Intensification of the Indonesian Throughflow in a Coupled GCM During the Last Interglacial

       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. In the context of global warming in the 21^st century, The Indonesian Throughflow are projected to be weaken (medium confidence) by CMIP6 simulations. As an analog of possible future warming, the Last Interglacial (LIG, Marine Isotope Stage 5e or Eemian), with global surface temperature reached about 2 °C above present, serves as an outstanding period to explore the climate response to the external forcing and the mechanisms behind it.

       We use the 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). During the LIG, the tropical western Pacific Ocean thermocline was deepened while the tropical eastern Indian Ocean thermocline was relatively shallowed, which was closely tied to the strengthening of the surface easterlies above the tropical western Pacific. Correspondingly, the gradient of the sea surface height between the tropical western Pacific and the tropical eastern Indian Ocean increased, causing pressure contrast between the two basins and probably contribute to the ITF strengthening. We also find that the thermocline gradient between the tropical western Pacific and tropical eastern Pacific was increased, suggesting a La Niña-like state during the LIG. Comparisons of models and proxies further support our conclusions. An examination of the changes in the thermocline water temperature (TWT) 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.

       Further analysis suggested that the strengthened ITF during the LIG is inconsistent with the weakened one in the 21^st century. While the future global warming is primarily driven by increased CO₂ levels, the climate changes during the LIG were principally caused by changes in orbital parameters.