Seasonal Climate Outlook: February 2014 - April 2014

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

International guidance indicates that ENSO-neutral conditions are the most likely outcome for the next three months (February – April 2014), with signs that El Niño conditions may develop by mid-year.

For the coming three months as a whole, mean sea level pressures are expected to be near normal over New Zealand, with lower pressures than usual to the north of the country and higher pressures than usual well to the east. This circulation pattern is expected to be associated with weak anomalous south-westerly flows over the South Island and easterly flows over the North Island.

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

Outlook summary

February-April temperatures are equally likely (40% chance) to be average or above average for all North Island regions, equally likely (40% chance) to be average or below average in the west of the South Island, and most likely (50% chance) to be near average in other South Island regions.

February-April rainfall totals are equally likely (40% chance) to be normal or above normal in the north of the North Island, equally likely (40% chance) to be normal or below normal in the north and west of the South Island, and most likely (40-50%) to be near normal for all remaining regions.

Soil moisture levels are most likely (40-50% chance) to be near normal for all regions except the east of the South Island where soil moisture levels are likely (35-40% chance) to be normal or below normal. River flows are likely (35-40% chance) to be normal or below normal in the north and west of the South Island, and most likely (45-50%) to be near normal for all remaining regions.

Regional predictions for the February to April 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 are equally likely (40% chance) to be in the near average or above average range.
  • Rainfall totals are equally likely (40% chance) to be in the near normal or above normal range.
  • Soil moisture levels are most likely (45%) to be in the near normal range.
  • River flows are most likely (50%) to be in the near normal range.

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

 

Temperature

Rainfall

Soil moisture

River flows

Above average

40

40

30

20

Near average

40

40

45

50

Below average

20

20

25

30

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 equally likely (40% chance) to be in the near average or above average range.
  • Rainfall totals, soil moisture levels and river flows are all most likely (50% chance) to be in the normal range.

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

40

20

20

20

Near average

40

50

50

50

Below average

20

30

30

30

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 (45-50% chance) to be in the near normal range. 

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

40

30

20

20

Near average

40

50

45

50

Below average

20

20

35

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

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

25

20

20

20

Near average

50

40

45

40

Below average

25

40

35

40

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 equally likely (40% chance) to be in the near average or below average range. 
  • Rainfall totals are equally likely (40% chance) to be in the normal or below normal range. 
  • River flows are likely (35-40% chance) to be in the near normal or below normal range. 
  • Soil moisture levels are likely (40% chance) to be in the near normal range. 

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

20

20

30

25

Near average

40

40

40

40

Below average

40

40

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 most likely (50% chance) to be in the near average range.
  • Rainfall totals are most likely (50% chance) to be in the near normal range.
  • River flows are most likely (45% chance) to be in the near normal range.
  • Soil moisture levels are likely (35-40% chance) to be in the near normal or below normal range. 

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

20

20

25

20

Near average

50

50

40

45

Below average

30

30

35

35

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

Background

The equatorial Pacific Ocean continued in a neutral ENSO-state (neither El Niño nor La Niña) in January 2013. The NIWA Southern Oscillation Index for January is +1.0 (estimated at 28-January). This brings the 3-month November-December-January value to +0.6. International guidance indicates that the tropical Pacific Ocean is very likely to remain neutral over the next three months (February-April). After that, development of weak El Niño conditions appears possible by the middle of 2014 with six out of fourteen models monitored by NIWA indicating warming that may approach El Niño thresholds by May-July 2014.

The monthly sea surface temperature (SST) anomaly for New Zealand was approximately +0.4°C in January. This is the 13th consecutive month that SSTs have been warmer than normal around New Zealand. However, there has been some weakening of these warmer waters in the central/eastern Tasman where SSTs are now closer to normal. The large region of warmer than normal water to the northeast of New Zealand has also weakened slightly from a peak value of +0.9°C in December. Sea surface temperatures are expected to be average or above average for the coming three months around New Zealand.

The remnants of Tropical Cyclone June affected much of the North Island in mid-January, bringing widespread rain and gusty wind to the New Zealand region as it moved from its genesis area in the Coral Sea to eventually south and east of the North Island. Historically, Ex-Tropical Cyclones (ETCs) approach within 550km of New Zealand for about 9 out of 10 years. However, the most likely period is during February-April so further events cannot be ruled out.  For ENSO-neutral years (the current situation), any ETCs approaching the North Island are twice as likely to pass to the east of Auckland as to the west of the city.

Finally, at the end of January 2014 soil moisture levels continue lower than normal across the central North Island and western areas of the Northland and Waikato Regions. However, the soil moisture deficits are not as severe or as extensive as those a year ago before the 2013 drought set in.

For comment, please contact

Mr Darren King, Environmental Scientist, NIWA National Climate Centre

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

Dr Brett Mullan, Principal Scientist, NIWA National Climate Centre

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 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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Graphical representation of the regional probabilities, Seasonal Climate Outlook, Feb-Apr 2014. [NIWA]