Additional info from the International Research Institute for Climate and Society

El Niño refers to the occasional warming of the eastern and central Pacific Ocean around the equator. The warmer water tends to get only 1 to 3 degrees Celsius above average sea-surface temperatures for that area, although in the very strong El Niño of 1997-98, it reached 5 degrees or more above average in some locations. La Niña is the climatological counterpart to El Niño — a yin to its yang, so to speak. A La Niña is defined by cooler-than-normal sea-surface temperatures across much of the equatorial eastern and central Pacific. El Niño and La Niña episodes each tend to last roughly a year, although occasionally they may last 18 months or longer.

The Pacific is the largest ocean on the planet, so a significant change from its average conditions can have consequences for temperature, rainfall and vegetation in faraway places. In normal years, trade winds push warm water — and its associated heavier rainfall — westward toward Indonesia. But during an El Niño, which occurs on average once every three to five years, the winds peter out and can even reverse direction, allowing the rains to shift toward South America instead. This is why we typically associate El Niño with drought in Indonesia and Australia and flooding in Peru. These changing climate conditions, combined with other factors, can have serious impacts on society, such as reduced crop harvests, wildfires, or loss of life and property in floods. There is also 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 and where disease control is limited.

During either an El Niño or a La Niña, we also observe changes in atmospheric pressure, wind and rainfall patterns in different parts of the Pacific, and beyond. An El Niño is associated with high pressure in the western Pacific, whereas a La Niña is associated with high pressure in the eastern Pacific. The “seesawing” of high pressure that occurs as conditions move from El Niño to La Niña is known as the Southern Oscillation. The oft-used term El Niño-Southern Oscillation, or ENSO, reminds us that El Niño and La Niña episodes reflect changes not just to the ocean, but to the atmosphere as well.

ENSO is one of the main sources of the predictable part of the year-to-year variability in weather and climate on Earth and has significant socioeconomic implications for specific seasons for many regions around the world. However, ENSO conditions significantly affect only about 25% of the world’s land surface during any particular season, rather than most regions of the globe. It is important to note that ENSO does not necessarily increase the year-to-year variability of weather and climate. This variability would still occur without ENSO. Instead, El Niño or La Niña allow us to predict which regions are more likely to be unusually wet, dry, warm or cold.