

40

40

40

40

Below average

20

20

25

25

Click these links to view the historic rainfall and temperature ranges for this region 

Central North Island, Taranaki, Wanganui, Manawatu, Wellington

Seasonal  temperatures are equally likely to be in the near average or above average range.  Near normal early spring rainfall, soil moisture levels and river flows are likely. 

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

20

20

Near average

40

50

50

50

Below average

20

30

30

30

Click these links to view the historic rainfall and temperature ranges for this region 

Gisborne, Hawke’s Bay, Wairarapa

Seasonal  temperatures are equally likely to be in the near average or above average range.  August-October rainfall totals, soil moisture levels and river flows are all likely to be near normal. 

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

25

25

Near average

40

50

50

50

Below average

20

25

25

25

Click these links to view the historic rainfall and temperature ranges for this region 

Nelson, Marlborough, Buller

August to October temperatures are equally likely to be in the near average or above average range.  Near normal seasonal rainfall totals, soil moisture levels, and river flows are likely.

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

25

25

Near average

40

50

50

50

Below average

20

25

25

25

Click these links to view the historic rainfall and temperature ranges for this region 

West Coast, Alps and foothills, inland Otago, Southland

Seasonal  temperatures are equally likely to be in the near average or above average range.  Early spring rainfall, soil moisture levels, and river flows are all likely to be near normal. 

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

20

20

Near average

40

50

50

50

Below average

20

30

30

30

Click these links to view the historic rainfall and temperature ranges for this region 

Coastal Canterbury, east Otago

August to October temperatures are equally likely to be in the near average or above average range.  Below normal seasonal rainfall totals, soil moisture levels, and river flows are likely.

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

20

20

Near average

40

30

35

30

Below average

20

50

45

50

Click these links to view the historic rainfall and temperature ranges for this region 

Background

Sea surface temperatures continue to warm in the equatorial Pacific. Tropical sea temperatures currently verge on the accepted El Niño threshold.  The majority of climate models which NIWA monitors predict that the El Niño threshold will likely be exceeded during the August-October period.  However, the Southern Oscillation remained close to zero in July, indicating the ocean-atmosphere feedbacks necessary for El Niño development are not yet fully in place.

For comment, please contact

Georgina Griffiths, Senior Climate Scientist

Tel (09) 375 4506, Mobile (027) 293 6545

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

Tel  (04) 386 0508, Mobile (027) 294 1169

Notes to reporters and 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 per cent
  • Near average: 30 per cent
  • Below average: 10 per cent

We therefore concluded that above average temperatures were very likely.

4. This three-way probability means that a random choice would be correct only 33 per cent (or one-third) of the time. It would be like randomly throwing a dart at a board divided into three 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 per cent 'hit rate' is substantially better than guesswork, 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 US 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 online and is sent to about 3500 recipients of NIWA's newsletters, including many farmers. See www.niwa.co.nz/our-science/climate/publications/all/cu

7. All outlooks are for the three months as a whole. There will inevitably be wet and dry days, and 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 for the long-term mean, whereas for rainfall the "near normal" category lies between approximately 80 per cent and 115 per cent 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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