A double whammy of weird ocean behavior washed over the world in 1997. The Pacific Ocean had already succumbed to an exceptionally strong El Niño, and then the Indian Ocean was hit fiercely by El Niño’s close cousin: the so-called Indian Ocean Dipole. Surface waters off the coast of Indonesia cooled and the ocean’s predominant westerly winds reversed, leading to catastrophic weather. Fires raged across a drought-stricken Indonesia, and floods across east African nations killed thousands.
Climate change could make years like 1997 come more often, according to a new study of the Indian Ocean Dipole cycle, which alternates between two opposite extremes, positive and negative, just as El Niño does with La Niña. The study suggests that rising greenhouse gases will cause extreme positive dipole events—like the one that struck the Indian Ocean in 1997—to occur three times as often this century as they did in the 20th century, or about once every 6 years, as opposed to once every 17 years.
“The Indian Ocean Dipole affects a lot of poor countries,” says lead author Wenju Cai, a climate modeler at the Commonwealth Scientific and Industrial Research Organisation in Aspendale, Australia, who published the study with his colleagues online today in Nature. “We really need to build our capacity to deal with these kinds of events.” In January, Cai led a study that found that extreme El Niño events—a warming of tropical waters off the coast of Peru—were likely to double in frequency this century.
Shang-Ping Xie, a climate modeler at the Scripps Institution of Oceanography in San Diego, California, says he likes that the study team focused on providing information on a more local rather than global scale. “They took an important step in the direction of understanding regional climate extremes in a better way.”
Cai and his colleagues examined 31 global climate models and found that 23 were able to model the rainfall conditions in the Indian Ocean that they used to define an extreme positive dipole event. As a control, they ran the models from 1900 to 1999 to see how well they reproduced extreme events in 1961, 1994, and 1997. Then they ran the models forward from 2000 to 2099 under the “business-as-usual” projections for rising greenhouse gases. Out of the 23 cases, only two did not show a rise in extreme dipole events. “We have a very strong intermodel agreement,” Cai says. Climate change, he says, causes the waters of the western Indian Ocean to warm more than other parts of the ocean, and this preconditions the area to more extreme dipole events.
Lisa Goddard, director of the International Research Institute for Climate and Society at Columbia University in Palisades, New York, does not necessarily disagree with the researchers’ conclusions, but she is concerned that they model the Indian Ocean Dipole as being completely independent of the El Niño–Southern Oscillation (ENSO). Studies have shown how an El Niño event can trigger a dipole event, and Goddard notes that two out of the three extreme dipole events, in 1994 and 1997, were preceded by El Niño. “They’re downplaying the role of ENSO,” she says.
Cai insists that the dipole can arise independently of El Niño. He points out that in 2007 and 2008 there was a moderate dipole event even though La Niña was occurring, which ought to squelch the dipole event if it were so dependent on ENSO.
Regardless, the dipole has the attention of people in Australia, where it has been linked to major bushfires in 1982 to 1983 (Ash Wednesday) and 2009 (Black Saturday) that killed hundreds. “The Indian Ocean Dipole is arguably more important to us,” he says. “It can cause big damages to these economies.”