A declining ventilation rate of Antarctic Bottom Water within the Ross Gyre

The global abyssal ocean has warmed significantly over the past two decades, possibly linked to a decrease in the production rate of Antarctic Bottom Water (AABW), the cold dense water mass formed around Antarctica that feeds most of the global abyssal ocean. Here we use chlorofluorocarbons (CFCs), sulfur hexafluoride (SF6) and conservative tracers from repeat S4P sections along 67 S to quantify a decrease in the ventilation rate of Antarctic Bottom Water (AABW) in the Pacific sector of the Southern Ocean, and show the relative mass fractions of its end members has remained the same. Regionally generated AABW is a combination of cold dense Shelf Water (SW) formed over the continental shelf during sea ice production and partially modified by melting and freezing under the Ross Ice Shelf, with warmer Circumpolar Deep Water (CDW) entrained during flow down and along the continental slope. An optimum multi-parameter analysis shows no significant change in the relative fraction of these two end members between 1992 and 2011, suggesting their mixing processes have not changed over the past 20 years. However, an apparent shift in the relative contributions of higher salinity SW from the western Ross Sea and lower salinity SWs from the central and eastern Ross Sea is seen in the newest formed AABW found in the westward flowing boundary current at Cape Adare and within the interior of the Ross Gyre. CFC and SF6 concentration histories in the SW show its component within the Ross Gyre was 5-10 years older in 2011 than in 1992. The OMP and CFC analyses thus provide strong evidence that the rate of ventilation and circulation within the abyssal Pacific sector of the Southern Ocean have significantly decreased over the past 20 years.