Ocean activity is key controller of summer monsoons

Each summer, a climatic shift brings persistent wind and rain to much of Southeast Asia, in the form of a seasonal monsoon. The general cause of the monsoon is understood to be an increasing temperature difference between the warming land and the comparatively cool ocean. But for the most part, the strength and timing of the monsoon, on which millions of farmers depend each year, is incredibly difficult to predict.

Now 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 South Asian monsoon.

Their results, published today in the Journal of Climate, show that as the summertime sun heats up the Indian subcontinent, it also kicks up strong winds that sweep across the Indian Ocean and up over the South Asian land mass. As these winds drive 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.

They say this ocean heat transporting mechanism may be a new knob in controlling the seasonal South Asian monsoon, as well as other monsoon systems around the world.

"What we find is, the ocean's response plays a huge role in modulating the intensity of the monsoon," says John Marshall, the Cecil and Ida Green Professor of Oceanography at MIT. "Understanding the ocean's response is critical to predicting the monsoon."

Marshall's co-authors on the paper are lead author 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 Univeristy of Washington.

Damps and shifts

Scientists have traditionally focused on the Himalayas as a key influencer of the South Asian monsoon. It's thought that the massive mountain ridge acts as a barrier against cold winds blowing in from the north, insulating the Indian subcontinent in a warm cocoon and enhancing the summer time temperature difference between the land and the ocean.

"Before, people thought the Himalayas were necessary to have a monsoon system," Lutsko says. "When people got rid of them in simulations, there was no monsoon. But these models were run without an ocean."

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 corraling 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."

Seeing past a mountain

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 insolation, or 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 static and unmoving, 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.

Interestingly, their model did not include any sort of Himalayan structure; nevertheless, they were still able to produce a monsoon simply from the effect of the ocean and winds.

"We initially had a picture that we couldn't make a monsoon without the Himalayas, which was the established wisdom," Lutsko says. "But in our model, we had no such barrier, and we were still able to generate a monsoon, and we were excited about that."

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.

"One reason the South Asian monsoon is so strong is there's this big barrier to the north keeping the land warm, and there's an ocean to the south that's cooling, so it's perfectly situated to be really strong," Lutsko says.

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.

"What's interesting to me is, during times when the northern hemisphere was much colder, you see a collapse of the monsoon system," Lutsko says. "People don't know why that happens. But we feel we can explain this, using our minimal model."

The researchers also believe their new, ocean-based explanation for generating monsoons may help climate modelers to predict how, for example, the monsoon cycle may change in response to ocean warming due to climate change.

"We're saying you have to understand how the ocean is responding if you want to predict the monsoon," Lutsko says. "You can't just focus on the land and the atmosphere. The ocean is key."

This research is supported in part by the National Science Foundation and the National Oceanic and Atmospheric Administration.