Simultaneous ocean measurements taken by two orbiting NASA science instruments suggest that another weather-disrupting El Niño may be developing in the Pacific Ocean, with the potential of affecting global weather patterns next winter.

Sea-surface height measurements taken by the radar altimeter onboard the joint U.S.-French TOPEX/POSEIDON satellite and wind data collected by the NASA Scatterometer on Japan's Advanced Earth Observing Satellite (ADEOS) are being used together for the first time to diagnose changing oceanographic and atmospheric conditions in the tropical Pacific Ocean.

The El Niño phenomenon is thought to be triggered when steady westward-blowing trade winds weaken and even reverse direction. This wind shift allows the large mass of warm water 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 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.

"NSCAT has observed two episodes of the reversal of the trade winds in the western Pacific, one at the end of December and one at the end of February. Both generated warm water masses, called Kelvin waves, that traveled across the Pacific and were measured by TOPEX/POSEIDON," said Lee-Lueng Fu, TOPEX/POSEIDON project scientist at the Jet Propulsion Laboratory. "Kelvin waves are often a precursor to a warm state of the tropical Pacific, sometimes leading to an El Niño. Whether an El Niño event will occur cannot be determined by just examining the satellite data. A computer model that couples ocean-atmosphere data, like the one used by the National Oceanic and Atmospheric Administration (NOAA), is a necessary tool to issue scientifically based predictions. Now, for the first time, both TOPEX/POSEIDON and NSCAT are observing and providing the best, near real-time view of global ocean winds and sea level ever obtained. These observations will help NOAA's model to predict the occurrence of El Niño."

NOAA has issued an advisory about the early indications of El Niño conditions. A number of El Niño forecast activities supported by NOAA indicate the likelihood of a moderate or strong El Niño in late 1997. The forecast model operated at NOAA's National Centers for Environmental Prediction (NCEP) used data collected by the TOPEX/POSEIDON satellite.

"The use of TOPEX/POSEIDON data clearly improved our forecast for the winter of 1996-1997," said Ants Leetmaa, chief scientist at NCEP. "We currently use the data continuously for our operational ocean analyses and El Niño forecasts. The use of this data set enabled a clearer picture to be developed of the multi-year evolution of ocean conditions in the tropical Pacific that have resulted in the onset of the current warm episode. We have not yet had a chance to utilize the NSCAT data in the models but we anticipate that its use also will improve our forecast system."

"Since the beginning of the instrument's operation in September 1996, NSCAT has observed stronger than normal easterly winds in the central and western tropical Pacific, which might have piled up warm water in the west as indicated by the higher than normal sea level and sea surface temperature," said W. Timothy Liu, NSCAT project scientist at JPL. "This is usually a precursor of subsequent anomalous warming in the east. Kelvin waves moving across the Pacific do not necessarily mean El Niño, but we are studying how these seasonal phenomena like Kelvin waves are related to events like El Niño that occur over several years.

Unfortunately, NSCAT and other instruments including a Total Ozone Mapping Spectrometer were lost to scientists when the ADEOS satellite failed and was declared irrecoverable by NASDA, the Japanese space agency, on June 30 this year. "The data we have obtained to date are extremely valuable," said Jim Graf, NSCAT project manager at JPL. "If we knew we were limited to just nine months of data, we would have chosen the period we actually got. We obtained coverage over the summer and winter monsoon seasons and what may be the onset of an El Niño. Perhaps the largest loss is the discontinuity of the long-term data set, which is being used to understand interannual and decadal variations in our climate."

El Niño, a Spanish term for a "boy child," is so called because the warm current first appeared off the coast of South America around Christmas. Past El Niño events have caused unusually heavy rain and flooding in California, unseasonably mild winters in the Eastern United States and severe droughts in Australia, Africa and Indonesia. El Niño episodes usually occur approximately every two to seven years.

