Seasonal Climate Outlook: November 2014 - January 2015

Overview

At the end of October 2014, atmospheric and oceanic indicators in the tropical Pacific Ocean were at borderline El Niño thresholds.

International guidance continues to indicate that a weak El Niño is the most likely outcome (about 66 % chance) over the coming three months (November 2014 – January 2015).

During November 2014 – January 2015, weak anticyclonic anomalies are forecast to the west of the country and could be accompanied with weak southwesterly flow anomalies.

Sea surface temperatures around New Zealand for the coming three months are expected to be below average off the east coast of the North Island and near average elsewhere.

On average, New Zealand experiences at least one ex-tropical cyclone passing within 550km of the country every year. Over the next three months, the chances of an ex-tropical cyclone passing close to the country are slightly lower than normal, but become elevated in the latter half of the tropical cyclone season (February to April 2015).

Outlook Summary

November 2014 – January 2015 temperatures are likely (35-40% chance) to be average or below average in the north and west of the North Island. Seasonal temperatures are most likely (40-45% chance) to be near average for the remaining regions.

November 2014 – January 2015  rainfall totals are about equally likely (35-40% chance) to be in the normal or below normal range in the north and east of the North Island. Seasonal rainfall is most likely (40–45%) to be in the near normal range in all remaining regions of New Zealand.

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

Regional predictions for the November 2014 to January 2015 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 about equally likely (35-40% chance) to be average or below average.
• Rainfall totals are equally likely (40% chance) to be in the normal or below normal range.
• Soil moisture levels and river flows are about equally likely (35-40% chance) to be in the near normal or above normal range.

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

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

 

Central North Island, Taranaki, Wanganui, Manawatu, Wellington

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

• Temperatures are about equally likely (35-40% chance) to be average or below average.
• Rainfall totals, soil moisture  levels and river flows are all 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	25	30	30
Near average	40	45	45	45
Below average	35	30	25	25

 

Gisborne, Hawke’s Bay, Wairarapa

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 about equally likely (35-40% chance) to be in the normal or below 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	25	25	30	30
Near average	45	40	40	40
Below average	30	35	30	30

 

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 near average range.
• Rainfall totals are most likely (40% chance) to be in the normal range.
• Soil moisture and river flows are about equally 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	25	30	25	20
Near average	50	40	40	40
Below average	25	30	35	40

 

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

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	30	35	15	15
Near average	40	45	40	40
Below average	30	20	45	45

 

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 totals are most likely (45% chance) to be in the near normal range.
• Soil moisture levels and river flows are most likely (50% chance) to be in the below normal range.

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	30	25	15	15
Near average	45	45	35	35
Below average	25	30	50	50

Background

After having reached borderline El Niño conditions during September 2014, ocean and atmospheric indicators in the Pacific returned to more neutral levels towards the end of September and beginning of October, but recently increased again to close to or above El Niño thresholds. Sub-surface ocean temperature rose slightly in the central Pacific to reach anomalies of over +3°C at 100-150m depth near 170°W, and remained stable at about +2°C in the eastern Pacific at about 50m depth. Weak westerly wind anomalies prevailed recently all along the equatorial Pacific.

The NIWA Southern Oscillation Index (SOI) for October 2014 (preliminary value estimated on the 31 of October) is –0.9. This brings the 3-month August-September-October value to -0.9. Strongly negative SOI values (less than -1) are typically associated with  El Niño. The NASA ENSO Precipitation Index (ESPI) for the 30 days to the 30^th of October was +0.55 (on the El Niño side of neutral).

Altogether, these various signals indicate that ocean-atmosphere conditions in the Pacific might transition again towards a weak El Niño state.

The international guidance places the chances of El Niño developing over the November 2014 to January 2015 period at about 66%. This probability remains basically unchanged over the following three months (67% for February to April 2015). Again there is general consensus to indicate that this event, were it to occur, would be moderate at best.

Note that for New Zealand, El Niño events are related to stronger and/or more frequent westerly winds over the New Zealand region. Such a climate pattern typically leads to drier conditions in eastern areas and more rain in western areas of the country.

Meanwhile, waters surrounding New Zealand have cooled further compared to September. The  preliminary  monthly sea surface temperature anomaly for New Zealand was -0.2°C in October. Ocean models forecasts indicate that coastal SSTs are likely to be below normal to the east of the North Island and near normal elsewhere.

For comment, please contact;

Chris Brandolino, NIWA forecaster, 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).
   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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