Global atmospheric circulation experienced drastic changes during the Middle Miocene climate transition (MMCT~14.7–13.0 Ma) possibly related to the glaciation on West-Antarctica. Palaeoceanographic reconstructions have shown that upwelling in the western Arabian Sea due to summer South Asian monsoon (SAM) winds likely occurred since ~14.7 Ma, with fully modern-like monsoonal wind patterns after the end of the MMCT at around ~13 Ma. Whether the changes in monsoonal circulation since ~14.7 Ma are also associated with upper ocean hydrographic changes in the eastern Arabian Sea (EAS) is currently not known for the middle Miocene. For that reason, the difference in Mg/Ca-based temperatures of surface-dwelling with lower mixed layer/thermocline-dwelling and sub-thermocline-dwelling planktic foraminifera was reconstructed to estimate the upper ocean thermal gradient at Site NGHP-01-01 A in the EAS and thus changes in the upper surface water column structure. The Mg/Ca-temperature estimates from the upper mixed layer down to the sub-thermocline show a prominent cooling trend between ~14.2 Ma and 13.2 Ma. The upper water column reconstructions reveal weaker mixing, a shallower thermocline, and therefore a well stratified upper water column in the EAS after ~14.2 Ma. We suggest that the weaker mixing and shallower thermocline in the EAS after ~14.2 Ma are most likely due to an intensification of the summer SAM. A strong salinity stratification because of increased SAM rainfall and advection of low salinity water may have contributed to a weaker mixing and shallower thermocline in the EAS during the investigated time period. The change in the upper ocean hydrography in the EAS after ~14.2 Ma fits well into the emerging picture of monsoonal-driven upper ocean hydrographic changes in the equatorial and northern Indian Ocean due to an intensification of the summer SAM since ~14.7 Ma.

planktic foraminifera, stable oxygen isotopes, Mg/Ca palaeothermometry, upper ocean thermal gradient, mixed layer depth, thermocline depth