The South Asian summer monsoon has long been known to shape the Mediterranean climate thousands of kilometers away as part of a phenomenon known as the "monsoon–desert" mechanism.
Through this mechanism, intense monsoon convection during the summer releases large amounts of heat into the atmosphere, triggering large-scale circulation changes that promote sinking air over the Mediterranean. This process suppresses clouds and rainfall and contributes to the Mediterranean region's hot, dry climate.
Previous studies showed that heating associated with the South Asian monsoon helps drive this descending circulation. However, scientists have been unsure whether this long-distance atmospheric connection would persist under global warming.
To address this question, a research team led by Prof. ZHOU Tianjun at the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences (CAS) combined real-world observations, dozens of climate model simulations, and idealized numerical experiments to see how climate change under a future high-emissions scenario would affect this monsoon–Mediterranean link.
The study was published in Nature Geoscience.
The results showed a remarkably consistent signal. In 97.5% of the CESM1 simulations, the South Asian monsoon had a weaker effect on atmospheric circulation over the Mediterranean.
The relationship between monsoon heating and mid-tropospheric subsidence over the central and eastern Mediterranean declined from a correlation of about 0.4 to essentially zero by the second half of this century.
Underlying processes
To understand why the monsoon's remote influence on the Mediterranean is expected to weaken, the researchers examined the underlying atmospheric processes.
They identified one key change anticipated over South Asia. As the climate warms, deep monsoon convection shifts higher in the troposphere. In the CESM1 simulations, the characteristic level of monsoon convection shifts from about 452 hPa to 417 hPa, with a similar upward shift appearing across CMIP6 models. As a result, the warm response produced by monsoon heating spreads farther west and reduces the east-west temperature contrast over the Mediterranean, which in turn suppresses subsidence through atmospheric dynamics.
A second process occurs locally over the Mediterranean. The monsoon-related sinking motion weakens most strongly in the middle and upper troposphere. Through an intrinsic atmospheric relationship known as the "Sverdrup balance," this change also weakens the northerly wind response at mid-to-lower levels. The weaker northerlies further reduce the descending motion, creating a local feedback that reinforces the remote effect of changing monsoon heating.
These circulation changes also have implications for Mediterranean rainfall. Currently, stronger South Asian monsoon heating tends to generate stronger subsidence and thus less summer rainfall over Mediterranean land. Under future warming, this relationship is projected to largely disappear, with the fraction of rainfall variance explained by the monsoon falling from about 14.2% to 5.1%.
However, this does not mean that Mediterranean rainfall itself will become less variable. Rather, it indicates that the South Asian monsoon will account for a much smaller share of year-to-year rainfall variability, suggesting a shift in the factors controlling Mediterranean summers as the "monsoon–desert" coupling weakens.
"Global warming does not only change the mean state of temperature and rainfall. It can also reorganize the dynamical links between distant parts of the climate system," said Prof. ZHOU from IAP, corresponding author of the study.
According to Zhou, "The changing monsoon–desert coupling phenomenon across Eurasia provides a new dynamical perspective on how large-scale modes of climate variability and teleconnections may evolve in a warmer world. It also means that future projections of Mediterranean summer rainfall need to account for changes in the factors that control its year-to-year variability."
Dr. YU Hanzhao from IAP, lead author of the study, noted that atmospheric connections that appear robust today may not remain so under global warming.
"Understanding how such teleconnections change will be important for projecting future regional climate variability, especially in climate change hotspots such as the Mediterranean," he said.
