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La Niņa Episodes in the Tropical Pacific
 
At times ocean surface temperatures in the equatorial Pacific are colder than normal. These cold episodes, sometimes referred to as La Niņa episodes, are characterized by lower than normal pressure over Indonesia and northern Australia and higher than normal pressure over the eastern tropical Pacific. This pressure pattern is associated with enhanced near-surface equatorial easterly winds over the central and eastern equatorial Pacific.

During cold (La Niņa) episodes the normal patterns of tropical precipitation and atmospheric circulation become disrupted. The abnormally cold waters in the equatorial central give rise to suppressed cloudiness and rainfall in that region, especially during the Northern Hemisphere winter and spring seasons. At the same time, rainfall is enhanced over Indonesia, Malaysia and northern Australia. Thus, the normal Walker Circulation during winter and spring, which features rising air, cloudiness and rainfall over the region of Indonesia and the western Pacific, and sinking air over the equatorial eastern Pacific, becomes stronger than normal. 

By studying past cold episodes scientists have discovered precipitation and temperature anomaly patterns that are highly consistent from one episode to another. Significant departures from normal are shown in the accompanying figures for the Northern Hemisphere winter and summer seasons. During cold episodes, the colder than normal ocean temperatures in the equatorial central Pacific act to inhibit the formation of rain-producing clouds over that region. Wetter than normal conditions develop farther west over northern Australia, Indonesia and Malaysia, during the northern winter, and over the Philippines during the northern summer. Wetter than normal conditions are also observed over southeastern Africa and northern Brazil, during the northern winter season. During the northern summer season, the Indian monsoon rainfall tends to be greater than normal, especially in northwest India. Drier than normal conditions during cold episodes are observed along the west coast of tropical South America, and at subtropical latitudes of North America (Gulf Coast) and South America (southern Brazil to central Argentina) during their respective winter seasons.

Mid-latitude low pressure systems tend to be weaker than normal in the region of the Gulf of Alaska, during a cold episode winter. This favors the build-up of colder than normal air over Alaska and western Canada, which often penetrates into the northern Great Plains and the western United States. The southeastern United States, on the other hand, becomes warmer and drier than normal.

Since anomaly patterns during cold episodes tend to persist for several months, accurate long-range forecasts (1 to 3 seasons) are possible for the regions shown in the accompanying figures. For the latest information on the status of La Niņa, go to ENSO Advisory (issued when appropriate) or the latest monthly Climate Diagnostics Bulletin.

More technical information on the global patterns of abnormal precipitation and temperature related to cold episodes in the tropical Pacific can be found in Ropelewski and Halpert (1989, J. Climate, 2, 268-284), and Halpert and Ropelewski (1992, J. Climate, 5, 577-593).

source: Climate Prediction Center