This is a three-dimensional view of the Walker circulation. The Walker circulation consists of trade winds blowing from east to west across the tropical Pacific Ocean (blue arrow), bringing moist surface air to the west. In the western tropical Pacific, the moist air rises, forming clouds. The rising air becomes drier as much of its moisture falls to the surface as rain. Winds a few miles high blow from west to east, moving the now drier air toward South America. The air returns back to the surface in the eastern tropical Pacific, dry and relatively cloud free, completing the circulation loop.
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Image courtesy of NOAA Geophysical Fluid Dynamics Laboratory
The Walker circulation is an ocean-based system of air circulation that influences weather on the Earth. The Walker circulation is the result of a difference in surface pressure and temperature over the western and eastern tropical Pacific Ocean. Normally, the tropical western Pacific is warm and wet with a low pressure system, and the cool and dry eastern Pacific lie under a high pressure system. This creates a pressure gradient from east to west and causes surface air to move east to west, from high pressure in the eastern Pacific to low pressure in the western Pacific. Higher up in the atmosphere, west-to-east winds complete the circulation.
The warm waters of the western Pacific Ocean in East Asia heat the air above it and supply it with moisture. On average, the air rises, forms clouds, and then flows to the east across the Pacific, losing moisture to rainfall. The air then sinks off the west coast of South America and returns to the west along the surface of the ocean, back to the western Pacific Ocean.
The Walker circulation contributes to normal weather conditions in the tropical Pacific Ocean: warm, wet weather in the western Pacific and cool, dry weather in the eastern Pacific.
The Walker circulation reverses every few years, as part of a phenomenon called the El Niño-Southern Oscillation (ENSO). When the Walker circulation weakens, the winds also weaken and the warm water of the western Pacific spreads to the east. These conditions are called El Niño. During times when the Walker circulation is particularly strong, called La Niña, the winds are stronger across the Pacific. These strong winds cause cooler ocean temperatures because of upwelling in the eastern Pacific. El Niño and La Niña impact the weather in North and South America, Australia, and Southeast Africa, and can cause flooding, droughts, and increases or decreases in hurricane activity.
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