Weakening of the Indian Ocean Dipole in the mid-Holocene due to the mean oceanic climatology change

The Indian Ocean Dipole (IOD) is one of the leading modes of interannual climate variability in the tropical Indian Ocean (IO). Paleoclimate provides real climate scenarios to examine IOD behaviors and the linkage to basic states. Based on 18 models from the Paleoclimate Modelling Intercomparison Project phase 3 and 4 (PMIP 3/4), the IOD change from the preindustrial period to mid-Holocene is investigated. The multimodel mean reveals that the IOD variability weakens by 14% as measured by the standard deviation of the Dipole Mode Index, which is defined using the zonal sea surface temperature (SST) difference. Such attenuation is dominated by the spatially consistent suppression in the western-pole SST variability, while the eastern pole contributes little due to the opposite-signed changes in its northwestern and southeastern portions. The primary reason for the aforementioned changes comes from the altered climatic background, which displays a positive IOD-like pattern during IOD growing seasons, with intensified westward currents along the equator and northwestward currents in the southeastern equatorial IO. Such changes in the mean-state currents modulate the strength of the IOD-related anomalous advection and subsequently cause alterations in the IOD variability. Further analyses show that the IOD attenuation in the mid-Holocene is likely irrelevant to the concurrently subdued El Niño–Southern Oscillation in the tropical Pacific because of the diminished connections between the two oscillations themselves. The above simulated changes in both the IO mean climatology and IOD variability agree well with the available paleo-records in literature.