Strong El Niño conditions continue in the Tropical Pacific.
The latest monthly sea surface temperature anomalies exceed +2oC in the central and eastern Pacific. Sub-surface temperature anomalies in the eastern Pacific increased over the last month (November 2015) and exceed +7oC at 75-100m depth near 120oW. The SOI showed a – temporary – weakening in November (November value estimated on the 30th of November is -0.3), but overall the atmosphere remains well coupled with the ocean. In particular, strong westerly wind anomalies (weaker easterly trade-winds) dominate the western and central Pacific. Convection and rainfall are also much more intense than normal in the central and eastern Pacific, while overall large-scale convection was reduced over the Maritime Continent.
International guidance indicates that El Niño will continue (100% probability) over the next three months. Models have the event continuing through the following season, March-May 2016, and the majority predicts a rapid return to ENSO-neutral conditions in June-August 2016. By some measures, the current event is on par with the 1997/98 El Niño (the strongest since 1950).
For December 2015 - February 2016, above normal pressure is forecast to the north of New Zealand, while below normal pressure is expected to the south of the country. This circulation pattern is likely to be accompanied by anomalous westerly wind flows - a signature of El Niño conditions.
Outlook Summary
December 2015 - February 2016 temperatures are most likely (45% chance) to be near average for the north and east of the North Island as well as the east of the South Island. Temperatures are about equally likely to be near average (40% chance) or below average (40-45% chance) for all remaining regions of the country.
December 2015 - February 2016 rainfall is most likely (45%) to be below normal for the north and east of the North Island. Seasonal rainfall totals are about equally likely to be near normal (35-40% chance) or below normal (35-40% chance) for the west of the North Island as well as the north and east of the South Island. Rainfall is equally likely to be near normal (40% chance) or above normal (40% chance) for the west of the South Island.
December 2015 - February 2015 soil moisture levels are about equally likely to be normal (35-40% chance) or below normal (40-45% chance) in all regions of the North Island as well as the east of the South Island. Above normal soil moisture levels are most likely (45% chance) for the west of the South Island and below normal soil moisture levels are most likely (45% chance) for the north of the South Island.
December 2015 - February 2015 river flows are about equally likely to be normal (40-45% chance) or below normal (40-45% chance) in the north and west of the North Island. Seasonal river flows are most likely to be below normal (45-50% chance) in the east of the North Island and the north and east of the south Island, and most likely (45% chance) to be above normal in the west of the South Island.
Regional predictions for the December 2015 - February 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 most likely to be near average (45% chance).
- Rainfall totals are most likely to be below normal (45% chance).
- Soil moisture levels are equally likely to be near normal (40% chance) or below normal (40% chance).
- River flows are about equally likely to be near normal (40% chance) or below normal (45% chance).
Other outcomes cannot be excluded. The full probability breakdown is:
|
Temperature |
Rainfall |
Soil moisture |
River flows |
|
|
Above average |
25 |
20 |
20 |
15 |
|
Near average |
45 |
35 |
40 |
40 |
|
Below average |
30 |
45 |
40 |
45 |
Central North Island, Taranaki, Wanganui, Manawatu, Wellington
Probabilities are assigned in three categories: above average, near average, and below average.
- Temperatures are equally likely to be near average (40% chance) or below average (40% chance).
- Rainfall totals are about equally likely to be near normal (35% chance) or below normal (40% chance).
- Soil moisture levels and river flows are about equally likely to be in the near normal (40-45% chance) or below normal ranges (40% chance).
The full probability breakdown is:
|
Temperature |
Rainfall |
Soil moisture |
River flows |
|
|
Above average |
20 |
25 |
20 |
15 |
|
Near average |
40 |
40 |
40 |
45 |
|
Below average |
40 |
35 |
40 |
40 |
Gisborne, Hawke’s Bay, Wairarapa
Probabilities are assigned in three categories: above average, near average, and below average.
- Temperatures are most likely to be near average (45% chance).
- Rainfall totals are most likely to be below normal (45% chance).
- Soil moisture levels are about equally likely to be in the below normal (40% chance) or near normal (35% chance) ranges.
- River flows are most likely to be in the below normal range (45% chance).
The full probability breakdown is:
|
Temperature |
Rainfall |
Soil moisture |
River flows |
|
|
Above average |
25 |
20 |
25 |
20 |
|
Near average |
45 |
35 |
35 |
35 |
|
Below average |
30 |
45 |
40 |
45 |
Nelson, Marlborough, Buller
Probabilities are assigned in three categories: above average, near average, and below average.
