Seasonal climate outlook: November 2016 - January 2017

The tropical Pacific exhibits mixed ENSO (El Niño-Southern Oscillation) signals, with some indicating La Niña status and others indicating neutral conditions.

Sea surface temperatures (SSTs) in the central equatorial Pacific Ocean are below average, close to the threshold used to define La Niña events. Anomalously cold sub-surface waters are still present, but are now more confined to the central Pacific (near 140^oW) than in previous months.  

The atmospheric conditions are also mixed: stronger easterly trade winds in the west and enhanced convection over the Maritime Continent (islands of Indonesia and Papua New Guinea) are consistent with a weak La Niña. However, the Southern Oscillation Index (SOI) has dropped to weakly negative values which are indicative of ENSO-neutral conditions.

International guidance still slightly favours La Niña conditions (53% chance versus 46% for neutral) over the next three month period (November 2016 - January 2017). However, neutral conditions are now much more likely than La Niña by February – April 2017: 74% chance for neutral, and only 22% for La Niña. In summary, La Niña conditions are only slightly more likely than not over the next 3-month period, and become less likely as we progress into 2017.

Despite the current borderline La Niña conditions and the only modest probability for La Niña to develop by the end of the year, the circulation pattern expected over the coming three months for the New Zealand region is broadly consistent with the typical La Niña signature: higher pressure than normal is forecast to the south and southeast of the country, while lower pressures than normal are forecast to the north of the New Zealand, leading to more persistent easterly or north-easterly airflow than normal.

NIWA Forecaster - Ben Noll - discusses the Nov 2016 - Jan 2017 seasonal climate outlook.

Outlook summary

November 2016 - January 2017 temperatures are most likely to be above average (50-55% chance) for all regions of New Zealand. Sea surface temperatures around New Zealand are forecast to remain near or above normal over the next three months.

November 2016 - January 2017 rainfall totals are about equally likely to be above normal (40% chance) or near normal (35-40% chance) throughout the North Island. In the north and east of the South Island, rainfall is most likely to be near normal (40-50%), whereas rainfall in the west and south of the South Island is about equally likely to be near normal (40% chance) or below normal (35% chance).

November 2016 - January 2017 soil moisture levels are most likely to be near normal (45% chance) in the north of the North Island, whereas river flows are equally likely to be near normal (40% chance) or above normal (40% chance).  In the remainder of the North Island, soil moisture and river flows are about equally likely to be above normal (40% chance) or near normal (35% chance). Near normal soil moisture levels and river flows are most likely (45% chance) in the north of the South Island. For the rest of the South Island, soil moisture and river flows are about equally likely to be near normal (35-40% chance) or below normal (35% chance).

Regional predictions for the November 2016 – January 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 most likely to be above average (55% chance).
• Rainfall totals are equally likely to be in the near normal (40% chance) or above normal (40% chance) range.
• Soil moisture levels are most likely to be near normal (45% chance).
• River flows are equally likely to be near normal (40% chance) or above normal (40% chance).

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	55	40	35	40
Near average	30	40	45	40
Below average	15	20	20	20

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 above average (55% chance).
• Rainfall totals, soil moisture levels and river flows are all about equally likely to be above normal (40% chance) or near normal (35% chance).

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	55	40	40	40
Near average	30	35	35	35
Below average	15	25	25	25

Gisborne, Hawke’s Bay, Wairarapa

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

• Temperatures are most likely to be above average (50% chance).
• Rainfall totals are equally likely to be near normal (40% chance) or above normal (40% chance).
• Soil moisture levels and river flows are about equally likely to be above normal (40% chance) or near normal (35% chance).

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	50	40	40	40
Near average	30	40	35	35
Below average	10	20	25	25

Tasman, Nelson, Marlborough, Buller

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

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

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	50	25	30	30
Near average	30	50	45	45
Below average	20	25	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 most likely to be above average (50% 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:

	Temperature	Rainfall	Soil moisture	River flows
Above average	50	25	25	25
Near average	30	40	40	40
Below average	20	35	35	35

 Coastal Canterbury, east Otago

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

• Temperatures are most likely to be above average (50% chance).
• Rainfall totals are most likely to be in the near normal range (40% chance), but confidence is low.
• Soil moisture levels and river flows are equally likely to be near normal (35% chance) or below normal (35% chance), but again confidence is low.

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	50	30	20	30
Near average	30	40	35	35
Below average	20	30	35	35

 Graphical representation of the regional probabilities

Background

The tropical Pacific exhibits mixed ENSO signals, with some indicating La Niña status and other indicating neutral conditions. Sea surface temperatures (SSTs) in the central equatorial Pacific Ocean are below average, with the NINO3.4 index for October being close to the threshold of -0.5^oC used by NOAA (the US National Oceanic and Atmospheric Administration) to define La Niña events. In the sub-surface, the coolest anomaly has strengthened to about -4^oC at 140^oW, but the previously cool anomalies to the west have weakened substantially to near normal.

The Southern Oscillation Index (SOI) was negative for October 2016, with an estimated value of -0.5 for the month, and so 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) did not show any clear displacement from its climatological position: a southwest displacement would be expected during La Niña events. Thus, the atmosphere also exhibits mixed ENSO signals.

International guidance still slightly favours La Niña conditions (53% chance) over neutral conditions (46% chance), during the next three-month period. The Australian Bureau of Meteorology remains on La Niña ‘Watch’, and NOAA’s Climate Prediction Center has reinstituted its La Niña Watch alert status after dropping it in September. On 19 October, the World Meteorological Organization updated its ENSO Advisory, indicating that a La Niña-like circulation pattern may be developing in the Pacific. However, the window for such a development is narrow: the climate models predict ENSO-neutral conditions are now much more likely than La Niña by February – April 2017: 74% chance for neutral, and only 22% for La Niña. In summary, La Niña conditions are only slightly more likely than not over the next three-month period, and become less likely as we progress into 2017.

Coastal waters around the country remain warmer than normal, and ocean waters in the western Tasman Sea and to the north and northeast of New Zealand are much warmer than normal. The dynamical model’s forecasts for SSTs indicate that this pattern is likely to persist over the next three months and coastal waters around the country are expected to be near or above normal.

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 season 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).
   1. Each month, NIWA publishes an analysis of how well its outlooks perform. This is sent to about 3500 recipients of NIWA’s newsletters, including many farmers.
   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.
   4. Where probabilities are within 5% of one another, the term “about equally” is used.