46 El Niño and La Niña-Mid

El Niño and La Niña FOCUS Question How does water in the Pacific Ocean impact climates around the world?

The Ocean/Air Connection The Pacific Ocean is the largest body of water on Earth. The heat and energy stored within its waters are so great that this vast ocean affects climates around the globe. Energy in the form of heat is transferred from the ocean to the atmosphere and from the atmosphere back to the ocean. These cycles of heat transfer work together like the gears of a machine to create global climate patterns. In turn these patterns create weather events such as droughts or flooding that affect countries surrounding the Pacific Ocean. One important pattern in the Pacific Ocean is the El Niño-Southern Oscillation (ENSO).

The ENSO is a natural climate cycle that moves heat energy throughout the ocean and between the ocean and the atmosphere. This exchange of heat happens deep underwater, at the surface of the water, and in the atmosphere above the ocean. The ENSO cycle is made up of three parts, or phases: the neutral phase, the El Niño phase, and the La Niña phase. One phase leads into another. About half the time, the Pacific Ocean is in the neutral phase. This is the part of the ENSO cycle that scientists consider the normal conditions of the Pacific Ocean. Word Wise To oscillate means to move back and forth between two points, like some electric fans do. In ENSO, it means to move between the El Niño and La Niña phases.

Neutral Phase Across our planet, air moves from areas of high pressure, to areas of low pressure which creates wind. The equator is an area of low pressure, so wind moves toward it from both the Northern and Southern Hemispheres. Due to Earth's rotation, winds arriving at the equator move from east to west (see diagram). They are called trade winds. Across the Pacific Ocean, trade winds move warm water at the ocean's surface from east to west. The zone between warmer surface water and colder deep-ocean water is called the thermocline (see page 5). As the surface water moves west, deeper, cooler water comes up to take its place. This upwelling creates clear differences, in water temperature, with cooler water off the coast of South America and warmer water near Australia.

When the warm surface water reaches land in the western Pacific, the heat energy within the ocean is transferred to the atmosphere. Warm, moist air rises from the ocean, creating thunderstorms. After the heavy rainfall, the air becomes drier and cooler. This dry, cool air moves back toward the east where it sinks and joins the trade winds again. This cycle of air movement in the atmosphere is called the Walker Circulation. The Walker Circulation is driven by convection—the movement of heat through air or water. A convection cycle also occurs underwater as warm surface water. moves west and cold, deep water rises up to take its place.

El Niño Phase When cool water off the coast of South America is replaced by warm water, an El Niño phase begins. The atmosphere and ocean work together to form an area of warmer-than-normal water in the Pacific. The Walker Circulation weakens or even reverses. Less cool dry air sinks in the eastern Pacific. The trade winds also weaken. Now, warm surface water builds up near South America. This warm water blocks the upwelling of cold water, making the surface water even warmer. With warm water now centered in the eastern Pacific, rain clouds form farther east and bring more rain to Central and North America.