Changes in wind patterns play the leading role in influencing often-devastating tropical rainfall changes rather than simply the warming atmosphere holding more moisture, according to new research.
Climate scientists have discovered that land-driven climate processes induced by human activities lead to shifts in wind patterns and strongly influence current tropical rainfall trends.
They warn that this means current climate models that are used to project future tropical rainfall changes in regions such as Africa, Southeast Asia, India, Central and South America, and Australia may not be accurate.
The study, published in Nature Communications , was led by Ligin Joseph, a postgraduate researcher in physical oceanography at the University of Southampton, alongside Dr Pascal Terray and Dr Sebastien Masson from Sorbonne University in Paris and Dr K P Sooraj from the Centre for Climate Change Research at the Indian Institute of Tropical Meteorology.
Mr Joseph said: "Tropical rainfall trends are not simply driven by a warmer atmosphere holding more moisture. Changes and shifts in the winds are key.
"We analysed satellite and reanalysis data from the past 45 years and conducted experiments using climate models to understand how rainfall has changed in recent decades due to climate change.
"We found that the observed rainfall trends do not follow the expected 'wet-gets-wetter' behaviour that suggests climate change intensifies existing rainfall patterns. Instead, they are largely controlled by changes in atmospheric circulation, where the winds converge and produce rain."
The researchers found that stronger warming over land - especially over deserts in the Northern Hemisphere such as the Sahara in Africa and the Thar Desert in India and Pakistan - together with warming and extension of the Indo-Pacific Warm Pool (the world's largest and warmest tropical surface waters in the western Pacific Ocean and eastern Indian Ocean) is altering wind patterns, which in turn is redistributing rainfall across the tropics.
Dr P Terray explained: "The link between climate change and temperature is relatively straightforward: as carbon dioxide in the atmosphere increases, most of the planet warms.
"But rainfall is much more complicated, as warming can increase floods in some regions while causing droughts in others. Natural climate variability, such as El Niño and La Niña, can also mask long-term rainfall trends."
The research found three main driving forces behind changes in atmospheric circulation that leads to tropical rainfall trends:
- Faster warming of land compared with the ocean
- Ongoing warming and expansion of the Indo-Pacific Warm Pool
- Increased warming and desertification in the Northern Hemisphere.
Together, these factors change global temperature contrasts and wind patterns, which then shift rainfall across the tropics.
"Many current climate models struggle to reproduce the rainfall patterns we observe today for reasons which are still uncertain," said Dr P Terray. "The models tend to show current rainfall trends that resemble El Niño-related rainfall patterns as in future climate projections, whereas real-world observations over recent decades look different. These discrepancies raise concerns about the accuracy of these projections."
Mr Joseph added: "Understanding these mechanisms is important because tropical rainfall directly affects water resources, agriculture, and billions of people, especially in regions such as Asia and Africa. If the processes driving rainfall changes are different from what our models assume, then our projections of future droughts and floods may be off."
