Half the world's population is affected by Asian monsoons, yet the rainy seasons are notoriously hard to predict. But U.S. researchers have put together a 700-year record of the Asian monsoons they hope will guide forecasters.
Every summer, India, East Asia, eastern Africa, Indonesia and northern Australia are drenched by moist air masses called monsoons, which are pulled in by a high pressure area over the Indian Ocean and a low pressure area to the south.
A monsoon will typically begin with no warning sometime during June, July or August, or instead of non-stop rain there may be drought conditions, which can spell disaster for subsistence farmers.
Edward Cook of the Lamont-Doherty Earth Observatory at Columbia University in New York says the complex, interrelated nature of the climate systems across Asia makes monsoons hard to predict. He says climate records date to 1950, too recent and not detailed enough to be of much use.
So researchers led by Cook spent more than 15 years traveling across Asia locating trees old enough to provide long-term records. They measured the rings inside of the trunks of thousands of ancient trees at more than 300 sites.
The investigators put together a Monsoon Asia Drought Atlas documenting monsoons over a 700-year period, beginning in the 1300s.
Cook says the tree-ring data indicate periods of wet and dry conditions.
"If the monsoon basically fails or is very weak one year, the trees affected by the monsoon at that location might put on a very narrow ring," explains Cook. "But if the monsoon is very strong, the trees affected by that monsoon might put on a very wide ring for that year. So, the wide and narrow ring widths of the tree chronology that we developed in Asia provide us with a measure of monsoon variability."
Armed with such a sweeping set of data, researchers say they now can begin to refine climate computer models to try to predict the behavior of monsoons, according to Cook.
"Because the system is kind of coupled, in other words, there is nothing really truly, completely independent about those areas with respect to monsoon variability, having the information in any given year about the monsoon variability in all those regions at the same time might help us produce a more robust model for explaining how the Asian monsoon system works," he said.
Eugene Wahl is with the U.S. National Oceanic and Atmospheric Administration's paleoclimate branch, which studies weather patterns over the history of the Earth.
Wahl says the Monsoon Asia Drought Atlas is a major accomplishment, filling in a major data gap on monsoons, which affect the lives of one half of the world's population.
"There has been widespread famine and starvation and human dying in the past in large droughts. And on the other hand if the monsoon is particularly heavy it can cause extensive flooding. So, to get a sense of what the regional moisture patterns have been, dryness and wetness over such a long period of time in great detail, I would call it a kind of a victory for paleoclimate science," he said.
An article describing the Monsoon Asia Drought Atlas, and accompanying commentary by Eugene Wahl, is published this week in the journal Science.