Study indicates Indian Ocean monsoon similar to El Niño phenomenon in Pacific

September 22, 1999

A research campaign in the Indian Ocean indicates there are ocean-atmosphere interactions similar to El Niño events in the Pacific, but are not always tied to El Niño as previously had been believed by climate researchers.

Peter Webster, director of the University of Colorado at Boulder's Program in Atmospheric and Oceanic Sciences, or PAOS, who is directing the project, said the 1997 to 1999 findings were somewhat surprising. "Our research indicates the Indian Ocean has it's own El Niño -like phenomenon characterized by an east to west oscillation of warm waters that affect other parts of the world," said Webster.

During 16 of the years from 1950 to 1998, the equatorial sea-surface temperature gradients reversed -- much like El Niño -- substantially warming the western portion of the Indian Ocean. But only three of the years were active El Niño years, he said.

Although the Indian Ocean's summer monsoon was strongly linked to the El Niño/Southern Oscillation system, or ENSO, from 1960 to 1988, there has been no significant link since that time, said Webster. ENSO triggers movement of warm water from the western Pacific eastward every two to 10 years, wreaking havoc on the environment through the proliferation of droughts and floods worldwide.

A paper on the subject by Webster, Andrew Moore and Johannes Loschnigg from CU-Boulder's PAOS and Robert Leben from CU-Boulder's aerospace engineering department was published in the Sept. 23 issue of Nature.

In 1997 and 1998 the Indian Ocean monsoon triggered the wettest year on record in East Africa, sparking an outbreak of mosquito-borne Rift Valley Fever and associated cholera and malaria in livestock and people. "This event occurred completely independent of El Niño activity," said Webster.

"I think people have become a bit 'El Niñocentric,'" he said. "Certainly El Niño plays a major role in climate variation on Earth, but there also is a lot of independent climate variability. If we can better understand the engine driving the Indian Ocean monsoon, we will be better able to forecast its onset and impacts."

The Indian Ocean study, known as the Joint Air-Sea Monsoon Interaction Experiment, or JASMINE, is a joint venture of CU, the University of Washington, the University of Hawaii, the National Oceanic and Atmospheric Administration, the National Science Foundation, NASA and several Australian agencies. A principa; aim of Jasmine was to study the transitions between the active and break periods of the monsoon.

Last summer, researchers used the NOAA ship Ron Brown to make a number of north-south passes in India's Bay of Bengal totaling about 10,000 miles. The team measured gradient changes in water temperatures and salinity to more than 1,500 feet deep, and used six types of radar, six to eight weather balloons each day, and data from European satellites to measure atmospheric temperatures and wind speeds from the sea surface to about 12 miles high.

The Indian Ocean generally has three "active periods" during the annual monsoon and three relatively calm periods, said Webster. As the Indian Ocean builds toward an active period, waves up to 15 feet high replace calm water and the surface water warms significantly, pumping large amounts of moisture into the atmosphere and creating rainfall over India, East Africa, South Asia and Australia.

"Roughly 65 percent of the world's population lives in monsoon regions," said Webster. "This is the fastest growing region on the planet. By the year 2025, it is anticipated this number will grow to 75 percent."

The Bay of Bengal is one of the "freshest" seas in the world due to large discharges from the Ganges and Bramaputra Rivers and the heavy monsoon rains each year, said Webster. "Because it is fresher, the water is less dense, warms more quickly and stays warm for longer periods."

In one 1998 measurement made 150 feet below the surface, researchers on the NOAA ship found the water temperature cooled by 5 degrees F in just three vertical feet of water.

"Our goal is to understand the basic physics that underpin these variations in the Indian Ocean monsoons and allow us to predict them so people in the region will have warning weeks or months in advance of drought or floods," he said. "It is the variability on these time scales that really impact agriculture and society, and the annual Indian monsoon is a major player in these climate fluctuations."

University of Colorado at Boulder

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