Goodbye El Niño: watch for La Niña
Developing atmospheric conditions in the equatorial Pacific Ocean point to La Niña weather conditions prevailing over New Zealand by Christmas according to NIWA senior climate scientist Dr Brett Mullan.
6 July 1998
These conditions tends to bring to New Zealand more easterly and north easterly winds, with above average temperature everywhere, and drier conditions in the south and west of the South Island. It is wetter in the north and east of the North Island.
Dr Mullan said today the El Niño weather pattern of the past 15 months was now finished and all evidence pointed to a shift to La Niña conditions - the opposite of El Niño - during spring.
“The latest evidence marked a definite change from the situation in April. The Southern Oscillation Index (SOI), which measures whether the atmosphere is in the “El Niño“ or “La Niña“ state has increased dramatically from near -3 in April to +1 in June“, said Dr Mullan. “This denotes the trend to the La Niña phase of the Southern Oscillation.“
The scientific evidence now coming in shows that easterly Trade Winds in the tropical Pacific have returned to near normal strength, and there is a large area in the equatorial central Pacific where ocean temperatures are cooler than average, although remnants of the warmer El Niño surface waters remain near the South American coast.
Further evidence of La Niña development is provided by climate prediction models that attempt to predict tropical atmospheric and oceanic changes out to 18 months ahead. Most of the overseas models are now in agreement that La Niña conditions should strengthen during spring and into summer. “This is the time of year when we tend to see the switch from El Niño to La Niña conditions occurring, and it is also the time of year when the predictive skill of the models starts to become useful”, said Dr Mullan.
However, warm waters in the Indian Ocean are still the main factor influencing New Zealand's winter climate patterns, and NIWA does not expect La Niña influences to be noticeable until spring.
The next two months will be critical in the development of a La Niña, and tropical conditions will be monitored closely by the National Institute of Water and Atmospheric Research
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