Tuesday, November 29, 2022

Ocean activity is key controller of summer monsoons

Results may help researchers interpret ancient monsoon variations, predict future activity in the face of climate change.

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Motivated by the observations of southward ocean heat transport (OHT) in the Northern Indian Ocean during summer, scientists observed the role of ocean in modulating monsoon circulations. They found that 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.

During summertime, sun heats up the Indian subcontinent and kicks up strong winds that sweep across the Indian Ocean and up over the South Asian land mass. As the wind flows to the northward, it pushes water to flow in the opposite direction, means southward. Such south-flowing water transfers heat and cooling the ocean and in effect increasing the temperature gradient between the land and sea.

This heat transferring mechanism may be a knob in controlling the seasonal South Asian monsoon, as well as other monsoon systems around the world.

John Marshall, the Cecil and Ida Green Professor of Oceanography at MIT said, “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.”

Earlier, scientists used to observe 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.

In this study, scientists used an atmospheric model to a slab ocean with an interactive representation of OHT and an idealized subtropical continent. 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.

Scientists concluded, 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.

During another experiment, scientists 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.

Moreover, scientists tested their 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.

The wind blowing around the land provoked ocean waters to stream the other way, diverting warmth from waters nearest to the land. This wind/ocean interaction significantly affected any monsoon that framed over the land: the stronger this interaction, or coupling among winds and ocean, the more extensive the distinction in land and ocean temperature, and the stronger the intensity of the ensuing monsoon.

Lead author Nicholas Lutsko said, “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.”

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.

Their results, published today in the Journal of Climate.

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