A recent study found that ocean activity, not the Himalayas, is responsible for the intense seasonal monsoon in South Asia.
Researchers from MIT revealed that the same summer heat that warms the Indian subcontinent also sends strong winds that sweep the Indian Ocean and South Asia. As the winds head northward, they push the ocean waters southward.
The south-bound water transport heat, therefore cooling the ocean. The process contributes to the difference between the temperature of land and ocean.
The study published in the Journal of Climate revealed that the ocean's response dictates the intensity of the South Asian monsoon and might be the key to predicting the occurrences of monsoon around the world.
The Ocean, Not The Himalayas Control Monsoon
Previous studies have focused on the Himalayas, the great mountain system in Asia.
"Before, people thought the Himalayas were necessary to have a monsoon system," shared Nicholas Lutsko, a postdoc at MIT and the lead author of the study. "When people got rid of them in simulations, there was no monsoon. But these models were run without an ocean."
Lutsko and co-author John Marshall, a professor of oceanography at MIT, had the idea to test if the ocean's dynamic would affect the monsoon based on a previous experiment they conducted on Inter Tropical Convergence Zone or ITCZ. Wind-driven ocean circulation minimized the shifts in the ITCZ, so they predicted that the ocean's dynamics would lessen the intensity of a monsoon. They found that the opposite is true.
For the study, they created a simulation of a planet covered entirely by an ocean with a rectangular mass to represent land. They also controlled the amount of sunlight to simulate how the winds and rains would respond to the changes in temperature.
The simulation was run under different scenarios, including one in which the ocean was allowed to circulate and respond to atmospheric winds. The interaction between the ocean and the winds had a significant effect on the monsoon that formed over the land. The stronger the interplay between the ocean and the winds, the greater the difference in land and ocean temperature, and the stronger the intensity of the monsoon.
The findings might explain why the South Asian monsoon is the strongest in the world.
"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," said Lutsko, "so it's perfectly situated to be really strong."
Monsoon Cycles Around The World
The researchers believe that their study could be used to predict how the monsoon cycles around the world would be affected by ocean warming due to climate change.
They also plan to use their observations to go back in time and analyze variations of the monsoon in the past.