We hear a lot in the news about El Niño (and its counterpart La Niña), but what is it? How and why does it affect New Zealand?

The El Niño/La Niña cycle is a natural part of how the climate works. After the regular changing of the seasons, it is the biggest variation in the global climate, on the year-to-year scale. It is based in the tropical Pacific but involves energy flows so large, its consequences can affect climates across almost all regions of the globe.

The El Niño cycle evolves as a conversation between the tropical Pacific Ocean and the atmosphere that overlies it. The way these two fluids – the sea and the air – interact, account for El Niño and La Niña patterns, as well as for normal conditions.

Normally, the wind blows from east to west across the tropics, air flowing from South America towards Australia and Indonesia. That flow of air is quite steady and is called the trade wind. The name comes from the days of sailing ships – these reliable winds helped establish international trade routes. The trade wind flow helps to confine the warmest ocean waters to the western Pacific, and helps to bring cold waters to the surface along the South American coast and along the Equator in the eastern Pacific. As a result, ocean surface waters off the coast of Ecuador are around 8°C colder than Equatorial waters around Indonesia and north of Australia.

The trade winds help maintain the sea temperature difference between South America and Indonesia, but in turn the sea temperature difference maintains the trade winds by fostering high pressures in the east, low pressures in the west, and a flow of air from high to low pressure. This is an example of a feedback in the climate system. The trade winds and sea temperatures rely on each other, in a ‘chicken-and-egg’ relationship. An El Niño is a breakdown of this normal picture. If the trade winds weaken, that helps reduce the sea temperature difference, which in turn weakens the east-west pressure difference and weakens the trade winds further. This back and forth between weakening trade winds and weakening sea temperature differences can balloon up over the course of several months into what we call an El Niño. At the peak of a strong El Niño such as the one we have in place this summer, the usual eight degree sea temperature difference can drop away to nearly zero, and the trade winds can cease to blow and may even reverse and blow from the west for a while.

When the waters warm near South America, upwelling slows, nutrient levels drop and the anchovy fishery along the Peruvian coast is badly affected. In the 19th century, Peruvian fishermen who experienced this dubbed the event “El Niño”, a Spanish term for “the boy child”, a reference to Christ since El Niño events typically reach peak strength around Christmas. In the 1960 and 70s, climate scientists realised that El Niño was connected to an irregular see-saw in air pressures and trade winds across the Pacific, which in the 1920s had been dubbed the “Southern Oscillation”. Hence, the full atmosphere-ocean variation is now known as ENSO, the El Niño-Southern Oscillation.

At one extreme, the ENSO cycle has El Niño, where eastern Pacific waters along the Equator warm by several degrees and the trade winds drop away. At the other extreme we have “La Niña” (Spanish for the girl child) where eastern Pacific waters cool even more than normal and the trade winds blow extra-hard. Either extreme develops and decays over the space of about 9 months, usually starting around June, peaking near Christmas, and decaying by April. In any given year, either El Niño or La Niña can occur, or there can be neither, just “normal” conditions. In the long run, each occurs about one third of the time so an El Niño comes along every three or four years on average.

In the Tropics, the heaviest rain occurs over the warmest water, as the air above warm water is buoyant and full of moisture. In normal conditions, the heaviest rain usually falls over Indonesia and northern Australia while the eastern tropical Pacific is usually dry. As the warm waters flow east during an El Niño, the rainfall follows, often leaving Indonesia and the east of Australia dry but bringing flooding rains to central Pacific nations such as Kiribati and even right across to western South America. Hence, drought and fire is more common in Australia and Indonesia during El Niño while La Niña can often bring very wet conditions to those regions.

As the tropical winds and rainfall patterns change, the whole tropical atmosphere has to adjust and rearrange itself. That rearrangement sends out ripples to higher latitudes, right around the Pacific basin and beyond, just as dropping a rock in the centre of a pond causes ripples to travel outwards in all directions. The “ripples” show themselves as change in the position and strength of the highs and lows that pepper our weather patterns. In an El Niño, New Zealand tends to experience more low pressures and storms, with stronger westerly winds, while the high pressures retreat over Australia or head south over the Southern Oceans. Exactly how an El Niño affects New Zealand depends in part on what else is happening with the weather, so its effects are not entirely predictable. But, more often than not, we see the increased westerlies and stormier conditions over New Zealand during an El Niño.

In an El Niño, when New Zealand has stormier weather and stronger westerlies, western parts of the country tend to be wetter than normal while the east often basks in dry warm weather, good news for holidaymakers but bad news for farmers. El Niño can bring summer drought to Canterbury, Hawkes Bay and Northland, as rainfalls stay low and the warm dry winds draw moisture out of the soil. This year’s El Niño is a very strong one and it is also one that is developing in the “textbook” way, bringing the stronger westerlies to New Zealand and helping to dry out soils in eastern regions. This El Niño developed during the second half of 2015 and is expected to fade out by about April of 2016 so its effect on New Zealand is likely to be felt right through the summer and into the autumn.

El Niño does have a global reach. An El Niño event warms the tropical eastern Pacific Ocean (and the air above it) so much that global average temperatures also rise. The global mean temperature for 2015 is guaranteed to be a lot higher than it was in 2014, or any year on record, because of the strong El Niño, on top of the continuing upward trend in global temperatures brought about by the ever-increasing blanket of “greenhouse gases” in the atmosphere. Because El Niño lingers, 2016 is very likely to be as warm as 2015, and may be even warmer still.

For more information on how El Niño and the ENSO cycle works, click on

http://www.pmel.noaa.gov/tao/elnino/nino-home.html This site has a very comprehensive set of information, including pages where you can track the progress of El Niño in real time. For more information on how El Niño affects New Zealand, see the description on the NIWA web site at https://www.niwa.co.nz/climate/information-and-resources/elnino and the link to El Niño’s impact on New Zealand’s climate.

For information on global mean temperature change and the role of El Niño, see https://www.wmo.int/media/content/wmo-2015-likely-be-warmest-record-2011-2015-warmest-five-year-period and http://www.bbc.com/news/science-environment-35121340