How El Nino Works

Although El Nino's effects can be felt across the globe, it all starts in the Pacific Ocean, right along the equator. Under normal conditions, a persistent breeze known as a trade wind blows westward across the ocean, pushing warm surface water off the coast of South America to Southeast Asia. This allows colder water to rise in the eastern Pacific in a process known as upwelling. As a result, the surface water off the coast of Peru is about 14 degrees Fahrenheit (8 degrees Celsius) colder than the surface water surrounding Indonesia [source: TAO]. Heavy rain falls over the warm waters of the western Pacific, while the east stays cool and dry.

Every two to seven years, however, El Nino makes an appearance. It begins when — for reasons scientists don't fully understand — the trade wind begins to weaken. With nothing pushing the warm surface waters westward, they shift east along the equator and bring the rains along for the ride. That makes for wet conditions and low surface pressure in the central and eastern Pacific, while the western Pacific is unusually dry under high pressure [source: Live Science].

Scientists start thinking El Nino when water in the tropical eastern Pacific is 0.9 degrees Fahrenheit (0.5 degrees Celsius) warmer than normal and the wind, surface pressure and rain are consistent with the event in the ways mentioned above. If this happens for five overlapping three-month periods (Jan.-Feb.-Mar., Feb.-Mar.-Apr., Mar.-Apr.-May, etc.), then it's officially an El Nino [source: NOAA].

Often, but not always, El Nino is followed by its sister phenomenon, La Nina. That's when the trade wind picks back up and actually gets stronger than normal, pushing the warm surface water and rain far into the western Pacific. Upwelling resumes, restoring colder water and cool, dry weather to the central and eastern Pacific [source: NOAA].

Both El Nino and La Niña typically last nine to 12 months, develop in the spring, peak in late autumn or winter, and weaken during the spring or early summer. Scientists describe the return to normal conditions between these events as a neutral phase. Together, these three events — El Nino (the warm phase), La Niña (the cool phase) and the neutral phase — make up what is known as the El Nino-Southern Oscillation [source: NOAA].

El Nino impacts the planet in two related ways. The first has to do with the way it influences weather patterns by altering the oceans and atmosphere, while the second relates to how those changes affect our property, food, water and health.

A strong El Nino can have a big impact on global weather patterns because it changes the way air circulates in the atmosphere. These circulations are determined by Earth's rotation, the angle of its axis relative to the sun, and the position of its landmasses and oceans. Significantly for El Nino, such air movements can also be influenced by dense tropical rainclouds that form over warm parts of the ocean [source: National Weather Service].

One circulation feature that you may have heard of is the jet stream — a narrow river of strong wind that circles the globe from west to east in our upper atmosphere. When El Nino shifts warm water to the eastern Pacific, the dense tropical rainclouds in the atmosphere above shift the jet stream as well, influencing weather hundreds or even thousands of miles down the line. These changes in atmospheric circulation and the altered weather patterns that result are just one example of what are called El Nino teleconnections [source: Barnston].

So how does all this wacky weather affect people's daily lives? Well, with all the fires, floods, storms and mudslides, property is certainly at risk. Take the 1997-1998 El Nino we mentioned earlier. By the time the warm waters retreated from the eastern Pacific in May 1998 it had caused some $33 billion in damage — a half billion in California alone [sources: Suplee, Linn and Mai-Duc].

Just as significant is the way El Nino interrupts the world's water supply, food production and human health. Many small Pacific Islands, for example, rely on rainfall and groundwater for their freshwater, so drought puts them at increased risk of coming up short [source: PacOOS]. Food crops also suffer from periods of drought, and unusually warm ocean water can disrupt fisheries. Anchovies, which are usually prolific in the cold waters off the coast of Peru, flee for colder waters to the south when things start to heat up [source: Suplee]. And if threatening our food and water isn't bad enough, El Nino can also be bad for your health: Warm, wet conditions are ideal for disease-carrying rodents and insects [source: WHO].

From 1950 to 2015, Earth has experienced 24 El Nino events [source: National Weather Service]. Some were stronger than others; it all just depended on how long and how much the eastern Pacific heated up. Two winters, 1982-1983 and 1997-1998, stand out as particularly strong, but 2015-2016 might end up the most powerful yet.

In 1982, El Nino wasn't the household word that it is today. Even some meteorologists had never heard of the term. So when an El Nino sent temperatures in the cool eastern Pacific to unusually high levels, scientists initially blamed the event on the recent eruption of Mexico's El Chichon volcano. By spring 1983 the water temperature had begun to return to normal and meteorologists decided it wasn't the volcano after all [source: Williams]. One thing was clear, though: This was the worst El Nino in at least a century if not longer. Drought dominated Australia, Africa and Indonesia, while some areas of Peru that normally received just 6 inches of rain (15 centimeters) were doused with 11 feet (3 meters)! In the end the climate event was blamed for $8 billion in damage and between 1,300 and 2,000 deaths [source: Gannon].

A monster El Nino reared its head again beginning in 1997. As the Pacific waters warmed, more than 24 million acres (9.7 million hectares) of parched Indonesian rainforest burned largely unchecked [source: Tacconi]. In Peru, heavy rains created ideal conditions for mosquitoes, and malaria infections increased threefold as a result. This El Nino, which released more energy than 1 million Hiroshima bombs during its eight-month reign, ultimately caused some $33 billion in damages and killed about 2,100 people [source: Suplee].

Yet another strong El Nino took hold during the winter of 2015-2016. By February, ocean temperatures along the equator in the eastern Pacific had been at or above record-high readings for several months, meaning it was probably even stronger than the 1982 or 1997 events. Still, those earlier El Ninos brought way more rain to southern California than anything the region saw in 2015 or early 2016. For reasons that aren't yet clear, this massive El Nino just didn't change the jet stream in the same way it had in the past. Instead, northern California and the Pacific Northwest caught the brunt of the precipitation [source: Swain].

People are always better prepared for weather events when they have some type of warning. Take tornado forecasting, for example. Major improvements over the past 10 or 20 years have saved countless lives because people know when to take cover. The same is true for El Nino: If meteorologists can warn people ahead of time, they can try to prepare for the crazy weather that follows.

Forecasting has come a long way since the El Nino of 1982-1983. That event caught scientists completely off guard and pushed them to install 70 buoys in the southern Pacific Ocean from the Galapagos Islands to Australia [sources: Williams, National Geographic]. These buoys, along with instruments carried by satellites and weather balloons, collect vast amounts of weather data from the oceans and atmosphere. Scientists process this information using complex computer models, which can often forecast an El Nino several months to a year ahead of time [source: NOAA].

The first time scientists were able to forecast El Niño was a few months prior to the 1997-1998 event. That allowed Peruvian farmers to plan ahead so they could graze cattle and grow crops in areas that were normally too dry. Fisherman gave up on the usual anchovy harvest to trap cold-water-loving shrimp instead. Government officials built storm drains and stockpiled emergency supplies, potentially saving lives thanks to the scientists' heads up [source: Suplee].

Of course, now the big question is: How will climate change affect El Nino? And the answer to that question is: Yes. Confused? Well, so are scientists. See, none of them really know how climate change will affect El Nino, but all know that it will. The best guess right now is that climate change will affect how heavy El Nino's impacts are, lessening or intensifying them based what the weather is like in a warmer climate. For example, Indonesia, which dries up and catches fire during El Nino, is also expected to get wetter as a result of climate change. So during El Nino it might still be dry, just not as dry. And while that sounds like good news, it works the other way, too: Areas that dry out during El Nino are in big trouble if they're also expected to be drier as a result of climate change [source: Di Liberto].