If the Asian monsoon were to become deregulated in the decades to come, the origin could well be in the massive release of fresh water in the North Atlantic by the melting of Greenland. Indeed, researchers recently demonstrated how the disturbances initiated in the North Atlantic basin led in the past to a reorganization of rain regimes in Southeast Asia. The results were published in the journal Nature Geoscience on November 18.
The Asian monsoon carries essential fresh water to more than one billion people between southeast China and Bangladesh, via India. This planetary wind system is organized by the temperature contrast that appears every summer between an overheated landmass to the north and a relatively cool ocean to the south.
Despite the distance between them, the Asian monsoon is modulated by the ocean currents of the Atlantic basin. We speak of AMOC (Atlantic Meridional Overturning Circulation) to summarize this circulation which transports a large amount of heat from the tropics to the pole. Also, any change in its intensity tends to affect monsoon areas and increase or decrease the amount of rain falling on a given region.
Study the past to better anticipate the future
However, with climate change, we expect AMOC to weaken, which would necessarily influence the evolution of monsoon regimes. However, at this time, the precise relationship between the two systems remains poorly understood. To better understand, a group of researchers decided to look for answers in Earth’s past.
The period chosen by scientists targets the end of the penultimate ice age, 147,000 to 125,000 years ago, at a time when numerous iceberg debacles weakened or even collapsed AMOC. Thanks to the study of stalagmites in southwestern China and an innovative methodology to decouple the influence of temperature from that of precipitation, the study authors were able to see how the monsoon behaved when the AMOC suffered from power cuts.
“In continental climates, there is nothing better than stalagmites as a climate record because they offer incomparably high dating accuracy over several millennia,” notes Hubert Vonhof, lead author of the paper.
A graduated response of the Asian monsoon to AMOC declines
The results of this work show that for weak disturbances, that is to say for a slightly slower Atlantic circulation, the Asian monsoon lasts a little less, but does not show a major change in the rainfall regime. Conversely, for major disturbances with a collapse of the AMOC, the monsoon is greatly reduced because of the weakening of the thermal contrast between the continent and the ocean linked to the drop in heat transport towards the pole by ocean currents.
“The study deciphers in unprecedented detail how monsoon climates reacted to meltwater impulses at that time,” emphasizes Hubert Vonhof. “We have thus taken a big step forward to better understand the global consequences of today’s man-made climate change.”
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