The Indonesian Throughflow (ITF) plays a pivotal role in large-scale ocean-atmosphere interactions in the tropics, regulating the heat and freshwater budget between the Pacific and Indian Oceans. Despite its importance in the climate system, how would the ITF change in the context of projected global warming remains poorly resolved in the literature, though simulations from CMIP6  (the Coupled Model Intercomparison Project Phase 6) suggest a weakening trend. Alternatively, geological past provides valuable time windows to explore the climate dynamics surrounding the ITF in a warmer-than-today world. For instance, the Last Interglacial (LIG, Marine Isotope Stage 5e or Eemian), with global surface temperature reached about 2 °C above present, serves as an outstanding analog to explore the climate response to the external forcing and the mechanisms behind it. In this regard, past proxy studies have suggested an intensification of ITF during the LIG. However, the mechanisms underlying this instensification, together with its environmental implications, are yet to be explored.
       Here, we present model outputs from a set of Last Interglacial snapshot simulations carried out by CAS-FGOALS (the Chinese Academy of Sciences Flexible Global Ocean–Atmosphere–Land System model) under the protocol of PMIP for four time periods at 130, 128, 125, and 115 ka. Compared to the piControl simulations (the annual mean ITF flux is 18.46Sv), an annual mean ITF flux increase of about 30.6% - 35.9% was found in the LIG snapshot simulations (24.11 - 25.08Sv). We found this enhancement of the ITF was closely correlated with a La Niña-like state during the LIG. During the LIG, there was an increased SST gradient between the tropical western Pacific and tropical central-eastern Pacific, accompanied by intensified easterlies over the tropical western Pacific. This increased SST gradient and intensified easterlies acted as positive feedback, reinforcing each other and resulting in a La Niña-like state with elevated sea levels in the western Pacific. This led to an asymmetric increase in sea level height between the tropical western Pacific and the tropical eastern Indian Ocean, which increased the pressure contrast between the two basins and subsequently strengthened ITF volume transport. Comparisons between models and proxies further support our conclusions. An examination of the changes in the thermocline water temperature record from the eastern Indian Ocean found an enhancement of ITF during MIS 5. Besides, the Maritime Continent was supposed to be more humid by pollen records from west Java and sediment composition from Halmahera Sea.

ITF,LIG,ENSO