Climate change researchers at the Pacific Northwest National Laboratory and the Indian Institute of Technology in the easterly city of Bhubaneswar in India have discovered an important climate link that is responsible for the success of India’s monsoon season—dust storms in Northwest Africa. The monsoon is India’s wettest season and is the lifeblood of a large agrarian economy that supplies the country’s full requirement of grain.
The research team is led by Dr. Phil Rasch, Chief Climate Scientist at the research laboratory and funded by the Department of Energy Office of Science. The team identified the correlation between the dust storms prevalent over large parts of North Africa and West Asia and the seasonal monsoon that sweeps over India, bringing much-needed rain to the subcontinent. The greater monsoon season is categorized as the south-west monsoon and the north-east monsoon and it extends from late May to November, bringing heavy precipitation to over 85 percent of the country with over 55 percent of India’s farmland relying on the timely arrival of the rain it brings. This season has been identified as very susceptible to climate change.
The team’s analysis of satellite data and photographs reveals that the natural dust aerosols generated over the large desert areas over Africa and the Arabian Peninsula absorb solar energy resulting in increased air temperatures. Increased temperatures subsequently drive moisture-laden winds towards the subcontinent, improving rainfall and the effectiveness of the monsoons. The researchers identified a climate change link that reveals that the Indian monsoon is strengthened by African dust storms after running a series of computer simulations that accounted for the aerosols produced by the desert sand, industrial pollution and natural salts from the ocean.
The simulations were performed with the Community Atmosphere Model (CAM5) and explored the effect on monsoon winds with and without the presence of man-made and natural aerosols. The studies confirmed that the increase in the density of these aerosols had a significant and positive impact on the monsoon in the east. The team also tested the effect of each type of aerosol to study its influence on the monsoon and concluded that dust particles were the key ingredient in improving rainfall. Dr. Rasch stated that, “The difference between a monsoon flood year or a dry year is about 10 percent of the average summer rainfall in central India and variations driven by dust may be strong enough to explain some of that year-to-year variation.” He went on to caveat the research findings by stating that the dust was just one of the climate change factors that affect the monsoon rainfall, although the decline of the monsoon rainfall across the last 50 years may be, in part, on account of declining natural dust aerosols in the atmosphere.
Further studies explored the time it took for the presence of dust in the atmosphere over parts of Saharan Africa and the Arabian Peninsula to influence monsoon rainfall. The researchers simulated the dissipation of atmospheric dust in a week and saw a considerable decline in rain-bearing winds over India. This finding is significant to climate change researchers because it reveals the far-reaching short-term effects of African dust storms on the rainfall during the regular monsoon season in India and the link between the two systems. The research program continues to be supported by the Office of Biological and Environmental Research Earth System Modeling from the U.S. Department of Energy and is expected to continue to investigate the short-term and long-term effect of natural and atmospheric phenomena that cause climate change.
By Grace Stephen