Seasonal Climate Outlook: February - April 2016

Strong El Niño conditions continued in the tropical Pacific during January 2016.

Sea surface temperature anomalies still exceed +2^oC in the central and eastern Pacific, but have weakened slightly from the peak values in November-December 2015. The sub-surface ocean temperature anomalies in the eastern Pacific have decreased further since December, and are now about +4^oC at 75-100m depth near 120^oW, compared to peak values of about +7^oC in November 2015. Conversely, the Southern Oscillation Index strengthened to about -2.0 for the month of January.

International guidance indicates that El Niño conditions will continue (96% probability) over the next three months (February – April 2016) and will rapidly decay thereafter, with a return to normal conditions or a transition to La Niña conditions by August – October 2016.

For February - April 2016, above normal pressure is forecast to the north 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 westerly wind flows – a signature consistent with El Niño.

Outlook Summary

February - April 2016 temperatures are most likely (45% chance) to be above average for the north and west of the North Island.  Temperatures are about equally likely to be near average (35% chance) or above average (40% chance) for all remaining regions of the country.

February - April 2016 rainfall is about equally likely to be below normal (35-40% chance) or in the normal range (40% chance) for the north and east of the North Island and for the east of the South Island. Seasonal rainfall totals are most likely to be near normal (45% chance) for west of the North Island and for the north and west of the South Island.

February - April 2016 soil moisture levels and river flows are about equally likely to be below normal (35-45% chance) or in the normal range (40% chance) for the north and east of the North Island and for the east of the South Island. Soil moisture levels and river flows are most likely to be near normal (45% chance) for west of the North Island and for the north and west of the South Island.

Regional predictions for the February - April 2016 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 most likely to be above average (45% chance).
• Rainfall totals and soil moisture levels are about equally likely to be below normal (35% chance) or near normal (40% chance). There is nevertheless the possibility that weather systems coming out of the tropics could produce substantial rainfall in parts of the region.
• River flows are equally likely to be below normal (40% chance) or near normal (40% chance).

The full probability breakdown is:

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

Central North Island, Taranaki, Wanganui, Manawatu, Wellington

Probabilities are assigned in three categories: above average, near average, and below average.

• Temperatures are most likely to be above average (45% chance).
• Rainfall totals, soil moisture levels and river flows are all most likely to be near normal (45% chance).

The full probability breakdown is:

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

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 above average (40% chance) or average (35% chance).
• Rainfall totals and river flows are equally likely to be below normal (40% chance) or near normal (40% chance).
• Soil moisture levels 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	40	20	15	20
Near average	35	40	40	40
Below average	25	40	45	40

Nelson, Marlborough, Buller

Probabilities are assigned in three categories: above average, near average, and below average.

• Temperatures are about equally likely to be above average (40% chance) or average (35% chance).
• Rainfall totals, soil moisture levels and river flows are all most likely to be normal (45% chance).

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	40	20	20	20
Near average	35	45	45	45
Below average	25	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 about equally likely to be above average (40% chance) or average (35% chance).
• Rainfall totals, soil moisture levels and river flows are all most likely to be in the normal range (45% chance).

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	40	35	25	30
Near average	35	45	45	45
Below average	25	20	30	25

Coastal Canterbury, east Otago

Probabilities are assigned in three categories: above average, near average, and below average.

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

The full probability breakdown is:

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

Graphical representation of the regional probabilities

Background

Strong El Niño conditions continued in the tropical Pacific during January 2016. Sea surface temperature anomalies still exceed +2^oC, but have weakened slightly from the peak values in November-December 2015. The sub-surface ocean temperature anomalies in the eastern Pacific have decreased further since December, and now reach about +4^oC at 75-100m depth near 120^oW (compared to peak values of about +7^oC in November 2015). Conversely, the Southern Oscillation Index (SOI) strengthened to about -2.0 for the month of January. Strong westerly wind anomalies (weaker easterly trade-winds) also continued to affect the western and central Pacific. The contrast in convective activity between the Maritime Continent and areas east of the Dateline also intensified: rainfall was well above normal along the equator from the Dateline to about 140^oW, while dryness was very pronounced over Indonesia and north of Australia during January 2016.

International guidance indicates that El Niño conditions will continue (96% chance) over the next three month period (February – April 2016), and forecast a swift decline in the following season  May – July 2016 (only 39% chance of El Niño conditions persisting over this period). For the August – October season, the models are forecasting an approximately equal chance of neutral or La Niña conditions. 

While sea temperature indicators of El Niño showed signs of weakening in the past month, other indicators (such as the SOI and westerly wind anomalies) have remained strong or intensified. Thus, the current event is not yet over, and the rainfall outlooks above suggest that typical El Niño impacts on New Zealand’s climate will likely persist into autumn. Fortunately, recent rainfall in both islands means that soil moisture deficits have improved significantly since the start of the year, though soils will dry out rapidly at this time of year if there is another sustained period of little or no rainfall.

El Niño events are typically (but not always) associated with stronger and/or more frequent westerly winds over summer in New Zealand, and more south-westerlies in autumn. Autumn and early winter is also the time of year when ENSO events typically decay. In the present instance, it appears that El Niño will not return in spring, but that the opposite La Niña pattern is a possibility by late in 2016.

The southwest Pacific tropical cyclone season continues through to April 2016. Ex-tropical weather systems have brought periods of significant rainfall to the northeast of the North Island from Northland to Bay of Plenty, alleviating incipient dry conditions. Further ex-tropical activity cannot be ruled out in the coming three months. 

Water temperatures surrounding New Zealand are currently close to normal to the west of New Zealand, but cooler than normal to the east. This pattern applies not only at the surface but down to about 1000m depth. Ocean models suggest that waters will remain cooler than normal to the east of the country for February – April 2016.

To find out more about normal conditions for this outlook period, refer to NIWA’s website, where daily updates on climate maps are available.

Contact

For comment, please 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. 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.

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