Atmospheric Stabilization Weakened Proto-Low-Level Jet over the IndianOcean during the Eocene Hothouse

The early Eocene represents one of the warmest periods in Earth’s history, with atmospheric CO₂ concentrations and global temperatures far higher than today. Studying this period offers a useful way to explore how monsoon systems behave under extreme greenhouse conditions. However, the markedly different paleogeography, including altered land–sea distributions and the absence of the Himalayas, makes direct comparison with the modern monsoon challenging. Here, we examine the behavior of low-level monsoonal circulation over the Indian Ocean during the early Eocene using five climate model simulations from the Deep-time Model Intercomparison Project (DeepMIP). All simulations show a coherent monsoon-like circulation, indicating that a proto-monsoon system existed during this warm climate state. We further identify low-level jet structures aligned with paleotopographic features over the Eastern African Rift and the Deccan Plateau, which we refer to as the Proto-LLJ. Despite enhanced land–sea temperature contrasts under elevated CO₂, the strength of the Proto-LLJ weakens across the simulations. This contrasts with present-day behavior, where a stronger land–sea contrast is often linked to intensified or poleward-shifted monsoon jets. Our results indicate that CO₂-driven warming leads to increased tropical atmospheric stability, reduced vertical temperature gradients, and weaker convective overturning. As a result, the vertical motion needed to sustain strong low-level monsoon winds is suppressed. These findings suggest that in very warm climates, increased atmospheric stability can outweigh thermal forcing and lead to weaker monsoonal circulation, highlighting a key control on paleo-monsoon dynamics under extreme greenhouse conditions.