Seasonal Climate Outlook: January - March 2012

La Niña through late summer: continuing wet in north and dry in south.

The NIWA National Climate Centre’s outlook for late summer, January to March 2012, indicates that seasonal rainfall is likely to be above normal for all North Island regions, normal or above normal in Nelson-Marlborough, and below normal or normal for the remainder of the South Island.  

Soil moisture levels and river flows are both predicted to follow a very similar regional pattern to rainfall: they are likely to be above normal for all North Island regions, normal or above normal in Nelson-Marlborough, and below normal for the remainder of the South Island. For the season as a whole, temperatures are likely to be average or above average for the eastern South Island, and above average in all other regions.

A moderate La Niña is in place in the tropical Pacific and should persist into early autumn 2012, according to the NIWA National Climate Centre.

For the January to March season, mean sea level pressures are likely to be above normal across the South Island, but below average to the north of New Zealand, with more north-easterly air flow than normal over the North Island.

For the tropical cyclone season through to May 2012, fewer than the normal number of cyclones is expected overall. January to March is typically the most active part of the cyclone season. However, the chance of an ex-tropical cyclone passing close to New Zealand is expected to be below the long-term average. On average, at least one ex-tropical cyclone passes within 500km of New Zealand in 9 out of 10 cyclone seasons.

Overall Picture

Temperature:

For the January-March period as a whole, air temperatures are likely to be above average throughout the North Island and in the north, west and south of the South Island. In the east of the South Island, temperatures are likely to be average or above average. Sea surface temperatures in the New Zealand region are likely to be near average or above average.  

Rainfall, soil moisture, and river flows:

The National Climate Centre projects that late summer rainfall totals are likely to be above normal throughout the North Island, and normal or above normal in the north of the South Island. For the west, south and east of the South Island, late summer rainfall is likely to be below normal or normal. Soil moisture levels and river flows are likely to be above normal in North Island regions, normal or above normal in Nelson-Marlborough, and below normal in the west, south and east of the South Island.

Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:

Late summer temperatures are likely to be above average. Seasonal rainfall totals, soil moisture levels, and river flows are all likely to be above normal. 

Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	50%	50%	55%	45%
Near average	40%	30%	30%	35%
Below average	10%	20%	15%	20%

Central North Island, Taranaki, Wanganui, Manawatu and Wellington: 

Seasonal temperatures are likely to be above average. Late summer rainfall totals, soil moisture levels, and river flows are likely to be above normal, for the three month season as a whole.

Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	50%	50%	50%	50%
Near average	40%	30%	35%	30%
Below average	10%	20%	15%	20%

Gisborne, Hawke’s Bay, Wairarapa:

Late summer temperatures are likely to be above average. Seasonal rainfall totals, soil moisture levels, and river flows are all likely to be above normal. 

Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	50%	50%	50%	45%
Near average	40%	30%	35%	35%
Below average	10%	20%	15%	20%

Nelson, Marlborough, Buller:

Temperatures over the late summer period are likely to be above average. Seasonal rainfall totals and river flows are equally likely to be near normal or above normal. Late summer soil moisture levels are likely to be above normal or normal for the time of year.

Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	50%	40%	45%	40%
Near average	40%	40%	40%	40%
Below average	10%	20%	15%	20%

West Coast, Alps and Foothills, Inland Otago, Southland:

Above average late summer temperatures are likely. Seasonal rainfall totals are likely to be below normal or normal. Soil moisture levels and river flows are likely to be below normal, for the period as a whole.

Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	50%	25%	20%	20%
Near average	40%	35%	35%	35%
Below average	10%	40%	45%	45%

Coastal Canterbury, East Otago:

Late summer temperatures are equally likely to be average or above average. Seasonal rainfall totals are likely to be below normal or near normal. Soil moisture levels and river flows are both likely to be below normal, for the three month season as a whole. 

Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	40%	25%	20%	15%
Near average	40%	35%	35%	35%
Below average	20%	40%	45%	50%

Background

The tropical Pacific is currently showing moderate La Niña conditions. During December 2011, atmospheric indices of La Niña conditions (Southern Oscillation Index, tropical easterly winds, patterns of convection over Indonesia and near the Date Line) all showed increasing trends. The colder than normal sea surface temperatures along the equatorial Pacific persisted, with the anomalies weakening slightly in the eastern Pacific and intensifying (becoming more negative) in the western Pacific near the Date Line. Global climate models predict a continuation of La Niña conditions into autumn 2012. The models agree on the peak intensity occurring during summer, but some delay the weakening of the La Niña event until early winter.

For comment, please contact:

Dr Brett Mullan, NIWA Principal Scientist, Climate Variability & Change Tel (04) 386 0508 (office DDI) Mobile (027) 294 1169

Dr James Renwick, NIWA Principal Scientist, Climate Variability & Change Mobile (021) 178 5550

© Copyright NIWA 2011. All rights reserved. Acknowledgement of NIWA as the source is required.

Notes to reporters & 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%
• Near average: 30%
• Below average: 10%

4. We therefore concluded that above average temperatures were very likely.

5. This three-way probability means that a random choice would only be correct 33% (or one-third) of the time. It would be like randomly throwing a dart at a board divided into 3 equal parts, or throwing a dice with three numbers on it. An analogy with coin tossing (a two-way probability) is not correct.

6. A 50% ‘hit rate’ is substantially better than guess-work, 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 U.S. (http://iri.ldeo.columbia.edu/) 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).

7. Each month NIWA publishes an analysis of how well its outlooks perform. This is available on-line and is sent to about 3,500 recipients of NIWA’s newsletters, including many farmers. See The Climate Update: www.niwascience.co.nz/ncc

8. All outlooks are for the three months as a whole. There will inevitably be wet and dry days, 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 from the long-term mean, whereas for rainfall the “near normal” category lies approximately between 80% and 115% of the long-term mean.

9. 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.