Land's greenness affects monsoon rains' strength

Scientists investigating the East Asian Summer Monsoon have found that the abundance of vegetation during winter and spring months is an important indicator of how much summer precipitation will fall.

The East Asian Summer Monsoon influences the lives of a quarter of the world's people and is critical for the cultivation and production of rice, East Asia's staple food. Forecasts of monsoon intensity and duration largely affect yearly agricultural planning.

“By including the land cover in our monsoon forecast models, we were able to explain about 80 percent of the variation in monsoonal rainfall. That's a huge improvement in how well models successfully predict the strength of the monsoon several months out,” says the study's lead author Eungul Lee.

Lee conducted the work as a Ph.D. student at the University of Colorado at Boulder's Cooperative Institute for Research in Environmental Sciences (CIRES). He's now at the University of Wisconsin-Madison.

Lee and his CIRES colleagues published their findings last week in Water Resources Research, a journal of the American Geophysical Union (AGU).

Earlier monsoon models, which base their predictions on ocean factors such as sea surface temperature, typically explain just 25 percent to 40 percent of variations in East Asian summer rainfall. Such low predictive skill makes it impossible to accurately forecast droughts or floods, says Lee.

The CIRES team doubled the predictive capability of these models by including both ocean factors and estimates of the greenness of the landscape. They find that land cover in certain regions of Asia correlates particularly well with the strength of the monsoon.

Specifically, Lee and his colleagues find that strong northern East Asian Summer Monsoons tend to follow verdant springs in southern Asia but are weaker when northern and central Asia are more green. Southern East Asian Summer Monsoons are strongest following springs with abundant vegetative growth in Mongolia and western Asia.

Vegetation may indicate high soil moisture, the researchers say.

“In northern Asia, high soil moisture is probably cooling the land as it evaporates, in the same way that sweating cools the body. This cooling decreases the temperature difference between the land and ocean, inhibiting the formation of a strong monsoon system,” says study coauthor Thomas Chase.

In contrast, higher soil moisture in southern Asia, near the ocean, appears to provide an additional moisture source to fuel monsoonal rains.

Earlier this year, the researchers found similar links between land cover and the Indian Monsoon.

“Findings from both regions enhance our hopes of identifying land- atmosphere relationships that will allow us to better predict the North American Monsoon, which serves as an important source of moisture for the U.S. Southwest,” says Balaji Rajagopalan, who participated in both studies.