How a Monster El Niño Transforms the World's Weather

From crippling drought in southern Africa to a record number of February tornadoes in the U.S. Southeast, an exceptionally strong El Niño has been making headlines around the globe as it tampers with the world’s weather.

While the event has begun its slow decline, those wide-ranging impacts will continue to be felt for weeks and months to come — good news for those in California, who need El Niño-fueled rains, but bad news for the many areas, like Indonesia, which is suffering from deep drought, food and water shortages, and wildfires.

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Through the interactive graphic below, Climate Central takes a look at how this major climate pattern typically influences the world’s weather, what it can mean for societies in the areas it affects, and what has actually been happening with this particular event, which ranks amongst the strongest on record. After all, no two El Niños are the same. For instance, while Southern California was drenched during the 1997-1998 El Niño, it has remained disconcertingly dry this time around.

There are two main circulation patterns that are affected. All around the tropics is a pattern of rising and sinking air — like a vertical loop — called the Walker Circulation. The rising air corresponds to areas of unsettled, rainy weather, while the sinking air creates a stable, dry clime. Normally in the tropical Pacific, a major area of rising air is found over the western portions, where the warmest waters are found. With the eastward shift from El Niño, that rising air (and its sinking counterpart) move eastward as well. This displacement shifts the other branches of the Walker Circulation around the tropics, pushing wetter weather over areas that might normally be dry and vice versa. These areas typically see some of the strongest impacts from El Niño because they are in a region more directly linked to it.

The changes in the Walker Circulation in turn cause shifts in another looping pattern called the Hadley Circulation that runs north-to-south to the Walker’s east-to-west. And those changes in the Hadley Circulation can affect the subtropical jet stream — an area of fast-moving air that guides storms — in both the Northern and Southern hemispheres. This is how El Niño can affect regions far from the tropics.

Indonesia, and Southeast Asia more broadly, is one of the areas most directly impacted by El Niño. The eastward shift in the Walker Circulation means the normal area of rising air and rainy weather is shifted eastward as well, leaving the Indonesian area high and dry during much of the year. This can lead to drought that can cause water shortages and crop and livestock losses from dried up fields. Those losses then lead to food shortages and rising prices, and can also increase the odds of large wildfires.

This year has seen major drought grip Indonesia, Vietnam, Thailand, the Philippines and much of the U.S. Pacific Island territories. In Indonesia there were major delays in the planting of the rice crop, leading to concerns over food shortages. And when farmers set their usual fires to clear brush for planting, the dry conditions caused many to burn out of control, causing one of the biggest burn years on record and leading to a pall of smoke over the region. In Vietnam, drought has led to salinization issues in the Mekong Delta region, home to some 20 million people and a major rice growing area.

So far this year, Australia has seen a mixed bag of El Niño impacts. While dry conditions have been in place in much of the usual areas, some of the inland regions of the eastern part of the country actually had fairly normal rains. That may be due to exceptionally warm Indian Ocean waters providing more moisture to storm systems. The southeast has seen some major heat waves, though, with temperatures sometimes climbing above 100°F (40°C). The hot and dry conditions have helped create more favorable wildfire conditions, with several burning in Victoria this year. One of the worst destroyed 116 houses around Christmas.

Warmer ocean waters in that area also provide more fuel for hurricanes in the central and eastern Pacific Ocean basins. The atmospheric changes El Niño brings also mean more favorable winds for storm formation. This year sawabove-normal hurricane activity in the region, including the strongest storm ever directly measured in the Western Hemisphere. Hurricane Patricia, which had winds that clocked in at 200 mph, hit the west coast of Mexico, though its effects were mostly confined to less populated areas.

Parts of South America are also more directly affected by El Niño. The warmer ocean waters off its west coast tend to bring warmer weather to coastal areas during the Southern Hemisphere winter. Those waters also make more moisture available to storms. Combined with changes to storm tracks linked to atmospheric circulation changes, this can mean wetter winter weather for central Chile. Several rounds of storms during this event had some benefits, bringing moisture to a region in drought and snows to ski resorts. But the rains also brought flooding and mudslides. A rare storm event in March saw parts of the bone dry Atacama Desert receive a year’s worth of rain in just one day. The Copiapó River — dry for nearly 17 years — filled to overflowing and flash floods swept through several towns. Come spring, that influx of water led to a surreal bloom of flowers across the desert that tends to coincide with rainy El Niño years.

The chain of changes to the Walker and Hadley circulations also tends to bring rainier weather to an area that includes Paraguay, northern Argentina, part of southern Brazil and Uruguay during the Southern Hemisphere summer. In December and January, heavy rains caused the worst flooding in the region in 50 years, displacing more than 200,000 people. The Paraguay River in Asunción reached more than 16 feet above its normal level in December. Thousands of acres of cropland were inundated and concerns were heightened for the spread of mosquito-borne disease like dengue fever and the Zika virus.

Further north, drier weather also tends to grip Central America and the Caribbean for the same reasons as northern South America. The area that spans Guatemala, Nicaragua, Honduras and El Salvador is experiencing one of its worst droughts in decades, according to the European Commission. Major crop losses have caused huge financial hits to farmers and left some 3.5 million people facing food shortages, the agency reported.

Not far away, though, El Niño brings the opposite conditions to Florida. The shifts in the Hadley Circulation affect the subtropical jet stream that crosses the U.S., pushing it southward and making it extend further eastward. This puts Florida in a prime location for storms during the winter months, which is normally its dry season. It also ups the odds for tornadoes thanks to the added energy from the jet stream.

The tweak to the jet stream also tends to favor wetter weather over Southern California — much needed this year after four years of deep drought. While storms have been hitting California, they have mostly affected the northern portions of the state because the jet stream has been slightly further north than is typical during an El Niño. The rains shut off almost completely in February, bringing hot and dry weather to the state, though storms have since returned and look to bring at least some rain to Southern California.

Across the northern tier of the U.S., from the Northeast all the way up into eastern Alaska, El Niño tends to favor warmer than normal conditions. That has indeed been the case this year, with nearly all northern states recording one of their top 10 warmest winters. Alaska saw its second-warmest winter in the books, which has contributed to a lack of snow in varied locations there. That dearth of snow can have ecological impacts and promote the spread of invasive species. El Niño wasn’t the only reason Alaska was warm, though, as the Arctic more broadly was exceptionally warm this winter.

Heavy rains in the area began in late October, inundating villages and agricultural lands. By January, more than 100,000 people had been displaced and 112 killed, according to the International Federation of Red Cross and Red Crescent Societies. Authorities are concerned about the spread of both waterborne diseases, such as cholera, and mosquito-borne ones, particularly Rift Valley Fever, which tends to occur in years with unusually heavy rainfall.

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Andrea Thompson is an associate editor at Scientific American, where she covers sustainability, energy and the environment. Prior to that, she was a senior writer covering climate science at Climate Central and a reporter and editor at Live Science, where she primarily covered Earth science and the environment. She holds a graduate degree in science health and environmental reporting from New York University, as well as a bachelor of science and and masters of science in atmospheric chemistry from the Georgia Institute of Technology.