- Temperatures are about equally likely to be average (35% chance) or below average (40% chance).
- Rainfall totals are equally likely to be normal (40% chance) or below normal (40% chance).
- Soil moisture levels and river flows are both most likely to be below normal (45% chance).
The full probability breakdown is:
|
Temperature |
Rainfall |
Soil moisture |
River flows |
|
|
Above average |
25 |
20 |
20 |
20 |
|
Near average |
35 |
40 |
35 |
35 |
|
Below average |
40 |
40 |
45 |
45 |
West Coast, Alps and foothills, inland Otago, Southland
Probabilities are assigned in three categories: above average, near average, and below average.
- Temperatures are about equally likely to be near average (40% chance) or below average (45% chance).
- Rainfall totals are equally likely to be near normal (40% chance) or above normal (40% chance).
- Soil moisture levels and river flows are most likely to be in the above normal range (45% chance).
The full probability breakdown is:
|
Temperature |
Rainfall |
Soil moisture |
River flows |
|
|
Above average |
15 |
40 |
45 |
45 |
|
Near average |
40 |
40 |
35 |
30 |
|
Below average |
45 |
20 |
20 |
25 |
Coastal Canterbury, east Otago
Probabilities are assigned in three categories: above average, near average, and below average.
- Temperatures are most likely to be near average (45% chance).
- Rainfall totals are about equally likely to be in the near normal range (40% chance) or below normal (35% chance) range.
- Soil moisture levels are about equally likely to be near normal (40% chance) or below normal (45% chance).
- River flows are most likely to be below normal (50% chance).
The full probability breakdown is:
|
Temperature |
Rainfall |
Soil moisture |
River flows |
|
|
Above average |
20 |
25 |
15 |
10 |
|
Near average |
45 |
40 |
40 |
40 |
|
Below average |
35 |
35 |
45 |
50 |
Graphical representation of the regional probabilities
Background
Strong El Niño conditions continue unabated in the Tropical Pacific. The latest monthly sea surface temperature anomalies exceed +2oC in the central and eastern Pacific. Sub-surface temperature anomalies in the eastern Pacific increased over the last month (November 2015) and exceed +7oC at 75-100m depth near 120oW.
The Southern Oscillation Index (SOI) showed large intra-seasonal variability in November 2015, with several excursions in the positive, and is currently weakly negative for the month of November as a whole (value of -0.3 estimated on the 30th of November). However large-scale atmospheric patterns continue to indicate that the atmosphere remains fully coupled with the atmosphere. International guidance indicates that El Niño conditions will continue (100% chance) to over the next three month period (December 2015 – February 2016) and are virtually certain (97% chance) to carry on through into autumn (March – May 2016).
Note that El Niño events are typically (but not always) associated with stronger and/or more frequent westerly winds over summer in New Zealand, following more southwesterlies in spring. Such a circulation pattern can lead to cooler conditions in some regions of the country along with wetter than normal conditions to the west of the Southern Alps and drier conditions in northern and eastern regions of both Islands. The next three months (December 2015 – February 2016) forecasts broadly reflect these typical impacts. In particular, the three previous strongest El Niño events since 1950 (1972/73, 1982/83 and 1997/98) all show very dry conditions (below 50% of the normal rainfall over summer) in northeast parts of the country: eastern Northland, Coromandel, coastal Bay of Plenty, southern Hawke’s Bay and coastal Wairarapa, and Marlborough. It’s important to remember that once El Nino begins to weaken, i.e. sea surface temperatures cool in the central and eastern equatorial Pacific, the impacts in New Zealand will persist well beyond into the autumn season.
On average, New Zealand experiences at least one ex-tropical cyclone passing within 550km of the country every year. For this tropical cyclone season (November 2015 - April 2016), the risk for New Zealand is slightly higher than normal. If an ex-tropical cyclone comes close to the country, the current background climate conditions suggest it has an equal probability of passing east or west of Auckland city. Significant rainfall, damaging winds and coastal impacts can occur leading up to and during these episodic events.
Water temperatures surrounding New Zealand are currently below normal, primarily to the southeast of the country. Ocean models suggest that waters will remain cooler than normal mainly, east of the South Island.
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
- 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.
- 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. 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.
- 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.
- 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).
- 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
- 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.
- 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.
- Where probabilities are within 5% of one another, the term “about equally” is used.
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