NIWA has published its climate outlook for the next three months (Dec – Feb):
“El Niño strengthens, with conditions likely to be cooler in south and drier in northeast”
The El Nino has strengthened during the last two months, and is expected to continue throughout the rest of summer. It is also likely that this summer will see stronger south-westerlies across New Zealand, due to mean sea lpressure levels being higher than normal to the north, and lower than normal to the south.
Climate indicators are expected to be in normal, or slightly below normal, ranges.
Overall Picture
Temperature:
Air temperatures are likely to be below average or average in all South Island regions and in the west of the North Island, and near average in other North Island regions. Sea surface temperatures near New Zealand are expected to be below average during December-February.
Rainfall, soil moisture, and stream flows:
Rainfall is likely to be normal or below normal in the north and east of the North Island, and in Nelson-Marlborough, and near normal elsewhere. River flows are likely to be below normal in the north and east of both Islands, with soil moistures also likely to be below normal in the east of the South Island. In all other regions, river flows and soil moistures are expected to be normal or below normal for the summer season as a whole.
Regional predictions for the next three months:
Northland, Auckland, Waikato, Bay of Plenty:
Temperatures are likely to be in the near average category. Seasonal (3-month) rainfall totals and soil moisture levels are expected to be in the normal or below normal range, with stream flows below normal.
Central North Island, Taranaki, Wanganui, Manawatu and Wellington:
Seasonal temperatures are projected to be most likely in the below average category. Three-month rainfall totals and soil moisture levels are expected to be near normal, with stream flows likely to be normal or below normal.
Gisborne, Hawke’s Bay, Wairarapa:
Three-month temperatures are likely to be in the near average category. Seasonal rainfall totals and soil moisture levels are expected to be in the normal or below normal range, and stream flows are most likely to be below normal.
Nelson, Marlborough, Buller:
Temperatures are most likely to be in the below average category. December-February rainfall and soil moisture levels are likely to be normal or below normal. Seasonal stream flows are likely to be below normal.
West Coast, Alps and Foothills, Inland Otago, Southland:
Temperatures are most likely to be in the below average category. Seasonal rainfall and soil moisture levels are expected to be near normal, while stream flows are likely to be normal or below normal.
Coastal Canterbury, East Otago:
Temperatures are equally likely to be in the near average or below average categories. Seasonal rainfall is expected to be in the normal range, while soil moisture levels and stream flows are most likely to be below normal overall.
Background
The El Niño has strengthened over the past two months in terms of increased surface and sub-surface equatorial ocean temperatures, although the Southern Oscillation Index weakened slightly in November from the October value of -1.6. Most of the El Niño forecast models predict El Niño to reach its maximum intensity over the summer before declining in autumn.
El Niño events can lead to dry conditions in eastern parts of New Zealand over the spring and summer seasons, and soil moistures are already very low at the end of November in Northland, Bay of Plenty and central Otago. The latest guidance suggests that drier conditions in the north and east of both islands are likely over the December 2009-February 2010 period. El Niño events are also often associated with colder than average conditions in the New Zealand region, and this is consistent with this latest outlook.