The following Internet sites can be accessed for more information:



NCEP: /index.html


Jupiter has both wet and dry regions, just as Earth has tropics and deserts, according to new images and data from the Galileo spacecraft. The data may explain why Galileo's atmospheric probe found much less water than scientists had anticipated when it dropped into the Jovian atmosphere in December 1995. "We had suspected that the probe landed in the `Sahara Desert of Jupiter," said Andrew Ingersoll, a professor at the California Institute of Technology and member of the Galileo science team. "But the new data show there is moisture in the surrounding areas. Jupiter is not as dry overall as we thought."

The probe apparently entered a clearing in the clouds — a dry spot through which deeper, warmer layers can be seen. By studying various areas, including those resembling the probe entry site, the Galileo orbiter has helped scientists understand the probe results. In fact, the air around a dry spot has 100 times more water than the dry spot itself, according to Robert Carlson of the Jet Propulsion Laboratory, principal investigator for the imaging spectrometer instrument onboard Galileo.

Such dry spots cover less than 1% of the atmosphere and appear to be regions where the winds converge and create a giant downdraft, according to Caltech graduate student Ashwin Vasavada. In fact, the water content of the giant, gaseous planet varies at least as much as the moisture varies from place to place on Earth.

"Winds rise from the deep atmosphere and lose water and ammonia," explained Glenn Orton, a Galileo interdisciplinary scientist at JPL and Photopolarimeter-Radiometer co-investigator. "At the top, when they converge and drop back down, nothing is left to condense back into clouds, and a dry clearing is created. These dry spots may grow and diminish, but they recur in the same places, possibly because of the circulation patterns on Jupiter."

Ingersoll said the dry spots are found in a northern hemisphere band at 5-7° latitude. When the Galileo probe was released near the tops of the clouds, it found dry air underneath. But at other locations, the weather might be rather Earth-like.

In the months since the probe's descent, Galileo mission scientists have debated whether the dry conditions it encountered were due to the downdraft concept, or whether Jupiter's water had somehow been concentrated deep in the gas planet's interior as it formed and evolved four billion years ago. "There was a cosmochemical explanation and a meteorological explanation, and our latest analysis clearly favors the idea that the dry spots are a consequence of weather-related activity," Ingersoll said. "Fifty miles below the cloud tops, we could expect thunderstorms, lightning, and rain. But in contrast to Earth, individual jovian storms and weather systems sometimes last for months, years, or even centuries. The Great Red Spot, for example, has existed for at least the 300 years that we've been aware of it."

Despite the relatively warm temperatures and the presence of water on Jupiter, Ingersoll said it is "highly unlikely" that the planet could sustain life in its thick, gaseous environment without any solid surface. He expressed the opinion that any jovian life forms would have to hover, and "while we might imagine an advanced life form that could adapt, prebiotic compounds would not survive in that environment and, therefore, evolution could not take place there."

The latest data from Galileo also shed new light on the auroras that glow in a narrow ring around the Jupiter's poles. The auroras are created when charged particles collide with atmospheric particles, causing them to light up. New images show the nightside aurora for the first time.

Scott Bolton of JPL, co-investigator for the Galileo plasma and plasma wave instruments, said the latest findings show "Jupiter's auroras are a lot like the auroras we see on Earth as the northern lights. However, we now know that the auroral arc on Jupiter is thin and patchy, and we can also estimate its altitude is between 300 and 600 kilometers."

These auroras had previously been viewed in ultraviolet light by the Hubble Space Telescope, and in infrared light by Earth-based telescopes, but Galileo was able to capture images of the auroras in visible light and from a closer vantage point.

Images and other data received from Galileo are posted on the Galileo home page on the World Wide Web.


Astronomers using the Hubble Space Telescope to track weather on Mars, and how it might affect the Pathfinder landing site in Ares Vallis, reported July 15 that a large dust storm seen south of the site only 12 days earlier had dissipated. However, a new dust storm has appeared in the polar region, about 2000 kilometers (1200 miles) due north of Pathfinder. The researchers conclude that Pathfinder landed during a period when rapid changes in the regional distributions of dust and clouds were taking place on Mars.

