Antarctic Bottom Water (AABW) export pathways across the Southern Ocean

The Antarctic Bottom Water (AABW) is the densest water mass of the global ocean that covers about 30 ~ 40% of the global ocean volume with temperatures ranging from -0.8 to 2°C and salinities from 34.6 to 34.7 psu. It forms around Antarctica and spreads into the Southern Ocean and then into the Atlantic, Indian, and Pacific Oceans, feeding the lower cell of the global meridional overturning circulation. Repeated hydrographic sections across the Southern Ocean showed that a significant contraction of the AABW volume occurred between the 1980s and 2000s with a freshening and warming of AABW observed throughout the Southern Ocean. This is consistent with the future projected slowdown of the AABW formation in response to increasing Antarctic meltwater discharge and decreasing sea-ice formation. As a result, the deep ocean carbon storage rate is projected to decrease throughout the 21st century, which is consistent with a decrease in the observed column-integrated storage rate of anthropogenic CO₂ in the Weddell Sea during 2004-2014 compared to 1994-2004.

Using a global ocean and sea-ice model with a horizontal resolution between ~11.5 km near the equator, and ~4 km near the Antarctic shelves, Solodoch et al. (2022) explored the export pathways of AABW across the Southern Ocean and the degree to which AABW originating from four distinct source regions, namely Weddell Sea, Ross Sea, Adélie Coast, and Prydz Bay. Two distinct export “conduits” are identified. Specifically, Weddell Sea- and Prydz Bay-sourced AABW are blended together and exported mainly to the Atlantic and Indian Oceans north of 30°S, while Ross Sea- and Adelie Coast-sourced AABW are exported mainly to the Pacific Ocean north of 30°S. It takes a minimum of 10 ~ 15 years for the tracers released in the source regions to cross 30°S, and about 30 ~ 40 years to have the AABW formation signal fully spread into the Atlantic, Indian, and Pacific Oceans north of 30°S. These findings imply that regional changes in the AABW formation may impact the three-dimensional structure of the global meridional overturning circulation, and thus impact the deep ocean heat and carbon storage in the Atlantic, Indian, and Pacific Oceans, differently.

Figure 1 in Solodoch et al. (2022). Snapshot of Antarctic bottom water tracer concentrations after 61 years of model integration. Tracer concentrations are evaluated at the seafloor-adjacent model cell, in regions where the density σ₂ ≥ 37.125 kg m^-3, that is, within the density range of northward flow of the Southern Ocean abyssal meridional overturning circulation branch: (a) Weddell Sea tracer, (b) Prydz Bay tracer, (c) Ross Sea tracer, (d) Adelie Coast tracer. Tracer source region masks are shown in red in each panel. Gray contours show the 1 and 3-km isobaths. Acronyms in panel (c) denote Drake Passage (DP), the Mid-Atlantic-Ridge (MAR), Walvis Ridge (WR), Southwest Indian Ridge (SWIR), Kerguelen Plateau (KP), Campbell Plateau (CP), and the Pacific-Antarctic Ridge (PAR).

Solodoch, A., Stewart, A. L., Hogg, A. M., Morrison, A. K., Kiss, A. E., Thompson, A. F., et al. (2022). How does Antarctic Bottom Water cross the Southern Ocean? Geophysical Research Letters, 49, e2021GL097211. https://doi.org/10.1029/2021GL097211