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Research Highlights

December 2012

Slow Trumps Fast in Changing the Summer Monsoon

Pollution's gradual impact on sea surface temperature more important than quicker impact on clouds, air and land

Slow Trumps Fast in Changing the Summer Monsoon
In South Asia, summer monsoons deliver about three quarters of the annual rainfall influencing the fresh water supply, agriculture, and energy production. Small changes in monsoons can have a very large impact on local living conditions, affecting crop yields, prolonging droughts, or fostering floods. Monsoons also affect the global circulation, producing world-wide impacts. Enlarge Image

Results: Researchers at Pacific Northwest National Laboratory traced the different ways pollution particles change summer monsoon rainfall in South Asia. They found that pollution's effect through "slow" processes, affecting the region over weeks to months, has a more extensive impact on the monsoon than the "fast" processes occurring in a matter of days. Monsoons are an important climatic feature of our planet, and understanding the factors that influence monsoon behavior is a fundamental challenge for climate science.  Their work was published in the Geophysical Research Letters.

"Our results show that the slower impact of aerosols-cooling down sea surface temperatures-is more profound than their faster direct impacts through atmospheric heating, changes in clouds, and cooling land surface when it comes to shaping the behavior of the monsoon system," said Dr. Dilip Ganguly, atmospheric scientist at PNNL and lead author of the study. "But both slow and fast effects are important."

Why It Matters: Monsoons affect about half the population of the Earth. Summer monsoons deliver about three quarters of the annual rainfall to South Asia, influencing the fresh water supply, agriculture, and energy production. Small changes in monsoons can have a large impact on local living conditions, affecting crop yields, prolonging droughts, or fostering floods. Monsoons also affect the global circulation, producing world-wide impacts. That's why it's important to understand factors that may change monsoons, including emissions of pollution particles (a.k.a. aerosols). In this study, PNNL researchers delved into the different ways that pollution particles influence the South Asian summer monsoon rainfall.  

Methods: PNNL researchers used the Community Earth System Model (CESM) to study the effect of human-caused pollution particles on the South Asian summer monsoon. The CESM is a three-dimensional global climate model that predicts changes in features of the atmosphere, ocean, land and ice from different climate forcing agents like greenhouse gases and pollution.

The researchers explored the climate response to aerosol particle emissions by varying the emissions between pre-industrial and present-day values and monitoring the model response to those changes.

While both fast and slow climate responses to pollution particles are important, they found the slow and fast changes influence precipitation patterns differently over South Asia.

"The processes changing sea surface temperatures influence the whole of South Asia, while the aerosol processes that affect land and atmosphere temperatures have a somewhat weaker impact and shift the precipitation to the west a bit," said Ganguly.

What's Next? Aerosols emissions from Asia have been increasing rapidly with the enormous regional changes in society and industry. Emissions are likely to change again from the many new technological advances and increasing local concerns about emissions' impact on health and ecosystems. Researchers are interested in comparing and assessing the impacts of emissions from Asia with those from other parts of the planet.

Acknowledgments: 

Sponsors: This research was supported by the U.S. Department of Energy's (DOE's) Office of Science Biological and Environmental Research Earth System Modeling Program.

Research Team: Dilip Ganguly, Philip J. RaschHailong Wang, and Jin-Ho Yoon of PNNL. They used the computing resources provided by the DOE's National Energy Research Scientific Computing Center (NERSC).

Research Area: Climate & Earth Systems Science

Reference: Ganguly D, PJ Rasch, H Wang, and J-H Yoon. "Fast and Slow Responses of the South Asian Monsoon System to Anthropogenic Aerosols." Geophysical Research Letters 39:L18804. DOI:10.1029/2012GL053043.


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Pollution Particles and Monsoons

Pollution particles can absorb sunlight, or scatter it back to space. Particles that absorb sunlight heat the air, and scattering and absorbing particles cool the Earth by reducing the amount of sunlight reaching the surface. Pollution can also change clouds because cloud droplets form on both natural and human-made particles, changing  the patterns of heating and cooling in the atmosphere and at the surface-over both land and ocean. These heating and cooling changes influence weather and climate, particularly precipitation patterns, including the onset and strength of monsoons.

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