Imminent rapid decline of the Indonesian Throughflow after reaching a turning point of CO2 concentration.

The Indonesian Throughflow (ITF) represents the only tropical inter-ocean pathway of global ocean circulation and plays a central role in the Earth's climate system. Climate models suggest that the ITF would be weakened under global warming, but historical observations indicate a contradictory trend. Here using CMIP6 simulations, we find that the response of the ITF to increasing atmospheric carbon dioxide (CO2) concentration is nonlinear and exhibits an abrupt rapid decline after surpassing a turning point. The ITF's decline can be explained by the response of its halosteric and thermosteric components to rising CO2 levels associated with changes in winds and precipitation. The current level of CO2 concentration has reached the turning point, indicating that a rapid decline of ITF and a transition of the Indo-Pacific state is likely imminent despite the remaining uncertainties. These findings have implications for understanding climate tipping elements and global ocean circulation under global warming.