Strong La Niña developing

Observations in the tropical Pacific confirm that a La Niña event is now under way and is continuing to strengthen. "It looks like this La Niña is shaping up to be the most significant since the 1988/89 La Niña", says NIWA climate scientist Dr Jim Renwick. "Sea temperatures in the critical region along the equator in the central Pacific are now well below average."

29 August 1998

NIWA research shows that during La Niña periods, New Zealand usually experiences a higher frequency of northeasterly winds. This increases the likelihood of warmer temperatures nationwide in spring and summer. In summer, there tend to be more frequent and heavier rain events in the north and east of the country and drier conditions in the west and south of the South Island.

The La Niña is following hard on the heels of the recent strong El Niño. La Niña events are roughly the opposite of El Niño events and have different climatic effects on New Zealand. "Once fully established, this event will stay in place for many months and will influence our spring and summer weather" says Dr Renwick.

The La Niña cooling in the equatorial Pacific Ocean is supported by other changes such as the movement of tropical convection from the date line towards Indonesia, stronger easterly trade winds, and atmospheric pressure changes as monitored by the Southern Oscillation Index (SOI). Over the last 30-day period, the SOI has risen to a high value of +1.8, which is typical of strong La Niña episodes.

All of the main overseas computer models used for long-term forecasting of tropical conditions are now consistent in their prediction of a strong La Niña pattern, peaking over the coming spring and summer seasons.

However, Dr Renwick cautioned that daily weather fluctuations add an unpredictable cumulative effect to New Zealand climate, which could alter the "typical" La Niña picture. "It’s better to assess the effects of La Niña in probabilistic terms, such as how it affects the chances of low or high rainfall, rather than making simple predictions of above or below average conditions", he said.

This latest evidence confirms the La Niña warning issued by NIWA two months ago. Scientists will continue to monitor the progress of La Niña conditions through the coming months.