Seasonal Climate Outlook: March - May 2013

Mild autumn on the cards.

The equatorial Pacific Ocean remains in a neutral state (neither El Niño nor La Niña), and neutral conditions are very likely to continue over the next three months (March-May).  Lower pressures than usual are likely over the north Tasman Sea during autumn, with slightly higher pressures than usual over and to the south of New Zealand.  Autumn temperatures are likely to be above average across the South Island, and average to above average in the North Island. 

Rainfall totals for the March – May period as a whole are likely to be in the near normal range for most regions, except for the West Coast of the South Island, where normal to above normal rainfall is likely.  However, because of the existing soil moisture deficits across the North Island and in the eastern South Island, soil moisture levels and river flows are expected to take some time to recover in these areas. 

Soil moisture levels and river flows are projected to be normal to below normal for the autumn season as a whole for the North Island, as well as the eastern South Island, and near normal elsewhere.

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

 

Overall picture

Rainfall, soil moisture and river flows

Rainfall totals for the March – May period as a whole are likely to be in the near normal range for most regions, except for the West Coast of the South Island, where normal to above normal rainfall is likely.  However, because of the existing soil moisture deficits across the North Island and in the eastern South Island, soil moisture levels and river flows are expected to take some time to recover in these areas. 

Soil moisture levels and river flows for autumn as a whole are projected to be normal to below normal for the North Island, as well as the eastern South Island, and near seasonal normal elsewhere.

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

Temperature

Autumn temperatures are likely to be above average in the South Island, and equally likely to be in the near average or above average range in the North Island. Sea surface temperatures around New Zealand are currently above average (especially near the South Island and to the south of the country), and this is forecast to continue for the coming three months.

 

Regional predictions for the next three months

Northland, Auckland, Waikato, Bay of Plenty

Autumn temperatures are equally likely to be in the near average or above average range.  March – May rainfall totals are likely to be in the near normal range.  Seasonal soil moisture levels and river flows are likely to be in the near 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

40

30

15

20

Near average

40

50

45

40

Below average

20

20

40

40

Click these links to view the historic rainfall and temperature. 

Central North Island, Taranaki, Wanganui, Manawatu, Wellington

March- May temperatures are equally likely to be in the near average or above average range.  Seasonal rainfall totals are likely to be in the near normal range.  Seasonal soil moisture levels and river flows are projected to be in the near 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

40

30

15

20

Near average

40

50

45

40

Below average

20

20

40

40

Click these links to view the historic rainfall and temperature. 

Gisborne, Hawke’s Bay, Wairarapa

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

40

30

15

20

Near average

40

50

45

40

Below average

20

20

40

40

Click these links to view the historic rainfall and temperature. 

Nelson, Marlborough, Buller

March - May temperatures are likely to be above average.  Seasonal rainfall totals, soil moisture levels, and river flows are all likely 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

50

25

20

20

Near average

30

55

45

45

Below average

20

20

35

35

Click these links to view the historic rainfall and temperature. 

West Coast, Alps and foothills, inland Otago, Southland

Autumn temperatures are likely to be above average.  Seasonal rainfall totals are equally likely to be in the near normal or above normal range. March-May soil moisture levels and river flows are likely to be at near normal levels.

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

50

40

20

20

Near average

30

40

45

45

Below average

20

20

35

35

Click these links to view the historic rainfall and temperature. 

Coastal Canterbury, east Otago

March-May temperatures are likely to be above average.  Seasonal rainfall totals are projected to be near normal, with soil moisture levels and river flows likely to be in the below normal or 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

50

20

15

15

Near average

30

45

45

40

Below average

20

35

40

45

Click these links to view the historic rainfall and temperature. 

Background

Atmospheric indicators of ENSO, such as the Southern Oscillation Index (SOI), trade winds, and tropical cloud patterns continue at neutral levels. The estimated NIWA SOI for February was -0.6, and the 3-month December-February estimate was -0.5.  Tropical Pacific Ocean temperatures have remained steady over the last fortnight, after several months of gradual cooling.  Climate models and current observations indicate this neutral state is likely to continue well beyond the southern hemisphere autumn. 

Intense anticyclones (‘highs’) dominated in the New Zealand region over the last three months (summer), with associated widespread dryness in many regions.  In the coming season, the intensity of these higher pressures likely to lie over and to the south of New Zealand is expected to be much reduced, in comparison – and that is why model guidance indicates near normal rainfall for many regions of the country.  However, confidence in the rainfall forecasts remains low.

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 2053, Mobile (027) 9348940

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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