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

Changes in Indian Ocean sea surface temperature (SST) gradients play an important role in determining the strength of the South Asian summer monsoon (SASM). We show that, during a past warm climate interval—the middle Miocene—the Arabian Sea exhibited a zonal SST gradient that was reversed compared to today. Meridional SST gradients across the Indian Ocean were also greatly reduced. These patterns are not typically reproduced in Miocene climate simulations, raising the possibility that systematic regional SST biases hinder the models' ability to simulate the SASM. We assess the impact of these SST gradient changes on the SASM, by conducting three prescribed-SST experiments using early-to-middle Miocene boundary conditions along with the paleoclimate-calibrated Community Earth System Model version 2. The control experiment uses SSTs from our published coupled Miocene simulation. The second and third experiments modify Indian Ocean SSTs based on proxy-derived data from 14 to 12 million years ago. Specifically, the second simulation imposes a reversed zonal SST gradient; the third imposes warmer southern subtropical Indian Ocean SSTS, leading to reduced meridional SST gradients. The reversed zonal gradient experiment shows decreased SASM rainfall and reduced western Arabian Sea wind-driven upwelling, whereas the reduced meridional gradient experiment shows little impact on the SASM. We find that most Miocene climate simulations overestimate Indian precipitation, supporting our hypothesis that accurately simulating the reversed zonal Arabian Sea SST gradients will reduce model-data discrepancies. This study implies that these gradient changes, likely related to high‑latitude cooling in the late Middle Miocene, were an important precursor to the modern SASM. Thus, paleoclimate models may struggle to accurately simulate Miocene monsoon transitions, largely due to their persistent inability to reproduce the pattern of polar amplification characteristic of the Miocene and other warm climates.