Atmospheric Stabilization Weakened Proto-Low-Level Jet Over the Indian Ocean During the Eocene Hothouse

Simulations of the Eocene climate using state-of-the-art Earth system models provide a reference state for the future climate, as the Eocene was one of the warmest geological epoch with high atmospheric carbon dioxide (CO₂) concentration and global temperatures comparable to projections for the coming centuries. However, the paleogeographic configuration of the Eocene impact's distinct climate features. Here, we decompose the response of low-level monsoon dynamics over the Indian Ocean to the early Eocene hothouse using five climate model simulations from the Deep-time Model Intercomparison Project (DeepMIP). We see a circulation pattern resembling the paleo-monsoon across all models over the Indian Ocean. Surprisingly, we find low-level jet (LLJ) forming along the topographic barriers of the Eastern African Rift and the Deccan Plateau, which we refer to as the “Proto-LLJ.” Based on the analysis of the DeepMIP results, we find a reduction in the Proto-LLJ strength with elevated CO₂. Under present-day conditions, the northward shift of monsoonal LLJ is attributable to the increased land-sea contrast under global warming. Even though land-ocean temperature contrasts increased during the Eocene hothouse, Proto-LLJ weakened due to tropical atmospheric stabilization. This stabilization reduced vertical temperature gradients, suppressed convection, and weakened atmospheric overturning, limiting the upward motion needed to drive strong monsoonal winds under CO₂-induced warming.