El Niño and La Niña

“Climate” is the word used to describe the long-term weather patterns experienced over a region. There can be cycles or oscillations in the climate that can change the typical weather for an area over a period of months or even years. One of the most important of these cycles is ENSO. 

ENSO is short for El Niño-Southern Oscillation. It is a climate oscillation, or a long term see-saw in atmospheric and ocean conditions across the Pacific Ocean. There are three phases of ENSO: El Niño, La Niña, and Neutral. Each phase brings a change to the winds, sea surface temperatures, and rainfall across the globe and here in Aotearoa New Zealand. 

El Niño

During El Niño, easterly winds across the Pacific Ocean, called the Trade Winds, weaken, or sometimes even switch direction. Warmer sea surface temperatures move east towards South America, bringing higher rainfall to the region. Cooler sea surface temperatures are experienced in the western Pacific near South-East Asia, leading to more settled weather there.

Aotearoa New Zealand typically experiences more westerlies during El Niño and they can often be stronger. This leads to wetter and cooler conditions in the west of the country but drier and warmer conditions in the east.

La Niña

During La Niña, the easterly Trade Winds become stronger, pushing warmer ocean waters towards South-East Asia, leaving cooler waters in the eastern Pacific. Eastern areas tend to be drier than normal, whereas there is higher rainfall in the west.

Aotearoa New Zealand experiences more frequent northeasterly winds during La Niña, bringing more rainfall to the northeast of the North Island. This results in less than normal rain for the west and south of the South Island. Temperatures are generally warmer than normal around the country.

Neutral

The neutral phase is where there is neither one extreme nor the other. Sea surface temperatures, rainfall, and winds across the Pacific Ocean and Aotearoa New Zealand are near average.

Fun fact, the word El Niño comes from Spanish and means “the boy”. Early sailors off the coast of Peru in South America noticed that the sea usually got warmer from about Christmas time. They named this annual warming "the Christ child" or, in Spanish, El Niño. During periods of cooling, the opposite to El Niño occurred and the term "the girl" or La Niña was used.

Changes in sea surface temperatures across the Pacific are used to determine which phase ENSO is in. The gradual change between El Niño and La Niña occurs every couple of years but it is important to note that no two El Niño or La Niña events are the same.