The South Asian summer monsoon sustains billions of people today. For a long time, the prevailing scientific view has held that the formation and intensification of the South Asian summer monsoon were primarily controlled by the rapid uplift of the Tibetan Plateau. However, geological records present a long-standing puzzle from the Early to Middle Miocene (25 to 15 million years ago): the South Asian monsoon rainfall was remarkably strong, even though the Somali Jet—the primary wind system transporting moisture—was relatively weak.
Previously, Zixuan Han and colleagues first identified that African topographic evolution was a critical factor driving the decoupling of South Asian summer monsoon precipitation and the Somali Jet. However, that study did not fully address a core question: through what specific dynamic and thermodynamic pathways do changes in African topography achieve enhanced monsoon precipitation against the backdrop of a weakened Somali Jet? And how can the energy sources and their contributions be quantified?
To address these questions, a research team led by Dr. Zixuan Han from Hohai University, China, in collaboration with the University of Copenhagen and other institutions, recently published an article titled "Dynamic and thermodynamic mechanisms of Miocene South Asian summer monsoon rainfall enhancement driven by African topography evolution" in Atmospheric and Oceanic Science Letters , revealing the dynamic and thermodynamic mechanisms by which African topographic evolution drove the enhancement of South Asian summer monsoon precipitation during the Miocene.
Using the fully coupled Earth system model EC-Earth3, and combined with moisture budget diagnostics, moist static energy budget analysis, and the three-pattern decomposition of global atmospheric circulation, the team systematically diagnosed and quantitatively assessed the physical processes by which African topography modulates the South Asian monsoon.
The results indicate that alterations in African topography initially weaken the local African monsoon circulation, leading to reduced precipitation over Central Africa. This local perturbation, by exciting atmospheric Kelvin waves, triggers a coupled ocean–atmosphere response across the Indian Ocean: it generates anomalous cyclonic circulation in the western Indian Ocean and, in conjunction with the weakened Somali Jet, produces an anomalous structure in the eastern Indian Ocean resembling a positive phase of the Indian Ocean Dipole. This process significantly enhances moisture convergence over the Arabian Sea.
Driven by the climatological mean westerlies, this enriched moisture is continuously transported toward South Asia, where it strengthens convective activity through latent heat release. The advection of moist enthalpy, driven by latent heat, provides a crucial energy source for anomalous ascending motion, effectively offsetting the adverse effects of the weakened wind field and ultimately leading to a significant enhancement of South Asian precipitation during the Early to Middle Miocene.
Against the backdrop of persistently increasing greenhouse gases, climate models generally project a trend of increasing precipitation over North Africa in the future. If the regional climate regime in Africa undergoes significant adjustment, it warrants close attention to whether a "dynamic–thermodynamic teleconnection mechanism" similar to that operating during the Miocene might be reactivated and further influence the future evolution of the South Asian and even East Asian summer monsoon systems. This question holds considerable scientific significance for understanding future climate change and regional hydrological cycle responses.
