Antarctic Bottom Water formation and dynamics in a changing climate

Antarctic Bottom Water (AABW) is an important part of the climate system as it supplies the lower limb of the global overturning circulation. AABW is formed from dense waters on the Antarctic shelf which mix with surrounding waters while overflowing into the abyssal ocean. In recent decades, AABW has warmed, freshened, and declined in volume and AABW formation is also projected to decline in the future. The production and propagation of AABW and how these change are difficult to observe and numerical models still remain an important tool to investigate open questions.

For the first project, I used the ocean-sea ice model ACCESS-OM2-01 to investigate the interannual variability of AABW formation. The simulated formation and export of AABW exhibits strong interannual variability which is not correlated between the different formation regions. The main factor controlling years of high AABW formation are weaker upstream easterly winds, which reduce sea ice import into the AABW formation region, leaving increased areas of open water primed for air-sea buoyancy loss and convective overturning. This study highlights the variability of simulated AABW formation in all four formation regions, with potential implications for interpreting trends in observational data using only limited duration and coverage.

Modelling the formation and downslope flow of AABW represents an ongoing challenge for ocean and climate models due to the high horizontal resolution required. In my second project, we assessed the formation and export of AABW to the abyss and its sensitivity to horizontal model resolution in a circumpolar ocean-sea ice model available at horizontal resolutions of 1/10°, 1/20° and 1/40°. The AABW transport across the 1000 m isobath of the Antarctic continental slope increases by 27% with 1/20° resolution compared to 1/10°, but there is no further transport increase at 1/40° resolution. The higher AABW export at 1/20° compared to 1/10° resolution is due to formation of denser waters on the continental shelf and less diapycnal mixing during the downslope flow. This has effects downstream in the abyss of the Australian Antarctic Basin which is better ventilated in the 1/20° case.

Freshening of Antarctic shelf waters has occurred over the past five decades leading to a reduction of AABW volume. However, since the mid 2010s a rebound in salinity in the Ross Sea has been observed but the mechanisms have not yet been fully quantified. In my third project, we use the high-resolution ocean-sea ice model ACCESS-OM2-01 to isolate the effects of changes in winds and meltwater input on the salinity in the Ross Sea. Decreasing the zonal winds upstream of the Ross Sea by 50% or decreasing the meltwater input in the Amundsen Sea by 50% both increase the bottom salinity by ~0.07 psu in the western Ross Sea. Propagation of salinity anomalies into the Ross Sea occurs both via advection within 2-3 years and baroclinic waves within the first 2-3 months. Both decreasing the winds or decreasing the meltwater leads to a reduction of sea ice transport into the Ross Sea leaving increased areas of open water where dense shelf waters are formed.

Tags: Antarctic Bottom Water, Antarctic margin, global overturning circulation, ocean-sea ice modelling, physical oceanography, water masses
Organization: Australian Ocean Data Network