NIWA: Dry Conditions Likely As La Nina Returns

Seasonal Climate Outlook: October - December 2011

Dry Conditions Likely As La Nina Returns

The NIWA National Climate Centre’s outlook for late spring and early summer, October to December 2011, indicates that seasonal rainfall is likely to be normal or below normal in all regions. Soil moisture levels and river flows are likely to be below normal in all regions of the country, except for the west and south of the South Island where normal or below normal soil moisture levels are likely. The Centre notes that soils are already drier than normal for this time of year in north Canterbury, MacKenzie country and central Otago, as well as parts of the North Island.

Temperatures for October-December are likely to be average or above average in the North Island and northern South Island, and near average in the rest of the South Island.
La Niña conditions are redeveloping in the tropical Pacific, and the event is expected to build through spring and continue over the summer season, according to the NIWA National Climate Centre.

The outlook states that mean sea level pressures during the October-December period as a whole are likely to be above normal across New Zealand, with weaker westerlies over the country.

Overall Picture

Temperature:
For the October – December period as a whole, air temperatures are likely to be average or above average in the North Island and in Nelson-Marlborough. For the remainder of the South Island, temperatures are likely to be near average. Sea surface temperatures near New Zealand are expected to be close to normal or slightly above normal through the period.

Rainfall, soil moisture, and river flows:
The National Climate Centre says that late spring-early summer rainfall is likely to be normal or below normal for all regions of New Zealand. Soil moisture levels and river flows are likely to be below normal everywhere, except for the west and south of the South Island where normal or below normal soil moisture levels are likely. Soils are already drier than normal for this time of year in north Canterbury, MacKenzie country and central Otago, as well as parts of the North Island.

Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:
Temperatures are likely to be average or above average for the time of year. October to December rainfall totals are equally likely to be in the normal or below normal range. Soil moisture levels and river flows are likely to be below 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 40% 20% 15% 15%
Near average 40% 40% 35% 35%
Below average 20% 40% 50% 50%

Central North Island, Taranaki, Wanganui, Manawatu and Wellington:
Temperatures in the late spring and early summer period are likely to be average or above average. Rainfall totals are equally likely to be in the normal or below normal range, while soil moisture levels and river flows are likely to be below 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 40% 20% 10% 15%
Near average 40% 40% 40% 35%
Below average 20% 40% 50% 50%

Gisborne, Hawke’s Bay, Wairarapa:
Temperatures are likely to be average or above average for the time of year, while rainfall is equally likely to be in the normal or below normal range. Soil moisture levels and river flows are likely to be below normal.Probabilities are assigned in three categories; above normal, near normal, and below normal. The full probability breakdown is:

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

Nelson, Marlborough, Buller:
Temperatures are likely to be average or above average for the time of year, while rainfall is equally likely to be in the normal or below normal range. Soil moisture levels and river flows are likely to be below 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 40% 20% 15% 15%
Near average 40% 40% 35% 35%
Below average 20% 40% 50% 50%

West Coast, Alps and Foothills, Inland Otago, Southland:
Temperatures are likely to be near average for the time of year. Seasonal rainfall totals and soil moisture levels are equally likely to be in the normal or below normal range, over the October to December period as a whole. River flows are likely to be below 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% 20% 20% 20%
Near average 50% 40% 40% 35%
Below average 20% 40% 40% 45%

Coastal Canterbury, East Otago:
Temperatures are likely to be near average, and rainfall is equally likely to be in the normal or below normal range, over the October to December period as a whole. Soil moisture levels and river flows are likely to be below 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% 20% 15% 15%
Near average 50% 40% 35% 30%
Below average 20% 40% 50% 55%

Background

Following the La Niña event over July 2010 to April 2011, the tropical Pacific returned to neutral conditions. However, over the past 4-6 weeks there is mounting evidence of a transition back to La Niña conditions. The Southern Oscillation Index has become more positive, sea surface temperature anomalies have become increasingly negative in the east-central equatorial Pacific, and the easterly trade winds have intensified near the Date Line. There has been a major shift in the consensus from global climate models which predict El Niño-Southern Oscillation conditions. The majority of these models are now forecasting further development of La Niña patterns over spring and a continuation through the summer of 2011/12. At this early stage, we cannot be confident about the intensity of the event.

© Scoop Media