Seasonal climate outlook: December 2016 - February 2017

Overview

The tropical Pacific exhibits mainly neutral ENSO (El Niño-Southern Oscillation) conditions.

Sea surface temperatures (SSTs) in the central Equatorial Pacific Ocean are marginally below average and do not exceed the threshold used by NOAA’s Climate Prediction Center to define La Niña events. Additionally, sub-surface temperatures in the central and eastern Pacific Ocean (150°W to 110°W) have continued to warm, a signal from the ocean that La Niña conditions are easing. The atmospheric setup is mixed, as stronger easterly trade winds in the western Pacific along with enhanced convection north and east of Papua New Guinea are consistent with weak La Niña conditions. However, the Southern Oscillation Index (SOI) remains weakly negative and is consistent with ENSO-neutral conditions.

International guidance still slightly favours La Niña conditions (53% chance) over the next three month period (December 2016 - February 2017). However, neutral conditions are much more likely than La Niña by March - May 2017: 80% chance for neutral, and only 16% for La Niña. In summary, La Niña conditions are only slightly more likely than not over the next 3-month period, and become significantly less likely as we progress into 2017.  

Consequently, the combination of easing La Nina conditions, along with the absence of other large-scale climate drivers this summer, results in no clear air flow anomaly. Typical summer conditions are expected of anticyclones crossing the North Island and prevailing westerly winds over the South Island.

NIWA Forecaster—Chris Brandolino—discusses the December 2016 - February 2017 seasonal climate outlook.

Outlook summary

December 2016 – February 2017 temperatures are most likely to be near average (45% chance) for the west of the North Island and are about equally likely to be near average or above average (40-45% chance) for the remainder of the country. Sea surface temperatures around New Zealand are expected to be near average or above average over the next three months. 

December 2016 – February 2017 rainfall totals are about equally likely to be near normal or below normal (35-40% chance) for the east of the South Island and mostly likely to be near normal (45% chance) for the rest of New Zealand.    

December 2016 – February 2017 soil moisture levels and river flows are most likely to be near normal (45-50% chance) for the north and east of the North Island, and equally likely (40% chance) to be near normal or above normal for the west of the North Island.  Near normal or above normal soil moisture and river flows are equally likely (40% chance) for the north of the South Island with near normal (45% chance) soil moisture and river flows the most likely outcome for the west of the South Island. Near normal or below normal soil moisture and river flows are about equally likely (35-40% chance) for the east of the South Island.

Regional predictions for the December 2016 – February 2017 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 to be near average (40% chance) or above average (45% chance). 
• Rainfall totals are most likely to be in the near normal range (45% chance). 
• Soil moisture levels and river flows are most likely to be near normal (45% chance).

The full probability breakdown is: 

Central North Island, Taranaki, Whanganui, Manawatu, Wellington 

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, soil moisture levels and river flows are all mostly likely to be in the near normal range (40-45% chance).

The full probability breakdown is: 

Gisborne, Hawke’s Bay, Wairarapa 

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

The full probability breakdown is: 

Tasman, Nelson, Marlborough, Buller 

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

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

The full probability breakdown is: 

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 (45% chance) or above average (40% chance). 
• Rainfall totals, soil moisture levels and river flows are all most likely to be in the near normal range (45% chance). 

The full probability breakdown is: 

Coastal Canterbury, east Otago 

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 above average (45% chance). 
• Rainfall totals, soil moisture levels and river flows are all about equally likely to be near normal (40% chance) or below normal (35% chance). 

The full probability breakdown is: 

Graphical representation of the regional probabilities 

Image: NIWA

Background 

The tropical Pacific exhibits mainly neutral ENSO (El Niño-Southern Oscillation) conditions. Sea surface temperatures (SSTs) in the central equatorial Pacific Ocean remain slightly below average, with the NINO3.4 index for November close to -0.3^oC, which does not exceed the threshold of -0.5°C that NOAA (the US National Oceanic and Atmospheric Administration) uses to define a La Niña event. In the sub-surface of the central and eastern Pacific Ocean (150°W to 110°W) waters have continued to warm. Collectively, the ocean is signaling that La Niña conditions have eased over the past month in the tropical Pacific. 

The Southern Oscillation Index (SOI) was weakly negative for November 2016, with an estimated value of -0.2 for the month, or in the ENSO-neutral range. However, the easterly trade winds remained slightly stronger than normal in the western Pacific (west of about 140^oW). The Intertropical Convergence Zone (ITCZ) was displaced north of its climatological position just north of the Equator in the central and eastern Pacific. Convection and rainfall were well above normal for the Maritime Continent (Indonesia, Papua New Guinea). These wind and convection anomaly patterns are consistent with a weak La Niña. 

The South Pacific Convergence Zone (SPCZ) appears to be disjointed around Fiji and displaced north and northeast of its climatological position east of the Date Line. Thus, the atmosphere reflects mixed ENSO signals. 

International guidance still slightly favours La Niña conditions (53% chance versus 47% for neutral, and 0% for El Niño) over the next three month period (December 2016 – February 2017). However, like last month, neutral conditions are much more likely than La Niña by March – May 2017: 80% chance for neutral, and only 16% for La Niña and 4% for El Niño. To sum up, La Niña conditions are only slightly more likely than not over the 3 month period, and become significantly less likely as we progress into 2017.  Given the aforementioned ENSO conditions/trends, international guidance and time of year – typically La Niña and El Niño events peak in December or early January – it’s likely that the window for additional La Niña conditions to develop has closed. 

While sea surface temperatures (SSTs) immediately around the New Zealand coast are near normal, there is some variance farther to the west and east. The Tasman Sea is exhibiting slightly below normal SSTs while, east of the country SSTs are slightly above normal for this time of the year. The dynamical models' forecasts indicate that near normal or above normal SSTs are likely over the next three months for New Zealand coastal waters. 

The 2016-17 tropical cyclone season is expected to produce near average activity across islands of the southwest Pacific. Refer to NIWA’s Tropical Cyclone outlook for more information. 

Contacts

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.
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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