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Monday, April 12, 2010

El Nino renewed by Kelvin waves

Kelvin Wave Renews El Niño

NASA's Earth Observatory, March 21, 2010.  Click on this link to see full graphic:
Kelvin Wave Renews El Niño
Color bar for Kelvin Wave Renews El Niño
  acquired January 15, 2010 - February 15, 2010
download large January 15th image (411 KB, PNG) acquired January 15, 2010
download large January 30th image (416 KB, PNG) acquired January 30, 2010
download large February 15th image (416 KB, PNG) acquired February 15, 2010
download animation (2 MB, QuickTime) acquired December 1, 2009 - March 4, 2010

The climate pattern known as the El Niño-Southern Oscillation, or “ENSO” for short, is the biggest cause of large-scale climate variability in the tropics. During an El Niño episode, the central and eastern Pacific Ocean are warmer than normal. The above-normal sea surface temperatures are maintained by gentle but giant waves of warm water that slosh across the Pacific from Indonesia toward South America.

This series of globes shows the eastward progression of one of these deep Kelvin waves in February 2010. The globes show sea surface height anomalies, which means places where the water surface is higher (red) or lower (blue) than average. A higher-than-average sea surface height at a given location indicates that there is a deeper-than-normal layer of warm water. Lower-than-average sea surface height indicates a shallower layer of warm water. The globes are based on 10 days of data centered on January 15, January 30, and February 15, 2010.

In January (left-hand globe), sea surface heights across the central and eastern equatorial Pacific were elevated (red), but not extremely so, potentially a sign that El Niño was weakening. But in early February, a strong sea level anomaly appeared northeast of Australia (center globe). This swell of deep, warm water is the start of the Kelvin wave, and by late February, it had spread eastward into the central Pacific (right-hand globe) and re-invigorated the current El Niño.

Where do Kelvin waves come from? Under normal conditions, the tropics’ prevailing easterly winds push Sun-warmed surface waters across the Pacific from the Americas toward Indonesia, creating a deep pool of warm water in the western Pacific. During an El Niño, the trade winds falter, and sometimes even reverse, for months. When the winds that maintain the warm pool falter, a large pulse of warm water from the western Pacific slides back toward the east. The most current sea surface height images are available on the JASON mission sea level Webpage.


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