Abstract

Antarctic Bottom Water (AABW) supplies the lower limb of the overturning circulation and ventilates the abyssal ocean. Observations show AABW has warmed, freshened and contracted. However, the drivers of these changes and their impact on abyssal circulation and ventilation are poorly understood. Here we analyse observed changes in AABW properties and transport in the well-ventilated Australian Antarctic Basin to assess the causes and consequences of abyssal ocean change. Freshening of dense shelf waters reduced the volume transport of AABW by -1.5±0.5 Sv/decade between 1994 and 2009. Decreased AABW export reduced oxygen transport into the abyssal ocean and caused well-ventilated layers to thin, driving deoxygenation of 3 μmol/kg/decade. AABW changes of opposite sign were observed when shelf water salinity increased between 2014 and 2018, confirming the link between shelf water salinity, AABW formation, and abyssal ventilation. These findings suggest that multi-decadal freshening of shelf waters, which has been attributed to increased glacial melt, has driven a marked slowdown of the deep overturning circulation and rapid deoxygenation of the abyssal ocean.