New results from the ocean-observing TOPEX/Poseidon satellite are challenging a fundamental oceanographic theory about the speed of large-scale ocean waves -- a finding that could ultimately revise science textbooks and improve global weather forecasting.
The large-scale ocean waves, with wavelengths of hundreds of kilometers from one wave crest to the next, are called Rossby waves. These waves carry a "memory" of weather changes that have happened at distant locations over the ocean, according to Dr. Dudley Chelton, a TOPEX/Poseidon science team member at the College of Oceanic and Atmospheric Sciences at Oregon State University in Corvallis.
Using data gathered by the satellite, scientists tracked the waves as they move through the open ocean and have determined that, at mid-latitudes, the Rossby waves are moving two to three times faster than previously thought, Chelton reports in today's issue of Science magazine.
Rossby waves are a natural result of the Earth's rotation and a key feature of large-scale ocean circulation. In animations of altimeter data from the TOPEX/Poseidon satellite, the waves appear as alternating positive and negative sea level features traveling throughout much of the world oceans.
"Every first-year student in physical oceanography learns about Rossby waves. However, observing them away from the Equator has been extraordinarily difficult, because they cause changes in sea level of 10 to 20 centimeters (4 - 8 inches), spread over hundreds of kilometers, and move westward so slowly that a wave may take more than 10 years to cross the Pacific at the latitude of Los Angeles, and more than 30 years at the latitude of Portland, Oregon," said Dr. Victor Zlotnicki, an oceanographer at NASA's Jet Propulsion Laboratory.
"Thanks to TOPEX/Poseidon, for the first time we can see these waves very clearly, and this research shows that they become more intense to the west of the great mountain chains on the ocean floor, and more fundamentally, that they travel much faster than the accepted theory predicts," Zlotnicki said.
Since Rossby waves can alter currents and their corresponding sea surface temperatures, the waves influence the way the oceans release heat to the atmosphere and thus are able to affect weather patterns.
For example, in 1994, oceanographers at the Naval Research Laboratory mapped a Rossby wave that they concluded was a remnant of the 1982-83 El Nino event. They found evidence that the Kuroshio current, located off the coast of Japan, was pushed northward, raising the temperature of the northwest Pacific. Some scientists blamed this shift for contributing to the flooding across the midwestern United States in 1993.
"If our traditional notions about the wave speeds are incorrect," Chelton said, "then the waves get from one side of the mid-latitude ocean to the other twice as fast, which means that the ocean evidently adjusts more rapidly that we previously thought."
This more precise information about how fast the waves are traveling may help forecasters improve their ability to predict the effects of El Nino events on weather patterns years in advance.
TOPEX/Poseidon, a joint program of NASA and the Centre National d'Etudes Spatiales, the French space agency, uses a radar altimeter to precisely measure sea-surface height. Scientists use the TOPEX/Poseidon data to produce global maps of ocean topography, which then can be used to identify Rossby waves.
TOPEX/Poseidon is part of NASA's Mission to Planet Earth, a coordinated, long-term research program to study the Earth as a global system. TOPEX/Poseidon's sea-surface height data are essential to a better understanding of the role oceans play in regulating global climate, one of the least understood areas of climate research.
The Jet Propulsion Laboratory manages the U.S. portion of the TOPEX/Poseidon mission for NASA.
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