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What causes La Niña?* Typically, a La Niña is preceded by a buildup of cooler-than-normal subsurface waters in the tropical Pacific. Eastward-moving atmospheric and oceanic waves help bring the cold water to the surface through a complex series of events still being studied. In time, the easterly trade winds strengthen, cold upwelling off Peru and Ecuador intensifies, and sea-surface temperatures (SSTs) drop below normal. During the 1988- 89 La Niña, SSTs fell to as much as 4 degrees C (7 degrees F) below normal. Both La Niña and El Niño tend to peak during the Northern Hemisphere winter. What's the difference between La Niña and El Niño?* Both terms refer to large-scale changes in sea-surface temperature across the central and eastern tropical Pacific. Usually, sea-surface readings off South America's west coast range from the 60s to 70s F, while they exceed 80 degrees F in the "warm pool" located in the central and western Pacific. This warm pool expands to cover the tropics during El Niño but shrinks to the west during La Niña. The El Niño/Southern Oscillation (ENSO) is the coupled ocean-atmosphere process that includes both El Niño and La Niña. What are the global impacts of La Niña? Both El Niño and La Niña impact global and U.S. climate patterns. In many locations, especially in the tropics, La Niña (or cold episodes) produces the opposite climate variations from El Niño. For instance, parts of Australia and Indonesia are prone to drought during El Niño, but are typically wetter than normal during La Niña. What are the U.S. impacts of La Niña? La Niña often features drier than normal conditions in the Southwest in late summer through the subsequent winter. Drier than normal conditions also typically occur in the Central Plains in the fall and in the Southeast in the winter. In contrast, the Pacific Northwest is more likely to be wetter than normal in the late fall and early winter with the presence of a well-established La Niña. Additionally, on average La Niña winters are warmer than normal in the Southeast and colder than normal in the Northwest.
What impacts do El Niño and La Niña have on tornadic activity across the country? Since a strong jet stream is an important ingredient for severe weather, the position of the jet stream determines the regions more likely to experience tornadoes. More Info! Tornado information from the National Weather Service Contrasting El Niño and La Niña winters, the jet stream over the United States is considerably different. During El Niño the jet stream is oriented from west to east over the northern Gulf of Mexico and northern Florida. Thus this region is most susceptible to severe weather. During La Niña the jet stream extends from the central Rockies east- northeastward to the eastern Great Lakes. Thus severe weather is likely to be further north and west during La Niña than El Niño.