UDMessenger

Volume 12, Number 1, 2003


Connections to the Colleges

As the World Turns

Most people who hear that the days are getting longer probably think in terms of having more hours of daylight as summer approaches. However, the phrase has a different meaning to one researcher in the College of Marine Studies.

Xiao-Hai Yan, professor of oceanography and co-director of the College's Center for Remote Sensing, says the movement of a body of warm water in the western Pacific actually can make the day longer than 24 hours.

Yan and other members of a nine-person research team have reported that the migration of this body of water, called the Western Pacific Warm Pool (WPWP), affects the Earth's rotation rate enough to lengthen the day by a few microseconds. Although no one may be aware of having this extra time, any changes in the length of day are an important indicator of climate changes on a global scale, Yan says.

The study was cited as one of the most significant findings of 2002 by the American Geophysical Union. "Even though the Western Pacific Warm Pool is the largest single expanse of warm water on Earth--it is approximately the size of Africa--its movement was thought to be too small to have a significant effect on the Earth's rotation," Yan says.

"Information on the movement of the warm pool can now be incorporated into models that predict global climate changes. In addition, this discovery leads to new questions that need to be answered. This can only add to our understanding of the processes that govern global climate changes."

Located roughly northeast of New Guinea, the WPWP has a surface temperature of 28 degrees Celsius or higher, which is 25 degrees warmer than most of the equatorial ocean. Seasonal and interannual variations in both atmospheric conditions and ocean currents cause the warm pool to migrate across the tropical ocean.

The researchers used satellite images of sea-surface temperatures from 1970-2000 to determine the size and location of the WPWP, which they then correlated to measurements of the length of the day. They found that the migration of the warm pool causes a redistribution of mass in the ocean, resulting in a corresponding change in the Earth's rotation rate.

"The effect is most pronounced during El Niño years," Yan says. "During an El Niño year, the eastward movement of the Western Pacific Warm Pool is the largest and most active."

Not only did Yan lead the way in arriving at this startling conclusion, but he also pioneered the technique that the researchers used. He was the first scientist to show that satellite imagery, instead of actual temperatures of the sea surface, could be used to precisely determine the size and location of the WPWP. His results were published by the American Geophysical Union and by Science magazine in 1992, and Yan continues to receive requests to use them.

Another area in which Yan has made significant contributions is in expanding the use of remote-sensing technology to study processes beneath the surface of the water. In 1992, he developed a computer model that calculates subsurface characteristics based on available surface information obtained from satellites. Predictions based on his mixed-layer remote-sensing model are so reliable that oceanographers all over the world have asked to use it. So have other scientists, such as marine biologists studying the growth of microscopic algae and climatologists working on computer models of the global climate system.

Yan joined the UD faculty in 1990. In 1994, he received the prestigious Presidential Faculty Fellow Award, an annual award that is bestowed by the president of the United States on 30 exceptionally promising scholars early in their academic careers. This award was followed by an honorary Cheung Kong Chair Professorship at the Ocean University of China in 2000. Sponsored by the Li Ka Shing Foundation of Hong Kong and the Ministry of Education in China, the permanent chair is awarded to only a few scientists throughout the world.

Yan maintains an active research group, writing grants that bring approximately $500,000 in external research support per year to UD. He teaches, publishes an average of 10 papers a year in peer-reviewed journals and speaks at international conferences. In addition, he says he places a high value on educating high-quality new scientists.

"I like to make learning more exciting and problem-based by bringing state-of-the-art techniques into the classroom," says Yan, who has advised 20 graduate students and eight undergraduate international exchange students. In describing his approach to teaching, he quotes a Chinese proverb: "Tell me, I may forget; show me, then I may remember; but involve me, and I'll understand."

"Working under Dr. Yan has been a great experience," says Brian Dzwonkowski, a master's degree student in oceanography, who joined the research group because of Yan's expertise in satellite oceanography. "He is open-minded about new avenues of research and is therefore very encouraging in the pursuit of new ideas. In addition, his obvious work ethic provides a good example to his students and puts high expectations on us."

How does Yan do it all?

"I enjoy being busy," he says. "I am most happy when I am in the throes of a new discovery and when I see my graduate students finish and succeed as scientists."

--Kari Gulbrandsen