Persistent Warming and Reduced Freshening of the Abyssal Southern Ocean

The coldest, densest water on Earth forms around Antarctica and fills the bottom of the global ocean. Over the past 40 years, observations have shown that this deep water layer is shrinking, warming, and becoming less salty (freshening). We combine historical ship-based data (1985–2024) with ocean models to understand these changes. We find that the deep layer is thinning by about 95 m per decade, allowing warmer water from above to take its place. The freshening occurs because meltwater from Antarctica's ice shelves mixes into the source waters before they sink. Model projections that include meltwater input reproduce the observed trends and show that thinning and warming continue as ice melt increases. However, we find a surprising shift: freshening of the deep ocean slows down and, in some areas, even reverses. Ongoing meltwater input makes the source waters formed on the continental shelf too fresh and light to sink to the deep ocean as before, weakening the shelf-to-deep-ocean connection. This disruption accelerates deep-ocean warming and increases the abyssal ocean's contribution to future sea level rise.