The Hadley circulation, a fundamental component of Earth's atmospheric system, plays a crucial role in global climate regulation by transporting energy and moisture between the tropics and higher latitudes. However, recent studies have revealed a puzzling discrepancy: while reanalysis data suggest a strengthening of the global zonal mean Hadley circulation intensity, climate model simulations predict a weakening trend. What causes this contradiction?
A new study, published in Atmospheric and Oceanic Science Letters, investigates this issue by examining regional Hadley circulation intensity trends in the Northern Hemisphere across six key regions: the eastern Pacific, western Pacific, Atlantic, Africa, Indian Ocean, and residual areas. The research team analyzed 6 reanalysis datasets and 13 state-of-the-art climate models to uncover the factors driving these conflicting results.
"We found that regional trends in the Indian Ocean are a key reason for the disagreement between models and reanalysis data," says the study's corresponding author, Prof. Bo Sun from Nanjing University of Information Science and Technology, China. "While reanalysis datasets indicate a strengthening trend in the Indian Ocean region, climate models consistently simulate a weakening trend."
The study attributes this discrepancy to how models and reanalysis datasets simulate diabatic heating and zonal friction—two critical factors influencing atmospheric circulation (see figure). Furthermore, the researchers used optimal fingerprint analysis to determine that greenhouse gas external forcing plays a suppressing role in most regions. Anthropogenic forcing and non-greenhouse gas external forcing are the primary drivers of regional Hadley circulation intensity trend changes. Notably, in the African region, the impact of anthropogenic external forcings, especially aerosol forcing, on regional Hadley circulation intensity trends surpasses that of other external forcings.
Understanding these regional variations is essential for improving climate model accuracy. "Our findings highlight the need for better representation of regional Hadley circulation intensity trends in climate models to enhance predictions of future atmospheric circulation changes", Prof. Sun adds.
This study explains the reasons, from a regional perspective, behind the discrepancies in Hadley circulation intensity trends between reanalysis data and state-of-the-art climate models. It also reveals the significant impact of human activities on the trends of Hadley circulation intensity across different regions of the Northern Hemisphere. It emphasizes the importance of accurately simulating physical processes in climate models and highlights the need for further model improvements to unravel the complex response of the Hadley circulation to global warming, reduce uncertainties in future projections, and inform adaptive strategies.
