NIWA Outlook: September – November 2017
Overview

Regional predictions for the next three months

Northland, Auckland, Waikato, Bay of Plenty
Central North Island, Taranaki, Whanganui, Manawatu, Wellington
Gisborne, Hawke’s Bay, Wairarapa
Tasman, Nelson, Marlborough, Buller
West Coast, Alps and foothills, inland Otago and Southland
Coastal Canterbury, east Otago
Background
Contacts
Notes to reporters and editors

ENSO (El Niño – Southern Oscillation) neutral conditions were still present in the tropical Pacific during August 2017. However, like July, several oceanic and atmospheric patterns, such as the Southern Oscillation Index (SOI) and decreasing sea surface temperatures (SSTs) in the tropical Pacific, leaned toward La Niña. The latest weekly sea-surface and ocean subsurface temperatures in the equatorial Pacific Ocean are slightly below normal (i.e. on the La Niña side of neutral). The Southern Oscillation Index (SOI) is currently positive (+0.4, i.e. on the La Niña side of neutral) and large-scale circulation anomalies along the equator are consistent with patterns usually associated with a La Niña state. On the other hand, La Niña-like rainfall and convection anomalies in the west Pacific were not as strong in August as they were in July.

International guidance favours a persistence of ENSO neutral conditions over the next three-month period (65% chance for September – November 2017). While ENSO-neutral conditions remain the most likely outcome through the end of 2017, recent observations indicate that the ocean and atmosphere may continue to periodically exhibit La Niña-like signatures.

For September – November 2017, the atmospheric circulation around New Zealand is forecast to be characterised by lower pressure than normal west of New Zealand and higher pressure than normal to the south and east of the country. This is expected to lead to northerly-quarter (ranging from northwest to northeast) flow anomalies over the next three months. Periodic easterly flow anomalies are also possible, consistent with a La Niña-like signal in the atmosphere. This type of atmospheric setup may lend itself to subtropical moisture connections associated with heavy rainfalls for New Zealand.

Outlook Summary

September – November 2017 temperatures are forecast to be above average for all regions of New Zealand (55% to 65% chance for above average temperatures). Nevertheless, frosts and cool snaps are still possible during spring. Coastal water temperatures around New Zealand are forecast to remain above average over the next three-month period.

September – November 2017 rainfall totals are about equally likely to be normal (35-40% chance) or above normal (35-40% chance) for the North Island and the north of the South Island and most likely to be near normal (45% chance) for all remaining regions of New Zealand.

September – November 2017 soil moisture levels and river flows are about equally likely to be near normal (35-40%) or above normal (35-40% chance) for the North Island and the north of the South Island. Soil moisture levels and river flows are most likely to be near normal (45% chance) in the east and west of the South Island.

Regional predictions for the September – November 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 very likely to be above average (60% chance).
• Rainfall totals are about equally likely to be in the above normal range (40% chance) or near normal range (35% 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 60 40 40 40
Near average 30 35 35 35
Below average 10 25 25 25

Central North Island, Taranaki, Whanganui, Manawatu, Wellington
Probabilities are assigned in three categories: above average, near average, and below average.
• Temperatures are very likely to be above average (60% chance).
• Rainfall totals are about equally likely to be near normal (40% chance) or above normal (35% chance).
• Soil moisture levels and river flows are about equally likely to be near normal (35-40% chance) or above normal (40% chance).

The full probability breakdown is:

Temperature Rainfall Soil moisture River flows
Above average 60 35 40 40
Near average 30 40 40 35
Below average 10 25 20 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 (55% chance).
• Rainfall totals are about equally likely to be above normal (40% chance) or near normal (35% chance).
• Soil moisture levels and river flows are about equally likely to be in the near normal (40% chance) or above normal (35% chance) range.
The full probability breakdown is:

Temperature Rainfall Soil moisture River flows
Above average 55 40 35 35
Near average 35 35 40 40
Below average 10 25 25 25

Tasman, Nelson, Marlborough, Buller
Probabilities are assigned in three categories: above average, near average, and below average.

