Seasonal Climate Outlook: November 2013 - January 2014

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

The recent cooler-than-normal sea-surface conditions (La Niña-like) in the eastern tropical Pacific have disappeared, but slightly warmer-than-normal sea-surface temperatures persist west of the Dateline. International guidance indicates that ENSO-neutral conditions are the most likely outcome for the next three months (November–January), and these are likely to persist into autumn 2014.

For the coming three months as a whole in the New Zealand region, lower pressures than normal are forecast in the Tasman Sea and across the North Island, and higher pressures than normal are expected to the south of the country. This circulation pattern is expected to produce disturbed northerly quarter flows over the north of the country, and slightly enhanced easterly flows over the South Island.

For November 2014, higher than normal pressures are expected to dominate over and around New Zealand, resulting in a marked change from the very disturbed westerly flow of October.  

Sea surface temperatures are expected to remain near average overall for the coming three months around New Zealand, except for above average temperatures east of the North Island.

Outlook Summary

Temperatures over the November–January period as a whole are equally likely (40% chance) to be near average or above average in all regions of New Zealand, except for the west of the North Island and east of the South Island, where near average temperatures are the most likely outcome (45% chance). The chances of below normal temperatures across all regions is about 20%.

Rainfall totals over the November–January period as a whole are most likely (45-50% chance) to be near normal in all regions except for the north and east of the North Island, where rainfall is equally likely (40% chance) to be near normal or above normal.

Soil moisture levels and river flows are most likely (45% chance) to be below normal in the north of the North Island, and most likely (50% chance) to be above normal in the east of the North Island, and likely to be near normal for all remaining regions.

Regional predictions for the November to January 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%) to be in the near normal or above normal range.
• Soil moisture levels and river flows are most likely (45%) to be in the below normal range. Note that this is quite different from the expected rainfall probability, because of the currently very dry soils in this region, especially in Northland.  

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

	Temperature	Rainfall	Soil moisture	River flows
Above average	40	40	25	25
Near average	40	40	30	30
Below average	20	20	45	45

 

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 near average range.
• Rainfall totals are most likely (50% chance) to be in the normal range
• 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	35	20	30	30
Near average	45	50	40	40
Below average	20	30	30	30

 

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 are equally likely (40%) to be in the near normal or above normal range.
• Soil moisture levels and river flows are most likely (50% chance) to be in the above normal range.

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	40	40	50	50
Near average	40	40	30	30
Below average	20	20	20	20

 

  Nelson, Marlborough, Buller

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, soil moisture 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	20	30	30
Near average	40	50	45	45
Below average	20	30	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 equally likely (40% chance) to be in the near average or above average range.
• Rainfall and river flows are most likely (45% chance) to be in the near normal range.
• Soil moisture levels are equally likely (35% chance) to be in the near normal or above normal range, but this is not greatly different from the 30% chance of below normal.

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	40	20	35	30
Near average	40	45	35	45
Below average	20	35	30	25

 

Coastal Canterbury, east Otago

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

• Temperatures are most likely (45% chance) to be in the near average range.
• Rainfall, soil moisture 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	35	20	25	25
Near average	45	50	45	45
Below average	20	30	30	30

Background

The equatorial Pacific Ocean reflects ENSO-neutral conditions at the end of October 2013.The NIWA Southern Oscillation Index for October is -0.3. This brings the 3-month August-September-October value to -0.1. International guidance indicates that the tropical Pacific Ocean is very likely (90% chance or above) to remain neutral over the next three months (November-January). Beyond this time, the probability of El Niño development starts to increase, and by April-June 2014 the probabilities of El Niño and neutral conditions are approximately equally likely, with only a small (~10%) chance of La Niña development by mid-2014. 

The monthly sea surface temperature (SST) anomaly for New Zealand was approximately +0.2°C in October. This is the 10^th consecutive month that SSTs have been warmer than normal around New Zealand, although there appears to be some weakening of warmer waters around the coast. The large region of warmer-than-normal water to the east-north-east of New Zealand (centered about 35ºS, 160ºW) is still persisting with little change, except it has propagated westward to lie slightly closer to the North Island. Sea surface temperatures are expected to remain near average overall for the coming three months around New Zealand, except for above average temperatures east of the North Island.

For the tropical cyclone season (November – April), 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.

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, NIWA National Climate Centre

Tel (09) 375 2053, Mobile (022) 122 8748

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).
   1. 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
   2. 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.
   3. 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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