ATLANTA--The force of the South Asian Monsoon - a weather pattern that affects the lives of several billion people - is more sensitive to warming in the southern hemisphere than scientists previously thought, according to a new study by an international team of climate researchers.
By analyzing ancient sediment cores and reconstructing one million years of precipitation history of the eastern Indian Ocean, the researchers also found that catalysts for the South Asian Monsoon's wind and precipitation patterns are more complex than assumed in the past. The discoveries are published in the journal Nature Communications.
The international research team includes scientists from Georgia State University, GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany, Woods Hole Oceanographic Institution and Brown University.
The South Asian Monsoon, which brings months of heavy rainfall every year from June to September followed by half a year of drought, has significant economic, social and environmental effects on nations, such as India, Nepal, Bangladesh, Bhutan, Pakistan and Sri Lanka. The monsoon's seasonally changing rainfall and wind directions have a crucial impact on agriculture and the food supply of people living in southern Asia, and the resulting floods and landslides in these densely populated areas can be catastrophic. However, the monsoon remains very difficult to predict.
In its simplest form, the South Asian Monsoon is driven by pressure and temperature differences between the Asian continent and the southern subtropical Indian Ocean. Previously, scientists believed the variability of the monsoon over recent geological time periods was driven by changes in solar insolation in the northern hemisphere caused by the regularly changing inclination of the Earth's axis.
"However, we were only able to associate 30 percent of the variability of monsoon precipitation in the eastern Indian Ocean with fluctuations in the Earth's axis inclination. This means that it only plays a subordinate role in the fluctuations of the monsoon," said Dr. Daniel Gebregiorgis, lead author of the study and a postdoctoral research associate in the Department of Geosciences at Georgia State who previously worked at GEOMAR Helmholtz Centre for Ocean Research Kiel. "Instead, our results pointed to important connections with warming phases in the southern hemisphere and moisture transport across the equator to the north. This process has hardly been considered so far."
For the first time, the research team has evaluated sediment cores from the eastern Indian Ocean that were obtained as part of the International Ocean Discovery Program. The chemical analysis of the shells of tiny plankton that settled and were preserved on the seafloor allows the reconstruction of temperature and the amount of fresh water at the sea surface during the organisms' lifetimes.
"Using this, we have been able to reconstruct precipitation in the eastern Indian Ocean for the past one million years," said Dr. Ed Hathorne, co-author of the study from GEOMAR Helmholtz Centre for Ocean Research Kiel.
Until now, the reconstruction of the monsoon's history was based mainly on two climate archives: sediment cores from the Arabian Sea and stalagmites from caves in China.
"The former, however, only provides information on wind conditions and not precipitation over the Indian subcontinent, while the latter has long been thought to reflect precipitation from the East Asian Monsoon," Hathorne said. "And both respond distinctly differently to changes in northern hemisphere summer insolation on orbital timescale."
Still, the researchers haven't completely solved the mystery of how this important climate system works on orbital time scale (10^4 to 10^5 years).
"The evaluation of the new climate archives shows that we have still not fully understood the monsoon," said Dr. Martin Frank, working group leader from GEOMAR Helmholtz Centre for Ocean Research Kiel. "As long as this is not the case, it is difficult to estimate the reactions of this important climate system to a globally warming atmosphere."
The study was funded by the German Research Foundation.
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