• Temperatures are very likely to be above average (60% chance).
• Rainfall totals are about equally likely to be above normal (40% chance) or near normal (35% chance).
• Soil moisture levels and river flows are about equally likely to be in the near normal (40% chance) or above normal (35% chance) range.

The full probability breakdown is:

Temperature Rainfall Soil moisture River flows
Above average 60 40 35 35
Near average 30 35 40 40
Below average 10 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 very likely to be above average (65% 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 in the near normal range (45% chance).

The full probability breakdown is:

Temperature Rainfall Soil moisture River flows
Above average 65 30 30 30
Near average 25 45 45 45
Below average 10 25 25 25

Coastal Canterbury, east Otago
Probabilities are assigned in three categories: above average, near average, and below average.

• Temperatures are very likely to be above average (60% 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 in the near normal range (45% chance).

The full probability breakdown is:

Temperature Rainfall Soil moisture River flows
Above average 60 25 30 25
Near average 30 45 45 45
Below average 10 30 25 30

Background

ENSO (El Niño – Southern Oscillation) neutral conditions persisted across the tropical Pacific during August 2017, but several oceanic and atmospheric indicators leaned toward La Niña. The SOI was, on average, positive during August (+0.4, i.e. on the La Niña side of neutral) and enhanced equatorial trade winds were reflective of a La Niña-like atmosphere.
Sea surface temperatures in the equatorial Pacific Ocean continued to trend cooler during August 2017 in both the central and eastern equatorial Pacific. The latest monthly SST anomaly, ending 27th August 2017, in the NINO3.4 region (in the central Pacific) is currently 0.02°C (was +0.43oC last month). Slightly cooler than average waters are also present in the far eastern equatorial Pacific.

Subsurface ocean temperatures have also cooled compared to last month, with the latest weekly data showing weak negative anomalies extending from the surface to about 150 meters depth in both the central and eastern Pacific. This is another oceanic indicator leaning in the La Niña direction.

The preliminary [value estimated on the 30th of August] Southern Oscillation Index (SOI) for the month of August 2017 is positive at +0.4, i.e. on the La Niña side of neutral. The SOI has now been positive for the past 2 months.
Zonal wind anomalies along the equator were predominantly negative in the central and western equatorial Pacific during August, indicating enhanced trade winds: a pattern which is consistent with a positive SOI. However, rainfall and convection anomalies in the tropical Pacific were not as La Niña-like as they were in July. This was due to decreased convection over parts of the western Pacific and Maritime Continent associated with the Madden Julian Oscillation.
Nevertheless, the ENSO Precipitation Index (ESPI) is currently negative with a value near -1.2. The ESPI was near -1.7 during July. The negative values over the past two months are indicative of a La Niña-like state.

In summary, while the ocean – atmosphere system in the tropical Pacific Ocean overall remains consistent with an ENSO-neutral state, La Niña-like signals became more prominent in the atmosphere during July and this trend continued during August 2017.

International guidance favours a persistence of ENSO neutral conditions over the next three-month period (65% chance for September – November 2017). The likelihood for La Niña peaks at 25% in the November 2017 – January 2018 period. While ENSO-neutral conditions remain the most likely outcome through the end of 2017, recent observations indicate that the ocean and atmosphere may continue to periodically exhibit La Niña-like signatures.

Coastal waters remain generally warmer than average all around the country and positive anomalies increased during the final week of August, especially near the eastern and northern North Island. The anomaly in the “NZ box” (160°E-170°W, 30-45°S) is currently nearing +0.6oC. Ocean waters are still much warmer than average across the Tasman Sea, particularly in the south and west.

The dynamical models’ forecasts indicate that warmer than average SSTs around New Zealand are likely to persist over the September – November 2017 period.

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