Environment

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-Fuelled rains, but bad news for the many areas, like Indonesia, which is suffering from deep drought, food and water shortages, and wildfires.

Already this year, El Niño-related weather has cost billions of dollars in damage and left some 100 million people facing food and water shortages.

 

First, the big picture: El Niño is most known for shifting a large pool of warm ocean waters from the western to the central and eastern tropical Pacific. That shift changes where heat is pumped into the tropical atmosphere, disrupting its typical circulation patterns. Those local disruptions cause a domino effect through the global atmosphere that can alter weather thousands of miles away.

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.

The changes in these main circulation patterns interact with other factors, like seasonal monsoons and other major climate patterns, which is why none of the typical impacts associated with El Niño are guaranteed. It merely shifts the odds in their favour.