A schematic view of the three-dimensional 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 descends back to the surface in the eastern tropical Pacific, dry and relatively cloud free, completing the circulation loop. Atmospheric sea level pressures are higher under the dry sinking air in the eastern Pacific than in the warmer and more humid west.
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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 beneath a high pressure system. The resulting east-west pressure gradient causes a circulation in the atmosphere along the equator, with surface air moving east to west from high pressure in the eastern Pacific to low pressure in the western Pacific. Higher up, 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 air returns to the west along the surface of the ocean, back to the western Pacific Ocean.
The surface wind pattern pushes Pacific water westward so it piles up in the western Pacific Ocean. Waters in the western Pacific are 50-60 cm higher than in the eastern Pacific due to these winds. The Walker circulation also contributes to normal weather conditions in this part of the 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 are the conditions we associate with El Niño. During periods when the Walker circulation is particularly strong, called La Niña, the winds are stronger across the Pacific, causing cooler ocean temperatures due to increased 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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