29 March 2014

Mighty Monsoon Reacts to Deserts' Whims

Science

 

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 to intensify 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.”

Approach

 

PNNL’s Rasch, V. Vinoj of the Indian Institute of Technology Bhubaneswar, India, 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 wanted to see if they could 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.”

Impact

 

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, bands of heat and moisture circling the globe and 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 and 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.”

Summary

 

The Indian summer monsoon results from a complex interplay between radiative heating, dynamics, and cloud-aerosol interactions in addition to various other factors, but despite increased scientific attention, the effect of aerosols on monsoons remains uncertain. Using satellite data and models, a team of U.S. DOE scientists at Pacific Northwest National Laboratory and researchers at the Indian Institute of Technology Bhubaneswar, India showedthat desert dust aerosol levels over the Arabian Sea, West Asia and the Saudi peninsula are positively correlated with the intensity of summer monsoon rainfall over India. They showed that dust and summer monsoon precipitation vary in concert over timescales of about a week. Global climate model simulations using the DOE/NSF-sponsored Community Atmosphere Model (CAM5) support this remote link and indicate that variability in dust aerosol loadings influence radiative heating rates that can induce larger scale atmospheric circulation changes, modulating moisture transport over the Arabian Sea and moisture convergence into India, thereby changing monsoon rainfall on relatively short time scales. These findings highlight the importance of natural aerosols in influencing the strength of the Indian summer monsoon. Such an aerosol induced remote link to monsoon was not known before, and the study clearly shows that aerosols of natural origin can have remote effects on large-scale circulations with important implications.

Contact
V Vinoj
Publications
Vinoj, V, PJ Rasch, H Wang, J Yoon, P Ma, K Landu, and B Singh.  2014.  "Short-term Modulation of Indian Summer Monsoon Rainfall by West Asian Dust."  Nature Geoscience.  https://doi.org/10.1038/ngeo2107.
Acknowledgments

 

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.