Bipolar Oceanic Processes Drive Indonesian Throughflow Decline Under Climate Warming

Abstract Climate models consistently project a weakening Indonesian Throughflow (ITF) under future warming, but its driving mechanisms remain incompletely explained. Prior studies primarily attribute the ITF decline to changes in the Atlantic Meridional Overturning Circulation (AMOC) and Indo‐Pacific winds, but these two processes account for only a fraction of the simulated ITF weakening (50%–100% across models). Here we show that Southern Ocean surface warming and freshening explain this missing fraction. Enhanced dense‐to‐light water mass transformation in the Southern Ocean produces upper‐ocean volume convergence that propagates globally via wave dynamics, slowing the ITF in a manner analogous to the AMOC‐driven impacts. We further quantified the combined AMOC and Southern Ocean contributions and find that these bipolar processes explain, on average, 89% of the projected ITF decline in climate models. Our results reveal critical linkages between high‐latitude climate change and low‐latitude circulation, positioning the ITF as an indicator of global overturning adjustments.