The study, published today (5 September) in the journal Science Advances, indicates how the heating in North America can trigger remote effects in Asia – this could be further exacerbated by anthropogenic global warming and human modification of the North American land surface. The authors of the study say their findings emphasise the importance of global cooperation in addressing climate change.
Using climate models, scientists from the University of Bristol and the Institute of Tibetan Plateau Research (ITP) from the Chinese Academy of Sciences (CAS) found changes over North America can drive atmospheric patterns that intensify South and East Asian rainfall, with an effect nearly half as strong as the Tibetan Plateau's influence on East Asian summer rainfall.
While researchers have long known the Asian monsoon can influence the climate far beyond Asia, this latest study is the first to reveal the reverse is also true - that the existence of North American continent has teleconnections across the Pacific Ocean and strengthens the East and South Asian summer monsoon rainfall.
Lead author Linlin Chen, PhD student in Physical Geography at the University of Bristol, said: "In the past few decades, when people talk about what factors would drive such a strong monsoon in Asia, they always looked at the Eurasian and African continents, especially the Himalaya and Tibetan region.
"These are indeed large influences. But we know Earth climate is closely connected, and now we have more evidence to show precisely how."
The study authors first modelled an idealised 'water' world with no continents, and then gradually added continents from Eurasia, Africa and India to produce a basic Asian monsoon system. Australia, Antarctica, North and South America, as well as a simplified Tibetan Plateau were added separately, to see how the Asian monsoon would respond.
Dr Alex Farnsworth, a Senior Research Associate at the University of Bristol, also affiliated with ITP, said: "We initially thought Australia would outperform, as it is the closest landmass not previously considered. But the results surprised everyone.
"North America turns out to be the most important extra continent impacting the Asian monsoon. It's always exciting when the model reveals something unexpected."
The team discovered the North American continent creates a heating centre in summer. This then creates atmospheric ripple effects, which strengthens the North Pacific subtropical high-pressure system and widens the north Hadley cell. This enhanced anticyclone pushes stronger westward flows toward Asia, causing a deeper convection and bringing more moisture from the ocean, thus intensifying rainfall over East and South Asia.
The Tibetan Plateau also plays a role but, remarkably, the North American impact is nearly half as strong as that of the Tibetan Plateau over the East Asian summer rainfall. The seasonal rains in Asia sustain more than a billion people. In recent years, people in Asia experienced more extreme flood and drought, which climate scientists suggest is due to global warming, which then causes more extreme climate events.
Co-author Paul Valdes, Professor of Physical Geography at the University of Bristol, added: "This study reveals how deeply connected the Earth's climate systems are: local changes can trigger global effects. Everyone is responsible for both the local and global climate changes."
