Antarctic Circumpolar Current slowdown precedes an abyssal overturning decline in response to added meltwater

Ice melt around Antarctica threatens to dramatically alter the Southern Ocean circulation, and thus disrupt the global redistribution of heat, freshwater, nutrients, and carbon. Past research has linked Antarctic ice melt to weakening of the Antarctic Circumpolar Current (ACC) and Antarctic Bottom Water (AABW) formation, but their relationship and regional timeline of response remain unclear. Here, we assess a high-resolution ocean projection applying future atmospheric and ice-melt forcing to quantify the response time scales of the ACC and AABW. We find that ACC weakening precedes AABW decline by ∼ 20 years, driven by different regional responses to meltwater. Along the West Antarctic sector west to Adélie Land, meltwater advected northward along near-surface isopycnals alters density gradients controlling ACC strength through a near-immediate barotropic response, while surface water mass transformation and AABW formation also decline rapidly. In contrast, global AABW decline lags because it is delayed by the dynamically stable Weddell and Prydz Bay sectors, despite surface freshening there. These results suggest ACC strength may serve as an early sign of future Southern Ocean overturning change. Plain Language Summary The ocean surrounding Antarctica is vital to our climate system. Vast quantities of water sink into the abyss via the overturning circulation, carrying dissolved carbon, oxygen, and anthropogenic heat into the deep ocean. The world’s strongest current, the Antarctic Circumpolar Current (ACC), circles Antarctica, isolating cold waters to the south from the rest of the ocean. Recent studies show that both the overturning circulation and the ACC may slow due to Antarctic ice melt, but the connections or timeline between these processes remains unexplored. Here, we assess a high-resolution ocean simulation forced by 50 years of future climate change including Antarctic meltwater. We find the ACC declines a full two decades before a global overturning slowdown is detected, because the ACC responds quickly to meltwater-induced density changes, while the global overturning signal only emerges once all sectors decline. This delay is driven by the Weddell and Prydz Bay sectors, which show little slowdown until the simulation’s final years, despite some local freshening. The West Antarctic sector west to the Adélie, by contrast, efficiently channel meltwater to the ACC and display a faster abyssal decline. Assessing these regional contributions is essential to understanding the Southern Ocean’s future response to climate change, and measuring the ACC may provide an early warning sign of impending overturning decline.