Eastern equatorial Indian Ocean (EEIO), being part of the Indo-Pacific Warm Pool (IPWP) plays a vital role in regional and global climate change on short-long time scales.  Therefore, it is crucial to understand the past variability of the IPWP and the interactions between Indonesian throughflow, Indian Ocean Dipole (IOD), Indo-Australian Monsoon, and other climate factors.  It is believed that the IOD mode is driven by seasonal variation of south easterlies (SE) and north westerlies (NW) monsoonal winds in the Australian-Indonesian monsoonal (AIM) regime. However, its connection with the Indian monsoon is yet to be thoroughly investigated. 

Here, we present a sub-centennial scale record of surface hydrography and productivity variations in EEIO during 4.7-13.07 ka based on the planktic foraminiferal assemblage and stable oxygen isotope records in a deep sediment core collected from offshore Java-Sumatra (8.40° S, 98.00°E; 3440m water depth). Temporal variations in abundances of ecologically sensitive, symbiont-barren, and symbiont-bearing planktic foraminifera combined with the oxygen isotope records of Globigerinoides ruber (surface species) and Pulleniatina obliquiloculata (thermocline species) are used to reconstruct past changes in the upper water-column structure and productivity in response to the variation in the seasonal monsoon wind intensity and ocean circulation. We integrated our proxy records from EEIO with the published Indian summer monsoon (ISM) record from the western Arabian Sea (WAS) to decipher the IOD-like mean state.

Our faunal and isotope proxy records suggest weakly stratified surface waters, reduced surface freshening, shoaling of the thermocline, and enhanced upwelling leading to high biological productivity during 12–9.2 ka, 8.0–7.3 ka, 6.4–5.6 ka, and 5.2–4.7 ka, attributed possibly to the intensified SE monsoonal winds over the EEIO; which were in phase with the enhanced upwelling in WAS driven by the strengthened ISM winds. By analogy with the modern scenario, this condition is more likely linked with the positive IOD-like mean state. Conversely, strongly stratified surface waters, increased freshening, and deep thermocline depth prevailing during 13-12 ka, 9.2-8.0 ka, 7.3-6.4 ka, and 5.6-5.2 ka might be related to the intensified NW monsoonal winds, and these intervals closely correspond to the periods of weakened ISM winds, suggesting a scenario of negative IOD-like mean state. Hence, the study demonstrates how alternating periods of shoaling and deepening of thermocline depths in the EEIO associated with changes in relative strengths of upwelling and salinity stratification modulated variation in IOD-like mean state and the Indian summer monsoon intensity during the Holocene.