Understanding Monsoons

Studies of the global environment are complex, involving interactions between oceans, solid earth, biological systems, and the atmosphere, over time scales ranging from nanoseconds to millions of years. Investigating and understanding these complicated and interconnected systems is the goal of Caltech's Ronald and Maxine Linde Center for Global Environmental Science. To that end, the center hosts workshops that bring together scientists from a range of disciplines to discuss current research and collaborate on solutions to pressing issues facing the global environment.

"The Linde Center workshops aim to provide a venue for a small group of scientists and engineers to discuss and put forward cutting edge, 'future-looking' plans for global environmental science," says Paul Wennberg, the R. Stanton Avery Professor of Atmospheric Chemistry and Environmental Science and Engineering and director of the Linde Center.

The topic of the center's latest workshop, held May 18–22, was monsoons, circulation patterns that develop over subtropical continents (up to around latitude 30 degree north and south of the equator) in response to seasonal variations in the amount of solar radiation received in these regions. Monsoons are characterized by seasonally reversing winds and summertime rainfall. Although monsoons occur across the globe, they are more often studied in Southeast Asian countries—where warm, moist air from the Indian Ocean brings humidity and rainfall during the summer, while winds from the northeast produce dry winters—and in West Africa. Because of their effects on the water supply, monsoons have a large impact on society, especially in densely populated countries and rapidly growing economies. And, as noted by the workshop organizers, "with projected increases in population and pressure for food and water security, understanding how anthropogenic climate change will affect monsoons is both a priority and a major challenge in climate science."

Indeed, the workshop—entitled "Monsoons: Past, Present, and Future" and co-led by monsoon researcher Simona Bordoni, assistant professor of environmental science and engineering at Caltech—was focused on understanding how monsoons have changed and how they will change in the future, across a variety of time scales, in response to different forcing agents—perturbations of Earth's energy balance caused by changing environmental parameters such as solar variability or human-induced greenhouse gas emissions.

"One of the central themes of the discussion," Bordoni says, "was how modern theories of the fundamental dynamics of monsoons can be used to better constrain future monsoon projections and past monsoon changes and shifts recorded by paleo-proxies"—media such as tree rings and ice cores that preserve information about past climates—"and how these paleo-reconstructions can provide support to emerging hypotheses and guide modeling studies. The implications of these modern theories are only now beginning to be explored."

Each section of the invitation-only workshop covered a particular subject area within monsoon research, including paleoclimate, aerosols, the intertropical convergence zone (the band of clouds encircling the equator), and thermal contrasts between land and sea. Speakers from institutions around the country gave talks on past and potential future changes in the monsoon cycle, the role of aerosols on monsoon circulation, and monsoon modeling, among other topics.

In a talk entitled "Monsoons on Idealized Continents," for example, Bordoni discussed how she uses models of "idealized" continental geometry to study how monsoons would develop on hypothetical planets—for example, a planet with land everywhere above 10 degrees north of the equator, and ocean everywhere south of that. Recently, Bordoni and her group also created simulations of an "aquaplanet"—a planet entirely covered with ocean. With the aquaplanet simulations, the team demonstrated that the rapid onset of large-scale monsoons, such as the Asian monsoon, results not from temperature differences between oceans and land, as previously believed. Instead, they found, the rapid appearance of this monsoon is controlled by the interaction between large swirling regions of turbulent air called eddies and the tropical circulation. These eddies, which are generated in mid-latitudes, propagate to lower latitudes towards the subtropics and interact with the tropical circulation, causing it to reverse rapidly, initiating the onset of the monsoon. Bordoni's group also studies the North American monsoon, which usually occurs during the summer over southwestern North America, when warm and moist air moving northwest from the Gulf of California meets similar air moving northwest from the Gulf of Mexico; the dynamics of the East Asian monsoon and its response to climate changes; the year-to-year variability of the Indian monsoon; and how mountain ranges such as those in Africa and Asia influence the larger-scale circulation of this monsoon.

The workshop was co-led by Timothy Merlis (Ph.D. '11), an assistant professor in atmospheric and oceanic sciences at McGill University. He gave a talk on tropical circulation changes influenced by various forcing agents. Other speakers from Caltech included Jess Adkins, professor of geochemistry and global environmental science, who gave a talk on historical precipitation variability over Borneo as measured in stalagmites; Salvatore Pascale, a NOAA Climate and Global Change postdoctoral scholar in environmental science and engineering; and Ho Hsuan Wei, a graduate student in environmental science and engineering. Hui Su, a JPL atmospheric scientist, gave a talk on the tropical Hadley cell (a pattern of atmospheric circulation in which warm air rises near the equator, cools as it travels at high altitude toward the poles, then sinks as cold air and warms as it travels toward the equator) and feedback from clouds. In addition, JPL scientist Christian Frankenberg—who will join the Caltech faculty in September as an associate professor of environmental science and engineering—discussed remote sensing of water isotopes.

The previous Linde Center workshop was held February 2–5 and focused on physical, chemical, and biological processes crucial to the circulation and ecosystems of the Southern Ocean around Antarctica.