Dust generated in North and West Africa can have a big impact on monsoons in India, according to new research published in Nature Geoscience.
Phil Rasch of the US Department of Energy's Pacific Northwest National Laboratory carried out the research with his colleagues, finding that as dust in the air absorbs sunlight west of India, it warms the air and strengthens the winds that carry moisture eastward. This results in more intense monsoon rainfall in India about a week later, according to the researchers, who suggest that the role of dust in the atmosphere could be used to explain changes in climate.
Summer monsoons are critical for life in India, and the new study highlights ways in which the monsoon season can be affected by what's going on in other parts of the world.
"The difference between a monsoon flood year or a dry year is about 10 percent of the average summer rainfall in central India. Variations driven by dust may be strong enough to explain some of that year-to-year variation," Rasch said.
Using satellite data, Rasch and his collaborators, including V. Vinoj of the Indian Institute of Technology in Bhubaneswar, India, established that a higher concentration of dust particles in the air above North Africa, West Asia, and the Arabian Sea seemed to be connected to stronger rainfall over India around the same time.
The went on to run experiments to test their theory, finding that dust was indeed a key ingredient that causes changes in Indian monsoons.
Using computer models, the researchers established that the atmospheric dust to the east led to relatively quick changes to monsoon intensity in India.
The researchers contend that this happens because dust can absorb sunlight that would normally reach the surface, warming the air instead. This warmer, dust-laden air pulls in moisture from the northern tropics, creating stronger winds that move moisture from the Arabian Sea to the India, where it lands as rainfall.
Rasch said that while African dust does influence monsoons, it does not overpower the other processes that affect the Indian monsoons, including temperature differences between land and ocean, pollution in India, global warming, and others.
"The strength of monsoons have been declining for the last 50 years," he said. "The dust effect is unlikely to explain the systematic decline, but it may contribute."
Climate Change Is Altering Rainfall Patterns Worldwide
Global precipitation patterns are being moved in new directions by climate change, a new study has found.
The research, published yesterday in the journal Proceedings of the National Academy of Sciences, is the first study to find the signal of climate change in global precipitation shifts across land and ocean.
"It's worth saying that this is another grain of sand on that vast pile of evidence that climate change is real and is occurring," said study co-author Kate Marvel, a climate scientist at Lawrence Livermore National Laboratory.
Climate models predict that the addition of heat-trapping gases in the atmosphere will shift precipitation in two main ways. The first shift is in a strengthening of existing precipitation patterns. This is commonly called "wet get wetter, dry get drier."
Warmer air traps more water vapor, and scientists expect that additional water to fall in already wet parts of the Earth.
"But because precipitation has to be balanced by evaporation, we expect a [corresponding] increase in dry regions," Marvel said.
The second shift is a change in storm tracks, which should move away from the equator and toward the poles as atmospheric circulation changes.
Wrestling with natural variability
Scientists have loOKed for these changes in rainfall patterns, but they are often difficult to distinguish because there is so much natural variability in precipitation.
For example, El Niño generally makes wet regions wetter and dry regions drier, so when scientists see that happening, it has been difficult to say whether it is climate change or simply El Niño.
"It's really hard to tease out that signal," Marvel said.
What Marvel and her co-author, Celine Bonfils, were able to do in this new study was loOK at both the expected shifts together.
"What we did in the study is say, OK, people have loOKed for one or the other in the observations, to see if it's happening. What if we loOK for both happening at the same time?" Marvel said.
Doing this was useful for the research because, while El Niño makes wet areas wetter, it also contracts storm tracks toward the equator. That's the opposite of what would happen due to climate change.
So loOKing for both increased rainfall in wet areas and a shift in storm tracks away from the equator helped the researchers separate the signal of climate change from the noise of natural variability.
"If you find wet get wetter, dry get drier, occurring increasingly in tandem with poleward expansion, there's just almost no way that can happen naturally," Marvel said.
Finding a signal amid the 'noise'
Similar to many studies that try to tease out climate change's signal, the researchers compared observed data with climate models.
The models were run without the influence of greenhouse gases, so that the scientists could compare those results with the observations from 33 years of satellite precipitation data.
The models tell the researchers whether it is at all likely that natural variations, without the influence of greenhouse gases, could have produced the rainfall patterns and shifts in storm tracks the satellites have observed.
When the answer to that question is no, then the greenhouse gases are implicated as the culprit in changing how precipitation is falling worldwide.
Gabi Hegerl, a climate researcher at the University of Edinburgh who was familiar with the work, called it "very innovative."
"As precipitation is so important, this is a very useful finding and an exciting method," she said.
Francis Zwiers, a climate scientist and director of the Pacific Climate Impacts Consortium who had seen the research presented, agreed.
"I think this paper strengthens, substantially, the available body of literature that we have concerning the question of whether humans are having an effect on precipitation at the global scale," Zwiers wrote in an email.
