Why this El Ni?o could be the biggest on record

If you’re wondering why you’re still wearing shorts in winter, meteorologists have a two-word answer: El Ni?o.

This year’s El Ni?o weather event – characterized by warming waters in the eastern Pacific Ocean – is already one of the three strongest ever recorded, and it could even rival the intensity of the record 1997 event that wreaked worldwide weather havoc, from mudslides in California to fires in Australia.

NASA says El Ni?o conditions are still strengthening, and Oxfam has warned that this El Ni?o could leave tens of millions of people exposed to disease and hunger.

The weather phenomenon largely became a part of the public vernacular during the 1997 El Ni?o. It caused devastating flooding in the western U.S. and drought in Indonesia. It was blamed for deadly virus outbreaks in Africa and rising coffee prices around the world.

Chris Farley even became El Ni?o personified on Saturday Night Live.

Pure chaos. But what is El Ni?o? Let us explain.

What is El Ni?o and what causes one?

Sea surface temperatures in October -- orange-red colors are above normal.

El Ni?o is a warming of the eastern Pacific Ocean, mainly along the Equator – see the thick red belt in the satellite photo, which indicates waters that are warmer than normal.

These warmer waters are normally confined to the western Pacific by winds that blow from east to west, pushing the warmer water toward Indonesia and Australia.

But during an El Ni?o, the winds slow down and can even reverse direction, allowing the warmer water to spread eastward all the way to South America.

El Ni?os occur every two to seven years in varying intensity, and the waters of the eastern Pacific can be up to 4 degrees Celsius (7 degrees Fahrenheit) warmer than usual.

What happens when there’s an El Ni?o?

(Swipe right to see ocean conditions in July 1997; swipe left to see July 2015)

A strong El Ni?o heats up the atmosphere and changes circulation patterns around the globe, especially the jet stream over the Pacific, which becomes stronger and dumps more frequent and intense storms over the western U.S., especially California. It also means more rain for the west coast of South America.

But the atmosphere is somewhat of a zero-sum game. More rain in North and South America comes at the expense of normally rainy Southern Asia and Australia, which become abnormally dry and experience droughts.

A strong El Ni?o also influences cyclone seasons around the planet. The warmer the East Pacific is, the more hurricanes it gets. The Atlantic Ocean sees fewer hurricanes, however, a result of increased upper level winds that prevent hurricanes from developing. That proved true for the 2015 hurricane season – the Atlantic saw another relatively quiet year, while the Pacific broke a number of records.

The weather isn’t the only thing that’s affected. Warmer surface waters in the eastern Pacific drive away the coldwater fish that are the backbone of the fishing industry in much of Latin America. It was here that the phenomenon was first noticed by fishermen, who named it “El Ni?o” – meaning “little boy” or “Christ child” in Spanish – since it would often appear around Christmas.

Will this El Ni?o be worse than 1997?

The Oceanic Ni?o Index (ONI) measures sea surface temperatures in the tropical Pacific. Zero is average, positives numbers are warmer, and negative numbers are colder. Anything above 0.5 is considered El Ni?o conditions – and anything above 1.5 is a strong El Ni?o.

The strongest El Ni?o was the 2.3 event in 1997-98. The current figure for this year’s El Ni?o is 2.0 and has been rising for months – and many are forecasting the strongest El Ni?o since records began in the 1950s.

What happened in 1997?

The prospect of a record-breaking El Ni?o is worrying, given that the 1997 edition created conditions that killed an estimated 23,000 people and caused as much as $45 billion in damage.

Parts of Latin America experienced torrential rain and mudslides. On the other side of the El Ni?o-fuelled Pacific, Indonesia had a record drought that led to uncontrollable forest fires in Southeast Asia. Thirty-five counties in California – currently enduring a catastrophic drought – were declared disaster areas after rain caused flooding and mudslides.

The world also heats up during an El Ni?o – 1998 became the warmest year on record at the time, and 2015 is almost certainly set to become the hottest year yet again.

Which areas will be hit the hardest?

Like snowflakes, no two El Ni?os are exactly alike, and they’re only one of many different large-scale weather patterns acting in tandem to influence global weather. The presence of “the blob,” as it has become known – an area of warmer water in the Northern Pacific – wasn’t there during the 1997 El Ni?o, which will likely influence the jet stream over North America, meaning this El Ni?o may not be the drought-buster that Californians are hoping for.

Nevertheless, if El Ni?o comes on as strong as current projections seem to indicate, we can say with some level of confidence that coastal South American countries will see above average rainfall and flooding in the coming months, while countries on the other side of the ocean like Australia and Indonesia will likely see drought.

Does climate change have anything to do with it?

The influence of climate change on El Ni?o is a matter of debate. Some research suggests that while the overall number of El Ni?os is unlikely to increase, the number of “super” El Ni?os are twice as likely to occur.

The U.N., for its part, says confidence is low that climate change will affect El Ni?os in the future. But just because El Ni?os may not change doesn’t mean their impacts won’t become more severe.

One of the most likely byproducts of global warming is more extreme precipitation events, as warmer temperatures can hold more water vapor in the atmosphere. This could make El Ni?o induced floods even more devastating.