Global warming could weaken the long distance atmospheric connection between the South Asian summer monsoon and the Mediterranean climate potentially changing the factors that influence summer rainfall variability in the region according to a new study published in Nature Geoscience.
Researchers led by scientists from the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences examined how the relationship between the South Asian monsoon and Mediterranean climate could change under a future high emissions scenario.
The South Asian summer monsoon is known to influence weather thousands of kilometres away through a mechanism often described as the “monsoon–desert” connection. During summer, strong monsoon convection releases large amounts of heat into the atmosphere. This can trigger large scale circulation changes that promote sinking air over the Mediterranean, suppressing clouds and rainfall and contributing to the region’s hot and dry summer conditions.
Scientists have previously identified the role of monsoon heating in driving this descending circulation but it remained uncertain whether the connection would remain strong as the planet warms.
The research team combined observations, dozens of climate model simulations and idealised numerical experiments to investigate the future of this atmospheric link.
The results showed a consistent weakening of the monsoon influence. In 97.5% of the CESM1 simulations examined by the researchers the South Asian monsoon had a weaker effect on atmospheric circulation over the Mediterranean.
The correlation between monsoon heating and mid atmospheric sinking motion over the central and eastern Mediterranean was projected to fall from about 0.4 to nearly zero by the second half of this century.
Researchers identified changes in the height of monsoon convection as one of the main reasons behind the weakening connection. As the climate warms deep monsoon convection is projected to shift higher into the atmosphere. In the CESM1 simulations the characteristic level of monsoon convection moved from about 452 hPa to 417 hPa, with a similar upward shift found in CMIP6 climate models.
This higher altitude heating changes how atmospheric warmth spreads westward and reduces the east west temperature contrast over the Mediterranean. The change weakens the atmospheric sinking motion that has traditionally linked the South Asian monsoon with Mediterranean summer conditions.
A second process occurs within the Mediterranean region itself. The weakening of monsoon-related sinking motion, particularly in the middle and upper atmosphere, also reduces northerly wind responses at lower levels. Researchers said this can further weaken descending air, creating a local feedback that reinforces the broader change.
The weakening connection could also affect the relationship between the South Asian monsoon and Mediterranean rainfall. At present, stronger South Asian monsoon heating tends to be associated with stronger atmospheric subsidence and reduced summer rainfall over Mediterranean land areas.
Under future warming, however researchers project that this relationship will largely disappear. The share of Mediterranean rainfall variability explained by the South Asian monsoon could decline from about 14.2% to 5.1%.
The researchers stressed that this does not necessarily mean Mediterranean rainfall will become less variable. Instead, it suggests that the South Asian monsoon will account for a much smaller part of year to year changes in Mediterranean summer rainfall while other climate factors become relatively more important.
The study highlights that climate change can alter not only average temperatures and rainfall but also the atmospheric connections between distant regions. The researchers said understanding how such teleconnections change will be important for improving projections of future regional climate variability particularly in climate sensitive regions such as the Mediterranean.