Pacific Ocean sea-surface height measurements and atmospheric water vapor information taken from two independent Earth-orbiting satellites are providing more convincing evidence that the weather-disrupting phenomenon known as El Nino is back.
"The new data collected since April 1997 confirm what we had earlier speculated upon and what the National Oceanographic and Atmospheric Administration has predicted - a full-blown El Nino condition is established in the Pacific," said Dr. Lee-Lueng Fu, project scientist for the U.S./French TOPEX/Poseidon satellite at NASA's Jet Propulsion Laboratory, Pasadena, CA.
The five years of global ocean topography observations made by TOPEX/Poseidon have been a boon for El Nino researchers, who have been able to track three El Nino events since the satellite's launch in August 1992.
"The recent data are showing us that a large warm water mass with high sea-surface elevations, about 15 centimeters (six inches) above normal, is occupying the entire tropical Pacific Ocean east of the international date line. In fact, the surface area covered by the warm water mass is about one and a half times the size of the continental United States," Fu said. "We watched this warm water mass travel eastward from the western Pacific along the equator earlier this spring. Right now, sea-surface height off of the South American coast is 25 centimeters (10 inches) higher than normal, which is comparable with the conditions during the so-called 'El Nino of the century' in 1982-83."
In addition, recent atmospheric water vapor data collected from NASA's Upper Atmosphere Research Satellite (UARS) show tell-tale signs of an El Nino condition in the tropical Pacific Ocean.
"The Microwave Limb Sounder experiment on UARS is detecting an unusually large build-up of water vapor in the atmosphere at heights of approximately 12 kilometers (eight miles) over the central-eastern tropical Pacific. Not since the last strong El Nino in the winter of 1991-92 have we seen such a large build-up of water vapor in this part of the atmosphere," said JPL's Dr. William Read. "Increased water vapor at these heights can be associated with more intense winter-time storm activity from the 'pineapple express,' a pattern of atmospheric motions that brings tropical moisture from Hawaii to the southwestern United States. This phenomena is an example of how the ocean and atmosphere work together to dictate the severity of El Nino events."
An El Nino is thought to be triggered when steady westward blowing trade winds weaken and even reverse direction. This change in the winds allows the 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. This displaced pool of unusually warm water affects evaporation, where rain clouds form and, consequently, alters the typical atmospheric jet stream patterns around the world. The change in the wind strength and direction also impacts global weather patterns.
In May, the National Oceanographic and Atmospheric Administration (NOAA) issued an advisory regarding the presence of the early indications of El Nino conditions. Subsequent El Nino forecast activities supported by NOAA indicate the likelihood of a moderate or strong El Nino in late 1997. The forecast model operated at NOAA's National Centers for Environmental Prediction used data collected by the TOPEX/Poseidon satellite.
"The added amount of oceanic warm water near the Americas, with a temperature between 21-30 degrees Celsius (70-85 degrees Fahrenheit), is about 30 times the volume of water in all the U.S. Great Lakes combined," said Dr. Victor Zlotnicki, a TOPEX/Poseidon investigator at JPL. "The difference between the current, abnormally high amount of heat in the near-surface waters and the usual amount of heat in the same area is about 93 times the total energy from fossil fuels consumed by the United States in 1995."
The climatic event has been given the name El Nino, a Spanish term for a "boy child," because the warm current first appeared off the coast of South America around Christmas. Past El Nino events have often caused unusually heavy rain and flooding in California, unseasonably mild winters in the Eastern United States and severe droughts in Australia, Africa and Indonesia. Better predictions of extreme climate episodes like floods and droughts could save the United States billions of dollars in damage costs. El Nino episodes usually occur approximately every two to seven years.
Developed by NASA and the Centre National d'Etudes Spatiales (CNES), the TOPEX/Poseidon satellite uses an altimeter to bounce radar signals off the ocean's surface to get precise measurements of the distance between the satellite and the sea surface. These data are combined with measurements from other instruments that pinpoint the satellite's exact location in space. Every 10 days, scientists produce a complete map of global ocean topography, the barely perceptible hills and valleys found on the sea surface. With detailed knowledge of ocean topography, scientist can then calculate the speed and direction of worldwide ocean currents.
The MLS instrument was originally designed to study atmospheric ozone depletion, but scientists have devised new ways of using the data to study atmospheric water vapor. The UARS satellite is completing its sixth year of operation after being designed for only a two-year mission, and is conducting an extended mission of longer-term global monitoring.
The Jet Propulsion Laboratory, a division of the California Institute of Technology, Pasadena, CA, manages the TOPEX/Poseidon mission and the MLS instrument for NASA's Mission to Planet Earth enterprise, Washington, DC. The UARS satellite is managed by NASA's Goddard Space Flight Center, Greenbelt, MD.