Wetter in west and drier in east likely to continue

Seasonal Climate Outlook

Wetter in west and drier in east likely to continue

The equatorial Pacific Ocean reflects ENSO-neutral conditions (neither El Niño nor La Niña). Global guidance indicates that these neutral conditions are very likely to persist into the Southern Hemisphere autumn. For the New Zealand region over the coming three months (January-March 2013), lower than normal pressures are expected in the south Tasman Sea and to the southeast of the Chatham Islands, with enhanced south-westerly winds over the country.

January-March rainfall is likely to be near normal or above normal in west and south of the South Island, but near normal or below normal in the east of the North Island. Near normal rainfall is likely for other regions. Late-summer soil moisture levels and river flows are likely to be below normal or normal in the east of the North Island, and near normal in all other regions of New Zealand.

January-March temperatures are likely to be near average or below average in the west of the North Island, and near average in all other regions. Sea surface temperatures around New Zealand are expected to remain close to average overall over the late-summer period.

For this tropical cyclone season (November – April), the risk of an ex-Tropical Cyclone approaching New Zealand is expected to be near normal. On average, one ex-Tropical Cyclone nears New Zealand each year.

Overall picture

Rainfall, soil moisture and river flows

January-March rainfall is likely to be near normal or above normal in west and south of the South Island, but near normal or below normal in the east of the North Island. Near normal rainfall is likely for remaining regions.

Late-summer soil moisture levels and river flows are likely to be below normal or normal in the east of the North Island, and near normal in all other regions of New Zealand.

Temperature

January-March temperatures are likely to be near average or below average in the west of the North Island, and near average in all other regions. Sea surface temperatures around New Zealand are expected to remain close to average overall over the late-summer period.

Regional predictions for the next three months

Northland, Auckland, Waikato, Bay of Plenty

January-March temperatures are likely to be in the near average range. Rainfall, soil moisture levels and river flows are all likely to be in the near normal range for the season as a whole.

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

Central North Island, Taranaki, Wanganui, Manawatu, Wellington

January-March temperatures are equally likely to be near average or below average. Rainfall, soil moisture levels and river flows are all likely to be in the near normal range for the season as a whole.

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

Gisborne, Hawke’s Bay, Wairarapa

Seasonal temperatures are likely to be in the near average range. Late summer rainfall totals, soil moisture levels and river flows are likely to be below normal or near normal for this time of year.

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

Nelson, Marlborough, Buller

Late summer temperatures are likely to be near average. Seasonal rainfall totals, soil moisture levels, and river flows are also all expected to be in the near normal range.

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

West Coast, Alps and foothills, inland Otago, Southland

Late summer temperatures are likely to be near average. Seasonal rainfall totals are equally likely to be in the near normal or above normal ranges. January-March soil moisture levels and river flows are likely to be in the normal range.

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

Coastal Canterbury, east Otago

January-March temperatures are likely to be near average. Seasonal rainfall, soil moisture levels and river flows are all likely to be in the near normal range.

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

Background

The equatorial Pacific Ocean as a whole reflects ENSO-neutral conditions (neither El Niño nor La Niña), although the Southern Oscillation Index for December dropped to -0.8. The 3-month SOI (October-December 2012) is -0.1.

International guidance indicates that the tropical Pacific Ocean is very likely to remain neutral over the next three months (January-March). All the 10 dynamical models and 5 statistical models monitored by NIWA (table below) forecast ENSO-neutral SST anomalies for January-March 2013 and also for the following April-June season.

For comment, please contact

Dr Brett Mullan, NIWA Principal Scientist, Climate Variability & Change

Tel (04) 386 0508, Mobile (027) 294 1169

Dr Andrew Lorrey, Climate Scientist

Tel (09) 375 2055, Mobile (021) 313 404

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.

Visit our media centre at: www.niwa.co.nz/news-publications/media-centre

ENDS

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