Seasonal Climate Outlook: July - September 2012

Drier late winter likely for some.

Neutral conditions (neither La Niña nor El Niño) presently exist in the tropical Pacific, but an El Niño is likely by spring if present sea temperature trends continue. Over the July to September period, circulation in the New Zealand region is likely to show a transition from the present north-easterly anomalies of the recent La Nina towards a westerly flow anomaly at the end of the season as El Niño conditions settle in. 

Rainfall in late winter to early spring is likely to be near normal in the north and west of the North Island, and normal or below normal in all other regions. Soil moisture levels are likely to be normal or below normal in the north and east of the South Island, and near normal elsewhere. River flows are likely to be below normal in eastern South Island, normal or below normal for Nelson/Marlborough, and near normal in other regions of New Zealand.

Sea temperatures around New Zealand are likely to be near normal for the season as a whole.  Late winter temperatures are likely to be near average overall for all regions except the west and south of the South Island, where average or above average temperatures are likely. Frosts and snowfalls typical of winter and early spring will occur from time to time.

Overall picture

Temperature

Seasonal temperatures are likely to be near average overall, except for the west and south of the South Island, where average or above average temperatures are likely. Frosts and snowfalls typical of winter will occur from time to time.

Rainfall, soil moisture and river flows

Late winter rainfall totals are likely to be normal or below normal in South Island regions and in the east of the North Island, but near normal in the north and west of the North Island. Soil moisture levels are likely to be normal or below normal in the north and east of the South Island, and near normal elsewhere. River flows are likely to be below normal in eastern South Island, normal or below normal for Nelson/Marlborough, and near normal in other regions of New Zealand.

Regional predictions for the next three months

Northland, Auckland, Waikato, Bay of Plenty

July to September temperatures are likely to be near average overall.  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 average, near average, and below average. The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

35

25

25

25

Near average

50

50

45

45

Below average

15

25

30

30

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

Central North Island, Taranaki, Wanganui, Manawatu, Wellington

Seasonal temperatures are likely to be near average.  Late winter rainfall totals, soil moisture levels and river flows are 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

35

25

25

20

Near average

50

50

45

50

Below average

15

25

30

30

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

Gisborne, Hawke’s Bay, Wairarapa

Late winter temperatures are likely to be near average overall, and seasonal soil moisture levels and river flows near normal for the time of year. Seasonal rainfall totals are 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

River flows

Above average

35

25

25

20

Near average

50

40

45

45

Below average

15

35

30

35

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

Nelson, Marlborough, Buller

July to September temperatures are likely to be near average overall.  Seasonal rainfall totals, soil moisture levels and river flows 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:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

35

20

20

20

Near average

50

40

40

40

Below average

15

40

40

40

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 near average or above average.  Near normal or below normal rainfall totals are likely, while soil moisture levels and river flows are likely to be near normal for the season overall.

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

25

20

Near average

45

40

45

45

Below average

10

40

30

35

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

Coastal Canterbury, east Otago

July to September temperatures are likely to be near average overall.  Seasonal rainfall totals and soil moisture levels are equally likely to be in the near normal or below normal range.  Late winter river 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

River flows

Above average

35

20

20

20

Near average

50

40

40

35

Below average

15

40

40

45

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. The sea temperatures do not yet exceed the accepted El Niño thresholds, and thus the Pacific is still ENSO-neutral (neither La Niña nor El Niño). However, if temperatures continue to rise at a similar rate, the El Niño threshold will be exceeded by the end of winter.

The Southern Oscillation is in its third consecutive month of negative values, and has dropped from   -0.3 in May to -1.2 in June.  However, patterns of enhanced convection and cloudiness in the tropical Pacific still show the remnants of a La Niña signature, indicating the ocean-atmosphere feedbacks necessary for El Niño development are not yet in place. Most of the global climate models that NIWA monitors indicate conditions are likely to approach or exceed El Niño thresholds by spring. 

For comment, please contact

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

Tel  (04) 386 0508 (office DDI), (027) 294 1169 (mobile)

Georgina Griffiths, Senior Climate Scientist

Mobile (027) 293 6545

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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