In a recent study, MIT scientists have found that an interplay between atmospheric winds and the ocean waters south of India has a major influence over the strength and timing of the seasonal South Asian monsoon. Their results show that as the summertime sun heats up the Indian subcontinent, it also creates strong winds that move across the Indian Ocean and up over the South Asian land mass. As these winds sweep northward, they also push ocean waters southward, much like a runner pushing against a treadmill's belt. The researchers found these south-flowing waters act to transport heat along with them, cooling the ocean and in effect increasing the temperature gradient between the land and sea.
John Marshall, the Cecil and Ida Green Professor of Oceanography at MIT, Nicholas Lutsko, a postdoc in MIT's Department of Earth, Atmospheric, and Planetary Sciences, and Brian Green, a former graduate student in Marshall's group who is now at the University of Washington, co-authored the paper. Lutsko and Marshall suspected that if they were to develop a model of the monsoon that included the ocean's dynamics, these effects would lessen the monsoon's intensity. Their hunch was based on previous work in which Marshall and his colleagues found that wind-driven ocean circulation minimized shifts in the Inter Tropical Convergence Zone, or ITCZ, an atmospheric belt near the equator that typically produces dramatic thunderstorms over large areas. This wide zone of atmospheric turbulence is known to shift seasonally between the northern and southern hemispheres, and Marshall found the ocean plays a role in caging these shifts.
"Based on the idea of the ocean damping the ITCZ shifts, we thought that the ocean would also damp the monsoon," Marshall says. "But it turns out it actually strengthens the monsoon."
The researchers came to this unexpected conclusion after drawing up a simple simulation of a monsoon system, starting with a numerical model that simulates the basic physics of the atmosphere over an "aqua planet" -a world covered entirely in an ocean. The team added a solid, rectangular mass to the ocean to represent a simple land mass. They then varied the amount of sunlight across the simulated planet, to mimic the seasonal cycles of sunlight and also simulated the winds and rains that result from these seasonal shifts in temperature.
They carried out these simulations under different scenarios, including one in which the ocean was inactive and another in which the ocean was allowed to circulate and respond to atmospheric winds. They observed that winds blowing toward the land prompted ocean waters to flow in the opposite direction, carrying heat away from waters closest to the land. This wind/ocean interaction had a significant effect on any monsoon that formed over the land: the stronger this interplay, or coupling between winds and ocean, the wider the difference in land and sea temperature, and the stronger the intensity of the ensuing monsoon.
Ultimately, their work may help to explain why the South Asian monsoon is one of the strongest monsoon systems in the world. The combination of the Himalayas to the north, which act to warm up the land, and the ocean to the south, which takes heat away from nearby waters, sets up an extreme temperature gradient for one of the most intense, persistent monsoons on the planet.
In future work, the researchers plan to apply their newfound observations of the ocean's role to help interpret variations in monsoons much farther back in time.
