Greenhouse forcing and paleogeographic controls on Tropical Indian Ocean surface temperature variability during the Miocene

The spatial variability of sea surface temperature (SST) in the Tropical Indian Ocean (TIO) plays a key role in controlling the South Asian monsoon and influencing global climate. Modern-day variability in SST is modest, but projections under future greenhouse warming suggest an increase in SST gradients across the basin. Yet, the magnitude and drivers of such changes remain poorly constrained. The Miocene archives, characterized by elevated atmospheric CO2 and warmer global temperatures, offer a valuable equilibrium climate analog for future conditions. However, limited SST reconstructions from the southern Bay of Bengal (SBoB), a region in the eastern Indian Ocean with sustained high SSTs, have hindered understanding of past spatial variability. Here, we reconstruct Miocene SSTs from the SBoB using carbonate clumped isotope (Δ47) thermometry on planktic foraminifera. Our results show surface cooling driven by upwelling during the Miocene Climatic Optimum (MCO; 17–14.7 Ma) and surface warming linked to weakened upwelling during the Mid-Miocene Climate Transition (MMCT; 14.7–13 Ma). Comparison with other TIO SST records reveals that while mean SSTs remained broadly decoupled from atmospheric CO2, the standard deviation of SSTs (σSST), a proxy for spatial heterogeneity, covaried with CO2 fluctuations, except between 13.75 and 14.25 Ma. We attribute this transient decoupling to regional paleogeographic changes, particularly the closure of the Mesopotamian Seaway and constriction of the Indo-Pacific Gateway. These findings highlight the sensitivity of Indian Ocean SST spatial structure to both atmospheric CO2 and paleogeographic boundary conditions, with implications for understanding ocean-atmosphere interactions in a warmer future climate.