Altered Ocean Temperature Gradients Are Key to Miocene South Asian Monsoon Evolution

Abstract Indian Ocean sea surface temperature (SST) gradients influence South Asian summer monsoon (SASM) strength. Middle Miocene proxy records show reversed Arabian Sea zonal gradients (warmer west than east) and reduced Indian Ocean meridional gradients compared to today—patterns coupled models cannot reproduce, hindering accurate SASM simulation. To assess these impacts, we conducted three prescribed‐SST experiments with mid‐Miocene boundary conditions: a control using coupled‐model SSTs and two cases imposing reversed zonal and reduced meridional gradients based on proxy‐derived SSTs. Reversed zonal gradients decreased SASM rainfall; reduced meridional gradients showed minimal impact. Since most mid‐Miocene simulations with modern‐like zonal SST gradients overestimate Indian precipitation, capturing zonal gradients is critical for reducing model–data discrepancies. These zonal gradient shifts from reversed mid‐Miocene toward modern‐like late mid‐Miocene conditions, likely driven by high‐latitude cooling, represent key precursors to the modern SASM. Model biases in simulating warm‐climate polar amplification likely underlie their inability to reproduce Miocene monsoon transitions.