Atmospheric Sciences & Global Change
Indian Monsoon Reacts to Deserts' Whims
Study shows distant desert dust strengthens Asian summer monsoon
The satellite image shows features of the Earth that are playing a role in the remote link between dust and Indian monsoon rainfall. This study shows that atmospheric heating induced by light-absorbing dust can modify winds that transport moisture from the ocean into India, where it falls as monsoon rainfall. The influence on monsoon rainfall over India occurs within a week. The clear sky, true-color globe image of the region is courtesy of NASA's Blue Marble project, with vectors (arrows) indicating summer winds (from the NASA MERRA Reanalysis). A blow-up of the North Arabian Sea region shows an example of dust to the west, and clouds to the east. (July 15, 2006 satellite image by Jesse Allen, NASA Earth Observatory). Enlarge image.
Results: In a new study published in Nature Geoscience, a research team led by Dr. Phil Rasch at Pacific Northwest National Laboratory found that desert dust plays a hand in intensifying the summer Asian monsoon rainfall. The study shows that dust in the air absorbs sunlight west of India, warming the air and strengthening the winds carrying moisture eastward, translating to more monsoon rainfall about a week later in India.
"The strength of monsoons has been declining for the last 50 years," said Rasch, chief scientist for climate at PNNL and corresponding author of the paper. "We found that dust plays a role in strengthening the monsoon, although this natural phenomenon does not overpower other influencing effects such as temperature differences between land and ocean, land use changes, and the local effect of pollution around India."
Why It Matters: India heavily relies on summer monsoon rains for the yearly fresh water supply. Even small changes to the summer monsoon can have a large impact on crop yields, and through extended droughts or floods. Monsoons also influence the global circulation affecting weather and climate in distant regions. This paper shows that natural airborne particles can influence rainfall in unexpected ways, with changes in one location rapidly affecting weather thousands of miles away.
"The difference between a monsoon flood year or a dry year is about 10 percent of the average summer rainfall in central India," said Rasch. "Variations driven by dust may be strong enough to explain some of that year-to-year variation."
Methods: PNNL's Rasch, V. Vinoj of the Indian Institute of Technology Bhubaneswar, India who worked with Rasch as a postdoctoral fellow, and their coauthors wanted to explore a correlation that appeared in satellite records: higher amounts of small particles called aerosols over North Africa, West Asia, and the Arabian Sea seemed to be connected to stronger rainfall over India around the same time. The team tried to verify this and determine how those particles might affect rainfall.
With the Community Atmosphere Model (CAM5), they explored this connection by including manmade aerosols from pollution, and natural sea salt and dust aerosol particles. The simulations showed a similar connection-more aerosols in the west translated to more rainfall in the east. When they systematically turned off the contribution of each aerosol type, they found that dust was the condition that showed a stronger rainfall connection in India as a result of increased dust from North Africa and the Arabian Peninsula.
The team then ran shorter simulations to determine how quickly dust worked. They found that after turning off dust in the model, dust dissipated within a week compared to the simulation with dust, and rainfall declined in central India as well, indicating that the dust effect happens over a short period of time.
To learn more, see PNNL news release "The rush to rain."
Sponsors: The research was supported by the PNNL Laboratory Directed Research and Development program and the U.S. Department of Energy's Office of Biological and Environmental Research Earth System Modeling program.
Research Area: Climate & Earth Systems Science
Reference: Vinoj V, PJ Rasch, H Wang, J-H Yoon, P-L Ma, K Landu, and B Singh. 2014. "Short-term Modulation of Indian Summer Monsoon Rainfall by West Asian Dust." Nature Geoscience, advance online. DOI:10.1038/NGEO2107.