Recent reduced abyssal overturning and ventilation in the Australian Antarctic Basin

Dense water formed near Antarctica, known as Antarctic bottom water (AABW), drives deep ocean circulation and supplies oxygen to the abyssal ocean. Observations show that AABW has freshened and contracted since the 1960s, yet the drivers of these changes and their impact remain uncertain. Here, using observations from the Australian Antarctic Basin, we show that AABW transport reduced by 4.0 Sv between 1994 and 2009, during a period of strong freshening on the continental shelf. An increase in shelf water salinity between 2009 and 2018, previously linked to transient climate variability, drove a partial recovery (2.2 Sv) of AABW transport. Over the full period (1994 to 2017), the net slowdown of −0.8 ± 0.5 Sv decade −1 thinned well-oxygenated layers, driving deoxygenation of −3 ± 2 μmol kg −1 decade −1 . These findings demonstrate that freshening of Antarctic shelf waters weakens the lower limb of the abyssal overturning circulation and reduces deep ocean oxygen content.