La Niña ending, but easterly circulation regime to persist.
The La Niña event is dissipating, and ENSO-neutral conditions are returning to the tropical Pacific. However, lower pressures than normal are expected to persist north of the country during the April to June period, with more high pressure systems and ridges over and east of southern New Zealand.
The NIWA National Climate Centre’s outlook for late autumn, April to June 2012, indicates air temperatures are likely to be average or above average in the west and south of the South Island, and near average elsewhere. Seas around New Zealand are likely to remain slightly cooler than normal.
Late autumn rainfall is likely to be normal or above normal in the north and east of the North Island, and near normal in all other regions. Soil moisture levels and river flows are likely to be near normal in the South Island and normal or above normal in North Island districts.
Overall picture
Temperature
Seasonal temperatures are likely to be average or above average in the west and south of the South Island, and near average elsewhere. Despite the overall pattern of near average seasonal temperatures in most places, frosts typical of late autumn will occur from time to time with the anticyclonic conditions.
Rainfall, soil moisture and river flows
Late autumn rainfall is likely to be normal or above normal in the north and east of the North Island, and near normal in all other regions. Soil moisture levels and river flows are likely to be near normal in the South Island and normal or above normal in all North Island districts.
Regional predictions for the next three months
Northland, Auckland, Waikato, Bay of Plenty
Late autumn temperatures are likely to be near average. Seasonal rainfall totals and river flows are likely to be in the near normal or above normal range. Soil moisture levels are projected to be above normal during April-June.
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 |
20% |
35% |
45% |
40% |
|
Near average |
50% |
40% |
35% |
40% |
|
Below average |
30% |
25% |
20% |
20% |
Click these links to view the historic rainfall and temperature ranges for this region
Central North Island, Taranaki, Wanganui, Manawatu, Wellington
Seasonal temperatures are likely to be near average. Late autumn rainfall totals and river flows are projected to be near normal for the three month season as a whole, while soil moisture levels are likely to be near normal or 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 |
20% |
20% |
35% |
30% |
|
Near average |
50% |
50% |
40% |
45% |
|
Below average |
30% |
30% |
25% |
25% |
Click these links to view the historic rainfall and temperature ranges for this region
Gisborne, Hawke’s Bay, Wairarapa
Seasonal temperatures are likely to be in the near average range. Late autumn rainfall totals are equally likely to be in the near normal or above normal range, while soil moisture levels and river flows are likely to be above normal or 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 |
30% |
40% |
45% |
45% |
|
Near average |
40% |
40% |
40% |
40% |
|
Below average |
30% |
20% |
15% |
15% |
Click these links to view the historic rainfall and temperature ranges for this region
Nelson, Marlborough, Buller
Late autumn temperatures are likely to be near average. Seasonal rainfall totals, river flows, and soil moisture levels 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:
|
Temperature |
Rainfall |
Soil moisture |
River flows |
|
|
Above average |
30% |
20% |
30% |
25% |
|
Near average |
45% |
50% |
45% |
45% |
|
Below average |
25% |
30% |
25% |
30% |
Click these links to view the historic rainfall and temperature ranges for this region
West Coast, Alps and foothills, inland Otago, Southland
Late autumn temperatures are equally likely to be near average or above average. Seasonal rainfall totals are very likely to be in the near normal range. Soil moisture levels and river flows are likely to be in the normal range, for the three months 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% |
10% |
15% |
15% |
|
Near average |
40% |
60% |
50% |
50% |
|
Below average |
20% |
30% |
35% |
35% |
Click these links to view the historic rainfall and temperature ranges for this region
Coastal Canterbury, east Otago
Seasonal temperatures are likely to be near average. Late autumn rainfall totals, river flows, and soil moisture levels 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:
|
Temperature |
Rainfall |
Soil moisture |
River flows |
|
|
Above average |
30% |
25% |
25% |
25% |
|
Near average |
45% |
50% |
50% |
50% |
|
Below average |
25% |
25% |
25% |
25% |
Click these links to view the historic rainfall and temperature ranges for this region
Background
The recent La Niña event weakened during February-March, and present conditions in the tropical Pacific are near ENSO-neutral. The Southern Oscillation Index (SOI) is close to zero for the second month in a row. Sea surface temperatures are now warmer than normal in the far eastern tropical Pacific.
Almost all global climate models predict neutral conditions in the tropical Pacific over late autumn (April-June), with a majority of them continuing this situation over the subsequent three months. However, about a quarter of the models predict the evolution of an El Niño state during the July-September period. NIWA will continue to monitor the global climate situation.
In spite of the collapse of La Niña in the tropical Pacific, the easterly circulation regime that has been dominating New Zealand climate patterns since December 2011 is expected to persist through the April to June 2012 period.
For comment, please contact
Dr James Renwick, NIWA Principal Scientist, Climate Variability & Change
Tel (021) 178 5550 (mobile), (04) 386 0343 (office DDI)
Dr Brett Mullan, NIWA Principal Scientist, Climate Variability & Change
Tel (04) 386 0508 (office DDI), 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.
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