Current status of ocean observation, ensemble reanalysis and CMIP6 models in describing Antarctic Bottom Water

Abstract. Antarctic Bottom Water (AABW), a key component of the global meridional overturning circulation, forms in distinct regions around Antarctic coasts. The properties of AABW from different sources can be distinguished by their temperature and salinity properties. This study proposes a classification scheme to divide AABW into three subtypes based on their properties and geographic regions: Weddell Sea and Prydz Bay-originated AABW (WPBW), Ross Sea-originated AABW (RSBW), and Adélie Land-originated AABW (ALBW). These three subtypes are clearly identified in the World Ocean Atlas 2023 (WOA23) dataset. Taking WOA23 as a benchmark, we then systematically evaluate the ability of other datasets, including ensemble ocean reanalysis and CMIP6 models, to represent the spatial distribution and thermohaline properties of these AABW subtypes. Results indicate that the ensemble reanalysis performs well, albeit with an overestimation of ALBW. Nearly all CMIP6 models fail to capture these AABW subtypes, except after the correction of systematic temperature and salinity biases. Notably, CESM1-CAM5-SE-LR is the only model reproducing the three AABW subtypes without the correction, likely due to that the implemented overflow parameterization fits tuned physical processes. Comparison with its high resolution counterpart indicates that increased model resolution may not necessarily improve AABW simulations. This refined classification framework, along with the evaluation of various datasets, provides a foundation for more detailed investigations into the AABW formation, variability, and trends under climate change.