March 10, 2005
"It was the best of times, it was the worst of times...." The same event can have dramatically different outcomes. For some, an El Niño means a welcome respite from bitter winter weather. For others, it can bring lashing rains and devastating floods.
All El Niños don't run the same course either. Though the El Niño of 2002-03 affected climate around the world, it was much milder than the unusually large El Niño of 1997-98. What made one so much stronger than the other?
University of Maryland researcher Eric Hackert and three colleagues at the university's Earth System Science Interdisciplinary Center took a close look at these two El Niños to find out. Using satellite data of ocean temperature and sea-surface height along with computer models, they analyzed the two events to determine how they were alike and, more importantly, how they were different. One of the things they discovered was that a particular kind of wave helped make one El Niño much stronger and longer lasting than the other.
Hackert and his colleagues first studied how the two events began to see if something in their initial states might have foreshadowed the difference in their ultimate intensity. "In many respects they started the same," says Hackert. "With these initial conditions, the prediction for both of these scenarios would have been for mild events."
The striking differences between the two El Niños became evident after the winds, what scientists call forcing, came into play. Winds drive ocean currents and excite waves. In an El Niño, a breakdown in the easterly (east to west) trade wind system spawns two different kinds of waves: Kelvin waves and Rossby waves. Kelvin waves travel from west to east along the equator. Rossby waves move in the opposite direction from east to west on either side of the equator. These waves create a change in ocean circulation.
When Hackert and his colleagues dissected ocean height satellite data to isolate the individual effects of the Kelvin and Rossby waves, they began to see differences in how the two El Niños developed.
They found that the Kelvin wave component for the two El Niños was similar at first but then strengthened for the 1997 El Niño and weakened for the 2002 El Niño. Even more striking was the influence of the Rossby waves. They had little effect on the development of the 2002 El Niño but made a large contribution to the strength and duration of the 1997 El Niño.
"Rossby waves alone contributed up to half of the sea surface temperature signal in the central Pacific during the key period of the build-up of the 1997 event and served to sustain the warm temperatures during the spring of 1998 in the far eastern Pacific," says Hackert. "One-third of the total sea level signal-the seesaw effect with sea level down in west and up in the east--is accounted for by Rossby waves."
"Our study is the first time that anyone has actually broken the components into Kelvin and Rossby waves and done data assimilation to try to separate the role of the two," says Hackert. "It is an example of how satellite data in combination with numerical ocean models can be used to investigate theories on how an El Niño develops." For their study, Hackert and his colleagues used sea-surface temperature data from National Oceanic and Atmospheric Administration satellites and sea-surface height measurements from the Topex/Poseidon and Jason satellites, joint missions of NASA and the French space agency.
"This was just one step toward improving our understanding of how an El Niño works and what makes the atmosphere and the ocean couple to create a strong event," Hackert says.