La Niña continues, with mild conditions nationally, and likely dry in the south
NIWA – A moderate La Niña in the tropical Pacific is expected to continue through autumn before returning to ENSO-neutral conditions by the start of winter (ENSO = El Niño Southern Oscillation). NIWA’s National Climate Centre says the late summer and early autumn of 2009 is likely to be warmer than average in many areas, with dry conditions likely in the south.
The centre’s Seasonal Climate Outlook for February to April 2009 indicates that average or above average temperatures are the most likely outcome across the whole country. Normal or below normal rainfall is likely in the west, south, and east of the South Island, while normal or above normal rainfall is likely over the north of the North Island.
Soils are presently drier than normal in many regions of the country, except for the southwest of the South Island. Normal or below normal soil moisture levels and streamflows are likely throughout the South Island through to April, while near normal conditions are likely in the North Island.
Mean sea-level pressures are likely to be higher than normal over southern New Zealand and to the east, with more easterly wind episodes than normal over the North Island, and lighter winds than normal over the South Island.
For the tropical cyclone season (through to May 2009), there is an average chance of an ex-tropical cyclone passing within 500 km of the country.
Air temperatures are likely to be above average in the North Island, and average or above average in the South Island. Sea surface temperatures around New Zealand are expected to remain above normal through the season.
Rainfall, soil moisture, and stream flows:
Rainfall is predicted to be normal or above normal in the north of the North Island, normal for other regions of the North Island, and normal or below normal for the South Island. Normal soil moisture levels and streamflows are likely in the north and west of the North Island, and below normal conditions are expected in the east of the South Island. In other regions, normal or below normal soil moisture levels and streamflows are likely.
[Reporters please note: Probabilities are assigned in THREE categories; above average, average, and below average. See end for more explanation.]
Regional predictions for the next three months:
Northland, Auckland, Waikato, Bay of Plenty:
Above average temperatures are likely. Rainfall is likely to be normal or above normal, with soil moisture and stream flows likely to be normal for the season as a whole.
Central North Island, Taranaki, Wanganui, Manawatu and Wellington:
Above average temperatures are likely. Normal rainfall, soil moisture levels and stream flows are likely for the three-month period.
Gisborne, Hawke’s Bay, Wairarapa:
Above average temperatures are likely for the three months. Normal rainfall is likely overall, with normal or below normal soil moisture levels and stream flows.
Nelson, Marlborough, Buller:
Average or above average temperatures are likely. Rainfall is expected to be near normal, with normal or below normal soil moisture levels and stream flows.
West Coast, Alps and Foothills, Inland Otago, Southland:
Average temperatures are likely. Rainfall is likely to be normal or below normal, as are soil moisture levels and stream flows.
Coastal Canterbury, East Otago:
Average or above average temperatures are likely for February-April. Rainfall is likely to be normal or below normal, with below normal soil moisture levels and stream flows.
Climate and Oceans:
In the New Zealand region, mean sea level pressures are likely to be higher than normal over southern New Zealand and to the east, with more easterly wind episodes than normal over the North Island, and lighter winds than normal over the South Island. Sea surface temperatures around New Zealand are currently about 0.5°C above normal, and are likely to remain near this level through the coming season.
La Niña conditions, which redeveloped in the tropical Pacific during December, are expected to continue through autumn, and then dissipate.
The tropical cyclone season for the southwest Pacific runs from November to May. With current conditions in the Pacific, there is an average chance of an ex-tropical cyclone passing within 500 km of the country between February and May. The districts at highest risk are Northland and Gisborne.
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, average conditions, and below average conditions for rainfall, temperature, soil moisture, and stream flows. For example, for winter (June-July-August) 2007, for all the North Island, we assigned the following probabilities for temperature:
o Above average: 60%
o Average: 30%
o Below average: 10%
We therefore conclude that above average temperatures were very likely.
4. 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.
5. 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).
6. 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
7. 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.
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.