Seasonal Climate Outlook: December 2013 - February 2014

The equatorial Pacific Ocean continues in a neutral state (neither El Niño nor La Niña) in November 2013.

International guidance indicates that ENSO-neutral conditions are very likely to persist for the next three months (December 2013 – February 2014). These ENSO-Neutral conditions are forecast to continue throughout the summer and early autumn, but El Niño development becomes increasingly likely as we approach winter, reaching approximately 50/50 percent chance along with ENSO neutral from May-July 2014.

For the coming three months as a whole in the New Zealand region, lower pressures than normal are forecast to the north of the country in the western Pacific, while anticyclonic conditions are forecast over the south and to the east of the New Zealand. This circulation pattern is expected to be associated with slightly less westerly and southwesterly airflow than usual over the summer.

Sea surface temperatures are expected to remain above average around the whole of New Zealand for the coming three months.

Outlook summary

Temperatures over the December–February period as a whole are equally likely (40% chance) to be near average or above average in the north and east of the North Island and the east of the South Island, while above average temperatures are the most likely outcome (45 to 50 % chance) for the remaining regions of the New Zealand.

Rainfall totals over the December–February period as a whole are most likely (45% chance) to be near normal in all regions except for the north of the North Island, where rainfall is equally likely (40% chance) to be near normal or above normal.

Soil moisture levels are most likely (45 %) to be near normal in all regions but for the east of the South Island, where normal and below normal soil moisture levels are equally likely (40 %). River flows for December 2013 – February 2014 are forecast to be near normal (35 to 40 %) or below normal (35 %) in the north and west of the North Island, as well as in the north and west of the South Island. Normal river flows are most likely (40 %) in the east of the North Island, while below normal river flows are most likely in the east of the South Island (45 % chance). On the basis of this forecast, coastal Canterbury and east Otago are most likely to experience drier than normal conditions over the summer.

Regional predictions for the December to February season

Northland, Auckland, Waikato, Bay of Plenty

The table below shows the probabilities (or percent chances) for each of three categories: above average, near average, and below average. In the absence of any forecast guidance there would be an equal likelihood (33% chance) of the outcome being in any one of the three categories. Forecast information from local and global guidance models is used to indicate the deviation from equal chance expected for the coming three month period, with the following outcomes the most likely (but not certain) for this region:

  • Temperatures and rainfall totals are equally likely (40% chance) to be in the near average or above average range.
  • Soil moisture levels are most likely (45%) to be in the near normal range.
  • River flows are equally likely (35%) to be in the below normal or near normal range. Note that this is not very different from the probability of above normal river flows (30 %), meaning that the uncertainty in the forecast is high for the December – February period as a whole.

Other outcomes cannot be excluded. The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

40

40

25

30

Near average

40

40

45

35

Below average

20

20

30

35

Click here to view the historic rainfall and temperature for this region

Central North Island, Taranaki, Wanganui, Manawatu, Wellington

Probabilities are assigned in three categories: above average, near average, and below average.

  • Temperatures are most likely (45% chance) to be in the above average range. 
  • Rainfall totals and soil moisture levels are most likely (45% chance) to be in the normal range.
  • River flows are almost equally likely to be in the near normal or below normal range (40% and 35% respectively). 

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

45

20

20

25

Near average

35

45

45

40

Below average

20

35

35

35

Click here to view the historic rainfall and temperature for this region

Gisborne, Hawke’s Bay, Wairarapa

Probabilities are assigned in three categories: above average, near average, and below average.

  • Temperatures are equally likely (40% chance) to be in the near average or above average range.
  • Rainfall totals, soil moisture levels and river flows are most likely (40 -45% chance) to be in the near normal range. 

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

40

35

25

30

Near average

40

45

45

40

Below average

20

20

30

30

Click here to view the historic rainfall and temperature for this region

Nelson, Marlborough, Buller

Probabilities are assigned in three categories: above average, near average, and below average.

  • Temperatures are most likely (50% chance) to be in the above average range. 
  • Rainfall and soil moisture are most likely (40-45% chance) to be in the near normal range.
  • River flows are almost equally likely to be in the near normal or below normal range (40% and 35% respectively).

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

50

25

25

25

Near average

35

45

45

40

Below average

15

30

30

35

Click here to view the historic rainfall and temperature for this region

West Coast, Alps and foothills, inland Otago, Southland

Probabilities are assigned in three categories: above average, near average, and below average.

  • Temperatures are most likely (50% chance) to be in the above average range. 
  • Rainfall and soil moisture are most likely (45% chance) to be in the near normal range. 
  • River flows are almost equally likely to be in the near normal or below normal range (40% and 35% respectively).

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

50

30

25

25

Near average

35

45

45

40

Below average

15

25

30

35

Click here to view the historic rainfall and temperature for this region

Coastal Canterbury, east Otago

Probabilities are assigned in three categories: above average, near average, and below average.

  • Temperatures are almost equally likely to be in the above average or near average range (45% and 40% chance respectively). 
  • Rainfall is most likely (45% chance) to be in the near normal range.
  • Soil moisture levels are equally likely (40%) to be in the near normal or below normal range.
  • River flows are most likely (45% chance) to be in the below normal range. 

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

45

25

20

20

Near average

40

45

40

35

Below average

15

30

40

45

Click here to view the historic rainfall and temperature for this region

Background

The equatorial Pacific Ocean reflects ENSO-neutral conditions at the end of November 2013.The NIWA Southern Oscillation Index estimate for November is +0.5. This brings the 3-month September-October-November estimate to +0.2. International guidance indicates that the tropical Pacific Ocean is very likely (90% chance or above) to remain neutral over the next three months (December-January). Beyond this time, the probability of El Niño development starts to increase, and by May-July 2014 the probabilities of El Niño and neutral conditions are approximately equally likely (~ 50 % chance).

The monthly sea surface temperature (SST) anomaly for New Zealand was approximately +0.6°C in November. This is the 11th consecutive month that SSTs have been warmer than normal around New Zealand. The large region of warmer-than-normal water to the east-north-east of New Zealand (centered about 35ºS, 160ºW) has propagated further southwestward, while weakening over its northern edge. Sea surface temperatures are expected to remain above average overall for the coming three months around New Zealand.

During the remaining of the tropical cyclone season (ending April 2014), the risk of an ex-Tropical Cyclone (ETC) approaching New Zealand is expected to be close to normal. Based on the long-term record, ETCs come within 550km of New Zealand for 9 out of every 10 years (averaging close to one event per year). These systems typically occur during the latter part of the Tropical Cyclone season (February – April). For ENSO-neutral years, ETCs are twice as likely to pass to the east of Auckland than to the west of the city.

Currently (as of the 1st of December 2013), soil moisture levels are significantly lower than normal in Northland, parts of the Waikato and Central Otago.

For comment, please contact

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

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

Dr Darren King, Climate Scientist, NIWA National Climate Centre.

Tel (09) 375 2086, Mobile (021) 399 637

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 our New Zealand Climate Update.
  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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