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NIWA Outlook: January – March 2017

NIWA Outlook: January – March 2017

Overview
The tropical Pacific continues to exhibit mainly ENSO-neutral conditions. Sea surface temperatures (SSTs) in the central Equatorial Pacific Ocean remain marginally below average. In the sub-surface ocean, the pockets of cooler than average temperatures that were present in November 2016 have contracted and warmed. Thus, it appears that any La Niña-like oceanic signal has peaked and is subsiding back toward neutral. The atmospheric setup is mixed as stronger easterly trade winds in the central Pacific along with enhanced convection over parts of the Maritime Continent are consistent with La Niña conditions. The Southern Oscillation Index (SOI) is weakly positive and is consistent with ENSO-neutral conditions.

International guidance now strongly favours ENSO-neutral conditions (77% chance) over the next three month period (January - March 2017). By March-May 2017, the chance for neutral conditions rises to 87% with a meagre 8% chance for El Niño conditions and 5% chance for La Niña conditions. In summary, ENSO-neutral conditions are likely over the next 3-month period and should continue into autumn.

Consequently, the combination of ENSO-neutral conditions, along with the absence of other large-scale climate drivers for the remainder of summer into early autumn, may result in only weak air flow anomalies. Mostly typical summer conditions are expected with high pressure systems crossing the North Island and prevailing westerly winds over the South Island.
Outlook Summary
January – March 2017 temperatures are about equally likely to be near average (40% chance) or above average (35-45% chance) for the north and east of the North Island and for the east of the South Island. Near average temperatures are most likely (40% 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.

January – March 2017 rainfall totals are about equally likely to be near normal (40% chance) or below normal (40-45% chance) for all North Island regions. Rainfall totals are about equally likely to be near normal (40% chance) or below normal (35% chance) for the north of the South Island. Near normal rainfall is most likely (40% chance) for the east and west of the South Island.
January – March 2017 soil moisture levels and river flows are most likely (50% chance) to be below normal for the north of the North Island and about equally likely to be normal (40% chance) or below normal (45% chance) for the east and west of the North Island and east of the South Island. Near normal soil moisture levels and river flows are most likely (45-50% chance) for the north and west of the South Island.

Regional predictions for the January – March 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 equally likely to be near average (40% chance) or above average (40% chance).
• Rainfall totals are about equally likely to be in the near normal (40% chance) or below normal (45% chance) range.
• Soil moisture levels and river flows are most likely to be below normal (50% chance).
The full probability breakdown is:
Temperature Rainfall Soil moisture River flows
Above average 40 15 10 10
Near average 40 40 40 40
Below average 20 45 50 50



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 (40% chance).
• Rainfall totals are equally likely to be in the near normal (40% chance) or below normal range (40% chance).
• Soil moisture levels and river flows are about equally likely to be in the near normal (40% chance) or below normal range (45% chance).
The full probability breakdown is:
Temperature Rainfall Soil moisture River flows
Above average 30 20 15 15
Near average 40 40 40 40
Below average 30 40 45 45

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 equally likely to be in the near normal (40% chance) or below normal (40% chance) range.
• Soil moisture levels and river flows are about equally likely to be near normal (40% chance) or below normal (45% chance).
The full probability breakdown is:
Temperature Rainfall Soil moisture River flows
Above average 45 20 15 15
Near average 40 40 40 40
Below average 15 40 45 45

Tasman, Nelson, Marlborough, Buller
Probabilities are assigned in three categories: above average, near average, and below average.
• Temperatures are most likely to be near average (40% chance).
• Rainfall totals are about equally likely to be in the near normal range (40% chance) or below normal (35%) 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 30 25 20 20
Near average 40 40 45 45
Below average 30 35 35 35

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 near average (40% chance).
• Rainfall totals are most likely to be in the near normal (45% chance) range.
• Soil moisture levels and river flows are most likely to be near normal (50% chance) range.
The full probability breakdown is:
Temperature Rainfall Soil moisture River flows
Above average 30 30 15 15
Near average 40 45 50 50
Below average 30 25 35 35

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 (35% chance).
• Rainfall totals are most likely to be in the near normal range (45% chance).
• Soil moisture levels and river flows are about equally likely to be below normal (45% chance) or near normal (40% chance).
The full probability breakdown is:
Temperature Rainfall Soil moisture River flows
Above average 35 30 15 15
Near average 40 45 40 40
Below average 25 25 45 45


Background
The tropical Pacific continues to exhibit mainly ENSO-neutral conditions. Sea surface temperatures (SSTs) in the central Equatorial Pacific Ocean remain marginally below average, with a value of -0.3oC in the NINO3.4 region, and thus do not exceed the threshold used by NOAA’s Climate Prediction Centre (CPC) to define La Niña events. In the sub-surface ocean, the pockets of cooler than average temperatures that were present in November 2016 have contracted and warmed. An expanding tongue of warmer than average sub-surface ocean temperatures is noted between about 160oE and 160oW. It appears that any La Niña-like oceanic signal has peaked and is subsiding back toward neutral.
The thirty-day Southern Oscillation Index (SOI) trended in a weakly positive direction (+0.2), and is currently in the ENSO-neutral range. During December, stronger than normal easterly trade winds continued in the central Pacific along with enhanced rainfall over parts of the Maritime Continent. Furthermore, the Intertropical Convergence Zone (ITCZ) was displaced north of its climatological position along the Equator in the central Pacific. These wind and convection anomaly patterns are consistent with La Niña-like conditions.
The South Pacific Convergence Zone (SPCZ) is better defined than last month around Fiji, extending south and east from there. It is displaced slightly south and southwest of its climatological position east of the Date Line. Consequently, the atmosphere reflects mixed ENSO signals.
International guidance now strongly favours ENSO-neutral conditions (77% chance) over the next three month period (January - March 2017). By March-May 2017, the chance for neutral conditions rises to 87% with a meagre 8% chance for El Niño conditions and 5% chance for La Niña conditions. In summary, ENSO-neutral conditions are likely over the next 3-month period and should continue into autumn.
Sea surface temperatures (SSTs) are warmer than average to the east and north of New Zealand, while to the west over the Tasman Sea they are slightly cooler than normal. The dynamical model forecasts for SSTs indicate that this pattern is likely to persist over the next three month period (January – March 2017), with some warming possible over the Tasman Sea. Thus, 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 most islands of the southwest Pacific. Refer to NIWA’s Tropical Cyclone outlook at https://www.niwa.co.nz/news/southwest-pacific-tropical-cyclone-outlook for more information.

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. See www.niwa.co.nz/our-science/climate/publications/all/cu
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.
Visit our media centre at: www.niwa.co.nz/news-publications/media-centre

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