FY 2024

Antarctic bottom water warming, freshening, and contraction in the eastern Bellingshausen Basin

Johnson, G.C., A.K.M. Sadman Mahmud, A.M. Macdonald, and B.S. Twining

Geophys. Res. Lett., 51(13), e2024GL109937, doi: 10.1029/2024GL109937, View open access article at AGU/Wiley (external link) (2024)

Antarctic Bottom Water has been warming in recent decades throughout most of the oceans and freshening in regions close to its Indian and Pacific sector sources. We assess warming rates on isobars in the eastern Pacific sector of the Southern Ocean using CTD data collected from shipboard surveys from the early 1990s through the late 2010s together with CTD data collected from Deep Argo floats deployed in the region in January 2023. We show cooling and freshening in the temperature-salinity relation for water colder than ∼0.4°C. We further find a recent acceleration in the regional bottom water warming rate vertically averaged for pressures exceeding 3,700 dbar, with the 2017/18 to 2023/24 trend of 7.5 (±0.9) m°C yr⁻¹ nearly triple the 1992/95 to 2023/24 trend of 2.8 (±0.2) m°C yr⁻¹. The 0.2°C isotherm descent rate for these same time periods nearly quadruples from 7.8 to 28 m yr⁻¹.

Plain Language Summary. Cold winds blowing over polynyas (areas of ice-free water) on the Antarctic continental shelf create sea ice, forming very cold and somewhat salty, hence very dense, waters. These dense shelf waters descend the continental slope to the abyss, mixing with adjacent waters to form Antarctic Bottom Water (AABW). AABW spreads northward from there, filling much of the global abyssal ocean as it mixes with warmer, lighter waters above. AABW has been warming on pressure surfaces, freshening and cooling on density surfaces, and reducing in volume (contracting). These changes are likely a result of melting Antarctic ice sheets, which freshen the shelf waters, making them less dense, hence less able to sink to the bottom. We compare profiles of ocean temperature and salinity in the eastern Pacific sector of the Southern Ocean collected in 2023 and 2024 by robotic freely drifting profilers to data collected from ships from the early 1990s to the late 2010s. We find all of the above listed changes, but also acceleration of the warming, with the rate from 2017/18 to 2023/24 being nearly triple the rate from 1992/95 to 2023/24. The contraction rate has nearly quadrupled. This acceleration has been predicted by high-resolution climate model simulations.