Antarctic Bottom Water in a changing climate

Antarctic Bottom Water (AABW) is derived from dense water that sinks from the Antarctic continental shelf to the deep ocean. The sinking of AABW is balanced by a return flow of lighter water, and the resulting overturning circulation determines the density stratification of the deep ocean, regulates ocean storage of heat and carbon, and supplies oxygen to the deep sea. In this Review, we highlight progress in understanding how and why AABW is changing and the consequences for the deep overturning circulation. Since the mid-1980s, ocean heat content below 4,000 dbar has increased at a rate of 12.9 (±1.8) trillion watts, and the AABW has thinned by more than 50 dbar decade−1, with more rapid thinning observed closer to the sources of AABW. The abyssal overturning circulation has slowed in response to freshening of shelf waters by glacial melt and changes in sea ice formation. Numerical model simulations indicate that these trends will accelerate under projected increases in meltwater input. Future research priorities include sustained observations in the deep ocean and on the Antarctic continental shelf; exploration of feedbacks between ocean circulation, sea ice, dense water formation and ice shelf melt; and improved representation of AABW in ocean and climate models.