El Niño for Middle School Science

Feb 21

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Have you ever heard people blaming unusual weather on El Niño or La Niña? These weather patterns are caused by the trade winds and the movement of ocean water. The first people to notice these changes were fishermen in South America. They named the phenomenon El Niño after the Baby Jesus because it happened around Christmas.

If you would like a copy of the picture book, you can get it along with the reading passage, flashcards, and comprehension activities at Teachers Pay Teachers.

What is an El Niño?

Usually, the trade winds between the Tropic of Cancer and the Tropic of Capricorn flow from east to west. The trade winds carry the warm surface water of the Pacific Ocean from the east coast of South America to Asia and Australia. The movement of the water means the water is about 20 inches higher and 45° Fahrenheit warmer in the western Pacific than the eastern Pacific.

When the warm ocean water evaporates, it leads to heavy rains and storms in southeast Asia and northern Australia. South America stays relatively dry, and the moving surface water brings cold water from deeper in the ocean up to the surface. This deep water is full of nutrients like phosphorus and nitrogen that algae need to grow. The rest of the animals up the food chain benefit from the increased algae growth.

Calmer trade winds mean less warm surface water is carried from South America to Asia and Australia. The warm water stays in the eastern Pacific Ocean. Heavy rains fall along the coast of South America, and drought hits southeastern Asia and northern Australia. Deep nutrient-rich water isn't forced to the surface, so the fish populations along the coast of South America either migrate away or die from starvation.

The fishermen of South America were the first to notice these changes in the ocean. Sometimes around Christmas time, the ocean waters got warmer, and the fish disappeared. They named the event an El Niño after the baby Jesus.

The Pacific Ocean makes up about one-third of the Earth's surface, so changes in the Pacific Ocean affect the entire globe. The changes in the ocean and wind currents during an El Niño create unusual weather far beyond the Tropics of Cancer and Capricorn. El Niño years typically result in a milder winter in the northern United States and a wetter winter in the southern United States.

The opposite of an El Niño is a La Niña. During a La Nina, the trade winds become stronger and push stormy weather farther west over Indonesia. Cold water spreads from the east coast of South America to the middle of the Pacific Ocean. Like an El Niño, a La Niña affects more than the Pacific Ocean. La Niñas cause wetter winters in the northern United States and warmer winters in the southern United States.

Scientists do not know what causes El Niños or La Niñas. They happen every two to seven years and can last between nine months and two years. Luckily, scientists can predict a coming El Niño or La Niña and measure the strength of it so that people can prepare for the changes to the oceans and atmosphere.