The Dynamic Pacific Ocean
The first image of the Pacific Ocean, above (fig. 1), was produced using sea surface height (SSH) measurements taken by the U.S./French TOPEX/Poseidon satellite. The image shows SSH relative to normal ocean conditions during a 10-day period centered on 10 November 1997.
The volume of warmer surface water (corresponding to higher SSH in the white and red areas) in the core of the El Niño peaked during this time. The area of lower sea level (purple and blue areas) decreased somewhat from late October 1997.
The white and red areas indicate higher heat storage in the waters of the eastern Pacific Ocean; in the white areas, the sea surface is between 14 centimeters and 32 cm (6 inches to 13 inches) above normal. The green areas indicate normal conditions, while purple (in the western Pacific) indicates at least 18 centimeters (7 inches) below normal sea level.
The surface area covered by the warm water mass in the east is about 50 percent larger than the continental United States. The added amount of oceanic warm water near the Americas, with a temperature between 21 to 30 degrees Celsius (70 to 85 degrees Fahrenheit), is about 30 times the volume of water in all the U.S. Great Lakes combined.
The El Niño phenomenon is thought to be triggered when the steady westward blowing trade winds over the Pacific Ocean weaken and even reverse direction. This change in the winds allows a large mass of warm water that is normally located near Australia to move eastward along the equator until it reaches the coast of South America.
The displacement of so much warm water affects evaporation. Rain clouds form and, consequently, alter the typical atmospheric jet stream patterns around the world. Using these global data, limited regional measurements from buoys and ships, and a forecasting model of the ocean-atmospheric system, the National Centers for Environmental Prediction (NCEP) of the National Oceanic and Atmospheric Administration (NOAA) are able to issue advisories indicating the consequences of strong El Niño conditions throughout the following winter.
Boosted by the influence of a larger climate event in the Pacific, one of the strongest La Niñas in many years slowly weakened as seen in this April 2008 image from the U.S.-French Jason-1 oceanographic satellite (fig. 2).
This La Niña, which persisted for nearly a year, is indicated by the blue area in the center of the image along the equator. Blue indicates lower than normal sea surface height (SSH) and cooler water.
The image also shows that this La Niña is occurring within the context of a larger climate event, the early stages of a cool phase of the basin-wide Pacific Decadal Oscillation. The Pacific Decadal Oscillation is a long-term fluctuation of the Pacific Ocean that waxes and wanes between cool and warm phases approximately every five to 20 years. In the cool phase, higher than normal sea-surface heights caused by warm water form a horseshoe pattern that connects the north, west and southern Pacific, with cool water in the middle. During most of the 1980s and 1990s, the Pacific was locked in the oscillation's warm phase, during which these warm and cool regions are reversed. For an explanation of the Pacific Decadal Oscillation and its present state, see: http://jisao.washington.edu/pdo/ and http://www.esr.org/pdo_index.html.
A La Niña is essentially the opposite of an El Niño. During La Niña, trade winds are stronger than normal. Cold water that usually sits along the coast of South America is pushed to the middle of the equatorial Pacific. A La Niña changes global weather patterns and is associated with less moisture in the air, and less rain along the coasts of North and South America.
Jason's follow-on mission, the Ocean Surface Topography Mission/Jason-2, launched in June 2008 will extend to two decades the continuous data record of sea surface heights begun by Topex/Poseidon in 1992.