Key Concepts:

Satellite Altimetry

• TOPEX/Poseidon (satellite) was developed to measure the ocean's sea-surface height. By knowing the sea-surface height, scientists are able to develop very accurate maps of ocean currents and sea-surface heights related to heat content.
  
• TOPEX/Poseidon and its follow-up satellite--Jason--are the result of a joint program between the U. S. and French space agencies.
  
• Satellites, such as TOPEX/Poseidon, give scientists early clues to an El Niño or La Niña event. By examining the satellite images, sea-surface height maps give scientists an almost biweekly look at the ocean and its changes. It is actually the sea-surface height anomally that scientists use to detect events that can lead to El Niño or La Niña.
  

Variations in Atmospheric Pressure

• As the warm water in the tropical Pacific shifts along the equator, the areas of high pressure and low pressure shift as well. Just a slightly lowered atmospheric pressure can cause a change in the trade-winds.
  
• An index, known as the Southern Oscillation Index, is used to compare the barometric pressure in Tahiti and Darwin, Australia. During a normal year, the index is zero. During an El Niño, the index will be a negative number because the pressure in Australia is higher than in Tahiti. This causes the trade-winds to weaken or even reverse (westerly wind burst).
  
• Changes in the wind field can also be used to determine when El Niño conditions are beginning or to judge the severity of the current El Niño.
  

Buoys

• To provide important data, NOAA operates a network of buoys which measure temperature, currents, and winds in the equatorial band. These buoys daily transmit data which are available to researchers and forecasters around the world in real time.
  
• The TAO (Tropical Atmosphere Ocean) array consists of approximately 70 ATLAS and current meter moorings in the tropical Pacific Ocean, telemetering oceanographic and meteorological data in real time via the Argos satellite system. Designed to improve detection, understanding, and prediction of El Niño, TAO is a major component of the global climate observing system. The array is presently supported by an international consortium, involving cooperation between the United States, France, Japan, and Taiwan.
  
• Measurements from buoys and ships describe the bulk temperature of the ocean several meters below the surface.