East-west contrasting changes in Southern Indian Ocean Antarctic Bottom Water salinity over three decades.

Antarctic Bottom Water (AABW) characteristics, derived from multiple water sources with various properties, are significantly affected by and contribute to climate change. However, the underlying causes of changes in AABW characteristics are not well-understood. In this study, we aimed to analyse the east-west contrasting pattern of AABW characteristics in the Southern Indian Ocean (SIO) over the last three decades. We show that AABW has become warmer and more saline in the western SIO (WSIO) but warmer and fresher in the eastern SIO (ESIO) in 2010s than in 1990s. The warming and salinification of WSIO AABW are primarily explained by changes in source water mixing ratios, although the source water properties also significantly contribute to the observed changes. In contrast, the warming and freshening of ESIO AABW cannot be explained without considering changes in the source water properties as the direction of AABW salinity change due to source water mixing ratios is opposite (salinification) to that of observations (freshening). The east-west contrasting pattern of AABW salinity changes and more rapid warming in the ESIO have important consequences for poleward AABW transport and sea-level rise within and beyond the SIO.