Introduction

Copyright by Harvey Schwadron;
licensed through cartoonstock.com.

For years, people have been pointing to El Niño as the culprit behind floods, droughts, famines, economic failures, and record-breaking global heat. Can a single climate phenomenon really cause all these events? Is the world just a step away from disaster when El Niño conditions develop?

What Is El Niño?
El Niño refers to conditions when the sea’s surface in an area along the equator in the central and eastern Pacific Ocean gets warmer than usual. The average water temperature in that area is typically just 1-3°C (about 2-5°F) warmer than normal, but it has the effect of adding huge amounts of heat and moisture to the atmosphere, ultimately affecting patterns of air pressure and rainfall across the Pacific and beyond.

La Niña is the climatological counterpart to El Niño—a yin to its yang, so to speak. La Niña refers to periods when sea-surface temperatures in the equatorial central and eastern Pacific are cooler than normal. The lower surface temperatures suppress transfer of heat and moisture to the atmosphere, again affecting air pressure and precipitation patterns across a large region.

These global maps centered on the Pacific Ocean show patterns of sea surface temperature during El Niño and La Niña episodes. The colors along the equator show areas that are warmer or cooler than the long-term average. Images courtesy of Steve Albers, NOAA.

El Niño and La Niña episodes tend to recur every three to seven years, with each set of conditions settling in for about a year. The “seesaw” of high pressure in the western Pacific associated with El Niño shifting to high pressure in the eastern Pacific associated with La Niña is known as the Southern Oscillation. The oft-used term El Niño-Southern Oscillation, or ENSO, describes the alternating states of the linked ocean-atmosphere system as well as the normal state that occurs between them.

El-Niño-related drought can reduce food availability in regions where agriculture is dependent on rain. Photo courtesy of United Nations Environmental Programme.

Why Does it Matter?
Outside of normal seasonal variation, ENSO is one of the main sources of year-to-year variability in Earth’s weather and climate, with significant socioeconomic implications for many regions around the world. In normal years, trade winds push warm water and its associated heavy rainfall westward from the central Pacific toward Indonesia. During an El Niño, the winds die down and can even reverse direction, pushing the rains toward South America instead. This is why people in Indonesia and Australia typically associate El Niño with drought, while people in Peru connect El Niño with floods. These changing climate conditions, combined with other factors, can have serious impacts on society: impacts include reduced crop harvests, wildfires, or loss of life and property in floods. Public health records also offer evidence that El Niño conditions increase the risk of certain vector-borne diseases, such as malaria, in places where they don’t occur every year.