Marvel noted the findings were not a surprise, but rather a confirmation of what scientists have long said would be the effects of climate change. "It would have been a much more surprising and exciting result if we had found that this wasn't happening," she said.
sources: [http://en.twwtn.com/Bignews/62581.html]
Phil Rasch of the US Department of Energy's Pacific Northwest National Laboratory carried out the research with his colleagues, finding that as dust in the air absorbs sunlight west of India, it warms the air and strengthens the winds that carry moisture eastward. This results in more intense monsoon rainfall in India about a week later, according to the researchers, who suggest that the role of dust in the atmosphere could be used to explain changes in climate.
Summer monsoons are critical for life in India, and the new study highlights ways in which the monsoon season can be affected by what's going on in other parts of the world.
"The difference between a monsoon flood year or a dry year is about 10 percent of the average summer rainfall in central India. Variations driven by dust may be strong enough to explain some of that year-to-year variation," Rasch said.
Using satellite data, Rasch and his collaborators, including V. Vinoj of the Indian Institute of Technology in Bhubaneswar, India, established that a higher concentration of dust particles in the air above North Africa, West Asia, and the Arabian Sea seemed to be connected to stronger rainfall over India around the same time.
The went on to run experiments to test their theory, finding that dust was indeed a key ingredient that causes changes in Indian monsoons.
Using computer models, the researchers established that the atmospheric dust to the east led to relatively quick changes to monsoon intensity in India.
The researchers contend that this happens because dust can absorb sunlight that would normally reach the surface, warming the air instead. This warmer, dust-laden air pulls in moisture from the northern tropics, creating stronger winds that move moisture from the Arabian Sea to the India, where it lands as rainfall.
Rasch said that while African dust does influence monsoons, it does not overpower the other processes that affect the Indian monsoons, including temperature differences between land and ocean, pollution in India, global warming, and others.
"The strength of monsoons have been declining for the last 50 years," he said. "The dust effect is unlikely to explain the systematic decline, but it may contribute."
Climate Change Is Altering Rainfall Patterns Worldwide
Global precipitation patterns are being moved in new directions by climate change, a new study has found.
The research, published yesterday in the journal Proceedings of the National Academy of Sciences, is the first study to find the signal of climate change in global precipitation shifts across land and ocean.
"It's worth saying that this is another grain of sand on that vast pile of evidence that climate change is real and is occurring," said study co-author Kate Marvel, a climate scientist at Lawrence Livermore National Laboratory.
Climate models predict that the addition of heat-trapping gases in the atmosphere will shift precipitation in two main ways. The first shift is in a strengthening of existing precipitation patterns. This is commonly called "wet get wetter, dry get drier."
Warmer air traps more water vapor, and scientists expect that additional water to fall in already wet parts of the Earth.
"But because precipitation has to be balanced by evaporation, we expect a [corresponding] increase in dry regions," Marvel said.
The second shift is a change in storm tracks, which should move away from the equator and toward the poles as atmospheric circulation changes.
Wrestling with natural variability
Scientists have loOKed for these changes in rainfall patterns, but they are often difficult to distinguish because there is so much natural variability in precipitation.
For example, El Niño generally makes wet regions wetter and dry regions drier, so when scientists see that happening, it has been difficult to say whether it is climate change or simply El Niño.
"It's really hard to tease out that signal," Marvel said.
What Marvel and her co-author, Celine Bonfils, were able to do in this new study was loOK at both the expected shifts together.
"What we did in the study is say, OK, people have loOKed for one or the other in the observations, to see if it's happening. What if we loOK for both happening at the same time?" Marvel said.
Doing this was useful for the research because, while El Niño makes wet areas wetter, it also contracts storm tracks toward the equator. That's the opposite of what would happen due to climate change.
So loOKing for both increased rainfall in wet areas and a shift in storm tracks away from the equator helped the researchers separate the signal of climate change from the noise of natural variability.
"If you find wet get wetter, dry get drier, occurring increasingly in tandem with poleward expansion, there's just almost no way that can happen naturally," Marvel said.
Finding a signal amid the 'noise'
Similar to many studies that try to tease out climate change's signal, the researchers compared observed data with climate models.
The models were run without the influence of greenhouse gases, so that the scientists could compare those results with the observations from 33 years of satellite precipitation data.
The models tell the researchers whether it is at all likely that natural variations, without the influence of greenhouse gases, could have produced the rainfall patterns and shifts in storm tracks the satellites have observed.
When the answer to that question is no, then the greenhouse gases are implicated as the culprit in changing how precipitation is falling worldwide.
Gabi Hegerl, a climate researcher at the University of Edinburgh who was familiar with the work, called it "very innovative."
"As precipitation is so important, this is a very useful finding and an exciting method," she said.
Francis Zwiers, a climate scientist and director of the Pacific Climate Impacts Consortium who had seen the research presented, agreed.
"I think this paper strengthens, substantially, the available body of literature that we have concerning the question of whether humans are having an effect on precipitation at the global scale," Zwiers wrote in an email.
Marvel noted the findings were not a surprise, but rather a confirmation of what scientists have long said would be the effects of climate change. "It would have been a much more surprising and exciting result if we had found that this wasn't happening," she said.
sources: [http://en.twwtn.com/Bignews/62581.html]
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