John Marshall, Cecil and Ida Green Professor of Oceanography, recently accepted the 2014 Sverdrup Gold Medal of the American Meteorological Society for his “fundamental insights into water mass transformation and deep convection and their implications for global climate and its variability."
Marshall is an oceanographer with broad interests in climate and the general circulation of the atmosphere and oceans, which he studies through mathematical and numerical models of physical and biogeochemical processes. His research has focused on problems of ocean circulation involving interactions between motions on different scales, using theory, laboratory experiments, and observations as well as innovative approaches to global ocean modeling pioneered by his group at MIT.
The Sverdrup Gold Medal recognizes Marshall’s influential ideas about deep convection in the ocean, the process by which, in certain polar regions, cooling water descends, transporting properties such as oxygen, salt, carbon, and heat into the ocean’s deep interior. Marshall, in a 1990s collaboration with his graduate students Sonya Legg and Helen Hill, née Jones, and the late Professor Friedrich Schott of the University of Kiel, in Germany, demonstrated that the convective process in the ocean occurs slowly enough for it to be influenced by Earth’s rotation. This insight overturned the prevailing view that convection in the ocean was an upside-down version of atmospheric convection.
Marshall’s work on rotating convection in water mass transformation triggered a vast amount of research, including the Labrador Sea Deep Convection Experiment, a major field program in 1996 that provided the most comprehensive set of measurements of ocean convection. The dataset collected on this international expedition led to insights into the convective process in the ocean and its representation in models in light of Marshall’s theoretical descriptions. This body of work also motivated the development of the MIT General Circulation Model (MITgcm), which Marshall’s group first used to simulate deep convection fluid dynamics at high resolution. The algorithms used to represent convection drive the modern-day MITgcm, one of the most widely used global ocean models in the world.
To gain a broader understanding of Earth’s fluid dynamical system, Marshall shifted focus to contemporary issues in global ocean circulation. “I’ve always tried to move forward,” Marshall says. “Even though this work on water mass transformation was enjoyable, I stopped it and moved on to study the role of the Southern Ocean in climate.” Marshall has now spent 10 years revising the scientific understanding of the Antarctic Circumpolar Current (ACC). In particular, his updated modeling shows that the ACC brings up deep water and buried carbon to the surface around Antarctica, leading him and colleagues to suggest that the Southern Ocean is the window by which the interior of the ocean connects to the atmosphere, and is thus a powerful mediator of climate.
Professor Marshall received a PhD in atmospheric sciences from Imperial College London in 1980. He joined MIT’s Department of Earth, Atmospheric and Planetary Sciences in 1991 as an associate professor and has been a professor in the department since 1993. He was elected a Fellow of the Royal Society in 2008. He is coordinator of Oceans at MIT, a new umbrella organization dedicated to all things related to the ocean across the Institute, and director of MIT’s Climate Modeling Initiative (CMI).