Warm ocean water to affect monsoons

Washington: New research finds ocean activity may control the strength and intensity of summer monsoons. The results could help researchers predict how monsoons will change with warming oceans.

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.

According to the study published in the Journal of Climate, the 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 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 also 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. Understanding the ocean's response is critical to predicting the monsoon," said one of the researchers, John Marshall.

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 summertime temperature difference between the land and the ocean.

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

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

Ultimately, researchers 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 acts 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 added.

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. People don't know why that happens. But we feel we can explain this, using our minimal model," Lutsko concluded.