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HOME > El Niño/La Niña > The ENSO Cycle > Mean Rainfall
December to February Normal Conditions

Ocean surface temperatures across the tropical Pacific contribute significantly to the observed patterns of tropical rainfall and tropical thunderstorm activity. The heaviest rainfall is typically observed across Indonesia and the western tropical Pacific, and least rainfall is normally found across the eastern equatorial Pacific.

The mean patterns of sea surface temperature and equatorial rainfall are accompanied by low level easterly winds (east- to- west flow) and upper level westerly winds across the tropical Pacific. Over the western tropical Pacific and Indonesia this wind pattern is associated with low air pressure and ascending motion, while over the eastern Pacific it is accompanied by high pressure and descending motion. Collectively, these conditions reflect the equatorial Walker Circulation, which is a primary large-scale circulation feature across the Pacific.

The subsurface ocean structure is characterized by a deep layer of warm water in the western tropical Pacific, and by a comparatively shallow layer of warm water in the eastern Pacific. This warm water is separated from the cold, deep ocean waters by the oceanic thermocline, which is normally deepest in the west and slopes upward toward the surface farther east. The resulting east-west variations in mean upper-ocean temperatures result in east-west variations in sea level height, which is higher in the west than in the east.

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Page last modified: December 19, 2005
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