Hubble pictures taken with the Wide Field Planetary Camera 2 recorded remarkable daily changes in the behavior of dust and water-ice clouds on the red planet as followed over a three-day period, from July 9 to 11. The variability seen in just three consecutive days of Mars observations shows that weather changes are very rapid — possibly chaotic — so it may be impractical to make an accurate weather forecast for Mars, say researchers. "Mars never ceases to amaze us," says Steve Lee of the University of Colorado at Boulder.

The team suggests that the dust seen in the skies over Pathfinder may have diffused toward the landing site from the now dissipated dust storm in Valles Marineris. The newly erupted 600-mile-long dust streamer coming off the northern polar cap and extending southward may also be sending dust toward the Pathfinder landing site.

The Hubble Mars Team consists of Phil James and Mike Wolff (University of Toledo), Steve Lee (University of Colorado), Todd Clancy (Space Science Institute), and James Bell III (Cornell University.)


NASA Hubble Space Telescope images of Mars, taken on June 27, 1997 (left) and July 11, 1997 (right), document the dissipation of a large dust storm during the 12 days separating the two observations.

The images were taken to monitor the weather conditions near Ares Vallis, the site where NASA's Pathfinder spacecraft landed on July 4. Maps of the equatorial region were constructed from the images and are shown at the bottom of the figure; a green cross marks the Pathfinder landing site. (All images are oriented with north to the top.)

These two sets of observations show a number of dramatic changes in the planet's atmosphere. At about the 7 o'clock position on the June 27 image, the eastern end of the Valles Marineris canyon system is just coming into daylight and can be seen to be filled with yellowish dust. The dust appears to be confined to the canyons, which can be as much as 8 kilometers deep and hundreds of kilometers wide. Estimates of the quantity of dust involved in this storm indicate that 96% of the incoming sunlight is being blocked from reaching the surface by the dust clouds. Note that on the July 9 image, the dust storm appears to be subsiding; it is estimated that the dust quantity in most of the visible canyon system has dropped to only 10% to 20% of that seen on June 27.

However, on July 9 a streamer of dust is visible in the North polar region, extending about 1200 kilometers southward from the dark sand dunes surrounding the polar ice cap; diffuse dust is visible over much of Acidalia, the dark region to the north of the Pathfinder landing site. The extent of clouds visible across the planet has also changed considerably between the two dates. Just to the west (left) of the July 9 dust streamer, a very bright area of water-ice clouds is seen; this area was considerably cloudier on June 27.

These images dramatically show that atmospheric conditions can change rapidly on Mars. Observations such as these will continue to be made over the next several months, allowing the detailed surface observations made by Pathfinder to be placed into the broader context of the global images available from HST.


NASA Hubble Space Telescope images of Mars, obtained over three consecutive days between July 9 and 11, 1997, dramatically show that the behavior of dust and water-ice clouds exhibit substantial daily variations. The full-disk images are shown along the top (Pathfinder landing site marked by the green crosses), and maps of the north polar region are shown along the bottom. (The maps are oriented with 0° longitude to the bottom, and extend from 40° N latitude to the pole; latitude circles are shown at 40°, 60°, and 80°, and lines of longitude are shown every 45°.) About 24 hours separates each of the images.

The polar maps document the movement of bright water-ice clouds as they are seen to progress eastward, perhaps driven by a passing weather front on Mars. Between July 9 and 10, the polar cloud seen near 60° N latitude is measured to have moved about 550 kilometers eastward over a period of about 24 hours, corresponding to a velocity of about 22 kilometers/hour. The cloud has dissipated considerably by July 11.

Clouds in the southern hemisphere seem to thicken considerably over this three-day period as well. The bluish south polar hood, composed of water-ice clouds, is seen along the bottom of the images. We cannot see the south polar cap, since the north polar cap is tilted toward us during this season and the south cap is in winter darkness.

On the July 9 image and map, a streamer of dust is visible in the north polar region, extending about 1200 kilometers southward from the dark sand dunes surrounding the polar ice cap; diffuse dust is visible over much of Acidalia, the dark region to the north of the Pathfinder landing site. This dust cloud is apparently diffusing with time, as it seems to become less distinct in each successive image. This dust storm may be supplying some of the dust seen overhead by the Pathfinder spacecraft.