What Is El Nino?

Often blamed for any and all out-of-the-ordinary weather, El Niño is a naturally occurring climate event and the warm phase of the El Niño-Southern Oscillation (ENSO) during which sea surface temperatures in the eastern and equatorial Pacific Ocean are warmer than average.

How much warmer? An increase of 0.5 C or more in average sea surface temperatures lasting 3 months in a row suggests the onset of an El Niño episode.

Meaning of the Name

El Niño means "the boy," or "male child," in Spanish and refers to Jesus, the Christ Child. It comes from South American sailors, who in the 1600s, observed the warming conditions off the Peruvian coast at Christmastime and named them after the Christ Child.

Why El Niño Happens 

El Niño conditions are caused by a weakening of the trade winds. Under normal circumstances, the trades drive surface waters towards the west; but when these die down, they allow the warmer waters of the western Pacific to seep eastward toward the Americas.

Frequency, Length, and Strength of Episodes

A major El Niño event generally occurs every 3 to 7 years, and lasts for up to several months at a time. If El Niño conditions will appear, these should begin to form sometime in the late summer, between June and August. Once they arrive, conditions typically reach peak strength from December to April, then subside from May to July of the following year. Events are categorized as either neutral, weak, moderate, or strong.

The strongest El Niño episodes occurred in 1997-1998 and 2015-2016. To date, the 1990-1995 episode is the longest-lasting on record.

What El Niño Means for Your Weather

We've mentioned that El Niño is an ocean-atmosphere climate event, but how do warmer-than-average waters in the far-off tropical Pacific Ocean affect weather? Well, these warmer waters warm up the atmosphere above it. This leads to more rising air and convection. This excess heating intensifies the Hadley circulation, which in turn, disrupts circulation patterns around the globe, including things like the position of the jet stream.

In this way, El Niño triggers a departure from our normal weather and rainfall patterns including:

• Wetter-than-normal conditions along coastal Ecuador, northwestern Peru, southern Brazil, central Argentina, and equatorial eastern Africa (during the months of December, January, February); and over the inter-mountainous U.S. and central Chile (June, July, August).
• Drier-than-normal conditions over northern South America, Central America, and southern Africa (December, January, February); and over eastern Australia, Indonesia, and the Philippines (June, July, August).
• Warmer-than-normal conditions in southeast Asia, southeast Africa, Japan, southern Alaska, and west/central Canada, SE Brazil, and SE Australia (December, January, February); and along South America's west coast, and again SE Brazil (June, July, August).
• Cooler-than-normal conditions along the U.S. Gulf coast (December, January, February).