Seasonal Climate Outlook: June - August 2016

Ocean-atmosphere conditions in the tropical Pacific have now returned to near normal after a rapid demise of the El Niño event that peaked late 2015. Sea surface temperatures in the central Pacific are close to average, and slightly cooler than normal ocean temperatures have emerged along the equator in the eastern Pacific.

Atmospheric circulation along the equatorial Pacific has returned to near-normal, and the Southern Oscillation Index (SOI) has also returned to near neutral values. Cooler than normal sub-surface ocean waters have continued to intensify and spread eastward along the equatorial Pacific, suggesting the possibility of La Niña conditions developing later this year.

International guidance indicates that neutral ENSO (El Niño - Southern Oscillation) conditions are most likely (54% chance) over the next three month period (June – August 2016), as a whole, but a transition to La Niña is also possible over the same period (43% chance). The likelihood of La Niña conditions establishing in the Pacific increases later in the year, with a 58% chance in September-November 2016 and 61% chance in December 2016 – February 2017.

For June – August 2016, higher pressures than normal are forecast to the north and northeast of New Zealand, with lower pressures than normal expected to the south and the southeast of the country. Westerly to northwesterly wind flow anomalies are expected to affect the country.

Seasonal Climate Outlook Video

In the video below NIWA Forecaster Chris Brandolino describes what New Zealanders can expect, in broad terms, in the way of rainfall and temperature over the coming winter season:

Summary

June – August 2016 temperatures are very likely (65-80% chance) to be above average in all regions of the country. Nevertheless, frosts and cold snaps will occur from time to time in cooler locations. Sea surface temperatures are forecast to be above normal over the next three months, especially to the west of New Zealand.

June – August 2016 rainfall is likely to be above normal (50% chance) in the north and west of the North Island and the west of the South Island. Seasonal rainfall totals are equally likely to be near normal (40% chance) or above normal (40% chance) in the east of the North Island and the north and east of the South Island.

June – August 2016 soil moisture levels and river flows are most likely (45-50% chance) to be in the above normal range in the north and west of the North Island and the west of the South Island and most likely (40% chance) to be in the near normal range in the east of the North Island. In the North of the South Island, soil moisture levels and river flows are about equally likely to be in the near normal range (40% chance) or above normal range (35% chance). Soil moisture levels and river flows are both about equally likely to be in the near normal (40% chance) or below normal (35% chance) range in the east of the South Island.

Regional predictions for the June – August 2016 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 very likely to be above average (80% chance).
  • Rainfall totals are most likely to be above normal (50% chance).
  • Soil moisture levels and river flows are most likely (45-50% chance) to be in the above normal range.

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

               80

50

45

50

Near average

10

30

35

30

Below average

10

20

20

20

Central North Island, Taranaki, Wanganui, Manawatu, Wellington

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

  • Temperatures are very likely to be above average (80% chance).
  • Rainfall totals are most likely to be above normal (50% chance).
  • Soil moisture levels and river flows are most likely (45-50% chance) to be in the above normal range.

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

80

50

45

50

Near average

10

30

35

30

Below average

10

20

20

20

Gisborne, Hawke’s Bay, Wairarapa

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

  • Temperatures are very likely to be above average (80% chance).
  • Rainfall totals are equally likely to be normal (40% chance) or above normal (40% chance).
  • Soil moisture levels and river flows are most likely (40% chance) to be in the near normal range.

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

80

40

30

30

Near average

10

40

40

40

Below average

10

20

30

30

Nelson, Marlborough, Buller

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

  • Temperatures are very likely to be above average (80% chance).
  • Rainfall totals are equally likely to be normal (40% chance) or above normal (40% chance). Note that this outcome is for the north of the South Island region as a whole, but there was some indication that the region could be split between a wetter than normal western side and drier than normal eastern side.
  • Soil moisture levels and river flows are about equally likely to be in the near normal (40% chance) or above normal (35% chance) range.

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

80

40

35

35

Near average

10

40

40

40

Below average

10

20

25

25

West Coast, Alps and foothills, inland Otago, Southland

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

  • Temperatures are very likely to be above average (70% chance).
  • Rainfall totals are most likely to be in the above normal range (50% chance).
  • Soil moisture levels and river flows are both most likely (50% chance) to be in the above normal range.

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

70

50

50

50

Near average

20

30

35

35

Below average

10

20

15

15

Coastal Canterbury, east Otago

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

  • Temperatures are very likely to be above average (65% chance).
  • Rainfall totals are equally likely to be normal (40% chance) or above normal (40% chance).
  • Soil moisture levels and river flows are about equally likely to be in the near normal (40% chance) or below normal (35% chance) range.

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

65

               40

25

25

Near average

25

40

40

40

Below average

10

20

35

35

Graphical representation of the regional probabilities

Background

Ocean-atmosphere conditions in the tropical Pacific have now returned to a near normal state after a rapid demise of the El Niño event that peaked late 2015. Sea surface temperatures (SSTs) in the central Pacific are close to average, and slightly cooler than average SSTs have emerged in the eastern equatorial Pacific. Both the NINO 3 and 3.4 SST indices now indicate ‘neutral’ El Niño – Southern Oscillation (ENSO) conditions, i.e below +0.5oC anomaly.

Enhanced convective activity and rainfall near and east of the Dateline in the equatorial Pacific (usually associated with El Niño events) has weakened further in May 2016. Atmospheric circulation along the equatorial Pacific has returned to near-normal, and the Southern Oscillation Index (SOI) has also returned to neutral values (value for May 2016 estimated to the 1st of June 2016: +0.1). Cooler than normal sub-surface ocean waters have continued to intensify and spread eastward along the equatorial Pacific, suggesting the possibility of La Niña conditions developing later this year.

International guidance indicates that neutral ENSO conditions are most likely (54% chance) over the next three month period (June – August 2016), as a whole, but a transition to La Niña is also possible over the same period (43% chance). The likelihood of La Niña conditions establishing in the Pacific increases later on, with a 58% chance in September-November 2016 and a 61% chance in December 2016 – February 2017. Note however that there is a rather large spread between the models, and the larger-scale background climate state suggests that a La Niña event, even if it does indeed develop later this year, might not be associated with typical regional impacts.

Waters surrounding New Zealand remain exceptionally warm, and ocean models predict that these warm conditions will persist through June – August 2016. Given the higher moisture source from the warmer surface waters, together with more troughs and low pressure systems that typically enter the Tasman Sea in the winter season, it is possible that New Zealand will experience more severe storms than usual this winter. The warmer sea surface waters are also a major driving influence behind the forecast of very likely above average temperatures across the country over the next three months.

To find out more about normal conditions for this outlook period, refer to NIWA’s website, where daily updates on climate maps are available.

Contact

For comment, please contact:

Chris Brandolino, Principal Scientist – Forecasting, NIWA National Climate Centre
Tel (09) 375 6335, Mobile (027) 886 0014

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 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.
  9. Where probabilities are within 5% of one another, the term “about equally” is used.

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