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Warming cuts the South Asian monsoon's influence on Mediterranean summer rain in climate models
Chinese Academy of Sciences researchers project warming will cut the South Asian monsoon's share of Mediterranean summer rain variance from 14.2% to 5.1%. The high-emissions projection is about what causes rain to swing from one summer to the next, and it does not say whether the region's summers will get wetter.
The Scientist · Science desk
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What happened
- In 97.5% of CESM1 climate model simulations, the South Asian monsoon had a weaker effect on atmospheric circulation over the Mediterranean.
- By the second half of this century, the correlation between monsoon heating and mid-level sinking air over the central and eastern Mediterranean falls from about 0.4 to essentially zero.
- Deep monsoon convection rises from about 452 hPa to 417 hPa in CESM1, so its warming spreads west and flattens the Mediterranean's east-west temperature contrast.
- A local feedback over the Mediterranean, acting through the Sverdrup balance, weakens mid-to-low-level northerly winds and cuts the sinking motion further.
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Why it matters
- decision A statistical forecast of Mediterranean summer rain that uses monsoon strength as an input would lose most of that input's value in a high-emissions future and would need other predictors.
- constraint The runs model a high-emissions pathway, so planners working to lower-emission scenarios cannot drop the 5.1% figure straight into their own assumptions.
- precedent Other long-distance climate links that regional projections treat as fixed now need the same check on whether they hold as the atmosphere warms.
The coupling explained only part of the variation even before any warming. By the study's figures, South Asian monsoon heating accounts for about 14.2% of the year-to-year variance in summer rainfall over Mediterranean land [9]. Everything else, 85.8%, already comes from other drivers [1]. The circulation link is modest too. A correlation of about 0.4 between monsoon heating and mid-tropospheric sinking over the central and eastern Mediterranean [5] means the monsoon accounts for roughly 16% of the variance in that sinking [3].
In the high-emissions projections the monsoon's share of rainfall variance falls to 5.1% [9], a relative drop of about 64% [2]. According to the report, Mediterranean summer rainfall does not become less variable as a result. The monsoon just explains a smaller share of the swings [10]. Part of the reason the region's summers are dry is that monsoon heat drives sinking air, and that sinking air suppresses cloud and rain [1]. The thing this doesn't tell you is whether the average summer ends up wetter or drier. The results as reported concern how closely that sinking tracks the monsoon from year to year [5][10].
Scientists had been unsure whether the connection would last under warming [14]. The study, published in Nature Geoscience [3], combined observations with dozens of climate model simulations and idealized numerical experiments [2]. "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 Zhou Tianjun of the Institute of Atmospheric Physics, the corresponding author [11]. "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," he said [12].
Two conditions apply to these numbers. The projections use a future high-emissions scenario [2]. The 97.5% consistency figure comes from CESM1 [4]. The report cites CMIP6 models for a similar upward shift in monsoon convection [6], and that shift is the first step in the westward spread of warming the authors describe [7]. In my view the direction of change is well supported for a high-emissions future, which is the future these runs model [2].
Yu Hanzhao of IAP, the lead author, noted that atmospheric connections that appear robust today may not remain so under global warming [15]. "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 [13].
What to watch
- Runs under moderate or low emissions scenarios, which would show whether the weakening needs high-emissions warming to appear.
- Whether model families beyond CESM1 reproduce the fall of the monsoon-subsidence correlation to near zero, and not only the upward shift in convection.
- Whether observed summers in coming decades show the monsoon-subsidence correlation drifting down from about 0.4.