"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 and high heating bills. For others, it can bring lashing rains, mudslides 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 higher than normal sea-surface height in the western Pacific near East Asia and lower than normal sea-surface heights along the equator in the eastern Pacific near South America. 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 two graphs show the separate signals for the Kelvin and Rossby components of the two El Ninos. Red and orange represent faster than normal currents. Blues and purples are slower than normal currents.The Rossby patterns for 1997 are much larger than for 2002.