New Zealand is not usually affected as strongly by El Niño conditions as are parts of Australia, but there is nevertheless a significant influence.
In El Niño years, New Zealand tends to experience stronger or more frequent winds from the west in summer, leading to drought in east coast areas and more rain in the west. In winter, the winds tend to be more from the south, bringing colder conditions to both the land and the surrounding ocean.
In spring and autumn southwesterlies tend to be stronger or more frequent, providing a mix of the summer and winter effects.
La Niña events which occur at the opposite extreme of the Southern Oscillation Index cycle have weaker impacts on New Zealand’s climate. New Zealand tends to experience more northeasterly winds, which bring more moist, rainy conditions to the northeast parts of the North Island, and reduced rainfall to the south and south-west of the South Island. Therefore, some areas, such as central Otago and South Canterbury, can experience drought in both El Niño and La Niña.
Warmer than normal temperatures typically occur over much of the country during La Niña, although there are regional and seasonal exceptions. The last La Niña event was in 2007/08.
Although El Niño has an important influence on New Zealand’s climate, it accounts for less than 25% of the year to year variance in seasonal rainfall and temperature at most New Zealand measurement sites. East coast droughts may be common during El Niños, but they can also happen in non El Niño years (for example, the severe 1988-89 drought).
Serious east coast droughts do not occur in every El Niño, and the districts where droughts occur can vary from one El Niño to another (although some are more consistently affected than others). However the probabilities of the climate variations discussed above happening in association with El Niño are sufficient to warrant management actions and planning to be taken when an El Niño episode is expected or in progress.