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This study presents modeling evidence that the Indian Ocean Dipole (IOD) is enhanced in its frequency and amplitude under mid-Holocene (8-ka) conditions. This enhancement is identified in a global climate model simulation driven by the 8-ka insolation, in which negative sea surface temperature anomalies grow more than the present-day simulation in the eastern Indian Ocean. The mechanism of IOD amplification is explained in terms of a positive feedback among sea surface temperature, precipitation, and wind speeds over the eastern Indian Ocean. A series of idealized experiments in which the present-day insolation seasonal cycle over the tropics is systematically perturbed in the model demonstrates that the IOD frequency and amplitude are proportional to the seasonal cycle strength of the atmospheric circulation off Sumatra-Java, consistent with proxy records in the mid-Holocene. Greater surface warming over Eurasia due to increased insolation in boreal summer is not critical to the amplification of the IOD.

Plain Language Summary Indian Ocean Dipole (IOD) mode is a prominent coupled air-sea variability in the Indian Ocean, which affects weather over the surrounding regions. In order to clarify the sensitivity of the IOD activity to changing climate condition from the past to present, we compared two long climate simulations: one with the present-day forcing and the other with the mid-Holocene (8,000 years ago) forcing that is characterized by a strong seasonal cycle of insolation. The mid-Holocene climate simulation clearly showed an enhancement of the IOD activity in its amplitude and frequency. The insolation changes during the mid-Holocene are complex, including intensified seasonal cycle and annual mean over the globe, so that for further exploring mechanisms for the enhanced IOD, we conducted a series of idealized experiments in which perturbations in insolation were confined to the seasonal cycle over the tropics. The intensification of IOD is similarly seen in the experiment, and the degree of the enhancement is found proportional to the rate of seasonal cycle change over the northern Indian Ocean. The process that gives rise to the IOD modulation is mainly controlled by the strength of mean surface winds off Sumatra during the monsoon onset season.