WEATHER

La Nina gone by Easter; somewhat stormy Tasman Sea

Wednesday 29 February 2012, 3:23PM
By NIWA
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A mature La Niña event in the tropical Pacific should be gone by Easter, according to the NIWA National Climate Centre.  This means that local sea temperatures around our coast, and low pressure activity over the Tasman Sea, are likely to most influence our autumn climate.

The NIWA National Climate Centre’s outlook for Autumn, March to May 2012, indicates cooler than usual seas around New Zealand, and a somewhat stormy Tasman Sea. Lower pressures than normal are expected over the Tasman Sea for March to May as a whole, resulting in more frequent northeast winds than usual over the country.

Autumn temperatures are likely to be average or below average in eastern areas of both Islands, and near average elsewhere. Autumn rainfall is likely to be normal or above normal in the north and east of the North Island, as well as the north of the South Island. In all other regions, near normal seasonal rainfall totals are likely.

 

Overall picture

Temperature

Seas around New Zealand are likely to be cooler than usual during autumn.  Seasonal temperatures are likely to be average or below average in eastern areas of both Islands, and near average elsewhere.

Rainfall, soil moisture and river flows

The National Climate Centre projects that autumn rainfall and river flows are likely to be normal or above normal in the north and east of the North Island, as well as the north of the South Island.  In all other regions, near normal seasonal rainfall totals and river flows are likely.  Autumn soil moisture levels are projected to be above normal in the northern North Island, normal or above normal in the east of the North Island and north of the South Island, and near normal elsewhere.

 

Regional predictions for the next three months

Northland, Auckland, Waikato, Bay of Plenty

Autumn temperatures are likely to be near average. Seasonal rainfall totals and river flows are equally likely to be in the near normal or above normal range.   Autumn soil moisture levels are projected to be 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% 40% 45% 40%
Near average 50% 40% 35% 40%
Below average 30% 20% 20% 20%

 
Central North Island, Taranaki, Wanganui, Manawatu, Wellington

Seasonal temperatures are likely to be near average. Autumn rainfall totals, river flows, and soil moisture levels are projected to be near normal for the three month season 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 20% 30% 35% 35%
Near average 50% 50% 40% 45%
Below average 30% 20% 25% 20%

 

Gisborne, Hawke’s Bay, Wairarapa

Seasonal temperatures are equally likely to be in the near average or below average range.   Autumn rainfall totals, river flows, and soil moisture levels are projected to be in the near normal or above normal category.

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% 40% 40% 40%
Near average 40% 40% 40% 40%
Below average 40% 20% 20% 20%

 
Nelson, Marlborough, Buller

Autumn temperatures are likely to be near average. Seasonal rainfall totals, river flows, and soil moisture levels are equally likely to be in the normal or above 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 20% 40% 40% 40%
Near average 50% 40% 40% 40%
Below average 30% 20% 20% 20%

 

West Coast, Alps and foothills, inland Otago, Southland

Autumn temperatures are projected to be near average. Seasonal rainfall totals, river flows and soil moisture levels are 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:

  Temperature Rainfall Soil moisture River flows
Above average 30% 30% 25% 20%
Near average 50% 50% 45% 45%
Below average 20% 20% 30% 35%

 
 
 
Coastal Canterbury, east Otago

Seasonal temperatures are equally likely to be near average or below average. Autumn rainfall totals, river flows and soil moisture levels are projected to be near 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% 30% 35% 30%
Near average 40% 50% 45% 50%
Below average 40% 20% 20% 20%

 

Background

Mature La Niña conditions in the tropical Pacific are expected to dissipate in early autumn 2012. The Southern Oscillation Index (SOI) eased sharply to around +0.1 in February (down from +1.1 in January). Sub-surface sea temperature/heat content anomalies are also weakening in the eastern equatorial Pacific region. Both are signs of the weakening of the La Niña event. 

Most global climate models predict neutral conditions in the tropical Pacific as we start winter (June-August), but a handful of models now develop an El Niño state during the winter period.  NIWA will continue to monitor the global climate situation.