A schematic view of the three-dimensional Walker Cell circulation. The Walker Cell 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.
Image courtesy of NOAA Geophysical Fluid Dynamics Laboratory