NIWA Outlook: November 2015 - January 2016

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:

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:

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:

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:

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:

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:

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

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