Seasonal climate outlook November 2017 - January 2018

Overview

The tropical Pacific is still officially in a ENSO (El Niño Southern Oscillation) neutral state, but some indicators have leaned more towards La Niña conditions during the course of October 2017. After a brief period of warming early in the month, sea surface temperatures (SST) in the eastern equatorial Pacific Ocean have cooled significantly, especially off the South American coast. The NIWA Southern Oscillation Index has been positive since July 2017 and is currently at +0.9.

The international consensus is that the tropical Pacific Ocean will cool further over the next 3 months (November 2017 – January 2018), with La Niña conditions likely to be met over the same period (with 70% chance). However, the models indicate that if La Niña does develop, it is likely to remain in the weak category and be short-lived: a return to neutral conditions is most likely (58% chance) over the February-April 2018 period.

For November 2017 – January 2018 as a whole, the atmospheric circulation around New Zealand is forecast to be characterized by higher pressure than normal to the south-east of the country, and lower pressure than normal to the north. This pressure pattern is expected to be associated with north-easterly to easterly flow anomalies, a pattern which is consistent with regional conditions typically observed during La Niña events.

For the coming tropical cyclone season (November 2017 to April 2018), outlooks indicate that the risk for New Zealand is normal or above normal. On average, at least one ex-tropical cyclone passes within 550km of New Zealand each year. If an ex-tropical cyclone comes close to the country, it has equal probability of passing east or west of Auckland and the North Island. Significant rainfall, damaging winds and coastal damage can occur leading up to and during these events.

Outlook Summary

November 2017 – January 2018 temperatures are forecast to be above average for all regions of New Zealand (60% to 70% chance for above average temperatures). Coastal water temperatures around New Zealand are forecast to remain above average over the next three-month period, especially along the east coast of the South Island.

November 2017 – January 2018 rainfall totals are about equally likely to be normal (35% chance) or above normal (40% chance) for the north and east of the North Island, and most likely to be near normal (45% chance) in the west of the North Island and the north and east of the South Island. Rainfall totals for the next three months are about equally likely to be below normal (40% chance) or near normal (35% chance) in the west of the South Island.

November 2017 – January 2018 soil moisture levels and river flows are most likely to be below normal (45% chance) in the west and east of the South Island and most likely to be above normal (45% chance) in the east of the North Island. Soil moisture levels and river flows are equally likely to be near normal (35% chance) or above normal (35% chance) in the north of the North Island. In the west of the North Island and the north of the South Island, soil moisture levels and river flows are most likely to be in the near normal range (40% chance).

Regional predictions for the November 2017 – January 2018 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 equally likely to be above normal (35% chance) or near normal (35% chance).

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	60	40	35	35
Near average	30	35	35	35
Below average	10	25	30	30

 

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 (65% chance).
• Rainfall totals, soil moisture levels and river flows are all most likely to be in the near normal range (40-45% chance).  

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	65	30	30	30
Near average	25	45	40	40
Below average	10	25	30	30

 

Gisborne, Hawke’s Bay, Wairarapa

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 most likely to be in the above normal range (45% chance).

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	60	40	45	45
Near average	30	35	30	30
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 most likely to be near normal (45% chance).
•  Soil moisture levels and river flows are most likely to be in the near normal range (40% chance).

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	60	30	30	30
Near average	30	45	40	40
Below average	10	25	30	30

 

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 (70% chance).
• Rainfall totals are about equally likely to be below normal (40% chance) or near normal (35% chance).
• Soil moisture levels and river flows are most likely to be in the below normal range (45% chance).

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	70	25	20	20
Near average	20	35	35	35
Below average	10	40	45	45

 

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 below normal range (45% chance).

 The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	60	25	20	20
Near average	30	45	35	35
Below average	10	30	45	45

Graphical representation of the regional probabilities

Image: NIWA

Attribution, Non-Commercial, No Derivative Work

Background

The tropical Pacific currently remains in a ENSO (El Niño Southern Oscillation) neutral state, but some indicators have leaned more towards La Niña conditions during October 2017.

After a brief period of warming early in October, sea surface temperatures (SSTs) in the central and eastern equatorial Pacific Ocean cooled substantially during the remainder of the month. This cooling was particularly significant in the far eastern Pacific, off the South American coast, where SST anomalies are now colder than -1^oC (NINO1 index currently at -1.07^oC). The latest monthly SST anomalies (as of 29 October) are reaching -0.2 ^oC in the NINO3.4 index region (with the latest weekly anomalies being -0.3 ^oC). The NINO3 region is also negative at -0.3^oC, and the NINO4 index (in the western Pacific) is close to zero.

Subsurface ocean waters in the first 150 metres of the ocean in the central and eastern equatorial Pacific (east of about 150^oW) remain cooler than normal and these anomalies have extended further eastward during  October. The heat content anomalies integrated over the first 300m of the ocean have become significantly more negative since late September and also expanded eastward. Heat content anomalies now exceed -1^oC from about 160^oW to 110^oW.

The Southern Oscillation Index (SOI) for the month of October 2017 (value estimated using data to the 30^th of October) is positive at +0.9; i.e., just short of the conventional La Niña threshold. In keeping with a positive SOI, trade winds along the equator are stronger than normal in the western half of the Pacific Ocean. Rainfall and convection anomalies in the tropical Pacific, which had been displaying a relatively strong La Niña-like pattern over the past few months, have however eased up. The ENSO Precipitation Index (ESPI), while still negative at -0.9, has weakened from values hovering around -1.5 in September 2017.

In summary, as was the case last month, the tropical Pacific Ocean has consistent La Niña-like signals in both the ocean and atmosphere. Some of these signals – notably SSTs in the eastern Pacific, and the SOI – have intensified since early October 2017, making it increasingly likely that the tropical Pacific will transition towards a La Niña state before the end of the year.

The international consensus now indicates that La Niña is the most likely outcome over the next three months period (November 2017 – January 2018) with 70% chance, a sharp increase compared to forecasts issued last month (with 50% chance over October – December 2017). Several institutions (e.g., the Australian BoM, and the NCEP CPC) are therefore in a ‘La Niña Watch’ status. However, there are clear indications that if La Niña does develop, the event is likely to remain weak and short-lived: a return to neutral conditions is most likely in the first quarter of 2018 (58% chance for neutral conditions over the February-April 2018 period).

La Niña events are typically associated with north-easterly flow anomalies over the country, leading to warmer than normal temperatures overall, above average rainfall over the northeast of the North Island and reduced rainfall in the south-west of the South Island. The expected circulation pattern over the next three months and the temperature and rainfall outlooks are broadly consistent with La Niña-like patterns.

Coastal waters remain generally warmer than average around New Zealand, especially along the east coast of the South Island, where the anomaly for the month of October (estimated using data to the 28^th October) exceeds +1^oC. On the other hand, waters around the North Island have cooled compared to last month. The warmest ocean waters in the New Zealand region remain around and south of the Chatham Islands.

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).
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.
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.