Climate Connections

A UMD researcher may have answered a 134-year-old climate riddle, providing a plausible explanation for why less snow in one part of the world could potentially lead to stronger rains months later and hundreds of miles away.

In 1884, British meteorologist Henry Blanford noted a connection between springtime snow cover on the Tibetan Plateau and Himalaya mountain range and the intensity of the summer monsoon season in India, suggesting that less snow meant a wetter monsoon season. Hundreds of studies since then have supported this relationship—the India Meteorological Department even uses it to develop its summer monsoon forecast—but scientists struggled to explain the connection, until now.

New research led by William Lau, a research scientist at the University of Maryland’s Earth System Science Interdisciplinary Center (ESSIC), suggests that the cause is dust blown from the deserts of the Middle East, more than a thousand miles away.

Using a powerful NASA-developed atmospheric model, Lau found that large quantities of dark aerosols (airborne particles like dust and soot that absorb sunlight) settle on the Tibetan Plateau’s snowpack in spring before the monsoons begin. As a result, the snow absorbs more sunlight and melts faster. The model findings suggest that windblown dust from the Middle East has the most powerful effect of making snow darker and more able to absorb heat.

In years with heavy springtime dust deposition, the end result is reduced snow cover across the Tibetan Plateau, which leads to warmer temperatures on the ground and in the air above. This sets off a series of interconnected feedback loops that intensify India’s summer monsoon. A paper describing the research, co-authored by Kyu-Myung Kim of NASA’s Goddard Space Flight Center, was published online this week in the journal Atmosphere.

“Blanford knew that snow cover on the Tibetan Plateau wasn’t the only phenomenon that influenced the monsoon, but he knew it was important,” Lau said. “By adding knowledge of the physical mechanism responsible for this relationship, our study may help to develop more accurate monsoon forecasts.”

Lau and Kim acknowledge the need to move beyond modeling and investigate the connections between dark aerosols, heating and the monsoon cycle using other methods and new observations. But they are confident that their results—which used real-world data to seed the GEOS-5 model—could help inform monsoon prediction efforts now.

“This could be extremely important for agriculture. Farmers have to plan around the monsoon season to decide when to plant and when to harvest,” Lau said. “In order to understand how human influences like climate change and land use affect the monsoon, we have to understand the basics.”