Seasonal Climate Outlook: December 2012 - February 2013

Normal to dry summer for the north & east of North Island.

The equatorial Pacific Ocean remains warmer than normal, especially around and west of the Dateline, but the atmospheric conditions continue to be near neutral.  Global guidance indicates that continuing neutral conditions (neither El Niño nor La Niña) are the most likely outcome over the next three months (December-February).  In the New Zealand region, lower than normal pressures are expected southeast of the Chatham Islands, with enhanced south-westerly winds over New Zealand. 

Summer rainfall is likely to be near normal or below normal in the north and east of the North Island, and near normal elsewhere.  Summer soil moisture levels and river flows are likely to be below normal in the north and east of the North Island, near normal for the southwest of the North Island and Nelson-Marlborough, and near normal or below normal over the remainder of the South Island. 

Summer temperatures are likely to be near average or below average in western areas of both Islands, as well as the east of the South Island, and near average elsewhere. Sea surface temperatures around New Zealand are currently about 1 degree colder than normal, and are expected to continue below normal over the summer period.

For this tropical cyclone season (November – April), the risk of an ex-Tropical Cyclone approaching New Zealand is expected to be near normal.  On average, one ex-Tropical Cyclone nears New Zealand during the season. 

Overall picture

Rainfall, soil moisture and river flows

Summer rainfall is likely to be near normal or below normal in the north and east of the North Island, and near normal elsewhere. 

Summer soil moisture levels and river flows are likely to be below normal in the north and east of the North Island, near normal for the southwest of the North Island and Nelson-Marlborough, and near normal or below normal over the remainder of the South Island. 

Temperature

Summer temperatures are likely to be near average or below average in western areas of both Islands, as well as the east of the South Island, and near average elsewhere. Sea surface temperatures around New Zealand are currently about 1 degree colder than normal, and are expected to continue below normal over the summer period.

Regional predictions for the next three months

Northland, Auckland, Waikato, Bay of Plenty

Summer temperatures are likely to be in the near average range.  Seasonal rainfall totals are likely to be near normal or below normal, while summer soil moisture levels and river flows are projected to be below normal for this time of 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

15

20

10

10

Near average

50

40

40

40

Below average

35

40

50

50

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

Central North Island, Taranaki, Wanganui, Manawatu, Wellington

Summer temperatures are equally likely to be near average or below average.  Rainfall, soil moisture levels and river flows are all likely to be in the near normal range for the season as a whole.

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

10

20

10

10

Near average

45

50

50

50

Below average

45

30

40

40

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

Gisborne, Hawke’s Bay, Wairarapa

Seasonal temperatures are likely to be in the near average range.  Summer rainfall totals are likely to be near normal or below normal, while soil moisture levels and river flows are projected to be below normal for this time of 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

15

20

10

10

Near average

50

40

40

40

Below average

35

40

50

50

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

Nelson, Marlborough, Buller

Summer temperatures are likely to be near average.  Seasonal rainfall totals, soil moisture levels, and river flows are also all expected to be in the near 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

15

20

10

10

Near average

50

50

50

50

Below average

35

30

40

40

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

West Coast, Alps and foothills, inland Otago, Southland

Summer temperatures are likely to be near average or below average.  Seasonal rainfall totals are likely to be in the near normal range, while summer 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

10

25

20

20

Near average

45

45

40

40

Below average

45

30

40

40

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

Coastal Canterbury, east Otago

Summer temperatures are equally likely to be near average or below average.  Seasonal rainfall is projected to be near normal, while summer 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

20

25

20

20

Near average

40

45

40

40

Below average

40

30

40

40

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

Background

The equatorial Pacific Ocean remains warmer than normal, especially around and west of the Dateline, which is not a location usually associated with El Niño development.  Other El Niño indicators such as low level winds in the tropical Pacific, upper ocean heat content, and convection in the Indonesian region, are all near normal.  The Southern Oscillation Index for November was slightly positive, reflecting the fact that the atmospheric conditions remain close to neutral. 

International guidance indicates that the tropical Pacific Ocean is likely to remain neutral over the next three months (December-February).  Only one of the 10 dynamical models monitored by NIWA forecasts El Niño SST anomalies within the forecast period (down from 3 last month and 5 the one before).

For comment, please contact

Georgina Griffiths, Climate Scientist

Tel (09) 375 4506, Mobile (027) 2936545

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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