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Archive for the ‘Trade winds’ Category

Your Worst Fears About El Niño

Posted by feww on March 21, 2010

Worst fears about El Niño may come true

The El Niño, formally known as the El Niño-Southern Oscillation, or “ENSO” for short, is the most significant cause of large-scale climate variability in the tropics. El Niño episodes bring warmer than normal waters to the central and eastern Pacific Ocean from Indonesia in the western end to South America in the eastern end of the  ocean, helping to maintain the above-normal sea surface temperatures.

Figure below shows one of these Kelvin Waves progressing across the Pacific in February 2010.

Kelvin Wave Renews El Niño

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.

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. NASA Earth Observatory images by Jesse Allen, Kevin Ward, and Robert Simmon. Caption by Rebecca Lindsey, based on interpretation provided by Josh Willis and Bill Patzert, NASA JPL.

Related Links

  • El Niño [Main Page, Links to Weekly Updates Archive]

Posted in El Niño episode, ENSO, Equatorial Pacific, Kelvin Waves, Trade winds | Tagged: , , , , | 2 Comments »

Speaking of El Niño, OLR Anomalies in Australia

Posted by feww on November 19, 2009

Another Human Induced Planetary Antiphase Event

El Niño is experiencing a late-fall resurgence

Recent measurements of sea level height from the Ocean Surface Topography Mission (OSTM)/Jason-2 oceanography satellite showed that a strong wave of warm water, known as a Kelvin wave, had spread from the western to the central and eastern Pacific. This warm wave appears as the large area of higher-than-normal sea surface heights in the area between 170 degrees east and 100 degrees west longitude.

This image was created with data collected OSTM/Jason 2 during a 10-day period centered on November 1, 2009. Red and white areas in the central and eastern equatorial Pacific were 100 to 180 millimeters (4 to 7 inches) above normal. In the western equatorial Pacific, blue and purple areas show where sea levels were between 80 and 150 millimeters (3 and 6 inches) below normal.

Sea surface height is an indication of temperature.The elevated sea levels in the central and eastern Pacific are equivalent to sea surface temperatures more than one to two degrees Celsius above normal (2 to 4 degrees Fahrenheit).

The Kelvin wave was triggered by a large-scale, sustained weakening of trade winds in the western and central equatorial Pacific during October. The change in winds disturbs not only the surface currents but also the deeper ocean circulation. The disturbances reverberate along the thermocline—the boundary between warm, surface water and cold, deep water—as large, slow-moving waves. Similar, weaker events that began in June 2009 initially triggered and have sustained the present El Niño.

NASA image by the Jet Propulsion Laboratory Ocean Surface Topography Team. Caption adapted from the Planetary Photojournal. Edited by FEWW

Related Links:

Posted in eastern Pacific, ENSO, Kelvin wave, thermocline, Trade winds | Tagged: , , , , , | 1 Comment »