27 December 2012

Fast and Slow Responses of the South Asian Monsoon System to Anthropogenic Aerosols

Science

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.”

Approach

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.

Impact

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.

Summary

Using a global climate model with a fully predictive aerosol life cycle, DOE researchers from Pacific Northwest National Laboratory investigated the fast and slow responses of the South Asian monsoon system to anthropogenic aerosol forcing. Their results show that the feedbacks associated with sea surface temperature (SST) change caused by aerosols play a more important role than the aerosol’s direct impact on radiation, clouds, and land surface (rapid adjustments) in shaping the total equilibrium climate response of the monsoon system to aerosol forcing. Inhomogeneous SST cooling caused by anthropogenic aerosols eventually reduces the meridional tropospheric temperature gradient and the easterly shear of zonal winds over the region, slowing down the local Hadley cell circulation, decreasing the northward moisture transport, and causing a reduction in precipitation over South Asia. Although total responses in precipitation are closer to the slow responses in general, the fast component dominates over land areas north of 25⁰N. The results also show an east-west asymmetry in the fast responses to anthropogenic aerosols causing increases in precipitation west of 80⁰E but decreases east of it. 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.

Contact
Peter M Caldwell
Funding
Acknowledgments

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.