Seasonal Climate Outlook: October - December 2012

Borderline or weak El Niño forecast for late spring.

The Pacific Ocean is close to El Niño thresholds, but the atmosphere has yet to show patterns typical of El Niño. The seasonal forecast models indicate that the conditions are likely to remain close to these thresholds, or transition towards a weak and short-lived El Niño event over the next three months. For New Zealand, it is not expected to result in the enhanced southwesterly and westerly airflow that is usually associated with El Niño events. Over the coming season (October-December), higher than normal pressures are expected south of Australia and New Zealand, and lower pressures are forecast to the north of the North Island, resulting in a weaker than normal westerly flow over the country.

Air temperatures are likely to be near average or above average in all regions. However as is typically the case in Spring, cold snaps may still occur from time to time. Sea surface temperatures are expected to be warmer than normal around the South Island, and cooler than normal to the east and north of the North Island.

Rainfall is likely to be above normal or near normal in the north and west of the North Island, as well as in Nelson-Marlborough, and near normal in the rest of the country. Soil moisture and river flows are forecast to be above normal in the southwest of the North Island, near normal or above normal in the north of the North Island and in Nelson-Marlborough, and near normal elsewhere.

Overall picture

Temperature

Air temperatures are likely to be near average or above average in all regions for the season as a whole. Sea surface temperatures are expected to be warmer than normal around the South Island, and cooler than normal to the east and north of the North Island.

Rainfall, soil moisture and river flows

October to December rainfall is likely to be above normal or near normal in the north and west of the North Island as well as in Nelson-Marlborough, and near normal elsewhere. Seasonal soil moisture levels and river flows are expected to be above normal in the southwest of the North Island, near normal or above normal in the north of the North Island and in Nelson-Marlborough, and near normal elsewhere.

Regional predictions for the next three months

Northland, Auckland, Waikato, Bay of Plenty

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

40

40

40

Near average

40

40

40

40

Below average

25

20

20

20

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 or above average. Near normal or above normal late spring rainfall is expected, while soil moisture levels and river flows are likely to be above 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

40

45

45

Near average

40

40

35

35

Below average

25

20

20

20

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

 Gisborne, Hawke’s Bay, Wairarapa

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

30

30

30

Near average

35

40

40

40

Below average

25

30

30

30

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

Nelson, Marlborough, Buller

October to December temperatures are likely to be near average or above average. Seasonal rainfall totals, soil moisture levels, and river flows are all expected to be near normal or above 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

40

40

35

Near average

35

40

35

40

Below average

25

20

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 likely to be in the near average or above average range. Late spring rainfall, soil moisture and river flows are likely to be close to normal values for this time of the year.

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

30

30

Near average

35

45

40

40

Below average

25

35

30

30

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

Coastal Canterbury, east Otago

October to December temperatures are 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

20

20

20

Near average

35

45

45

45

Below average

25

35

35

35

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

Background

Sea surface temperatures in the central equatorial Pacific are still close to the El Niño threshold, but the atmospheric components of the phenomenon are not in place and show signals atypical of El Niño.  Other international forecasting institutions indicate that borderline El Niño are present and the climate models that NIWA monitors predict these conditions to either persist over the late spring and summer season, or transition towards a weak and probably short-lived El Niño. The Southern Oscillation is currently weakly positive (+0.4).

For comment, please contact

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

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

Dr. Nicolas Fauchereau, Climate Scientist

Tel (09) 375 4553, Mobile (021) 177 5320

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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