Pools of warm water known as Kelvin waves can be seen traveling eastward along the equator (black line) in this Sept. 17, 2009, image from the NASA/French Space Agency Ocean Surface Topography Mission/Jason-2 satellite. El Niņos form when trade winds in the equatorial western Pacific relax over a period of months, sending Kelvin waves eastward across the Pacific like a conveyor belt.

Since May 2009, the tropical Pacific Ocean has switched from a cool pattern of ocean circulation known as La Niņa to her warmer sibling, El Niņo. This cyclical warming of the ocean waters in the central and eastern tropical Pacific generally occurs every three to seven years and is linked with changes in the strength of the trade winds. El Niņo can affect weather worldwide, including the Atlantic hurricane season, Asian monsoon season and northern hemisphere winter storm season. But while scientists agree that El Niņo is back, there's less consensus about its future strength. One of the characteristics that signal a developing El Niņo is a change in average sea surface height compared to normal sea level. The NASA/French Space Agency Jason-1 and Ocean Surface Topography Mission/Jason-2 satellites continuously observe these changes in average sea surface height, producing near-global maps of the ocean's surface topography every 10 days.

Recent data on sea-level height from the Jason-1 and Ocean Surface Topography Mission/Jason-2 satellites show that most of the equatorial Pacific is near normal (depicted here as green). The exceptions are the central and eastern equatorial Pacific, which are exhibiting areas of higher-than-normal sea surface heights (warmer-than-normal sea-surface temperatures) at 180 and 110 degrees west longitude.

This Jason-2 image reflects a 10-day data cycle centered around September 17, 2009. It shows a series of warm "bumps" visible along the equator, denoted by a black line. Known as Kelvin waves, these pools of warm water were triggered when the normally westward-blowing trade winds weakened in late July and again in early September, sending them sliding eastward from the western Pacific toward the Americas. The Kelvin waves are 5 to 10 centimeters (2 to 4 inches) high, a few hundred kilometers wide and a few degrees warmer than surrounding waters. Traveling east at about 3 meters per second (6 miles per hour), they are expected to reach the coast of Peru in October. (An animation of the evolution of Pacific Ocean conditions since January 2006 is at: http://www.jpl.nasa.gov/videos/earth/elnino20090928.mov).