NIWA Outlook: November 2015 - January 2016
Overview
Atmospheric and oceanic anomalies in the equatorial Pacific reflect strong El Niño conditions. Sea surface temperature
anomalies in the central and eastern Pacific have increased since September and are close to or exceed +2.5oC in places,
whereas sub-surface temperature anomalies exceed +6oC in the far eastern tropical Pacific. The atmosphere is well
coupled to these ocean anomalies: the Southern Oscillation Index (SOI) is strongly negative (-2.1 for October 2015), and
westerly wind anomalies (implying weaker easterly trade-winds) have intensified further since September. Convection and
rainfall is suppressed over Indonesia, while enhanced convective activity and rainfall are observed in the central and
eastern equatorial Pacific.
International guidance indicates that El Niño is certain (100% chance) to continue over the next three months. The
current event is slightly weaker than the 1997/98 El Niño (the strongest since 1950) at this stage, but is expected to
intensify further and peak in the summer months.
For November 2015 - January 2016, above normal pressure is forecast to the north and west 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 west-southwesterly wind flows - a signature of El Niño conditions. Sea surface temperatures are forecast to be
normal or below normal to the west of the country, and below normal to the east of New Zealand.
Outlook Summary
November 2015 - January 2016 temperatures are equally likely to be average (40% chance) or below average (40% chance)
for the north and east of the North Island, and most likely to be near average (45% chance) for the east of the North
Island and north of the South Island. Below average temperatures are most likely (45% chance) for the west and east of
the South Island.
November 2015 – January 2016 rainfall is most likely to be below normal (50% chance) in the north and east of the North
Island, but equally likely to be near normal (40% chance) or above normal (40% chance) in the west of the South Island.
Near normal rainfall is the most likely outcome (45% chance) for the west of the North Island and north of the South
Island. There is low confidence in seasonal rainfall in the east of the South Island.
November 2015 - January 2016 soil moisture levels are most likely to below normal in the north of the North Island (55%
chance) and in the east of the South Island (45% chance). Soil moisture levels are likely to be near normal (40% chance)
or below normal (35-45% chance) in other regions, except for the west of the South Island where the outlook is for near
normal (40% chance) or above normal (40% chance) soil moistures.
November 2015 - January 2016 river flows are most likely to be in the below normal range (45-50% chance) for the north
and east of the North Island, as well as the north of the South Island. River flows are about equally likely to be in
the near normal (35-40% chance) or below normal (40% chance) ranges for the west of the North Island and east of the
South Island. River flows are likely to be normal (35% chance) or above normal (35% chance) in the west of the South
Island.
Regional predictions for the October to December 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 below average (40% chance).
• Rainfall, soil moisture levels and river flows are all most likely to be below normal (50-55% chance).
Other outcomes cannot be excluded. The full probability breakdown is:TemperatureRainfallSoil moistureRiver flowsAbove average20151515Near average40353035Below average40505550
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 most likely to be in the near normal range (45% chance).
• Soil moisture levels and river flows are about equally likely to be in the near normal (40% chance) or below
normal (35-40% chance) ranges.
The full probability breakdown is:TemperatureRainfallSoil moistureRiver flowsAbove average2025 2520Near average404540 40Below average403035 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 (50% chance).
• Soil moisture levels are about equally likely to be in the below normal (45% chance) or near normal (40% chance)
ranges.
• River flows are most likely to be in the below normal range (45% chance).
The full probability breakdown is:TemperatureRainfallSoil moistureRiver flowsAbove average3015 15 20Near average453540 35 Below average255045 45
Nelson, Marlborough, Buller
Probabilities are assigned in three categories: above average, near average, and below average.
• Temperatures are most likely to be average (45% chance).
• Rainfall totals are most likely to be in the near normal range (45% chance) for the region as a whole, but with
a west-east gradient such that Marlborough is more likely to be normal or below normal.
• Soil moisture levels are equally likely to be in the near normal (40% chance) or below normal (40% chance)
ranges.
• River flows are most likely to be below normal (45% chance).
The full probability breakdown is:TemperatureRainfallSoil moistureRiver flowsAbove average202020 20 Near average454540 35Below average353540 45
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 below average (45% chance).
• Rainfall totals and soil moisture levels are equally likely to be in the near normal (40% chance) or above
normal (40% chance) ranges.
• River flows are equally likely to be in the near normal (35% chance) or above normal (35% chance) ranges. The
tercile distribution is close to a climatological spread (33% in each tercile), indicating low confidence in this
forecast.
The full probability breakdown is:TemperatureRainfallSoil moistureRiver flowsAbove average204040 35Near average354040 35 Below average45202030
Coastal Canterbury, east Otago
Probabilities are assigned in three categories: above average, near average, and below average.
• Temperatures and soil moisture levels are most likely to be below average (45% chance).
• Rainfall totals and river flows are about equally likely to be in the near normal range (35% chance) or below
normal range (35-40% chance).
The full probability breakdown is:TemperatureRainfallSoil moistureRiver flowsAbove average203025 25 Near average353530 35 Below average453545 40
Background
Sea surface temperature (SSTs) in the eastern and central Pacific continued to warm during October 2015, and now exceed
+2°C over much of the tropical belt east of the International Dateline. Sub-surface ocean temperatures in the eastern
Pacific have also remained high, exceeding +6oC off the South American coast at 120oW and 75-100m depth.
The Southern Oscillation Index (SOI) has remained strongly negative since July 2015, and is at -2.1 for the month of
October 2015. Episodes of intense westerly wind anomalies continued to dominate the central and western Pacific Ocean.
Consistent with these circulation patterns, convection and rainfall along the Equator were shifted eastward: Indonesia
and large parts of the Maritime Continent experienced much drier than normal conditions, while much more intense than
normal rainfall was observed in the central and eastern Pacific. Collectively, these atmospheric and oceanic features
reflect fully coupled El Niño conditions.
International guidance indicates that El Niño conditions are certain (100% chance) to continue over the next three month
period (November 2015 – January 2016) and virtually certain (95-100% chance) to carry on through early autumn (February
– April 2016), before a likely rapid weakening during the winter of 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 south-westerlies in spring. Such a circulation pattern usually leads to
cooler conditions in most regions of the country, 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 (November 2015 – January 2016)
forecasts broadly reflect these typical impacts.
There is an elevated risk of drought later this summer, in particular in the north and east of both islands. 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.
Waters surrounding New Zealand are currently slightly cooler than normal, especially to the east of the South Island.
Ocean models generally suggest that ocean temperature anomalies around New Zealand will become more negative as the
season progresses.
To find out more about normal conditions for this outlook period, refer to NIWA’s website, where daily updates on climate maps are available.
ENDS